Positive electrode active material for non-aqueous electrolyte secondary battery and method for producing same

Abstract

Provided is a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery that makes it possible to configure a non-aqueous electrolyte secondary battery having excellent output characteristics. The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery includes: preparing a lithium transition metal composite oxide having a layered structure, in which the ratio D50 / DSEM of the 50% particle size D50 of the cumulative particle size distribution based on volume to the average particle size DSEM based on electron microscope observation is 1-4 inclusive, and the ratio of the number of mol of nickel is 0.3 to less than 1 and the ratio of the number of mol of cobalt is 0 to less than 0.5 relative to the total number of mol of metals other than lithium; bringing the lithium transition metal composite oxide into contact with a cobalt compound to obtain an accretion; and heat treating the accretion at a temperature of more than 700°C to less than 1100°C to obtain a heat treatment product.

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a method for producing the same. [Background technology]

[0002] High output power is required for electrode active materials for nonaqueous electrolyte secondary batteries used in large power equipment such as electric vehicles. A positive electrode active material having a secondary particle structure in which many primary particles aggregate is considered effective for achieving high output power. However, in such positive electrode active materials, cracks may occur in the secondary particles due to pressure treatment during electrode formation, expansion and contraction during charge and discharge, and the like. In relation to this, a method for producing a positive electrode active material containing lithium transition metal composite oxide particles in which the number of primary particles constituting a single particle or one secondary particle is reduced has been proposed (see, for example, JP 2017-188443 A).

[0003] On the other hand, a technology has been proposed in which a lithium transition metal composite oxide containing nickel is used as a core material and is coated with a lithium transition metal composite oxide containing cobalt, which is said to improve stability while maintaining capacity characteristics (see, for example, JP 2006-199229 A). Summary of the Invention [Problem to be solved by the invention]

[0004] An object of one aspect of the present invention is to provide a positive electrode active material for a non-aqueous electrolyte secondary battery that can be used to construct a non-aqueous electrolyte secondary battery having excellent output characteristics, and a method for producing the same. [Means for solving the problem]

[0005] A first aspect of the present invention is a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery. This method is based on electron microscopic observation and is characterized in that the average particle diameter D SEM 50% particle size D of cumulative particle size distribution based on volume 50 Ratio of D 50 / D SEMthe ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.3 or more and less than 1, and the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is 0 or more and less than 0.5; contacting the lithium transition metal composite oxide with a cobalt compound to obtain a deposit; and heat-treating the deposit at a temperature greater than 700°C and less than 1100°C to obtain a heat-treated product.

[0006] A second aspect of the present invention is a positive electrode active material for a non-aqueous electrolyte secondary battery. This positive electrode active material has an average particle diameter D SEM 50% particle size D of cumulative particle size distribution based on volume 50 Ratio of D 50 / D SEM The lithium transition metal composite oxide constituting the positive electrode active material includes a lithium transition metal composite oxide having a composition in which the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 1 or more and less than 4, the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.3 or more and less than 1, and the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is 0.01 or more and less than 0.5. In the lithium transition metal composite oxide constituting the positive electrode active material, the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.2 or more in a first region that is 500 nm deep from the particle surface and is 0.06 or more in a second region that is 10 nm or less deep from the particle surface, and the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is greater in the second region than in the first region. [Effects of the Invention]

[0007] According to one aspect of the present invention, it is possible to provide a positive electrode active material for a non-aqueous electrolyte secondary battery that can constitute a non-aqueous electrolyte secondary battery having excellent output characteristics, and a method for producing the same. [Brief explanation of the drawings]

[0008] [Figure 1] 2 is an example of a scanning electron microscope (SEM) image of the positive electrode active material according to Example 1. [Figure 2] 10 is an example of an SEM image of a positive electrode active material according to Comparative Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0009] In this specification, the term "process" refers not only to an independent process, but also to a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved. Furthermore, the content of each component in a composition refers to the total amount of the multiple substances present in the composition, unless otherwise specified, when multiple substances corresponding to each component are present in the composition. Hereinafter, embodiments of the present invention will be described in detail. However, the embodiments described below are intended to exemplify a positive electrode active material for a non-aqueous electrolyte secondary battery and a method for producing the same, in order to embody the technical concept of the present invention. The present invention is not limited to the positive electrode active material for a non-aqueous electrolyte secondary battery and a method for producing the same described below.

[0010] Method for producing positive electrode active material for non-aqueous electrolyte secondary battery The method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery (hereinafter also simply referred to as a positive electrode active material) is to measure the average particle diameter D SEM 50% particle size D of cumulative particle size distribution based on volume 50 Ratio of D 50 / D SEM The method includes a preparation step of preparing a lithium transition metal composite oxide having a layered structure in which the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.3 or more and less than 1, and the ratio of the number of moles of cobalt is 0 or more and less than 0.5; an attachment step of contacting the prepared lithium transition metal composite oxide with a cobalt compound to obtain a deposit having the cobalt compound attached thereto; and a heat treatment step of heat treating the obtained deposit at a temperature of more than 700°C and less than 1100°C to obtain a heat-treated product.

[0011] D 50 / D SEMA positive electrode active material that can achieve excellent output characteristics in a non-aqueous electrolyte secondary battery is produced by attaching a cobalt compound to a single particle or a lithium transition metal composite oxide particle having a small number of primary particles constituting one secondary particle (hereinafter collectively referred to as "single particle"), where the number of primary particles is between 1 and 4, and then heat-treating the particle at a specific temperature. This can be attributed to the presence of a high concentration of cobalt near the surface of the lithium transition metal composite oxide particle, for example, due to the cobalt compound attached to the surface.

[0012] Preparation process In the preparation process, D 50 / D SEM The lithium transition metal composite oxide has a layered structure, and the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.3 or more and less than 1, and the ratio of the number of moles of cobalt is 0 or more and less than 0.5. The lithium transition metal composite oxide contains at least lithium, nickel, and cobalt, and may further contain at least one metal element selected from the group consisting of manganese, aluminum, and the like. The lithium transition metal composite oxide may be appropriately selected from commercially available products, or may be prepared by manufacturing a lithium transition metal composite oxide having the desired composition and structure.

[0013] The ratio of the number of moles of nickel to the total number of moles of metals other than lithium in the lithium transition metal composite oxide prepared in the preparation step is, for example, 0.3 or more and less than 1. The lower limit of the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is preferably 0.31 or more, and more preferably 0.32 or more. The upper limit of the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is preferably 0.98 or less, more preferably 0.8 or less, and particularly preferably 0.6 or less. When the mole ratio of nickel is within the above-mentioned range, it is possible to achieve both good charge / discharge capacity at high voltage and good cycle characteristics in a nonaqueous electrolyte secondary battery.

[0014] The ratio of the number of moles of cobalt to the total number of moles of metals other than lithium in the lithium transition metal composite oxide prepared in the preparation step is, for example, 0 or more and less than 0.5, and from the viewpoint of charge / discharge capacity, is preferably 0.15 or more and 0.45 or less, and more preferably 0.3 or more and 0.4 or less.

[0015] The lithium transition metal composite oxide prepared in the preparation step contains at least one metal element M selected from the group consisting of manganese and aluminum. 1 The lithium transition metal composite oxide may further contain a metal element M. 1 When including lithium, M relative to the total number of moles of metals other than lithium 1 The molar ratio is, for example, 0 or more and less than 0.5, and from the viewpoint of safety, is preferably 0.15 or more and 0.45 or less, and more preferably 0.3 or more and 0.4 or less.

[0016] The lithium transition metal composite oxide prepared in the preparation step contains at least one metal element M selected from the group consisting of boron, sodium, magnesium, silicon, phosphorus, sulfur, potassium, calcium, titanium, vanadium, chromium, zinc, strontium, yttrium, zirconium, niobium, molybdenum, indium, tin, barium, lanthanum, cerium, neodymium, samarium, europium, gadolinium, tantalum, tungsten, bismuth, etc. 2 The lithium transition metal composite oxide may further contain a metal element M. 2 When including lithium, M relative to the total number of moles of metals other than lithium 2 The ratio of the number of moles of is, for example, 0 or more and 0.1 or less, and preferably 0.001 or more and 0.05 or less.

[0017] The ratio of the number of moles of lithium to the total number of moles of metals other than lithium in the lithium transition metal composite oxide prepared in the preparation step is, for example, 0.95 or more and 1.5 or less, and preferably 1 or more and 1.3 or less.

[0018] When the lithium transition metal composite oxide prepared in the preparation step contains manganese in addition to nickel and cobalt, the molar ratio of nickel, cobalt, and manganese is, for example, nickel:cobalt:manganese=(0.3 to 0.95):(0 to 0.5):(0 to 0.5), preferably (0.3 to 0.6):(0.15 to 0.45):(0.15 to 0.45), and more preferably (0.3 to 0.4):(0.3 to 0.4):(0.3 to 0.4).

[0019] The composition of the lithium transition metal composite oxide prepared in the preparation step may be, for example, a composition represented by the following formula (1): Here, the composition of the lithium transition metal composite oxide refers to the composition of the lithium transition metal composite oxide as a whole. Li p Ni x Co y M 1 z M 2 w O2(1) 0.95≦p≦1.5, 0.3≦x<1, 0≦y<0.5, 0≦z<0.5, 0≦w≦0.1, x+y+z+w≦1, M 1 is at least one selected from the group consisting of Al and Mn, 2 is at least one element selected from the group consisting of B, Na, Mg, Si, P, S, K, Ca, Ti, V, Cr, Zn, Sr, Y, Zr, Nb, Mo, In, Sn, Ba, La, Ce, Nd, Sm, Eu, Gd, Ta, W, and Bi. x+y+z+w may be 0.9≦x+y+z+w.

[0020] The lithium transition metal composite oxide prepared in the preparation step may be in the form of a so-called single particle, which is composed of, for example, four or less primary particles. The lithium transition metal composite oxide has a 50% particle size D in the cumulative particle size distribution based on volume. 50 Average particle size D based on electron microscope (SEM) observation SEM Ratio to D 50 / D SEM may be 1 or more and 4 or less.

[0021] In the lithium transition metal composite oxide prepared in the preparation step, D 50 / D SEM When D is 1, it indicates a single particle, and the closer it is to 1, the fewer the number of primary particles that make up the particle. 50 / D SEM is preferably 1 or more and 4 or less from the viewpoint of durability, and is preferably 3.5 or less, more preferably 3 or less, even more preferably 2.5 or less, and particularly preferably 2 or less from the viewpoint of output density.

[0022] The lithium transition metal composite oxide prepared in the preparation step has an average particle size D SEM From the viewpoint of durability, it is, for example, 0.1 μm or more and 20 μm or less, and from the viewpoint of power density and electrode plate packing, it is preferably 0.3 μm or more, more preferably 0.5 μm or more, and is preferably 15 μm or less, more preferably 10 μm or less, even more preferably 8 μm or less, and particularly preferably 5 μm or less.

[0023] Average particle size D based on electron microscope observation SEM is the average spherical equivalent diameter of primary particles measured from scanning electron microscope (SEM) images. SEM Specifically, is calculated as follows: Observation is performed using a scanning electron microscope at a magnification ranging from 1000x to 10000x depending on the particle size. 100 primary particles whose particle outlines can be confirmed are selected. Image processing software is used to trace the outlines of the selected primary particles to determine the outline length. The spherical equivalent diameter is calculated from the outline length, and the average particle size D is determined as the arithmetic mean of the obtained spherical equivalent diameters. SEM is required.

[0024] In addition, the 50% particle size D of the lithium transition metal composite oxide prepared in the preparation process 50 is, for example, 1 μm or more and 30 μm or less, preferably 1.5 μm or more, more preferably 3 μm or more, and from the viewpoint of power density, preferably 10 μm or less, more preferably 5.5 μm or less.

[0025] 50% particle size D 50is determined as the particle size corresponding to the cumulative 50% from the small diameter side in the cumulative particle size distribution on a volume basis measured under wet conditions using a laser diffraction particle size distribution analyzer. Similarly, the 90% particle size D 90 and 10% particle size D 10 are calculated as the particle diameters corresponding to 90% and 10% cumulatively from the small diameter side, respectively.

[0026] The 90% particle size D in the cumulative particle size distribution based on the volume of the lithium transition metal composite oxide prepared in the preparation process 90 10% particle size D 10 The ratio of D to D indicates the spread of particle size distribution, and the smaller the value, the more uniform the particle size. 90 / D 10 may be, for example, 4 or less, and from the viewpoint of power density, is preferably 3 or less, and more preferably 2.5 or less. 90 / D 10 The lower limit may be, for example, 1.2 or more.

[0027] D prepared in the preparation process 50 / D SEM For lithium transition metal composite oxides in which is 1 or more and 4 or less, reference can be made to, for example, JP 2017-188443 A (U.S. Patent Publication No. 2017-0288221), JP 2017-188444 A (U.S. Patent Publication No. 2017-0288222), JP 2017-188445 A (U.S. Patent Publication No. 2017-0288223), etc.

[0028] The lithium transition metal composite oxide prepared in the preparation step contains nickel in its composition. From the viewpoint of initial efficiency in nonaqueous electrolyte secondary batteries, the lithium transition metal composite oxide preferably has a nickel disorder of 4.0% or less, more preferably 2.0% or less, and even more preferably 1.5% or less, as determined by X-ray diffraction. Here, the disorder of nickel refers to a chemical disorder in the transition metal ions (nickel ions) that should occupy their original sites. In lithium transition metal composite oxides with a layered structure, this is typically represented by an interchange between an alkali metal ion that should occupy a site represented by 3b in the Wyckoff notation (3b site, hereinafter the same) and a transition metal ion that should occupy a 3a site. The smaller the disorder of nickel, the better, since this improves initial efficiency.

[0029] The disorder of nickel element in lithium transition metal composite oxide can be determined by X-ray diffraction. For the lithium transition metal composite oxide, X-ray diffraction spectrum is measured using CuKα radiation. The composition model is (Li 1-d Ni d )(Ni x Co y Mn z )O2(x+y+z=1), and based on the obtained X-ray diffraction spectrum, perform structural optimization by Rietveld analysis. The percentage of d calculated as a result of structural optimization is taken as the value of disorder of the nickel element.

[0030] Specifically, the lithium transition metal composite oxide prepared in the preparation step can be prepared as follows: The method for preparing a lithium transition metal composite oxide may include, for example, a precursor preparation step of preparing a precursor, and a synthesis step of synthesizing a lithium transition metal composite oxide from the precursor and a lithium compound.

[0031] In the precursor preparation step, a precursor containing a composite oxide containing nickel and cobalt (hereinafter simply referred to as composite oxide) is prepared. The precursor may be prepared by appropriately selecting from commercially available products, or by preparing a composite oxide having the desired composition using a conventional method. Methods for obtaining a composite oxide having a desired composition include a method in which raw material compounds (hydroxides, carbonates, etc.) are mixed according to the target composition and decomposed into a composite oxide by heat treatment, and a coprecipitation method in which solvent-soluble raw material compounds are dissolved in a solvent, and precipitates having the target composition are obtained by adjusting the temperature, pH, adding a complexing agent, etc., and the precipitates are then heat-treated to obtain the composite oxide. An example of a method for producing a composite oxide is described below.

[0032] A method for obtaining a composite oxide by coprecipitation can include a seed generation step of adjusting the pH of a mixed solution containing metal ions in a desired composition ratio to obtain seed crystals, a crystallization step of growing the generated seed crystals to obtain a composite hydroxide having desired properties, and a step of heat-treating the obtained composite hydroxide to obtain a composite oxide. For details of the method for obtaining such a composite oxide, see, for example, JP 2003-292322 A and JP 2011-116580 A (U.S. Patent Publication No. 2012-270107 A).

[0033] In the seed generation process, a liquid medium containing seed crystals is prepared by adjusting the pH of a mixed solution containing nickel ions and cobalt ions in a desired composition ratio to, for example, 11 to 13. The seed crystals may contain, for example, a hydroxide containing nickel and cobalt in a desired ratio. The mixed solution can be prepared by dissolving a nickel salt and a cobalt salt in water in a desired ratio. Examples of nickel salts and cobalt salts include sulfates, nitrates, and hydrochlorides. In addition to the nickel salt and the cobalt salt, the mixed solution may also contain other metal salts in a desired composition ratio as needed. The temperature in the seed generation process can be, for example, 40°C to 80°C. The atmosphere in the seed generation process can be a low-oxidizing atmosphere, and for example, the oxygen concentration can be maintained at 10% by volume or less.

[0034] In the crystallization step, the generated seed crystals are grown to obtain a precipitate containing nickel and cobalt with desired properties. The seed crystals can be grown, for example, by adding a mixed solution containing nickel ions and cobalt ions, and optionally other metal ions, to a liquid medium containing the seed crystals while maintaining the pH at, for example, 7 to 12.5, preferably 7.5 to 12. The time for adding the mixed solution is, for example, 1 to 24 hours, preferably 3 to 18 hours. The temperature in the crystallization step can be, for example, 40 to 80°C. The atmosphere in the crystallization step is the same as that in the seed generation step. The pH in the seed generation step and crystallization step can be adjusted using an acidic aqueous solution such as an aqueous sulfuric acid solution or an aqueous nitric acid solution, or an alkaline aqueous solution such as an aqueous sodium hydroxide solution or aqueous ammonia.

[0035] In the step of obtaining a composite oxide, the composite hydroxide-containing precipitate obtained in the crystallization step is heat-treated to obtain the composite oxide. The heat treatment in the step of obtaining a composite oxide can be carried out by heating the composite hydroxide at a temperature of, for example, 500°C or lower, preferably 350°C or lower. The heat treatment temperature can be, for example, 100°C or higher, preferably 200°C or higher, and the heat treatment time can be, for example, 0.5 to 48 hours, preferably 5 to 24 hours. The heat treatment atmosphere can be either air or an oxygen-containing atmosphere. The heat treatment can be carried out using, for example, a box furnace, a rotary kiln furnace, a pusher furnace, a roller hearth kiln furnace, or the like.

[0036] The resulting composite oxide contains nickel and cobalt as well as other metal elements M 1 Other metal elements M 1 Examples of the ions include Mn and Al, and at least one selected from the group consisting of these is preferred, and it is more preferred that the ions contain at least Mn. When the composite oxide contains other metals, the other metal ions may be added in a desired composition to the mixed aqueous solution from which the precipitate containing nickel and cobalt is obtained. This allows the precipitate to contain nickel, cobalt, and other metals, and the precipitate can be heat-treated to obtain a composite oxide having the desired composition.

[0037] The average particle size of the composite oxide is, for example, 2 μm to 30 μm, preferably 3 μm to 25 μm. The average particle size of the composite oxide is a volume-average particle size, which is the value at which the volume-integrated value from the small particle size side in the volume-based particle size distribution obtained by a laser scattering method becomes 50%.

[0038] In the synthesis step, a lithium-containing mixture obtained by mixing the composite oxide with a lithium compound is heat-treated to obtain a heat-treated product. The heat-treated product has a layered structure and includes a lithium transition metal composite oxide containing nickel and cobalt.

[0039] Examples of the lithium compound to be mixed with the composite oxide include lithium hydroxide, lithium carbonate, lithium oxide, etc. The particle size of the lithium compound to be mixed is, as the 50% average particle size of the cumulative particle size distribution on a volume basis, for example, 0.1 μm to 100 μm, and preferably 2 μm to 20 μm.

[0040] The ratio of the total number of moles of lithium to the total number of moles of metal elements constituting the composite oxide in the mixture is, for example, 0.95 to 1.5. The composite oxide and the lithium compound can be mixed using, for example, a high-speed shear mixer.

[0041] The mixture contains no other metal elements M other than lithium, nickel and cobalt. 2 It may further contain other metal elements M 2 Examples of the metals include B, Na, Mg, Si, P, S, K, Ca, Ti, V, Cr, Zn, Sr, Y, Zr, Nb, Mo, In, Sn, Ba, La, Ce, Nd, Sm, Eu, Gd, Ta, W, and Bi, with at least one selected from the group consisting of these being preferred. When the mixture contains other metals, the mixture can be obtained by mixing the other metals as simple substances or metal compounds with the composite oxide and the lithium compound. Examples of metal compounds containing other metals include oxides, hydroxides, chlorides, nitrides, carbonates, sulfates, nitrates, acetates, and oxalates.

[0042] When the mixture contains other metals, the ratio of the total number of moles of the metal elements constituting the composite oxide to the total number of moles of the other metals is, for example, 1:0.015 to 1:0.1, and preferably 1:0.025 to 1:0.05.

[0043] The heat treatment temperature of the mixture is, for example, 550°C to 1100°C, preferably 600°C to 1080°C, and more preferably 700°C to 1080°C. The heat treatment of the mixture may be performed at a single temperature, but is preferably performed at multiple temperatures in terms of discharge capacity at high voltage. When heat treatment is performed at multiple temperatures, for example, it is desirable to hold the first temperature for a predetermined time, then further increase the temperature, and hold the second temperature for a predetermined time. The first temperature is, for example, 850°C to 950°C, preferably 900°C to 940°C. The second temperature is, for example, 980°C to 1100°C, preferably 1000°C to 1080°C. The difference between the first and second temperatures is, for example, 30°C or more, preferably 100°C or more, and, for example, 250°C or less, preferably 180°C or less.

[0044] When heat treatment is performed at a single temperature, the heat treatment time is, for example, 1 hour to 20 hours, preferably 5 hours to 10 hours. When heat treatment is performed at multiple temperatures, the heat treatment time at the first temperature is, for example, 1 hour to 20 hours, preferably 5 hours to 10 hours. The heat treatment time at the second temperature is, for example, 1 hour to 20 hours, preferably 2 hours to 10 hours. The heat treatment time at the first temperature and the heat treatment time at the second temperature may be the same or different. When the heat treatment time at the first temperature and the heat treatment time at the second temperature are different, for example, the heat treatment time at the first temperature can be longer than the heat treatment time at the second temperature. Specifically, for example, the heat treatment time at the second temperature can be 1.05 to 2 times the heat treatment time at the first temperature, preferably 1.1 to 1.5 times. Here, the first heat treatment and the second heat treatment may be performed consecutively or independently. When the first heat treatment and the second heat treatment are performed consecutively, the temperature rise rate from the first temperature to the second temperature can be, for example, 5°C / min.

[0045] The heat treatment may be carried out in air or in an oxygen-containing atmosphere using, for example, a box furnace, a rotary kiln, a pusher furnace, a roller hearth kiln, or the like.

[0046] The heat-treated product is subjected to a dispersion treatment as needed. By dissociating the sintered primary particles through a dispersion treatment, rather than a pulverization treatment involving strong shearing force or impact, lithium transition metal composite oxide particles with a narrow particle size distribution and uniform particle size can be obtained. The dispersion treatment may be carried out in a dry or wet manner, with a dry method being preferred. The dispersion treatment can be carried out using, for example, a ball mill, a jet mill, or the like. The conditions for the dispersion treatment are, for example, the D of the lithium transition metal composite oxide particles after the dispersion treatment. 50 / D SEM can be set to be in a desired range, for example, 1 or more and 4 or less.

[0047] For example, when dispersion treatment is carried out using a ball mill, resin media can be used. Examples of resin media materials include urethane resin and nylon resin. Generally, alumina, zirconia, etc. are used as media materials for ball mills, and particles are pulverized by these media. In contrast, by using resin media, sintered primary particles are dissociated without pulverizing the particles. The size of the resin media can be, for example, φ5 mm to 30 mm. Furthermore, for the body (shell), for example, urethane resin, nylon resin, etc. can be used. The time for dispersion treatment is, for example, 3 to 60 minutes, and preferably 10 to 30 minutes. The conditions for dispersion treatment using a ball mill include the desired D 50 / D SEM The amount of media, rotation or amplitude speed, dispersion time, media specific gravity, etc. may be adjusted so that the above can be achieved.

[0048] For example, when dispersion is performed using a jet mill, the primary particles are not pulverized and the desired D 50 / D SEM The supply pressure, pulverization pressure, etc. may be adjusted so that the above can be achieved. The supply pressure may be, for example, 0.1 to 0.5 MPa, and the pulverization pressure may be, for example, 0.1 to 0.6 MPa. By the above preparation method, a single-particle lithium transition metal composite oxide can be efficiently produced.

[0049] Adhesion process In the adhesion step, the prepared lithium transition metal composite oxide is contacted with a cobalt compound to obtain an adhesion product in which the cobalt compound is attached to the surface of lithium transition metal composite oxide particles. The contact of the lithium transition metal composite oxide with the cobalt compound may be performed in a dry or wet manner. When performed in a dry manner, the lithium transition metal composite oxide and the cobalt compound can be mixed and contacted using, for example, a high-speed shear mixer. Examples of the cobalt compound include cobalt hydroxide, cobalt oxide, and cobalt carbonate.

[0050] When the wet method is used, the lithium transition metal composite oxide can be brought into contact with the cobalt compound by contacting the lithium transition metal composite oxide with a liquid medium containing the cobalt compound. The liquid medium may be stirred as needed. The liquid medium containing the cobalt compound may be a solution of the cobalt compound or a dispersion of the cobalt compound. Alternatively, the lithium transition metal composite oxide may be suspended in a solution of the cobalt compound, and the cobalt compound may be precipitated in the solution by adjusting the pH, temperature, or the like, thereby adhering the cobalt compound to the surface of the lithium transition metal composite oxide particles.

[0051] Examples of the cobalt compound contained in the solution include cobalt sulfate, cobalt nitrate, and cobalt chloride. Examples of the cobalt compound contained in the dispersion include cobalt hydroxide, cobalt oxide, and cobalt carbonate. The liquid medium may contain, for example, water, or may contain a water-soluble organic solvent such as alcohol in addition to water. The concentration of the cobalt compound in the liquid medium may be, for example, 1% by mass or more and 8.5% by mass or less.

[0052] The total amount of cobalt compounds brought into contact with the lithium transition metal composite oxide is, for example, 1 mol % to 20 mol % and preferably 3 mol % to 15 mol % based on cobalt relative to the lithium transition metal composite oxide.

[0053] The contact temperature between the lithium transition metal composite oxide and the cobalt compound can be, for example, 40° C. or more and 80° C. or less, and preferably 40° C. or more and 60° C. or less. The contact temperature may also be, for example, 20° C. or more and 80° C. or less. The contact time is, for example, 30 minutes or more and 180 minutes or less, and preferably 30 minutes or more and 60 minutes or less.

[0054] After contacting with the liquid medium containing the cobalt compound, the lithium transition metal composite oxide to which the cobalt compound is attached may be subjected to treatments such as filtration, washing with water, and drying, as necessary. Furthermore, a preliminary heat treatment may be performed depending on the type of cobalt compound to be attached. When the preliminary heat treatment is performed, the temperature is, for example, 100°C or higher and 350°C or lower, and preferably 120°C or higher and 320°C or lower. The treatment time is, for example, 5 hours or higher and 20 hours or lower, and preferably 8 hours or higher and 15 hours or lower. The atmosphere for the preliminary heat treatment may be, for example, an oxygen-containing atmosphere, or may be the air atmosphere.

[0055] Heat Treatment Process In the heat treatment step, the deposit obtained in the deposition step is heat-treated at a predetermined temperature of more than 700°C and less than 1100°C to obtain a heat-treated product. The heat-treated product obtained is a positive electrode active material containing a lithium transition metal composite oxide having a high cobalt concentration near the particle surfaces, and a nonaqueous electrolyte secondary battery constructed using this can achieve excellent output characteristics.

[0056] The deposit subjected to the heat treatment may be a mixture with a lithium compound. That is, the manufacturing method may include a mixing step of mixing the deposit with a lithium compound to obtain a mixture before the heat treatment step. By heat treating the deposit together with the lithium compound at a predetermined temperature, the output characteristics of the nonaqueous electrolyte secondary battery can be further improved.

[0057] Examples of the lithium compound to be mixed with the deposit include lithium hydroxide, lithium carbonate, and lithium chloride. The amount of lithium compound added is such that the molar ratio of lithium to cobalt (Li:Co) relative to the amount of cobalt deposited in the deposition step is, for example, 0.95 to 1.50:1, preferably 1.00 to 1.30:1. Mixing can be performed using, for example, a high-speed shear mixer.

[0058] The temperature for heat treatment of the deposit is, for example, more than 700°C and less than 1100°C. The lower limit of the heat treatment temperature is preferably 750°C or higher, more preferably 800°C or higher, and particularly preferably 860°C or higher. The upper limit of the heat treatment temperature is preferably 1080°C or lower, more preferably 1060°C or lower, even more preferably 1020°C or lower, and particularly preferably 1000°C or lower. The heat treatment time is, for example, 1 hour or longer and 20 hours or shorter, and preferably 3 hours or longer and 10 hours or shorter. The heat treatment atmosphere is, for example, an oxygen-containing atmosphere, and may be air atmosphere.

[0059] After the heat treatment, the heat-treated product may be subjected to further treatments such as crushing, pulverization, classification, and particle size regulation, if necessary.

[0060] The heat-treated product obtained as described above contains lithium transition metal composite oxide particles in the form of single particles, and the cobalt concentration is higher near the particle surface. That is, in the lithium transition metal composite oxide particles, the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.2 or more in a first region located approximately 500 nm deep from the particle surface, and is 0.06 or more in a second region located approximately 10 nm deep from the particle surface. Furthermore, the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is higher in the second region than in the first region. The depth of the first region from the particle surface can be, for example, 450 nm to 550 nm, and the depth of the second region from the particle surface can be, for example, 5 nm to 15 nm.

[0061] Cathode active material for non-aqueous electrolyte secondary batteries The positive electrode active material for a non-aqueous electrolyte secondary battery (hereinafter simply referred to as the positive electrode active material) has an average particle diameter D SEM 50% particle size D of cumulative particle size distribution based on volume 50 Ratio of D 50 / D SEMThe lithium transition metal composite oxide includes a lithium transition metal oxide having a layered structure in which the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.3 or more and less than 1, and the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is 0.01 or more and less than 0.5. In the lithium transition metal composite oxide, the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.2 or more in a first region that is 500 nm deep from the particle surface, and 0.06 or more in a second region that is 10 nm or less deep from the particle surface. In the lithium transition metal composite oxide, the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is greater in the second region than in the first region.

[0062] In the lithium transition metal composite oxide particles constituting the positive electrode active material, cobalt is unevenly distributed near the particle surface, resulting in a high cobalt concentration. This allows for improved output characteristics when a battery is constructed using such a positive electrode active material. The form of cobalt present near the particle surface is unclear, but possible forms include cobalt being dissolved in the surface of the lithium transition metal composite oxide particles, or a cobalt-containing compound coating the surface of the lithium transition metal composite oxide particles serving as the base material.

[0063] The effect of improving output characteristics due to the uneven distribution of cobalt near the particle surface is due to the fact that the primary particles are composed of a large number of agglomerated particles. 50 / D SEM Compared with the case of so-called agglomerated particles, where D is greater than 4, 50 / D SEM This is more effective in the case of single particles where the ratio is 4 or less. This can be explained, for example, as follows: A three-dimensional grain boundary network is formed in agglomerated particles, and it is thought that the output characteristics are improved by grain boundary conduction. On the other hand, with single particles, it is difficult to fully utilize grain boundary conduction, and it is thought that the improvement in lithium conductivity due to cobalt unevenly distributed near the surface of the particles is more effective, thereby further improving the output characteristics.

[0064] D of the lithium transition metal composite oxide particles contained in the positive electrode active material 50 / D SEM is, for example, 1 or more and 4 or less, and from the viewpoint of power density, is preferably 3.5 or less, more preferably 3 or less, even more preferably 2.5 or less, and particularly preferably 2 or less. SEM and 50% particle size D 50 The measurement method is as described above.

[0065] The average particle size D of the lithium transition metal composite oxide particles is based on electron microscope observation. SEM From the viewpoint of durability, the average particle diameter D is, for example, 0.1 μm or more and 20 μm or less. SEM From the viewpoint of power density and electrode plate packing, the lower limit is preferably 0.3 μm or more, more preferably 0.5 μm or more, and the upper limit is preferably 15 μm or less, more preferably 10 μm or less, even more preferably 8 μm or less, particularly preferably 5 μm or less.

[0066] 50% particle size D of lithium transition metal composite oxide particles 50 is, for example, 1 μm or more and 30 μm or less, preferably 1.5 μm or more, more preferably 3 μm or more, and from the viewpoint of power density, preferably 10 μm or less, more preferably 5.5 μm or less.

[0067] D of lithium transition metal composite oxide particles 90 / D 10 may be, for example, 4 or less, and from the viewpoint of power density, is preferably 3 or less, and more preferably 2.5 or less. 90 / D 10 The lower limit is, for example, 1.2 or more.

[0068] In the lithium transition metal composite oxide particles, the ratio of the number of moles of nickel to the total number of moles of metals other than lithium in the first region, which is approximately 500 nm deep from the particle surface (hereinafter simply referred to as the "nickel ratio"), is, for example, 0.2 or more, preferably 0.25 or more. The nickel ratio in the first region is, for example, 1 or less, preferably 0.5 or less. The nickel ratio in the second region, which is 10 nm or less deep from the particle surface, is, for example, 0.06 or more, preferably 0.1 or more. The nickel ratio in the second region is, for example, 0.9 or less, preferably 0.5 or less. The value obtained by dividing the nickel ratio in the second region by the nickel ratio in the first region is, for example, less than 1, preferably 0.9 or less or 0.8 or less. The value obtained by dividing the nickel ratio in the second region by the nickel ratio in the first region is, for example, 0.02 or more, preferably 0.03 or more or 0.07 or more. Here, the depth of the second region from the particle surface is, for example, 10 nm or less, but may be approximately 10 nm.

[0069] Furthermore, in the lithium transition metal composite oxide particles, the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium (hereinafter simply referred to as the "cobalt ratio") is higher in the second region than in the first region. The cobalt ratio in the first region is, for example, 0 or more, and preferably 0.2 or more. The cobalt ratio in the first region is, for example, 0.5 or less, and preferably 0.4 or less. The cobalt ratio in the second region is, for example, 0.3 or more, and preferably 0.5 or more. The cobalt ratio in the second region is, for example, 0.9 or less, and preferably 0.8 or less. The value obtained by dividing the cobalt ratio in the second region by the sum of the cobalt ratio in the first region and the cobalt ratio in the second region is, for example, greater than 0.5 and less than 1, and preferably 0.55 or more and 0.72 or less.

[0070] The nickel ratio and cobalt ratio in the first region and the second region can be calculated by measuring the cross section of the lithium transition metal composite oxide particle with SEM-EDX.

[0071] In the lithium transition metal composite oxide particles, the cobalt ratio may decrease continuously or discontinuously from the particle surface to the particle interior. The concentration gradient of cobalt, which is the absolute value of the difference between the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium in the first region and the second region divided by the difference in depth from the surface of the first region and the second region, is, for example, 0.00004 (nm -1 ) greater than 0.00122 (nm -1 ), preferably less than 0.00005 (nm -1 ) or more 0.0011(nm -1 ) or less or 0.00006 (nm -1 ) or more 0.00009(nm -1 Specifically, the cobalt concentration gradient is calculated by subtracting the cobalt ratio in the first region from the cobalt ratio in the second region, and dividing the result by the value obtained by subtracting the depth from the surface of the first region from the depth from the surface of the second region.

[0072] The composition of the lithium transition metal composite oxide contained in the positive electrode active material can be considered to be a composition obtained by adding the cobalt compound attached to the composition of the lithium transition metal composite oxide before the cobalt compound is attached in the above-described manufacturing method.

[0073] The ratio of the number of moles of nickel to the total number of moles of metals other than lithium in the composition of the lithium transition metal composite oxide contained in the positive electrode active material is, for example, 0.3 or more and less than 1. The lower limit of the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is preferably 0.31 or more, more preferably 0.32 or more. The upper limit of the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is preferably 0.98 or less, more preferably 0.8 or less, and particularly preferably 0.6 or less. When the molar ratio of nickel is within the above-mentioned range, it is possible to achieve both good charge / discharge capacity at high voltage and good cycle characteristics in a nonaqueous electrolyte secondary battery.

[0074] The ratio of the number of moles of cobalt to the total number of moles of metals other than lithium in the composition of the lithium transition metal composite oxide contained in the positive electrode active material may be, for example, more than 0 and less than 0.5, or 0.01 or more and less than 0.5, and from the viewpoint of charge / discharge capacity, is preferably 0.15 or more and 0.45 or less, and more preferably 0.3 or more and 0.4 or less.

[0075] The composition of the lithium transition metal composite oxide contained in the positive electrode active material is such that it contains at least one metal element M selected from the group consisting of manganese and aluminum. 1 The lithium transition metal composite oxide may further contain a metal element M. 1 When including lithium, M relative to the total number of moles of metals other than lithium 1 The molar ratio is, for example, 0 or more and less than 0.5, and from the viewpoint of safety, is preferably 0.15 or more and 0.45 or less, and more preferably 0.3 or more and 0.4 or less.

[0076] The composition of the lithium transition metal composite oxide contained in the positive electrode active material is such that it contains at least one metal element M selected from the group consisting of boron, sodium, magnesium, silicon, phosphorus, sulfur, potassium, calcium, titanium, vanadium, chromium, zinc, strontium, yttrium, zirconium, niobium, molybdenum, indium, tin, barium, lanthanum, cerium, neodymium, samarium, europium, gadolinium, tantalum, tungsten, bismuth, etc. 2 The lithium transition metal composite oxide may further contain a metal element M. 2 When including lithium, M relative to the total number of moles of metals other than lithium 2 The ratio of the number of moles of is, for example, 0 or more and 0.1 or less, and preferably 0.001 or more and 0.05 or less.

[0077] The ratio of the number of moles of lithium to the total number of moles of metals other than lithium in the composition of the lithium transition metal composite oxide contained in the positive electrode active material is, for example, 0.95 or more and 1.5 or less, and preferably 1 or more and 1.3 or less.

[0078] When the lithium transition metal composite oxide contained in the positive electrode active material contains manganese in addition to nickel and cobalt, the molar ratio of nickel, cobalt, and manganese is, for example, nickel:cobalt:manganese=(0.3 to 0.95):(0.01 to 0.5):(0 to 0.5), preferably (0.3 to 0.6):(0.15 to 0.45):(0.15 to 0.45), and more preferably (0.3 to 0.4):(0.3 to 0.4):(0.3 to 0.4).

[0079] When the lithium transition metal composite oxide contained in the positive electrode active material is expressed as a composition, for example, a lithium transition metal composite oxide having a composition represented by the following formula is preferred. Li q Ni r Co s M 1 t M 2 u O2 0.95≦q≦1.5, 0.3≦r<1, 0.01≦s<0.5, 0≦t<0.5, 0≦u≦0.1, r+s+t+u≦1, M 1 is at least one selected from the group consisting of Al and Mn, 2 is at least one element selected from the group consisting of B, Na, Mg, Si, P, S, K, Ca, Ti, V, Cr, Zn, Sr, Y, Zr, Nb, Mo, In, Sn, Ba, La, Ce, Nd, Sm, Eu, Gd, Ta, W, and Bi. 0.9≦r+s+t+u may be satisfied.

[0080] From the viewpoint of initial efficiency in a non-aqueous electrolyte secondary battery, the lithium transition metal composite oxide contained in the positive electrode active material preferably has a nickel disorder of 4.0% or less, more preferably 2.0% or less, and even more preferably 1.5% or less, as determined by X-ray diffraction. The nickel disorder is as described above.

[0081] [Nonaqueous electrolyte secondary battery electrode] The electrode for a non-aqueous electrolyte secondary battery includes a current collector and a positive electrode active material layer disposed on the current collector and containing the positive electrode active material for a non-aqueous electrolyte secondary battery manufactured by the manufacturing method described above. A non-aqueous electrolyte secondary battery equipped with such an electrode can achieve high output characteristics.

[0082] Examples of materials for the current collector include aluminum, nickel, and stainless steel. The positive electrode active material layer can be formed by applying a positive electrode mixture obtained by mixing the above-mentioned positive electrode active material, a conductive material, a binder, and the like with a solvent onto the current collector, followed by drying and pressure treatments. Examples of conductive materials include natural graphite, artificial graphite, and acetylene black. Examples of binders include polyvinylidene fluoride, polytetrafluoroethylene, and polyamide acrylic resin.

[0083] [Non-aqueous electrolyte secondary battery] The non-aqueous electrolyte secondary battery includes the above-mentioned electrodes for a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery is configured to include, in addition to the electrodes for a non-aqueous electrolyte secondary battery, a negative electrode for a non-aqueous secondary battery, a non-aqueous electrolyte, a separator, etc. For the negative electrode, non-aqueous electrolyte, separator, etc. of the non-aqueous electrolyte secondary battery, those for non-aqueous electrolyte secondary batteries described in, for example, JP 2002-075367 A, JP 2011-146390 A, JP 2006-12433 A (the disclosures of which are incorporated herein by reference in their entirety) can be appropriately used. [Example]

[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.

[0085] Example 1 Seed generation process 30 kg of water was placed in a reaction vessel and stirred while nitrogen gas was circulated, and the temperature inside the vessel was set to 40°C. After maintaining the oxygen concentration in the reaction vessel space at 10% by volume or less, 197 g of a 25% by mass aqueous sodium hydroxide solution was added, and the pH value of the solution in the reaction vessel was adjusted to 11 or more. Next, a nickel sulfate solution, a manganese sulfate solution, and a cobalt sulfate solution were mixed to prepare a mixed aqueous solution containing nickel ions, manganese ions, and cobalt ions in a molar ratio of 1:1:1, with a total ion concentration of nickel ions, manganese ions, and cobalt ions of 1.7 mol / L. While stirring the solution in the reaction vessel, 4.76 L of the prepared mixed aqueous solution was added to prepare a liquid medium containing seed crystals.

[0086] Crystallization process After the seed generation step, 452 moles of 25% by mass sodium hydroxide and 201 moles of the mixed aqueous solution were added to the reaction vessel at a constant flow rate over 18 hours or more while maintaining the temperature at 40°C. The pH was maintained between 11.0 and 12.0. The 50% particle size D50 of the resulting hydroxide containing nickel, manganese, and cobalt was 10.1 μm. The resulting precipitate was then washed with water and filtered to obtain a composite hydroxide. The resulting composite hydroxide was heat-treated in air at 320°C for 12 hours to obtain a composite oxide having a composition ratio of Ni / Co / Mn=0.33 / 0.33 / 0.33.

[0087] Synthesis process The obtained composite oxide and lithium carbonate were mixed so that the ratio of Li:(Ni+Co+Mn) was 1.15:1 to obtain a raw material mixture. The obtained raw material mixture was heat-treated in air at 925°C for 7.5 hours, and then at 1060°C for 4 hours to obtain a heat-treated product. The heat-treated product was subjected to a dispersion treatment to obtain a 50% particle size D 50 is 10.5 μm, and the composition formula is: Li 1.14 Ni 0.33 Co 0.33 Mn 0.33 A lithium transition metal composite oxide having a composition represented by O2 was obtained.

[0088] Adhesion process and heat treatment process 5 kg of the resulting lithium-transition metal composite oxide was suspended in 50 kg of water in a reaction vessel, and the temperature inside the vessel was set to 40°C. 2.2 kg of 8.1 wt% cobalt sulfate was used as the cobalt source. While carbon dioxide gas was blown in at 0.56 L / min, the pH was adjusted to 9.5 with 25% sodium hydroxide to obtain a cobalt deposit precursor. The amount of cobalt sulfate used was 6 mol% in terms of cobalt relative to the lithium-transition metal composite oxide. The resulting cobalt deposit precursor was then washed with water and filtered to obtain a composite hydroxide. The resulting composite hydroxide was heat-treated in air at 300°C for 12 hours to obtain a cobalt deposit in which a cobalt compound was attached to the lithium-transition metal composite oxide. Lithium hydroxide was then added to obtain a mixture with a molar ratio of cobalt to lithium of Li:Co = 1.15:1. The resulting mixture was then heat-treated in air at 1000°C for 3 hours. The heat-treated product was passed through a dry sieve to separate the Co-treated Li 1.14 Ni 0.313 Co 0.374 Mn 0.313 A positive electrode active material containing a lithium transition metal composite oxide having a composition represented by O2 was obtained. The physical properties of the obtained positive electrode active material are shown in Table 1.

[0089] Example 2 A positive electrode active material of Example 2 was produced in the same manner as in Example 1, except that the heat treatment temperature of the mixture was changed to 900° C. as shown in Table 1.

[0090] Example 3 A positive electrode active material of Example 3 was produced in the same manner as in Example 1, except that the heat treatment temperature of the mixture was changed to 850° C. as shown in Table 1.

[0091] (Comparative Example 1) The lithium transition metal composite oxide obtained in the synthesis step of Example 1 was used as the positive electrode active material of Comparative Example 1.

[0092] (Comparative Example 2) A positive electrode active material of Comparative Example 2 was produced in the same manner as in Example 1, except that the heat treatment temperature of the mixture was changed to 1100° C. as shown in Table 1.

[0093] (Comparative Example 3) A positive electrode active material of Comparative Example 3 was produced in the same manner as in Example 1, except that the heat treatment temperature of the mixture was changed to 700° C. as shown in Table 1.

[0094] Comparative Example 4 The composite oxide obtained in the crystallization step of Example 1 and lithium carbonate were mixed so that Li:(Ni+Co+Mn)=1.15:1 to obtain a raw material mixture. The obtained raw material mixture was heat-treated in air at 930°C for 12 hours to obtain a heat-treated product. The heat-treated product was then subjected to a dispersion treatment to obtain a 50% particle size D 50 is 9.6 μm, and the composition formula is: Li 1.14 Ni 0.33 Co 0.33 Mn 0.33 A positive electrode active material containing a lithium transition metal composite oxide having a composition represented by O2 was obtained.

[0095] (Comparative Example 5) 5 kg of the lithium transition metal composite oxide obtained in Comparative Example 4 was suspended in 50 kg of water in a reaction vessel, and the temperature in the vessel was set to 40°C. 2.2 kg of cobalt sulfate with a concentration of 8.1 mass% was used as a cobalt source, and while carbon dioxide gas was blown in at 0.56 L / min, the pH was adjusted to 9.5 with 25% sodium hydroxide to obtain a cobalt deposit precursor. The resulting cobalt deposit precursor was then washed with water and filtered to obtain a composite hydroxide. The obtained composite hydroxide was heat-treated in an air atmosphere at 300°C for 12 hours to obtain a cobalt deposit. Thereafter, lithium hydroxide was added to obtain a mixture with a ratio of added cobalt to lithium of Li:Co = 1.15:1. The obtained mixture was heat-treated in air at 900°C for 3 hours. The obtained heat-treated product was sieved to obtain a Co-treated Li hydroxide. 1.14 Ni 0.313 Co 0.374 Mn 0.313 A positive electrode active material containing a lithium transition metal composite oxide having a composition represented by O2 was obtained.

[0096] Particle size evaluation The physical properties of the positive electrode active material obtained above were measured as follows.50 The cumulative particle size distribution on a volume basis was measured using a laser diffraction particle size distribution analyzer (SALD-3100 manufactured by Shimadzu Corporation), and the particle size was calculated as the particle size corresponding to the cumulative 50% from the smallest diameter side. The average particle size D SEM was calculated as follows. 100 primary particles whose outlines could be confirmed were selected from images observed at 1000x to 10000x magnification using a scanning electron microscope (SEM). The outline length of the selected primary particles was determined by tracing the outlines of the selected primary particles using image processing software (ImageJ). The spherical equivalent diameter was calculated from the outline length, and the average particle diameter D based on electron microscope observation was determined as the arithmetic mean value of the obtained spherical equivalent diameters. SEM asked for.

[0097] Evaluation of cobalt and nickel distribution The positive electrode active material obtained above was evaluated for cobalt distribution and nickel distribution inside the particle. Specifically, the nickel content and cobalt content in the first region and the second region were evaluated as follows.

[0098] Composition analysis The positive electrode active materials obtained in Examples 1 to 3 and Comparative Examples 1 to 5 were each dispersed in epoxy resin and solidified. A cross-section polisher (manufactured by JEOL Ltd.) was then used to cross-section the secondary particles of the positive electrode active material to prepare measurement samples. At one point each in the first and second regions of the measurement sample, the intensity ratios of each metal component other than lithium were determined using a scanning electron microscope (SEM) / energy dispersive X-ray analyzer (EDX) (manufactured by Hitachi High-Technologies Corporation; accelerating voltage 3 kV). The cobalt ratio was the intensity ratio of cobalt to the sum of the intensity ratios of the metal components other than lithium, and the nickel ratio was the intensity ratio of nickel to the sum of the intensity ratios of the metal components other than lithium.

[0099] Scanning electron microscope observation SEM images were obtained using a scanning electron microscope (SEM; acceleration voltage 1.5 kV) for the positive electrode active materials obtained in Example 1 and Comparative Example 5. The SEM image of the positive electrode active material of Example 1 is shown in FIG. 1, and the SEM image of the positive electrode active material of Comparative Example 5 is shown in FIG.

[0100] Preparation of evaluation battery Using the positive electrode active material obtained above, a battery for evaluation was fabricated in the following manner.

[0101] Preparation of the positive electrode A positive electrode mixture was prepared by dispersing 90 parts by weight of the positive electrode active material, 5 parts by weight of acetylene black, and 5 parts by weight of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). The resulting positive electrode mixture was applied to an aluminum foil current collector, dried, compressed using a roll press, and then cut to a predetermined size to fabricate a positive electrode.

[0102] (Preparation of negative electrode) A negative electrode slurry was prepared by dissolving 97.5 parts by weight of artificial graphite, 1.5 parts by weight of carboxymethyl cellulose (CMC), and 1.0 part by weight of SBR (styrene butadiene rubber) in pure water. The resulting negative electrode slurry was applied to a copper foil current collector, dried, and then compression-molded using a roll press. The negative electrode was then cut to a specified size.

[0103] (Preparation of evaluation battery) After attaching lead electrodes to the positive and negative electrode current collectors, a separator was placed between the positive and negative electrodes, and the resulting assembly was housed in a bag-shaped laminate pack. This was then vacuum dried at 65°C to remove moisture adsorbed to each component. An electrolyte solution was then poured into the laminate pack under an argon atmosphere and sealed to prepare a test battery. The electrolyte solution used was a mixture of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) in a volume ratio of 3:7, with lithium hexafluorophosphate (LiPF6) dissolved to a concentration of 1 mol / L. The test battery thus obtained was placed in a thermostatic chamber at 25°C and aged with a weak current. The following evaluations were then performed.

[0104] (DC internal resistance measurement) The aged test battery was placed in a -25°C environment and its DC internal resistance was measured. After constant current charging to a charge depth of 50% at a full charge voltage of 4.75V, pulse discharge was performed at a specific current i for 10 seconds, and the voltage V at the 10th second was measured. The intersections were plotted with the current i on the horizontal axis and the voltage V on the vertical axis, and the slope of the line connecting the intersections was taken as the DC internal resistance (DC-IR). The currents i were set to 0.03A, 0.05A, 0.08A, 0.105A, and 0.13A. A low DC-IR indicates good output characteristics.

[0105] [Table 1]

[0106] As in the positive electrode active materials of Examples 1 to 3, D 50 / D SEM A battery comprising a positive electrode active material in which the ratio of nickel in the first region is 0.2 or more and the ratio of nickel in the second region is 0.06 or more, and the ratio of cobalt in the second region is greater than the ratio of cobalt in the first region, has improved output characteristics compared to Comparative Examples 1 to 3.

[0107] [Table 2]

[0108] [Table 3]

[0109] Tables 2 and 3 show the improvement rates of the output characteristics of lithium transition metal composite oxides obtained by a manufacturing method including a cobalt deposition step and a heat treatment step, relative to a lithium transition metal composite oxide obtained by a manufacturing method not including a cobalt deposition step and a heat treatment step. It was confirmed that the effect of the manufacturing method including a cobalt deposition step and a heat treatment step in the examples of Table 2 was greater than the effect of the manufacturing method including a cobalt deposition step and a heat treatment step using agglomerated particles in Table 3.

[0110] The disclosure of Japanese Patent Application No. 2020-010848 (filing date: January 27, 2020) is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. Average particle size D based on electron microscope observation ＳＥＭ 50% particle size D of cumulative particle size distribution based on volume ５０ Ratio D ５０ / D ＳＥＭ preparing a lithium transition metal composite oxide having a layered structure, wherein the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.3 or more and less than 1, and the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is 0 or more and less than 0.5; contacting the lithium transition metal composite oxide with a cobalt compound in a liquid medium and then adjusting the pH to obtain a deposit having the cobalt compound attached thereto; and heat-treating the deposit at a temperature of 800° C. or higher and 1000° C. or lower to obtain a heat-treated product.

2. The heat-treated product has a first region having a depth of 500 nm from the particle surface and a second region having a depth of 500 nm from the particle surface. The ratio of cobalt moles to the total number of moles of metals other than lithium in the second region having a depth of 10 nm or less The absolute value of the difference in the ratio of the number of moles of the first and second regions divided by the difference in the depth from the surface of the first and second regions is The relative value is 0.00041 (nm －１ ) or more 0.00079 (nm －１ 2. The method according to claim 1, wherein the total weight of the polymer is 1000 or less.

3. 3. The method according to claim 1, wherein the temperature of the heat treatment is 860°C or higher and 1000°C or lower.

4. 4. The method according to claim 1, wherein the total amount of the cobalt compounds to be contacted is 1 mol % or more and 20 mol % or less, based on cobalt, relative to the lithium transition metal composite oxide.

5. The method according to claim 1 , wherein the heat treatment of the deposit comprises mixing the deposit with a lithium compound to obtain a mixture, and heat treating the mixture.

6. The method according to claim 1 , wherein the lithium transition metal composite oxide has a composition represented by the following formula: Li ｐ Ni ｘ Co ｙ M １ ｚ M ２ ｗ O ２ (0.95≦p≦1.5, 0.3≦x<1, 0≦y<0.5, 0≦z<0.5, 0≦w≦0.1, x+y+z+w≦1, M １ is at least one selected from the group consisting of Al and Mn, ２ is at least one selected from the group consisting of B, Na, Mg, Si, P, S, K, Ca, Ti, V, Cr, Zn, Sr, Y, Zr, Nb, Mo, In, Sn, Ba, La, Ce, Nd, Sm, Eu, Gd, Ta, W, and Bi.

7. The method according to claim 1 , further comprising a preliminary heat treatment at 100° C. or higher and 350° C. or lower before heat treating the deposit to obtain the heat-treated product.

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