JP2024007364A - Positive electrode active material for secondary battery, manufacturing method thereof, positive electrode for secondary battery arranged by use thereof, and secondary battery - Google Patents

Abstract

To provide a manufacturing method of a positive electrode active material for a secondary battery, which enables construction of a battery improved in battery resistance.SOLUTION: A method for manufacturing a positive electrode active material for a secondary battery comprises the steps of: preparing powder of a lithium transition metal composite having a layered structure, in which the ratio of a mole number of nickel atoms to a total mole number of atoms of metal other than lithium is equal to or larger than 0.5 and smaller than one (1), and the ratio of a mole number of cobalt atoms to a total mole number of atoms of metal other than lithium is equal to or larger than zero (0) and smaller than 0.5; bringing the lithium transition metal composite powder into contact with a cobalt material to gain a cobalt-deposited composite oxide; performing a first thermal treatment on the cobalt-deposited composite oxide at a temperature of over 600°C and below 800°C to gain a first thermal treatment product; bringing the first thermal treatment product into contact with a niobium material to gain a niobium-deposited composite oxide; and performing a second thermal treatment on the niobium-deposited composite oxide at a temperature of over 300°C and below 500°C to gain a second thermal treatment product.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a positive electrode active material for a secondary battery, a method for manufacturing the same, and a positive electrode for a secondary battery and a secondary battery using the same.

Lithium-ion batteries are secondary batteries with excellent energy density and cycle characteristics.In recent years, all-solid-state batteries have been developed that use solid electrolytes instead of conventional organic solvents. It is expected that safety will be achieved. In particular, high conductivity has been achieved with sulfide-based solid charge materials that contain sulfur in the solid electrolyte, but side reactions between the positive electrode layer and the sulfide-based solid electrolyte and the interface between the positive electrode active material and the solid electrolyte may occur. There are issues such as the formation of a high resistance layer.

Patent Document 1 discloses that lithium niobate coats the surface of a positive electrode active material to suppress the formation of a high resistance layer at the contact interface between the sulfide solid electrolyte and the positive electrode active material, and further coats the surface of the positive electrode active material with lithium niobate. A technique for preventing side reactions between the positive electrode active material and the positive electrode active material has been disclosed.

JP2018-125214

However, lithium-transition metal composite oxides with high nickel content still have the problem of high battery resistance even if the surface is coated with niobium compounds such as lithium niobate, and further improvements in output characteristics are required. It is being One aspect of the present invention aims to provide a positive electrode active material for a secondary battery with reduced battery resistance, a method for manufacturing the same, and a positive electrode for a secondary battery and a secondary battery using the same.

The first aspect has a layered structure, the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.5 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. preparing a lithium-transition metal composite powder having a number ratio of 0 or more and less than 0.5; and contacting the lithium-transition metal composite powder with a cobalt raw material to obtain a cobalt-attached composite oxide; The cobalt-adhered composite oxide is first heat-treated at a temperature higher than 600°C and lower than 800°C to obtain a first heat-treated product, and the first heat-treated product is brought into contact with a niobium raw material to produce a niobium-adhered composite oxide. A positive electrode active material for a secondary battery, comprising: obtaining an oxide; and performing a second heat treatment on the niobium-attached composite oxide at a temperature of more than 300°C and less than 500°C to obtain a second heat-treated product. This is a manufacturing method.

The second aspect has 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 0.5 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. The lithium transition metal composite oxide contains a lithium transition metal composite oxide having a composition in which the number ratio is 0.01 or more and less than 0.5, and the lithium transition metal composite oxide is a secondary particle containing a niobium compound on at least a part of the particle surface. The first region has a surface and the depth from the surface of the secondary particle is around 60 nm, and the second region is around 10 nm from the surface of the secondary particle. This is also a positive electrode active material for a secondary battery in which the second region has a higher cobalt concentration.

According to one aspect of the present disclosure, it is possible to provide a positive electrode active material for a secondary battery that reduces battery resistance and a method for manufacturing the same.

SEM image of the positive electrode active material of Example 1 SEM-EDX line analysis results for the positive electrode active material of Example 1

In this specification, when there are multiple substances corresponding to each component in the composition, unless otherwise specified, the content of each component in the composition refers to the total amount of the multiple substances present in the composition. means. Embodiments of the present disclosure will be described in detail below. However, the embodiments shown below exemplify a positive electrode active material for a secondary battery and a method for manufacturing the same for embodying the technical idea of the present disclosure, and the present disclosure does not apply to the secondary batteries shown below. The present invention is not limited to the cathode active material for use and the manufacturing method thereof.

Method for manufacturing a positive electrode active material for a secondary battery A method for manufacturing a positive electrode active material for a secondary battery (hereinafter also simply referred to as "positive electrode active material") has a layered structure, and a Prepare a lithium transition metal composite powder in which the ratio of the number of moles of nickel is 0.5 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. and obtaining a cobalt-adhered composite oxide by contacting the lithium-transition metal composite powder with a cobalt raw material; heat-treating to obtain a first heat-treated product; contacting the first heat-treated product with a niobium raw material to obtain a niobium-attached composite oxide; and heating the niobium-attached composite oxide at a temperature exceeding 300°C to performing a second heat treatment at a temperature below °C to obtain a second heat-treated product. In the positive electrode active material obtained by this manufacturing method, the lithium ion conductivity within the positive electrode active material is improved by increasing the cobalt concentration near the secondary particle surface of the lithium transition metal composite powder having a high nickel ratio. Furthermore, by providing a coating portion with a niobium compound on the surface of the secondary particles of the positive electrode active material, formation of a high resistance layer between the solid electrolyte and the positive electrode is suppressed. Therefore, it is thought that an all-solid-state secondary battery including this can achieve high output characteristics.

Preparation Step In the preparation step, the lithium transition metal composite oxide has a layered structure, the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.5 or more and less than 1, and the number of moles of cobalt is A lithium transition metal composite powder having a composition in which the ratio of 0 to 0.5 is prepared. The lithium transition metal composite powder contains at least lithium and nickel, and may further contain at least one metal element selected from the group consisting of cobalt, manganese, and aluminum. The lithium transition metal composite powder may be appropriately selected from commercially available products, or may be prepared by manufacturing a lithium transition metal composite powder having a desired composition and structure.

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 powder prepared in the preparation step is 0.5 or more and less than 1. The ratio of the number of moles of nickel to the total number of moles of metals other than lithium is preferably 0.6 or more, more preferably 0.7 or more. The ratio of the number of moles of nickel to the total number of moles of metals other than lithium is preferably 0.95 or less, more preferably 0.92 or less, particularly preferably 0.9 or less. When the molar ratio of nickel is within the above-mentioned range, there is a tendency for the effect of improving the output characteristics of a secondary battery using the obtained positive electrode active material to be more pronounced.

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 powder prepared in the preparation step is 0 or more. From the viewpoint of output characteristics, 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 powder is preferably 0.01 or more, more preferably 0.03 or more. Further, 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 powder is, for example, 0.5 or less, preferably 0.3 or less from the viewpoint of charge/discharge capacity, and more. It is preferably 0.2 or less, more preferably 0.12 or less. Within the above range, output characteristics can be improved while keeping costs down.

The lithium transition metal composite powder prepared in the preparation step may further contain at least one metal element ^M1 selected from the group consisting of manganese and aluminum. When the lithium-transition metal composite powder contains the metal element ^M1 , the ratio of the number of moles of ^M1 to the total number of moles of metals other than lithium may be greater than 0, for example, and from the viewpoint of safety, it is preferably 0. 0.03 or more, more preferably 0.05 or more. The ratio of the number of moles of ^M1 to the total number of moles of metals other than lithium is, for example, 0.5 or less, preferably 0.4 or less from the viewpoint of charge/discharge capacity, and more preferably 0.3 or less.

The lithium transition metal composite powder prepared in the preparation process contains boron, sodium, magnesium, silicon, phosphorus, sulfur, potassium, calcium, titanium, vanadium, chromium, zinc, strontium, yttrium, zirconium, niobium, molybdenum, and indium. , tin, barium ^, lanthanum, cerium, neodymium, samarium, europium, gadolinium, tantalum, tungsten, and bismuth. When the lithium-transition metal composite powder contains the metal element ^M2 , the ratio of the number of moles of ^M2 to the total number of moles of metals other than lithium may be, for example, greater than 0, preferably 0.0005 or more, particularly preferably It is 0.001 or more. The ratio of the number of moles of ^M2 to the total number of moles of metals other than lithium is, for example, 0.1 or less, preferably 0.05 or less, particularly preferably 0.02 or less.

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 powder prepared in the preparation step is, for example, 0.95 or more, preferably 1 or more. The ratio of the number of moles of lithium to the total number of moles of metals other than lithium is, for example, 1.5 or less, preferably 1.3 or less.

The composition of the lithium-transition metal composite powder prepared in the preparation step may be, for example, a composition represented by the following formula (1).
Li _p Ni _x Co _y M ¹ _z M ² _w O ₂ (1)
(0.95≦p≦1.5, 0.5≦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 member selected from the group consisting of Al and Mn, and ^M2 is 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.)

The lithium transition metal composite powder prepared in the preparation step may be composed of secondary particles that are aggregates of more than 20 primary particles, but may be composed of secondary particles that are composed of 10 or less primary particles. It is preferable that the particles be composed of particles, that is, in the form of a so-called single particle. The lithium transition metal composite powder in the form of a single particle has a ratio D ₅₀ /D _SEM of the 50% particle diameter D ₅₀ in the volume-based cumulative particle size distribution to the average particle diameter D _SEM based on electron microscopy (SEM) observation. The number may be greater than or equal to 4.

In the lithium transition metal composite powder prepared in the preparation step, the closer D ₅₀ /D _SEM is to 1, the smaller the number of primary particles constituting the secondary particles contained in the lithium transition metal composite powder. , D ₅₀ /D _SEM of 1 indicates that the particle is almost composed of only a single particle. From the viewpoint of durability, D ₅₀ /D _SEM is preferably 1 or more and 4 or less, and from the viewpoint of output density, it is preferably 3.5 or less, more preferably 3 or less, even more preferably 2.5 or less, and especially 2 or less. is preferred. It can be expected that the closer the value of D ₅₀ /D _SEM is to 1, the more remarkable the effect of improving the output characteristics when the second region has a higher cobalt concentration than the first region. In this specification, independently existing particles are defined as secondary particles, regardless of whether D ₅₀ /D _SEM is different from 1 or 1.

In the lithium transition metal composite powder prepared in the preparation step, the average particle diameter D _SEM based on electron microscopy is, for example, 0.1 μm or more, and from the viewpoint of durability, preferably 0.3 μm or more, and 0.1 μm or more. More preferably, the thickness is .5 μm or more. The average particle diameter D _SEM based on electron microscopic observation is, for example, 20 μm or less, more preferably 10 μm or less from the viewpoint of output density and electrode filling property, further preferably 8 μm or less, and particularly preferably 5 μm or less.

The average particle diameter D _SEM based on electron microscopy is the average value of the sphere-equivalent diameter of primary particles measured from a scanning electron microscopy (SEM) image. Specifically, the average particle diameter D _SEM is determined as follows.
<1> Using a scanning electron microscope, set the magnification so that the number of secondary particles whose outlines can be confirmed is 10 or more and 20 or less. At this time, secondary particles whose particle size is less than half of _D10 are not included in the number.
<2> For all secondary particles observed at the above magnification, whose particle size is more than half of _D10 and whose outline can be confirmed, image processing software is applied to the primary particles constituting each secondary particle. The outline length of each primary particle is determined by tracing the outline of the primary particle using . Calculate the equivalent sphere diameter from the contour length.
<3> Repeat <1> and <2> above until the number of primary particles whose sphere equivalent diameter was calculated exceeds 100, and then repeat <1> and <2> to obtain the sphere equivalent diameter of the primary particles. The average particle size D _SEM is determined as the arithmetic mean value of the .

Further, the 50% particle size _D50 of the lithium transition metal composite powder prepared in the preparation step is, for example, 1 μm or more and 30 μm or less. From the viewpoint of handling properties, the thickness is preferably 1.5 μm or more, and more preferably 2.5 μm or more. Further, from the viewpoint of output density, the thickness is preferably 10 μm or less, more preferably 7 μm or less.

The 50% particle size D ₅₀ is determined as the particle size corresponding to 50% of the cumulative particle size from the small diameter side in the volume-based cumulative particle size distribution measured under wet conditions using a laser diffraction particle size distribution measuring device. Similarly, the 90% particle size D ₉₀ and the 10% particle size D ₁₀ , which will be described later, are determined as particle sizes corresponding to cumulative 90% and cumulative 10% from the small diameter side, respectively.

The ratio of the 90% particle size D ₉₀ to the 10% particle size D ₁₀ in the volume-based cumulative particle size distribution of the lithium transition metal composite powder prepared in the preparation process indicates the spread of the particle size distribution, and the smaller the value, the larger the particles. This shows that the particle sizes are uniform. D ₉₀ /D ₁₀ may be, for example, 4 or less, preferably 3 or less, and more preferably 2.5 or less from the viewpoint of output density. D ₉₀ /D ₁₀ may be, for example, 1.2 or more. The smaller the value of D ₉₀ /D ₁₀ is, the more uniform the particle diameters are, which can be expected to result in more uniform coating with the niobium compound.

Regarding the lithium transition metal composite powder prepared in the preparation step and having a D ₅₀ /D _SEM of 1 or more and 4 or less, for example, JP 2017-188443A (US Published Patent Publication 2017-0288221), JP 2017- 188444 (US Published Patent No. 2017-0288222), JP 2017-188445, A (US Published Patent No. 2017-0288223), etc. can be referred to.

The lithium transition metal composite powder prepared in the preparation step contains nickel in its composition. From the viewpoint of initial efficiency in an all-solid-state secondary battery, the lithium transition metal composite oxide preferably has a nickel element disorder of 4.0% or less, determined by X-ray diffraction method, and 2.0% or less. More preferred. Here, the disorder of nickel element means chemical disorder of transition metal ions (nickel ions) that should occupy original sites. In a lithium-transition metal transition metal composite oxide with a layered structure, an alkali metal ion that should occupy a site represented by 3b (3b site, the same applies hereinafter) when expressed using the Wyckoff symbol, and a transition metal that should occupy a 3a site. A typical example is the exchange of ions. The smaller the disorder of the nickel element, the better the initial efficiency tends to be.

The disorder of the nickel element in the lithium transition metal transition metal composite oxide can be determined by X-ray diffraction. The X-ray diffraction spectrum of the lithium transition metal transition metal composite oxide is measured using CuKα rays. The composition model is (Li _1-d Ni _d )( _Nix Co _y Mn _z )O ₂ (x+y+z=1), and the structure is optimized by Rietveld analysis based on the obtained X-ray diffraction spectrum. Let the percentage of d calculated as a result of structure optimization be the disorder value of the nickel element.

Specifically, the lithium transition metal composite powder prepared in the preparation step can be prepared as follows. The method for preparing a lithium transition metal composite powder 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.

In the precursor preparation step, a precursor containing a complex oxide containing nickel (hereinafter also simply referred to as complex oxide) is prepared. The precursor may be appropriately selected and prepared from commercially available products, or may be prepared by preparing a complex oxide having a desired structure by a conventional method. Methods for obtaining a composite oxide having a desired composition include a method in which raw materials (hydroxide, carbonate compounds, etc.) are mixed according to the desired composition and decomposed into a composite oxide by heat treatment, and a method in which raw materials soluble in a solvent are mixed. Examples include a coprecipitation method in which a precipitate having a desired composition is obtained by dissolving the precipitate in a solvent, adjusting the temperature, adjusting the pH, adding a complexing agent, etc., and then heat-treating the precipitate to obtain a composite oxide. An example of a method for producing a composite oxide will be described below.

The method of obtaining a composite oxide by the coprecipitation method includes a seed generation step in which seed crystals are obtained by adjusting the pH etc. of a mixed solution containing metal ions at a desired composition ratio, and a seed generation step in which the generated seed crystals are grown to obtain the desired crystal. The method can include a crystallization step for obtaining a composite hydroxide having characteristics, and a step for heat-treating the obtained composite hydroxide to obtain a composite oxide. For details of the method for obtaining such a composite oxide, reference can be made to, for example, JP-A No. 2003-292322, JP-A No. 2011-116580 (US Patent Publication No. 2012-270107), and the like.

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

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

In the step of obtaining a complex oxide, a complex oxide is obtained by heat-treating the precipitate containing the complex hydroxide obtained in the crystallization step. The heat treatment in the step of obtaining the composite oxide can be carried out by heating the composite hydroxide at a temperature of, for example, 500°C or lower, preferably at a temperature of 450°C or lower. Further, the temperature of the heat treatment is, for example, 100° C. or higher, preferably 200° C. or higher, and the time of the heat treatment can be, for example, 0.5 to 48 hours, preferably 5 to 24 hours. The atmosphere for the heat treatment may be the air or an atmosphere containing oxygen. The heat treatment can be performed using, for example, a box furnace, rotary kiln, pusher furnace, roller hearth kiln, or the like.

The resulting composite oxide may contain cobalt in addition to nickel. When the composite oxide contains other metals, the mixed solution may contain other metal ions in a desired configuration in the seed generation step and the crystallization step. Thereby, a composite oxide having a desired composition can be obtained by making the precipitate contain nickel, cobalt, and other metals, and heat-treating the precipitate.

The resulting composite oxide may contain other metal elements ^M1 in addition to nickel. Examples of the other metal element ^M1 include Mn, Al, etc., and at least one selected from the group consisting of these is preferable, and it is more preferable that at least Mn is included. When the composite oxide contains other metals, the mixed solution in the seed generation step and the crystallization step may contain other metal ions in a desired configuration. Thereby, a composite oxide having a desired composition can be obtained by making the precipitate contain nickel and other metals and heat-treating the precipitate.

The average particle size of the composite oxide is, for example, 2 μm or more and 20 μm or less, preferably 3 μm or more and 10 μm or less. The average particle size of the composite oxide is a volume average particle size, and is a value at which the volume integrated value from the small particle size side in the volume-based particle size distribution obtained by the laser scattering method is 50%.

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

Examples of the lithium compound to be mixed with the composite oxide include lithium hydroxide, lithium carbonate, and lithium oxide. The particle size of the lithium compound used for mixing is, for example, 0.1 μm or more and 100 μm or less, and preferably 2 μm or more and 20 μm or less, as a 50% average particle size of a volume-based cumulative particle size distribution.

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 or more and 1.5 or less. The composite oxide and the lithium compound can be mixed using, for example, a high-speed shear mixer.

The mixture may further contain metal elements M ¹ or M ² other than lithium, nickel, and cobalt. The other metal element ^M1 is preferably at least one metal element selected from the group consisting of manganese and aluminum. ^M2 includes 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, Bi, etc., and at least one selected from the group consisting of these is preferred. When the mixture contains other metals, the mixture can be obtained by mixing the other metals or metal compounds together with the composite oxide and the lithium compound. Examples of metal compounds containing other metals include oxides, hydroxides, chlorides, nitrides, carbonates, sulfates, nitrates, acetates, oxalates, and the like.

When the mixture contains other metals, the ratio of the total number of moles of the metals constituting the composite oxide to the total number of moles of the other metals is, for example, 1:0.001 to 1:0.3; :0.01 to 1:0.15 is preferable.

The heat treatment temperature of the mixture is, for example, 550°C or more and 1100°C or less, preferably 600°C or more and 1080°C or less, and more preferably 700°C or more and 1080°C or less. Although the heat treatment of the mixture may be performed at a single temperature, it is preferably performed at multiple temperatures from the viewpoint of discharge capacity at high voltage. When performing heat treatment at multiple temperatures, for example, it is desirable to hold the first temperature for a predetermined time, then further increase the temperature, hold the second temperature for a predetermined time, and then further heat treat at a third temperature lower than the second temperature. A heat-treated product may be obtained by heat treatment.

Further, by performing heat treatment at the first temperature or second temperature and then heat treatment at a third temperature during the temperature drop for a predetermined period of time, there is a tendency to obtain the effect of reducing the disorder value of the nickel element described above.

The first temperature is, for example, 300°C or more and 600°C or less, preferably 350°C or more and 550°C or less. Further, the second temperature is, for example, 800°C or more and 1100°C or less, preferably 850°C or more and 1050°C or less. Further, the third temperature is, for example, 600°C or more and 850°C or less, preferably 700°C or more and 800°C or less.

The heat treatment time in the case of heat treatment at a single temperature is, for example, 1 hour or more and 20 hours or less, preferably 5 hours or more and 15 hours or less. Further, when heat treatment is performed at a plurality of temperatures, the heat treatment time at the first temperature is, for example, 1 hour or more and 20 hours or less. The heat treatment time at the second temperature is, for example, 1 hour or more and 20 hours or less. The heat treatment times at each temperature may be the same or different, and may be performed continuously or independently.

The atmosphere for the heat treatment may be the air or an atmosphere containing oxygen. The heat treatment can be performed using, for example, a box furnace, rotary kiln, pusher furnace, roller hearth kiln, or the like.

The heat-treated product is subjected to a dispersion treatment if necessary. Lithium transition metal composite oxide particles with a narrow particle size distribution and uniform particle size can be obtained by dissociating the sintered primary particles through a dispersion process rather than a crushing process that involves strong shearing force or impact. The dispersion treatment may be carried out in a dry or wet manner, and is preferably carried out in a dry manner. The dispersion treatment can be performed using, for example, a ball mill, a jet mill, or the like. The conditions for the dispersion treatment can be set, for example, so that the D ₅₀ /D _SEM of the lithium-transition metal composite powder after the dispersion treatment falls within a desired range, for example, from 1 to 4.

For example, when performing the dispersion treatment with a ball mill, resin media can be used. Examples of the material of the resin media include urethane resin and nylon resin. Generally, alumina, zirconia, or the like is used as a material for the media of a ball mill, and particles are pulverized by these media. On the other hand, by using resin media, the sintered primary particles are dissociated without pulverizing the particles. The size of the resin media can be, for example, φ5 mm to 30 mm. Further, as the body (shell), for example, urethane resin, nylon resin, etc. can be used. The time for the dispersion treatment is, for example, 3 minutes to 60 minutes, preferably 10 minutes to 30 minutes. As conditions for the dispersion process using a ball mill, the amount of media, rotation or amplitude speed, dispersion time, media specific gravity, etc. may be adjusted so that the desired D ₅₀ /D _SEM can be achieved.

For example, when the dispersion treatment is performed using a jet mill, the supply pressure, the crushing pressure, etc. may be adjusted so that the desired D ₅₀ /D _SEM can be achieved without pulverizing the primary particles. The supply pressure can be, for example, from 0.1 MPa to 0.6 MPa, and the crushing pressure can be, for example, from 0.1 MPa to 0.6 MPa.

First adhesion step In the first adhesion step, the prepared lithium-transition metal composite powder and cobalt raw material are brought into contact to obtain a cobalt-adhered composite oxide in which the cobalt raw material is attached to the surface of the lithium-transition metal composite powder. The lithium-transition metal composite powder and the cobalt raw material may be brought into contact in a dry or wet manner.

When the first adhesion step is performed in a dry manner, the lithium transition metal composite powder and the cobalt raw material can be mixed and brought into contact with each other. Examples of cobalt raw materials include cobalt hydroxide, cobalt oxide, and cobalt carbonate. When the first adhesion step is performed in a dry manner, the number of steps is reduced compared to a wet method, and in addition, it can be expected to have the advantage of being able to suppress a decrease in the amount of lithium contained in the lithium-transition metal composite powder.

Examples of the mixing method include a high-speed shear mixer, a Henschel mixer, a high-speed mixer, a bead mill, and a ball mill.

When the first attachment step is performed wet, the lithium transition metal composite powder and the cobalt raw material can be brought into contact by bringing the lithium transition metal composite powder into contact with a liquid medium containing the cobalt raw material. At this time, the liquid medium may be stirred if necessary. The liquid medium containing the cobalt raw material may be a solution of the cobalt raw material or a dispersion of the cobalt raw material. Alternatively, the lithium transition metal composite powder may be suspended in a solution of the cobalt raw material, and the cobalt raw material may be precipitated in the solution by adjusting the pH, temperature, etc., and the cobalt raw material may be attached to the surface of the lithium transition metal composite powder. good.

When the first deposition step is carried out wet, examples of the cobalt raw material contained in the solution used for contacting the cobalt raw material include cobalt sulfate, cobalt nitrate, and cobalt chloride. Examples of the cobalt raw material contained in the dispersion liquid used for contacting with the cobalt raw material include cobalt hydroxide, cobalt oxide, and cobalt carbonate. The liquid medium may contain water, for example, and may contain a water-soluble organic solvent such as alcohol in addition to water. The concentration of the cobalt raw material in the liquid medium can be, for example, 1% by mass or more and 8.5% by mass or less.

The total number of moles of cobalt atoms contained in the cobalt raw material that is brought into contact with the lithium-transition metal composite powder is, for example, 0.5 with respect to the total number of moles of metal atoms other than lithium contained in the lithium-transition metal composite powder. The content is mol % or more and 15 mol % or less, preferably 1 mol % or more and 10 mol % or less.

The contact temperature between the lithium transition metal composite powder and the cobalt raw material is, for example, 40°C or more and 80°C or less, preferably 40°C or more and 60°C or less. Further, the contact temperature may be, for example, 20° C. or higher and 80° C. or lower. The contact time is, for example, 30 minutes or more and 180 minutes or less, preferably 30 minutes or more and 60 minutes or less.

After being brought into contact with a liquid medium containing a cobalt raw material, the cobalt-adhered composite oxide may be subjected to treatments such as filtration, washing with water, and drying, if necessary. Further, preliminary heat treatment may be performed depending on the type of cobalt raw material to be attached. When performing preliminary heat treatment, the temperature is, for example, 100°C or more and 350°C or less, preferably 120°C or more and 320°C or less. Further, the treatment time is, for example, 5 hours or more and 20 hours or less, preferably 8 hours or more and 15 hours or less. Further, the atmosphere for the preliminary heat treatment is, for example, an atmosphere containing oxygen, and may be an air atmosphere.

First heat treatment step In the first heat treatment step, the cobalt-attached composite oxide obtained in the first attachment step is heat treated at a predetermined temperature of more than 600° C. and less than 800° C. to obtain a heat treated product. It is possible to obtain a positive electrode active material containing a lithium-transition metal composite oxide having a desired cobalt concentration gradient depending on the heat treatment temperature, and it is possible to achieve excellent output characteristics in an all-solid-state secondary battery constructed using this material. can.

The manufacturing method may include, before the first heat treatment step, a mixing step of mixing the cobalt-attached composite oxide and the lithium compound to obtain a mixture.

Examples of the lithium compound to be mixed with the cobalt-attached composite oxide include lithium hydroxide, lithium carbonate, and lithium chloride. The amount of the lithium compound to be added is such that the molar ratio of lithium and cobalt (Li:Co) to the amount of cobalt deposited in the deposition step is, for example, 0.95 to 1.50:1, preferably 1.00 to 1. Mix at a ratio of 30:1. Mixing can be performed using, for example, a high-speed shear mixer.

The temperature of the first heat treatment of the cobalt-attached composite oxide is, for example, higher than 600°C and lower than 800°C. The first heat treatment temperature is preferably 650°C or higher, more preferably 675°C or higher. Further, the first heat treatment temperature is preferably 750°C or lower, more preferably 725°C or lower. The time for the first heat treatment is, for example, 1 hour or more and 20 hours or less, preferably 3 hours or more and 10 hours or less. The atmosphere for the heat treatment preferably contains oxygen. By including oxygen in the heat treatment atmosphere, the amount of residual lithium can be suppressed, for example, and sintering between particles can be suppressed more effectively. When the heat treatment atmosphere contains oxygen, its content is preferably 15% by volume or more, more preferably 30% by volume or more, and even more preferably 80% by volume or more.

The first heat-treated product after the first heat treatment may be subjected to treatments such as crushing, pulverization, classification operation, and sizing operation, as necessary.

In the surface composition of the first heat-treated product, the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium (hereinafter also simply referred to as "cobalt ratio") is larger, the higher the ratio is after the second adhesion step described later. The adhesion of niobium tends to be more uniform. The cobalt ratio in the surface composition of the first heat-treated product may be 10 mol% or more, preferably 15 mol% or more, and more preferably 25 mol% or more. Moreover, the cobalt ratio is preferably 40 mol% or less. If it exceeds 40 mol%, the discharge capacity may decrease.

The surface composition of the first heat-treated product can be determined by stirring the first heat-treated product in an acidic solvent for a short time and applying high-frequency inductively coupled plasma emission spectroscopy (ICP) to the eluate. Although detailed conditions will be described later, this method will be referred to as surface elution analysis in this specification.

The first heat-treated product obtained as described above contains a lithium transition metal composite oxide having a cobalt concentration gradient.

Second adhesion step In the second adhesion step, the first heat-treated product is brought into contact with a niobium raw material to obtain a niobium-adhered composite oxide. The contact between the first heat-treated product and the niobium raw material may be carried out in a dry manner or in a wet manner.

When carrying out the dry process, the first heat-treated product and the niobium raw material can be mixed and brought into contact with each other. Examples of the niobium raw material include niobium oxide.

Examples of the mixing method include a high-speed shear mixer, a Henschel mixer, a high-speed mixer, a bead mill, and a ball mill.

When performing the wet process, the first heat-treated product can be brought into contact with a liquid medium containing the niobium raw material. At this time, the liquid medium may be stirred if necessary. The liquid medium containing the niobium raw material may be a solution of the niobium raw material or a dispersion of the niobium raw material. Alternatively, the first heat-treated product may be suspended in a solution of the niobium raw material, and the niobium raw material may be precipitated in the solution by adjusting the pH, temperature, etc., and the niobium raw material may be attached to the surface of the first heat-treated product. The niobium raw material may be attached to the surface of the first heat-treated product using a fluidized bed dryer.

Examples of the niobium raw material contained in the solution include niobic acid, pentaethoxyniobium, niobium chloride, and the like. Examples of the niobium raw material contained in the dispersion include niobic acid, pentaethoxyniobium, and niobium chloride. The liquid medium may contain, for example, water, and may contain alcohol, hydrogen peroxide solution, ammonia water, etc. in addition to water. The concentration of the niobium raw material in the liquid medium can be, for example, 0.5% by mass or more and 3% by mass or less.

The total number of moles of niobium atoms contained in the niobium raw material that is brought into contact with the first heat-treated product is, for example, 0.1 mol% or more or more than 5 moles with respect to the total number of moles of metal atoms other than lithium contained in the first heat-treated product. % or less, preferably 0.5 mol% or more and 3 mol% or less.

The contact temperature between the first heat-treated product and the niobium raw material is, for example, 20°C or more and 200°C or less, preferably 40°C or more and 150°C or less. The contact time is, for example, 30 minutes or more and 180 minutes or less, preferably 30 minutes or more and 120 minutes or less.

After being brought into contact with the liquid medium containing the niobium raw material, the niobium-attached composite oxide may be subjected to treatments such as filtration, washing with water, and drying, if necessary.

Second heat treatment step In the second heat treatment step, the niobium-attached composite oxide obtained in the second attachment step is heat treated at a predetermined temperature of more than 300° C. and less than 500° C. to obtain a second heat treated product.

The niobium-attached composite oxide subjected to the second heat treatment may be a mixture with a lithium compound. That is, the manufacturing method may include, before the heat treatment step, a mixing step of mixing the niobium-adhered composite oxide and the lithium compound to obtain a mixture.

Examples of the lithium compound to be mixed with the cobalt-attached composite oxide include lithium hydroxide, lithium carbonate, and lithium chloride. The amount of the lithium compound added is such that the molar ratio of lithium to niobium (Li:Nb) to the amount of niobium deposited in the deposition step is, for example, 0.95 to 1.50:1, preferably 1.00 to 1. Mix at a ratio of 30:1. Mixing can be performed using, for example, a high-speed shear mixer.

The temperature of the second heat treatment of the niobium-attached composite oxide is, for example, higher than 300°C and lower than 500°C. The second heat treatment temperature is preferably 320°C or higher, more preferably 340°C or higher. Further, the second heat treatment temperature is preferably 450°C or lower, more preferably 400°C or lower. The time for the second heat treatment is, for example, 1 hour or more and 20 hours or less, preferably 3 hours or more and 10 hours or less. The atmosphere for the second heat treatment is, for example, an atmosphere containing oxygen, and may be an air atmosphere.

The heat-treated product after the second heat treatment may be subjected to treatments such as crushing, pulverization, classification operation, and sizing operation, as necessary.

The second heat-treated product obtained as described above contains a lithium-transition metal composite oxide, has a cobalt concentration gradient, and has a secondary particle surface containing a niobium compound on at least a portion of the particle surface. That is, 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 0 or more in the first region where the depth from the surface of the secondary particle is around 60 nm, In the second region where the depth from the surface of the secondary particle is around 10 nm, it is 0.15 or more. The depth of the first region from the surface of the secondary particle can be, for example, 50 nm to 70 nm, and the depth of the second region from the surface of the secondary particle can be, for example, 5 nm to 15 nm.

Positive electrode active material for secondary batteries The positive electrode active material for secondary batteries has a layered structure, the ratio of the number of moles of nickel to the total number of moles of metals other than lithium is 0.5 or more and less than 1, and the number of moles of metal other than lithium is 0.5 or more and less than 1. The lithium transition metal composite oxide includes a lithium transition metal composite oxide having a composition in which the ratio of the number of moles of cobalt to the total number of moles of metal is 0.01 or more and less than 0.5, and the lithium transition metal composite oxide has a A region having a secondary particle surface containing a niobium compound at least in part and having a depth of approximately 60 nm from the surface of the secondary particle is referred to as a first region, and a region approximately 10 nm from the surface of the coating portion is referred to as a second region. When this happens, the second region has a higher cobalt concentration than the first region.

The surface of a secondary particle includes a first region and a second region in which the ratio of moles of nickel in the composition is within a specific range, the cobalt concentration in the second region is higher than the cobalt concentration in the first region, and the surface contains a niobium compound. A positive electrode active material comprising a lithium-transition metal composite oxide having the following can achieve high charge/discharge capacity and excellent output characteristics in an all-solid-state secondary battery configured using the positive electrode active material. For example, by increasing the mole ratio of nickel in the composition, the charge/discharge capacity increases, but on the other hand, the lithium ion conductivity decreases, which can be alleviated by having a high cobalt concentration in the second region. This is thought to be due to the fact that Furthermore, by having a niobium compound on the surface of the secondary particles, it is possible to prevent a high resistance layer from being formed between the secondary particles and the solid electrolyte.

The ratio of the molar amount of niobium atoms contained in the niobium compound may be 0.1 mol% or more with respect to the total number of moles of metal atoms other than lithium contained in the lithium-transition metal composite oxide. From the viewpoint of suppressing side reactions with the solid electrolyte, the content may be 0.5 mol% or more, more preferably 0.8 mol% or more. The ratio of the molar amount of niobium atoms contained in the niobium compound may be 5 mol% or less with respect to the total number of moles of the lithium transition metal composite oxide, and from the viewpoint of resistance and capacity, it may be 4 mol% or less, more preferably 3 mol% or less. It may be contained in an amount of mol% or less.

Examples of the niobium compound include lithium niobate. The surface of the secondary particles of the lithium transition metal composite oxide is coated with a niobium compound such as lithium niobate, thereby preventing the formation of a high resistance layer at the interface between the lithium transition metal composite oxide and the sulfide solid electrolyte. It is expected to be effective and improve output. Furthermore, by suppressing side reactions between the lithium-transition metal composite oxide and the sulfide solid electrolyte, it is possible to suppress deterioration of the positive electrode and improve cycle characteristics.

The thickness of the niobium compound may be, for example, 30 nm or less, and preferably 20 nm or less from the viewpoint of resistance and capacity.

The composition of the lithium transition metal composite oxide contained in the positive electrode active material can be considered to be the composition of the lithium transition metal composite oxide before the niobium raw material is attached in the production method described above, with the niobium raw material deposited taken into account. I can do it.

In the lithium transition metal composite oxide constituting the positive electrode active material, cobalt is unevenly distributed in the second region, and its concentration is high. The form of cobalt present in the second region is not clear, but for example, cobalt is solid-solved in the second region of a lithium transition metal composite oxide, or a cobalt compound such as lithium cobalt oxide exists in the second region. Possible forms etc. Thereby, when a battery is constructed using such a positive electrode active material, output characteristics can be improved. Although the reason for this is not clear, it can be inferred as follows, as an example. As one aspect of the existence form of cobalt, when it exists as lithium cobalt oxide in the second region, the lithium ion conductivity of the lithium cobalt oxide is higher than that of the first region with a high nickel ratio or the coated part with a niobium compound. It is conceivable that lithium ions are more easily diffused throughout the positive electrode active material and the output characteristics are improved.

The effect of improving the output characteristics due to the uneven distribution of cobalt in the second region of the secondary particles is that compared to the case of so-called agglomerated particles, which are composed of a large number of primary particles agglomerated and have a D ₅₀ /D _SEM greater than 4, This is more effective in the case of single particles having a D ₅₀ /D _SEM of 4 or less. For example, this can be considered as follows. Since a three-dimensional grain boundary network is formed in agglomerated particles, it is thought that grain boundary conduction improves the output characteristics. On the other hand, with single particles, it is difficult to fully utilize grain boundary conduction, but cobalt unevenly distributed in the second region of the particle improves lithium conductivity more effectively, resulting in improved output characteristics. be able to.

The lithium transition metal composite oxide may be composed of secondary particles that are aggregates of more than 20 primary particles, but may be composed of secondary particles that are composed of 10 or less primary particles, so-called single particles. Preferably, it is in the form of particles. The lithium transition metal composite powder in the form of a single particle has a ratio D ₅₀ /D _SEM of the 50% particle diameter D ₅₀ in the volume-based cumulative particle size distribution to the average particle diameter D _SEM based on electron microscopy (SEM) observation. The number may be greater than or equal to 4.

D ₅₀ /D _SEM of the lithium transition metal composite oxide contained in the positive electrode active material is, for example, 1 or more and 4 or less, preferably 3.5 or less, more preferably 3 or less, from the viewpoint of output density; 2. It is more preferably 5 or less, particularly preferably 2 or less. It can be expected that the closer the value of D ₅₀ /D _SEM is to 1, the more remarkable the effect of improving the output characteristics when the second region has a higher cobalt concentration than the first region. The methods for measuring the average particle size D _SEM and 50% particle size D ₅₀ based on electron microscopic observation are as described above.

In the lithium transition metal composite oxide, the average particle diameter D _SEM based on electron microscopy is, for example, 0.1 μm or more and 20 μm or less from the viewpoint of durability. The average particle diameter D _SEM based on electron microscopy is preferably 0.3 μm or more, more preferably 0.5 μm or more from the viewpoint of output density and electrode filling property. Further, the average particle diameter D _SEM based on electron microscopic observation 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.

The 50% particle size _D50 of the lithium transition metal composite oxide 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 preferably 10 μm or less from the viewpoint of power density, and 5.5 μm. The following are more preferred.

D ₉₀ /D ₁₀ of the lithium transition metal composite oxide may be, for example, 4 or less, preferably 3 or less, and more preferably 2.5 or less from the viewpoint of output density. D ₉₀ /D ₁₀ is, for example, 1.2 or more.

In a 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 in the first region, which is approximately 60 nm deep from the surface of the secondary particle (hereinafter also simply referred to as "nickel ratio") ) is, for example, 0.5 or more, preferably 0.7 or more. The nickel ratio in the first region is, for example, 1 or less, preferably 0.95 or less. Further, the nickel ratio in the second region, which is approximately 10 nm deep from the surface of the secondary particle, is, for example, 0.8 or less, preferably 0.6 or less. Furthermore, 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. Further, the value obtained by dividing the nickel ratio in the second region by the nickel ratio in the first region is, for example, 0.3 or more, preferably 0.4 or more, or 0.5 or more. When the nickel ratio of the first region and the second region is within the above range, the effect of improving the output is more noticeable when the second region has a Co concentration gradient.

Furthermore, 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 (hereinafter also simply referred to as "cobalt ratio") is larger in the second region than in the first region. . The cobalt ratio in the first region is, for example, 0 or more, preferably 0.02 or more. The cobalt ratio in the first region is, for example, 0.5 or less, preferably 0.3 or less, more preferably 0.2 or less, still more preferably 0.1 or less, particularly preferably 0. 05 or less. Further, the cobalt ratio in the second region is, for example, 0.1 or more, preferably 0.2 or more. The cobalt ratio in the second region is, for example, 0.5 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 may be, for example, greater than 1, preferably 3 or more, and more preferably 5 or more.

The nickel ratio and cobalt ratio in the first region and the second region can be calculated by performing line analysis using SEM-EDX on a cross section of the lithium transition metal composite oxide.

In the lithium transition metal composite oxide, the cobalt ratio may decrease continuously or discontinuously from the surface of the secondary particle to the inside of the secondary particle. The absolute value of the difference in the ratio of the number of moles of cobalt to the total number of moles of metals other than lithium in the first and second regions divided by the difference in depth from the surface of the first and second regions. The concentration gradient of a certain cobalt may be, for example, 0.0002 (nm ⁻¹ ) or more, preferably 0.001 or more, and more preferably 0.002 or more. The cobalt concentration gradient, which is the absolute value of the value divided by the difference in depth from the surface of the first region and the second region, may be, for example, 0.2 (nm ⁻¹ ) or less, and preferably 0.08 (nm −1 ) or less ^{. 1} ) or less, more preferably 0.04 (nm ⁻¹ ) or less. 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 subtracting the depth from the surface of the second region from the depth from the surface of the first region. It is calculated by dividing by the value.

The composition of the lithium-transition metal composite oxide contained in the positive electrode active material can be considered to be the composition of the lithium-transition metal composite oxide before the cobalt raw material is deposited in the production method described above, with the cobalt raw material deposited taken into account. I can do it.

The uniformity of the niobium compound in the surface composition of the lithium-transition metal composite oxide can be evaluated using an index called SED standard deviation by analyzing SEMEDX data. Although a specific measurement method will be described later, it is preferable that the SED standard deviation value is lower because the niobium compound on the surface of the lithium transition metal composite oxide is more uniformly distributed. When the niobium compound is uniformly distributed, coating with the niobium compound is carried out in just the right amount, which can be expected to have the effect of preventing side reactions with the solid electrolyte in exposed areas and reduction in conductivity in overly coated areas. The SED standard deviation may be 6 or less, preferably 5 or less, more preferably 4 or less, even more preferably 3 or less.

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, 0.5 or more and less than 1. The ratio of the number of moles of nickel to the total number of moles of metals other than lithium is preferably 0.6 or more, more preferably 0.7 or more. The ratio of the number of moles of nickel to the total number of moles of metals other than lithium is preferably 0.95 or less, more preferably 0.92 or less, particularly preferably 0.9 or less. When the molar ratio of nickel is within the above range, it is possible to achieve both high voltage charge/discharge capacity and cycle characteristics in the all-solid-state secondary battery. The ratio of the number of moles of nickel is determined, for example, by analyzing the metal composition ratio of the positive electrode active material using an inductively coupled plasma emission spectrometer.

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 is, for example, 0 or more, and from the viewpoint of output characteristics, preferably 0.01 or more. and more preferably 0.03 or more. The ratio of the number of moles of cobalt to the total number of moles of metals other than lithium is, for example, 0.5 or less, preferably 0.3 or less from the viewpoint of charge/discharge capacity, and more preferably 0.2 or less.

The composition of the lithium transition metal composite oxide contained in the positive electrode active material may further include at least one metal element ^M1 selected from the group consisting of manganese and aluminum. When the lithium transition metal composite oxide contains the metal element M ¹ , the ratio of the number of moles of M ¹ to the total number of moles of metals other than lithium may be, for example, greater than 0, and from the viewpoint of safety, it is preferably 0. 0.03 or more, more preferably 0.05 or more. The ratio of the number of moles of ^M1 to the total number of moles of metals other than lithium is, for example, 0.5 or less, preferably 0.4 or less from the viewpoint of charge/discharge capacity, and more preferably 0.3 or less.

The composition of the lithium transition metal composite oxide contained in the positive electrode active material is boron, sodium, magnesium, silicon, phosphorus, sulfur, potassium, calcium, titanium, vanadium, chromium, zinc, strontium, yttrium, zirconium, niobium, molybdenum, It further contains at least one metal element ^M2 selected from the group consisting of indium, tin, barium, lanthanum, cerium, neodymium, samarium, europium, gadolinium, tantalum, tungsten, bismuth, and the like. The ratio of the number of moles of M ² to the total number of moles of metals other than lithium may be, for example, greater than 0, preferably greater than or equal to 0.005, particularly preferably greater than or equal to 0.01. The ratio of the number of moles of ^M2 to the total number of moles of metals other than lithium is, for example, 0.1 or less, preferably 0.05 or less, particularly preferably 0.03 or less.

^M2 contains niobium, and the ratio of the number of moles of niobium to the total number of moles of metals other than lithium in the lithium transition composite oxide is preferably 0.005 or more from the viewpoint of output characteristics, and 0.005 from the viewpoint of initial capacity. .03 or less is preferable.

When ^M2 contains zirconium, the ratio of the number of moles of zirconium to the total number of moles of metals other than lithium in the lithium transition composite oxide may be 0.001 or more and 0.01 or less from the viewpoint of output characteristics, and 0. It may be .002 or more and 0.005 or less.

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, preferably 1 or more and 1.5 or less. 3 or less.

When the lithium transition metal composite oxide contained in the positive electrode active material is expressed as a composition, a lithium transition metal composite oxide having a composition represented by the following formula is preferable, for example.
Li _p Ni _x Co _y M ¹ _z M ² _w O ₂
0.95≦p≦1.5, 0.5≦x<1, 0.01≦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, and ^M2 is B, Na, Mg, Si, P, S, K, Ca, Ti, V, Cr, Zn, Sr, Y, Zr, At least one member selected from the group consisting of Nb, Mo, In, Sn, Ba, La, Ce, Nd, Sm, Eu, Gd, Ta, W, and Bi.

From the viewpoint of initial efficiency in a secondary battery, the lithium transition metal composite oxide contained in the positive electrode active material preferably has a nickel element disorder of 4.0% or less as determined by an X-ray diffraction method; 2. More preferably 0% or less. The disorder of the nickel element is as described above.

The tap density of the positive electrode active material is preferably 1.7 g/cm ³ or more because the volume energy density becomes sufficiently high. More preferred is 2.0 g/cm ³ or more. There is no particular upper limit as long as the positive electrode active material can be taken as a powder. In reality, the upper limit is about 2.5 g/cm ³ .

The specific surface area of the positive electrode active material is, for example, 0.2 m ² /g or more and 3.0 m ² /g or less, preferably 0.3 m ² /g or more and 2.0 m ² /g or less. When the specific surface area is within the above range, the contact area between the positive electrode active material and the electrolyte increases, which tends to improve the output. The specific surface area of the positive electrode active material is measured by the BET method.

The ratio of the 90% particle size D ₉₀ to the 10% particle size D ₁₀ in the volume-based cumulative particle size distribution of the positive electrode active material indicates the spread of the particle size distribution, and the smaller the value, the more uniform the particle sizes are. . D ₉₀ /D ₁₀ may be, for example, 4 or less, preferably 3 or less, and more preferably 2.5 or less from the viewpoint of output density. D ₉₀ /D ₁₀ may be, for example, 1.2 or more.

From the viewpoint of durability, D ₅₀ /D _SEM of the positive electrode active material is preferably 1 or more and 4 or less, from the viewpoint of output density, it is preferably 3.5 or less, more preferably 3 or less, and even more preferably 2.5 or less. , particularly preferably 2 or less.

(solid electrolyte)
Preferably, the positive electrode layer contains a solid electrolyte. A positive electrode layer obtained by mixing a solid electrolyte with a positive electrode active material tends to exhibit higher ionic conductivity. As solid electrolytes, sulfide-based, oxide-based, halogen-based, and other solid electrolytes have been reported.

Examples of the crystal structure of the sulfide solid electrolyte include a Thio-LISICON type crystal structure, an LGPS type crystal structure, and an argyrodite type crystal structure.

The proportion of the solid electrolyte in the positive electrode layer may be, for example, 1% by weight or more and 50% by weight or less. Further, it may be 5% by weight or more and 40% by weight or less, and may be 10% by weight or more and 30% by weight or less.

(Positive electrode for all-solid-state secondary batteries)
The positive electrode for an all-solid-state secondary battery includes a current collector and a positive-electrode active material layer disposed on the current collector and containing a positive-electrode active material for an all-solid-state secondary battery manufactured by the above-described manufacturing method. An all-solid-state secondary battery including such an electrode can achieve high output characteristics.

Examples of the material of the current collector include aluminum, nickel, and stainless steel. The positive electrode active material layer is formed by applying a positive electrode mixture obtained by mixing the above-mentioned positive electrode active material, conductive material, binder, etc. with a solvent onto a current collector, and performing drying treatment, pressure treatment, etc. can be formed. Examples of the conductive material include natural graphite, artificial graphite, and acetylene black. Examples of the binder include polyvinylidene fluoride, polytetrafluoroethylene, and polyamide acrylic resin.

(All-solid-state secondary battery)
The all-solid-state secondary battery includes the above-mentioned positive electrode for a solid-state electrolyte secondary battery. An all-solid-state secondary battery includes, in addition to a positive electrode for an all-solid-state secondary battery, a negative electrode, a solid electrolyte, and the like. Regarding the negative electrode, solid electrolyte, etc. in all-solid-state secondary batteries, for example, WO 2018/038037, JP 2022-25903, and JP 2018-125214 (the entire disclosure content of which is incorporated herein by reference) Those for all-solid-state secondary batteries, which are described in (incorporated in the specification) etc., can be used as appropriate.

The invention according to the present disclosure may include, for example, the following aspects.
[1] Having a layered structure, the ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium is 0.5 or more and less than 1, and the ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium is cobalt atoms preparing a lithium-transition metal composite powder having a molar ratio of 0 or more and less than 0.5, and contacting the lithium-transition metal composite powder with a cobalt raw material to obtain a cobalt-attached composite oxide and obtaining a first heat-treated product by first heat-treating the cobalt-adhered composite oxide at a temperature of more than 600°C and less than 800°C, and bringing the first heat-treated product into contact with a niobium raw material to produce niobium. A positive electrode active for a secondary battery, comprising: obtaining an adhered composite oxide; and performing a second heat treatment on the niobium adhered composite oxide at a temperature of more than 300°C and less than 500°C to obtain a second heat-treated product. A method of manufacturing a substance.

[2] The lithium transition metal composite powder has a ratio D ₅₀ /D _SEM of 50% particle diameter D ₅₀ of the cumulative particle size distribution based on volume to the average particle diameter D _SEM based on electron microscopic observation, which is 1 or more and 4 or less. The method for producing a positive electrode active material for a secondary battery according to [1].

[3] The lithium transition metal composite powder has a ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium of 0.6 or more and less than 1. A method for producing a positive electrode active material for a next battery.

[4] For a secondary battery according to any one of [1] to [3], wherein obtaining the cobalt-adhered composite oxide includes dry mixing the lithium transition metal composite powder and the cobalt raw material. A method for producing a positive electrode active material.

[5] In obtaining the cobalt-attached composite oxide, the total molar amount of cobalt atoms contained in the cobalt raw material is relative to the total molar amount of metal atoms other than lithium contained in the lithium transition metal composite powder. The method for producing a positive electrode active material for a secondary battery according to any one of [1] to [4], wherein the content is 1 mol % or more and 20 mol % or less.

[6] The method for producing a positive electrode active material for a secondary battery according to any one of [1] to [5], wherein the cobalt raw material is cobalt oxide.

[7] The method for producing a positive electrode active material for a secondary battery according to any one of [1] to [6], wherein the temperature of the first heat treatment is 650°C or higher and 750°C or lower.

[8] In obtaining the niobium-adhered composite oxide, the total molar amount of niobium atoms contained in the niobium raw material is 0.0% relative to the total molar amount of metal atoms other than lithium contained in the first heat-treated product. The method for producing a positive electrode active material for a secondary battery according to any one of [1] to [7], wherein the content is 1 mol % or more and 5 mol % or less.

[9] The method for producing a positive electrode active material for a secondary battery according to any one of [1] to [8], wherein the temperature of the second heat treatment is 350°C or more and 450°C or less.

[10] The positive electrode active for a secondary battery according to any one of [1] to [9], wherein the niobium-attached composite oxide is obtained by contacting the first heat-treated product with a solution containing the niobium raw material. A method of manufacturing a substance.

[11] The method for producing a positive electrode active material for a secondary battery according to any one of [1] to [10], wherein the niobium raw material is niobic acid.

[12] The lithium transition metal composite powder is any one of [1] to [11], wherein the ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium is 0.6 or more and less than 0.8. A method for producing a positive electrode active material for a secondary battery according to claim 1.

[13] The lithium transition metal composite powder has a surface composition determined by surface elution analysis in which the ratio of the number of moles of cobalt atoms to the total number of moles of metal atoms other than lithium is 0.15 or more and 0.5 or less. The method for producing a positive electrode active material for a secondary battery according to any one of [1] to [11].

[14] The method for producing a positive electrode active material for a secondary battery according to any one of [1] to [13], wherein the lithium transition metal composite powder 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, 0.5≦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 member selected from the group consisting of Al and Mn, and ^M2 is 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.)

[15] Cobalt atoms have a layered structure, the ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium is 0.5 or more and less than 1, and the ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium The lithium transition metal composite oxide includes a lithium transition metal composite oxide having a composition in which the molar ratio of is 0.01 or more and less than 0.5. The first region has a secondary particle surface and the depth from the secondary particle surface is around 60 nm as a first region, and the region around 10 nm from the secondary particle surface as a second region. A positive electrode active material for a secondary battery in which the second region has a higher cobalt concentration than the second region.

[16] The positive electrode for a secondary battery according to [15], wherein the lithium transition metal composite oxide has a ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium of 0.6 or more and less than 1. active material.

[17] The lithium transition metal composite oxide has a ratio D ₅₀ /D _SEM of 50% particle diameter D ₅₀ of a volume-based cumulative particle size distribution to an average particle diameter D _SEM based on electron microscopic observation, which is 1 or more and 4 or less. The positive electrode active material for a secondary battery according to [15] or [16].

[18] The difference in the ratio of the number of moles of cobalt atoms to the total number of moles of metal atoms other than lithium in the first region and the second region is determined by the difference in depth from the surface of the first region and the second region. The positive electrode active material for a secondary battery according to any one of [15] to [17], wherein the absolute value of the difference obtained by dividing is 0.001 (nm ⁻¹ ) or more and 0.08 (nm ⁻¹ ) or less.

[19] The lithium transition metal composite oxide has a ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium of 0.6 or more and less than 0.8,
The positive electrode active material for a secondary battery according to any one of [15] to [18], wherein the surface of the secondary particles has an SED standard deviation of niobium of 5.0 or less as determined by SEMEDX measurement.

[20] The positive electrode active material for a secondary battery according to any one of [15] to [19], wherein the lithium transition metal composite oxide has a composition represented by the following formula (1).
(1) _Lip Ni _x Co _y M ¹ _z M ² _w O ₂
(0.95≦p≦1.5, 0.5≦x<1, 0.01≦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, and M ² is 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.)

[21] A positive electrode for a secondary battery, comprising a positive electrode active material layer containing the positive electrode active material for a secondary battery according to [20].

[22] A secondary battery comprising the positive electrode for a secondary battery according to [21], a negative electrode, and an electrolyte.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited to these Examples.

(Reference example 1)
A lithium transition metal composite powder having a composition represented by the composition formula: Li _1.05 Ni _0.887 Co _0.03 Mn _0.07 Al _0.01 Zr _0.003 O ₂ was prepared by a known method. Table 1 shows the composition ratios of metal elements other than Li.

(Example 1)
(First adhesion step and first heat treatment step)
1000 g of the lithium transition metal composite powder prepared in Reference Example 1 and 17.6 g of cobalt oxide (Co ₃ O ₄ ) were mixed for 5 minutes using a dry mixer. The number of moles of cobalt atoms contained in the cobalt oxide used was 2.0% with respect to the total number of moles of metal atoms other than lithium contained in the lithium-transition metal composite powder. After that, heat treatment is performed at 705°C for 6 hours in an oxygen atmosphere to obtain a composition represented by the composition formula Li _1.03 Ni _0.866 Co _0.051 Mn _0.07 Al _0.01 Zr _0.003 O ₂ A first heat-treated product was obtained.

(Second adhesion process and second heat treatment process)
Niobic acid (Nb ₂ O ₅ .H ₂ O), ammonia (NH ₃ ), and hydrogen peroxide (H ₂ O ₂ ) were dissolved in water as a solvent to obtain a niobium aqueous solution. The concentration of niobium in this aqueous niobium solution was 0.11 mol/L, the concentration of ammonia was 0.44 mol/L, and the concentration of hydrogen peroxide was 2.4 mol/L. 1000 g of the cobalt-adhered composite oxide obtained above was charged into a tumbling fluidized bed dryer, and fluidized at an air flow rate of 0.7 m ³ /hr and a set temperature of 140°C, and the above niobium aqueous solution was sprayed at a flow rate of 9 g/min. The liquid was sprayed for 100 minutes. Thereafter, after fluidizing and drying at an air flow rate of 0.7 m ³ /min and a set temperature of 140°C for 10 minutes, the obtained composite oxide was heat-treated at 350°C for 5 hours in an air atmosphere, and the composition A positive electrode active material according to Example 1 having a composition represented by the formula: Li _1.02 Ni _0.856 Co _0.051 Mn _0.07 Al _0.01 Zr _0.003 Nb _0.01 O ₂ was obtained. . Table 1 shows the composition ratios of metal elements other than Li. Note that Co in the compositional formula also includes Co in the second region. FIG. 1 shows a SEM image of the obtained positive electrode active material.

(Comparative example 1)
A positive electrode active material and a battery were produced in the same manner as in Example 1, except that the attachment step 1 and the heat treatment step 1 were not performed. Table 1 shows the composition ratio of metal elements other than Li.

Example 2
The lithium transition metal composite powder to be prepared is represented by Li _1.02 Ni _0.708 Co _0.081 Mn _0.202 Al _0.005 Zr _0.003 O ₂ and the temperature of the first heat treatment step is 660°C. The composition formula Li _1.03 Ni _0.688 Co _0.0981 Mn _0.196 Al _{0.005 Zr 0.003 Nb 0.01} O ₂ was prepared in the same manner as in Example 1 except that the temperature was A positive electrode active material according to Example 2 was obtained.

Example 3
The composition formula Li _1.03 Ni _0.688 Co _0.0981 Mn _0.196 Al _0.005 Zr _0.003 Nb ₀ was prepared in the same manner as in Example 2 except that the temperature of the first heat treatment step was 700°C. A positive electrode active material according to Example 3, expressed as _.01 O ₂ , was obtained.

Example 4
The composition formula Li _1.03 Ni _0.688 Co _0.0981 Mn _0.196 Al _0.005 Zr _0.003 Nb ₀ was prepared in the same manner as in Example 2 except that the temperature of the first heat treatment step was 740°C. A positive electrode active material according to Example 4, expressed as _.01 O ₂ , was obtained.

Example 5
The lithium transition metal composite powder to be prepared is represented by Li _1.02 Ni _0.723 Co _0.063 Mn _0.202 Al _0.005 Zr _0.003 O ₂ and the cobalt oxide used in the first attachment step. The composition formula Li _1.03 Ni _0.688 Co _0.0981 Mn _0.196 Al _0.005 Zr _0.003 Nb _0.01 O was prepared in the same manner as in Example 3 except that the amount of was 35.6 g. A positive electrode active material according to Example 5, represented by No. ₂ , was obtained.

Example 6
The composition formula Li _1.03 Ni _0.688 Co _0.0981 Mn _0.196 Al _0.005 Zr _0.003 Nb ₀ was prepared in the same manner as in Example 5 except that the temperature of the first heat treatment step was 740°C. A positive electrode active material according to Example 6, expressed as _.01 O ₂ , was obtained.

Example 7
The lithium transition metal composite powder to be prepared is represented by Li _1.00 Ni _0.739 Co _0.047 Mn _0.212 Al _0.005 Zr _0.003 O ₂ and the cobalt oxide used in the first attachment step. The composition formula Li _1.01 Ni _0.688 Co _0.0981 Mn _0.196 Al _0.005 Zr _0.003 Nb _0.01 O was prepared in the same manner as in Example 6 except that the amount of was 54.6 g. A positive electrode active material according to Example 5, represented by No. ₂ , was obtained.

Comparative example 2
In the same manner as Comparative Example 1, except that the lithium transition metal composite powder to be prepared is represented by Li _1.08 Ni _0.700 Co _0.10 Mn _0.20 Al _0.005 Zr _0.003 O ₂ , A positive electrode active material according to Comparative Example 2 represented by the composition formula Li _1.00 Ni _0.739 Co _0.047 Mn _0.212 Al _0.005 Zr _0.003 O ₂ was obtained.

Reference example 2
A positive electrode active material having a compositional formula of Li _1.08 Ni _0.700 Co _0.10 Mn _0.20 Al _0.005 Zr _0.003 O ₂ was prepared. Co coating and Nb coating were not performed.

Table 2 shows the lithium ratio, Co coating amount, and Nb coating amount for Examples 2 to 7 and Comparative Example 2.

(Evaluation of SED standard deviation)
SEM-EDX measurements using FlatQuad (manufactured by Bruker) were performed on each of the positive electrode active materials obtained in Example 2, Example 3, Example 4, Example 6, Example 7, and Comparative Example 2. Ta. The measurement voltage was 5 kV, and the current value/z coordinate was set so that the maximum count value was 1850±100 cps when a 50 μm thick Al foil was qualitatively analyzed at a magnification of 1000 times. The resolution during mapping measurement was 640 x 480 pixels. Mapping measurements were performed at a magnification such that the number of particles that fit into one screen was 1,000 to 3,000, and at a measurement time of 8,192 μsec/pixels. After the measurement, the raw data (Nb, O) of the EDX measurement was converted into text in CSV format. First, grain boundary separation was performed using the oxygen mapping results. After binarization using the minimum value between two peaks visible in the histogram as a threshold, the overlap was separated using the watershed method. Particle analysis was performed on the obtained images, and contour data of each particle was obtained. Particle analysis was performed for each particle in the Nb mapping image using contour data obtained from the oxygen mapping image, and the area (pixel) and average GRAY level of each particle were obtained. A histogram was created from the obtained results, with the horizontal axis representing the average gray level and the vertical axis representing the number of pixels. For gray level division, the step width was such that the distribution fell within 20 divisions. The average value and standard deviation were determined from the histogram, and the average value was taken as the average film thickness, the standard deviation was taken as the deviation between particles, and this was used as the evaluation index SED standard deviation for the uniformity of Nb. SEM images before mapping for Example 1 and Comparative Example 1 are shown in FIGS. 1 and 2, SEM images for Nb mapping images are shown in FIGS. 3 and 4, and the calculation results of the SED standard deviation are shown in Table 3.

Surface Composition Analysis The surface compositions of the lithium transition metal composite powders prepared in Examples 2, 3, 4, 6, 7, and Comparative Example 2 were determined according to the following procedure.
(1) 0.20 g of lithium transition metal composite powder was accurately weighed in a polybeaker.
(2) 10 mL of a buffer solution consisting of citric acid and trisodium citrate, having a pH of 5.8, and maintained at 20° C. was added to the polybeaker.
(3) By stirring the buffer solution for 4 minutes with a stirrer while maintaining the temperature of the buffer solution at 20° C., an eluate in which the metal on the surface of the lithium-transition metal composite powder was eluted was obtained.
(4) A filtrate was obtained by filtering the eluate using a plastic syringe equipped with a syringe filter.
(5) 0.5 mL of 6M HCl was added to 1 mL of the filtrate, and the mixture was diluted to 50 mL with pure water to obtain a diluted solution.
(6) The surface composition of the lithium-transition metal composite powder was determined by performing ICP measurement on the diluted solution.
The cobalt ratio (Co/Me) is the molar ratio of cobalt to the total number of moles of metal components other than lithium, and the nickel ratio (Ni/Me) is the molar ratio of nickel to the total number of moles of metal components other than lithium. The manganese ratio (Mn/Me) was defined as the molar ratio of manganese to the total number of moles of metal components other than lithium. The results are shown in Table 3.

Regarding the surface composition of the lithium-transition metal composite powder, the higher the Co/Me content, the lower the SED standard deviation, that is, the more uniform the coating with the niobium compound was. This is considered to be because the Co coating reduced the alkaline component due to excess lithium on the surface of the lithium-transition metal composite powder, and suppressed aggregation of the Nb compound due to the alkaline component.

(solid electrolyte)
An argyrodite-type sulfide having an average particle size of 10 μm and a composition of Li _5.4 PS _4.4 Cl _1.6 was used as the solid electrolyte.

A positive electrode composite material was obtained by mixing 70 parts by mass of the positive electrode active material obtained in Example 1 and Comparative Example 1, 27 parts by mass of solid electrolyte, and 3 parts by mass of VGCF (registered trademark), which is vapor grown carbon fiber. .

(Assembling the evaluation battery)
A cylindrical lower mold with an outer diameter of 11 mm was inserted into a cylindrical outer mold with an inner diameter of 11 mm from the lower part of the outer mold. The upper end of the lower mold was fixed at a position in the middle of the outer mold. In this state, 100 mg of solid electrolyte was poured from the upper part of the outer mold to the upper end of the lower mold. After charging, a cylindrical upper mold having an outer diameter of 11 mm was inserted from the upper part of the outer mold. After insertion, a pressure of 50 MPa was applied from above the upper die to mold the solid electrolyte into a solid electrolyte layer. After molding, the upper mold was pulled out from the upper part of the outer mold, and 20 mg of the positive electrode mixture was put into the upper part of the solid electrolyte layer from the upper part of the outer mold. After charging, the upper mold was inserted again, and this time a pressure of 600 MPa was applied to mold the positive electrode composite material to form a positive electrode active material layer. After molding, the upper mold was fixed, the lower mold was released and pulled out from the lower part of the outer mold, and the LiAl alloy as the negative electrode active material was introduced from the lower part of the lower mold to the lower part of the solid electrolyte layer. After charging, the lower mold was inserted again, and a pressure of 50 MPa was applied from below the lower mold to mold the negative electrode active material to form a negative electrode active material layer. The lower mold was fixed under pressure, and the positive electrode terminal and the negative electrode terminal were attached to the upper mold and the lower mold, respectively, to obtain an all-solid-state secondary battery for evaluation.

(D _SEM measurement)
D _SEM was determined for the positive electrode active materials of Example 1, Comparative Example 1, Reference Example 1, and Reference Example 2 according to the following procedure.
<1> Using a scanning electron microscope (Hitachi High-Technologies Corporation, SU8230), the magnification was set so that the number of secondary particles whose outlines could be confirmed was 10 or more and 20 or less. Specifically, the accelerating voltage was 1.5 kV and the magnification was 4000 times. At this time, secondary particles whose particle size was less than half of _D10 were not included in the number.
<2> For all the secondary particles whose particle size is half or more of _D10 , which are seen at the above magnification, use image processing software (ImageJ) to analyze the primary particles that constitute each of the primary particles. Find the contour length by tracing the contour. The equivalent sphere diameter was calculated from the contour length.
<3> The above <1> and <2> were repeated until the number of primary particles whose particle diameters were calculated exceeded 100, and the average particle diameter D _SEM was determined as the arithmetic mean value of the obtained sphere-equivalent diameters. Table 4 also shows the results of determining the ratio of D ₅₀ to D _SEM .

For each positive electrode active material, the BET specific surface area was measured using a BET specific surface area measuring device (manufactured by Mountec: Macsorb) by a gas adsorption method (one point method) using nitrogen gas. The results are shown in Table 5.

(Particle size evaluation)
Physical property values of each positive electrode active material were measured as follows. Using a laser diffraction particle size distribution measuring device (SALD-3100 manufactured by Shimadzu Corporation), the volume-based cumulative particle size distribution was measured, and _D50 was defined as the particle size corresponding to 50% of the cumulative size from the small diameter side. Similarly, D ₁₀ was determined as the particle size corresponding to 10% accumulation, D ₉₀ was determined as the particle diameter corresponding to 90% accumulation, and D ₉₀ /D ₁₀ was determined from the obtained values. The results are shown in Table 5.

(tap density)
A tapping type powder reduction meter TPM-3P (Tsutsui Rikagaku Kikai) was used to measure the tap density. 20 g of each positive electrode active material was placed in a 20 mL measuring cylinder as a measurement container, the number of shaking was set to 150, and the volume density after shaking was determined as the tap density. The results are shown in Table 5.

(Measurement of nickel disorder and crystallinity)
X-ray diffraction spectra (tube current 200 mA, tube voltage 45 kV) were measured using CuKα rays for each of the positive electrode active materials obtained in Example 1, Comparative Example 1, and Reference Example. Further, the crystallinity was calculated by substituting the peak position and integral width due to the lattice plane (104) obtained from the measured X-ray diffraction spectrum into the Scherrer equation. The disorder of the nickel element in the lithium-transition metal composite oxide can be determined by using the composition model as (Li _1-d Ni _d ) ( _Nix Co _y Mn _z )O ₂ (x+y+z=1) and using the obtained X-ray diffraction spectrum as follows: Based on this, it was determined by performing structural optimization using Rietveld analysis. The percentage of d in the above composition model calculated as a result of structure optimization was taken as the value of disorder of the nickel element. The results obtained are shown in Table 5.

(Measurement of Nb content)
The Nb content of the positive electrode active materials obtained in Example 1 and Comparative Example 1 was measured using an inductively coupled plasma emission spectrometer (ICP-AES; manufactured by PerkinElmer). Table 5 shows the determined mass content of Nb in the positive electrode active material.

(Measurement of unreacted lithium in positive electrode active material)
The amount of unreacted lithium in the positive electrode active materials obtained in Example 1, Comparative Example 1, and Reference Example was measured as follows. First, 10 g of each positive electrode active material was collected, placed in a container with a lid, added with 50 mL of pure water, covered with a lid, and stirred for 60 minutes. After standing still, the supernatant liquid was filtered. The filtrate was titrated with a 0.025 mol/L sulfuric acid standard solution, and the inflection point at around pH 8 was taken as the first end point, and the inflection point around pH=4 was taken as the second end point. The amount of LiOH and the amount of Li ₂ CO ₃ were calculated from the titration values at the first end point and the second end point based on the principle of the Warder method. The results obtained are shown in Table 5.

In Example 1, the amount of LiOH and the amount of Li ₂ CO ₃ are smaller than those in Comparative Example 1 and Reference Example. This is considered to be because cobalt oxide was consumed by reacting with excess lithium on the particle surface.

(SEM-EDX-ray analysis)
After each of the positive electrode active materials obtained in Example 1 was dispersed in an epoxy resin and solidified, a measurement sample was prepared by cross-sectioning the secondary particles of the positive electrode active material using a cross-section polisher (manufactured by JEOL). did. The cross-sectional measurement sample was subjected to line analysis using a scanning electron microscope (SEM)/energy dispersive X-ray analysis (EDX) device (manufactured by Hitachi High-Technologies, acceleration voltage 3 kV) to determine the surface area and internal area. A compositional analysis was conducted. FIG. 1 shows an SEM image of the surface region, FIG. 2 shows the results of compositional analysis by line analysis, and Table 6 shows the results of the Co concentration gradient determined from the analysis results.

(impedance measurement)
The all-solid-state secondary battery for evaluation was charged and set to a state of charge (SOC) of 50%. It was connected to an AC power source at 25° C. and resistance was measured using the AC impedance method. The frequency of the AC power source was varied logarithmically from 1 MHz to 0.1 Hz. Assuming an equivalent circuit, the diameter of an arc appearing in a frequency range of 1000 Hz or more and 5000 Hz or less was determined as the resistance derived from the positive electrode active material (resistance component in the impedance of the positive electrode/electrolyte interface) by fitting using the least squares method. The results are shown in Table 7.

(Charge/discharge evaluation)
The produced evaluation battery was charged and discharged under conditions of 2.2V to 4.0V using a charge/discharge test device (TOSCAT-3100, manufactured by Toyo System Co., Ltd.). The discharge current is the current value when taking out the 0.1C capacity, and after reaching the set voltage, the current is passed to keep the voltage constant, and when the current value reaches the equivalent of 0.02C, charging and discharging is started. It ended. Table 7 shows the obtained charging capacity, discharging capacity, and charging/discharging efficiency.

From the results in Table 7, the impedance of Example 1 was lower than that of Comparative Example 1. This is because the positive electrode active material according to Example 1 has a Co concentration gradient due to the second region, and it is considered that this improves the resistance. Example 1 exceeds Comparative Example 1 in charge capacity, discharge capacity, and charge/discharge efficiency, and this is considered to be due to a decrease in overvoltage due to a decrease in resistance.

Claims (22)

It has a layered structure, the ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium is 0.5 or more and less than 1, and the number of moles of cobalt atoms to the total number of moles of metal atoms other than lithium. preparing a lithium transition metal composite powder having a ratio of 0 or more and less than 0.5;
contacting the lithium transition metal composite powder with a cobalt raw material to obtain a cobalt-attached composite oxide;
Performing a first heat treatment on the cobalt-attached composite oxide at a temperature of more than 600°C and less than 800°C to obtain a first heat-treated product;
Bringing the first heat-treated product into contact with a niobium raw material to obtain a niobium-adhered composite oxide;
A method for producing a positive electrode active material for a secondary battery, comprising subjecting the niobium-adhered composite oxide to a second heat treatment at a temperature of more than 300°C and less than 500°C to obtain a second heat-treated product. 1. The lithium transition metal composite powder has a ratio D ₅₀ /D _SEM of 50% particle diameter D ₅₀ of a volume-based cumulative particle size distribution to an average particle diameter D _SEM based on electron microscopy observation of 1 or more and 4 or less. A method for producing a positive electrode active material for a secondary battery as described in . The positive electrode active material for a secondary battery according to claim 2, wherein the lithium transition metal composite powder has a ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium of 0.6 or more and less than 1. Production method. 4. The method for producing a positive electrode active material for a secondary battery according to claim 3, wherein obtaining the cobalt-attached composite oxide includes dry mixing the lithium transition metal composite powder and the cobalt raw material. In obtaining the cobalt-attached composite oxide, the total molar amount of cobalt atoms contained in the cobalt raw material is 1 mol% with respect to the total molar amount of metal atoms other than lithium contained in the lithium transition metal composite powder. The method for producing a positive electrode active material for a secondary battery according to claim 4, wherein the content is 20 mol% or less. The method for producing a positive electrode active material for a secondary battery according to claim 5, wherein the cobalt raw material is cobalt oxide. The method for producing a positive electrode active material for a secondary battery according to claim 6, wherein the temperature of the first heat treatment is 650°C or more and 750°C or less. In obtaining the niobium-attached composite oxide, the total molar amount of niobium atoms contained in the niobium raw material is 0.1 mol% with respect to the total molar amount of metal atoms other than lithium contained in the first heat-treated product. The method for producing a positive electrode active material for a secondary battery according to claim 7, wherein the content is 5 mol% or less. The method for producing a positive electrode active material for a secondary battery according to claim 8, wherein the temperature of the second heat treatment is 350°C or more and 450°C or less. 10. The method for producing a positive electrode active material for a secondary battery according to claim 9, wherein the niobium-attached composite oxide is obtained by contacting the first heat-treated product with a solution containing the niobium raw material. The method for producing a positive electrode active material for a secondary battery according to claim 10, wherein the niobium raw material is niobic acid. The positive electrode active for a secondary battery according to claim 1, wherein the lithium transition metal composite powder has a ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium of 0.6 or more and less than 0.8. A method of manufacturing a substance. The lithium transition metal composite powder has a surface composition determined by surface elution analysis, in which the ratio of the number of moles of cobalt atoms to the total number of moles of metal atoms other than lithium is 0.15 or more and 0.5 or less. 13. The method for producing a positive electrode active material for a secondary battery according to 12. The manufacturing method according to any one of claims 1 to 13, wherein the lithium transition metal composite powder 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, 0.5≦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 member selected from the group consisting of Al and Mn, and ^M2 is 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.) It has a layered structure, and the ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium is 0.5 or more and less than 1,
A lithium transition metal composite oxide having a composition in which the ratio of the number of moles of cobalt atoms to the total number of moles of metal atoms other than lithium is 0.01 or more and less than 0.5,
The lithium transition metal composite oxide has a secondary particle surface containing a niobium compound on at least a part of the particle surface,
When a region with a depth of around 60 nm from the surface of the secondary particle is defined as a first region, and a region with a depth of around 10 nm from the surface of the secondary particle as a second region, the second region is larger than the first region. A positive electrode active material for secondary batteries with a high cobalt concentration. 16. The positive electrode active material for a secondary battery according to claim 15, wherein the lithium transition metal composite oxide has a ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium of 0.6 or more and less than 1. The lithium transition metal composite oxide has a ratio D ₅₀ /D _SEM of 50% particle size D ₅₀ of the cumulative particle size distribution based on volume to the average particle size D _SEM based on electron microscopic observation, which is 1 or more and 4 or less. The positive electrode active material for a secondary battery according to claim 15. The difference obtained by dividing the difference in the ratio of the number of moles of cobalt atoms to the total number of moles of metal atoms other than lithium in the first region and the second region by the difference in depth from the surface of the first region and the second region. The positive electrode active material for a secondary battery according to claim 15, wherein the absolute value of is 0.001 (nm ^-1 ) or more and 0.08 (nm ^-1 ) or less. The lithium transition metal composite oxide has a ratio of the number of moles of nickel atoms to the total number of moles of metal atoms other than lithium of 0.6 or more and less than 0.8,
16. The positive electrode active material for a secondary battery according to claim 15, wherein the surface of the secondary particles has an SED standard deviation of niobium of 5.0 or less as determined by SEMEDX measurement. The positive electrode active material for a secondary battery according to any one of claims 15 to 19, wherein the lithium transition metal composite oxide has a composition represented by the following formula (1).
(1) _Lip Ni _x Co _y M ¹ _z M ² _w O ₂
(0.95≦p≦1.5, 0.5≦x<1, 0.01≦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, and M ² is 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.) A positive electrode for a secondary battery, comprising a positive electrode active material layer containing the positive electrode active material for a secondary battery according to claim 20. A secondary battery comprising the positive electrode for a secondary battery according to claim 21, a negative electrode, and an electrolyte.

Images