JP2023080310A - Positive electrode active material for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

To propose a positive electrode active material for a lithium ion secondary battery, in which the particle surface of lithium metal composite oxide is coated with tungsten at a high coverage, and which can reduce a positive electrode resistance (reaction resistance) to enhance the output characteristic while keeping a high battery capacity.SOLUTION: A positive electrode active material for a lithium ion secondary battery comprises: a particle of lithium metal composite oxide; and tungsten which coats at least part of the surface of the lithium metal composite oxide particle. According to measurement by X-ray photoelectron spectroscopy (XPS) on the positive electrode active material, the content of tungsten on the surface of the particle of the lithium metal composite oxide is over 50 mol% to a total content of all of metal elements, excluding lithium, on the particle surface.SELECTED DRAWING: Figure 1

Description

The present invention relates to a positive electrode active material for a lithium ion secondary battery and a lithium ion secondary battery.

In recent years, with the spread of mobile electronic devices such as smart phones and tablet PCs, there is a strong demand for development of small, lightweight non-aqueous electrolyte secondary batteries with high energy density. Further, there is a strong demand for the development of high-output secondary batteries as power sources for electric vehicles such as hybrid electric vehicles, plug-in hybrid electric vehicles, and battery-powered electric vehicles.

As a secondary battery that satisfies such requirements, there is a lithium ion secondary battery, which is a type of non-aqueous electrolyte secondary battery. This lithium-ion secondary battery is composed of a negative electrode, a positive electrode, an electrolyte, and the like, and a material capable of desorbing and intercalating lithium is used as an active material used for the negative electrode and the positive electrode. As the non-aqueous electrolyte, a non-aqueous electrolytic solution obtained by dissolving a lithium salt as a supporting salt in an organic solvent, a nonflammable solid electrolyte having ion conductivity, and the like are used.

For example, a lithium-ion secondary battery using a layered or spinel-type lithium metal composite oxide as a positive electrode active material can obtain a voltage of 4V class, so it is currently being researched and developed as a secondary battery having a high energy density. are being actively used, and some are even being put to practical use.

Examples of positive electrode active materials for lithium ion secondary batteries include lithium cobalt composite oxide (LiCoO ₂ ) which is relatively easy to synthesize, lithium nickel composite oxide (LiNiO ₂ ) using nickel which is cheaper than cobalt, Lithium-nickel-cobalt-manganese composite oxide (LiNi1 _{/ 3Co1} / _{3Mn1/ 3O2 )} , lithium-manganese composite oxide using manganese ( _LiMn2O4 ), lithium-nickel-manganese composite oxide (LiNi0 _{. 5} Mn _0.5 O ₂ ) have been proposed.

In order to obtain high output characteristics in a lithium ion secondary battery, it is important to reduce internal resistance, and for example, reduction of positive electrode resistance (reaction resistance) in the positive electrode is required. In particular, in an all-solid secondary battery using a solid electrolyte, further reduction of the interfacial resistance between the positive electrode active material and the solid electrolyte in the positive electrode is required.

As a technique for reducing the positive electrode resistance (reaction resistance) in the positive electrode active material, for example, the positive electrode active material is coated with a layer containing a different element (e.g., tungsten) other than the metal element contained in the lithium metal composite oxide. There are some suggestions to do.

For example, Patent Document 1 describes a coated active material used in a battery, which has an active material and a coating layer that coats the active material, and the coating layer is composed of a material containing a tungsten element. A coated active material characterized by According to Patent Document 1, an increase in interfacial resistance between an active material and a solid electrolyte material is caused by the reaction of the two to generate a high-resistance layer at the interface. said to be obtainable.

Further, Patent Document 2 discloses a positive electrode active material containing a first lithium metal composite oxide and a second compound containing lithium and tungsten, wherein the surface of the primary particles is coated with the second compound. and the tungsten content is 0.01% by mass or more and 1.0% by mass or less with respect to the entire positive electrode active material, and the positive electrode active material is dispersed in water, which is measured by a neutralization titration method. A positive electrode active material for a non-aqueous electrolyte secondary battery has been proposed in which the amount of lithium eluted when exposed to the entire positive electrode active material is 0.01% by mass or more and less than 0.4% by mass. According to Patent Document 2, a secondary battery using this positive electrode active material is said to have high output characteristics and charge/discharge capacity.

Further, Patent Document 3 discloses a mixing step of mixing lithium-nickel composite oxide particles and a lithium-free tungsten compound powder to obtain a tungsten mixture containing lithium-nickel composite oxide particles and tungsten compound particles, and Manufacture of positive electrode active material for non-aqueous electrolyte secondary battery having a heat treatment step of dispersing tungsten on the surface of primary particles by heat treatment to form lithium-nickel composite oxide particles in which a compound containing tungsten and lithium is provided on the surface of the primary particles A method is proposed. According to Patent Document 3, when used as a positive electrode material for a battery, a positive electrode active material capable of realizing high capacity and high output is obtained.

Further, in Patent Document 4, at least one selected from the group consisting of a fired powder made of a lithium metal composite oxide, an oxide not containing lithium, a hydrate of the oxide, and an inorganic acid salt not containing lithium mixing a first compound, which is also one species, with water; and drying the resulting mixture, wherein the first compound is mixed with lithium ions in the presence of water. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery, which is a compound capable of reacting to form a second compound containing lithium, has been proposed. According to Patent Document 4, it is said that a positive electrode active material can be obtained that can maintain battery capacity when a secondary battery is formed, high output characteristics, and excellent charge-discharge cycle characteristics at a high level. there is

WO2012/105048 WO2018/043515 JP 2017-134996 A WO2017/073238

In the above Patent Documents 1 to 4, a positive electrode active material is proposed in which the positive electrode resistance (reaction resistance) is reduced and the output characteristics are improved by coating with a dissimilar element containing tungsten. as a positive electrode active material, further reduction in positive electrode resistance (reaction resistance) is required while maintaining a high battery capacity.

As a result of intensive studies, the present inventors have found that a conventional positive electrode active material coated with a layer containing tungsten does not have a sufficient tungsten coverage. Then, the inventors found that by further improving the coverage of tungsten on the surface of the lithium metal composite oxide particles, it is possible to further reduce the positive electrode resistance (reaction resistance) while maintaining a high battery capacity.

In view of the above problems, the present invention has a high tungsten coverage, and when used as a positive electrode for a lithium ion secondary battery, while maintaining a high battery capacity, the positive electrode resistance (reaction resistance) is increased. An object of the present invention is to provide a positive electrode active material that can have a significantly reduced power output and high output characteristics. Another object of the present invention is to provide a method for producing such a positive electrode active material on an industrial scale easily and with high productivity.

According to a first aspect of the present invention, there is provided a positive electrode active material obtained by mixing particles of a lithium metal composite oxide and particles of a compound containing tungsten, wherein the particles of the lithium metal composite oxide and lithium and a layer containing tungsten that covers at least part of the surface of the metal composite oxide particles, and a tungsten content on the surface of the lithium metal composite oxide particles by measurement using X-ray photoelectron spectroscopy (XPS) A positive electrode active material for a lithium ion secondary battery is provided in which the content exceeds 50 mol % with respect to the content of all metal elements other than lithium on the surface of the particles.

Moreover, the content of tungsten with respect to the entire positive electrode active material is preferably more than 1% by mass and 4% by mass or less. At least part of the layer containing tungsten preferably has a thickness of 10 nm or less. In addition, the lithium metal composite oxide particles contain lithium (Li), nickel (Ni), cobalt (Co), and element M, and the material amount ratio (molar ratio) of these elements is Li:Ni:Co : M = s: (1-xy): x: y (where 0.95 ≤ s ≤ 1.30, 0 ≤ x ≤ 0.35, 0 ≤ y ≤ 0.35, M is Mn, one or more elements selected from Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, Nb, Hf, and Ta).

According to the second aspect of the present invention, in a decarboxylation atmosphere, the particles of the lithium metal composite oxide are mixed by spraying water to obtain a first mixture; the first mixture; and drying the second mixture to coat at least part of the surface of the lithium metal composite oxide particles with a layer containing tungsten The lithium metal composite oxide particles obtained after drying are measured by X-ray photoelectron spectroscopy (XPS), and the tungsten content on the surface of the particles is equal to the lithium on the surface of the particles Provided is a method for producing a positive electrode active material for a lithium-ion secondary battery, which has a content of all metal elements other than 50 mol %.

Moreover, water is preferably mixed in an amount of 2% by mass or more with respect to the whole particles of the lithium metal composite oxide. Moreover, it is preferable to mix the compound containing tungsten so that the amount of tungsten with respect to the whole particles of the lithium metal composite oxide is more than 1% by mass and 4% by mass or less. Further, the compound containing tungsten preferably contains tungsten oxide. The lithium metal composite oxide particles contain lithium (Li), nickel (Ni), cobalt (Co), and an element M, and the material amount ratio (molar ratio) of these elements is Li:Ni:Co:M = s: (1-xy): x: y (where 0.95 ≤ s ≤ 1.30, 0 ≤ x ≤ 0.35, 0 ≤ y ≤ 0.35, M is Mn, Mo, one or more elements selected from V, Mg, Ca, Al, Ti, Cr, Zr, La, Nb, Hf, and Ta). Moreover, drying is preferably performed at 50° C. or higher and 300° C. or lower.

According to a third aspect of the present invention, there is provided a lithium ion secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode contains the positive electrode active material described above.

In the positive electrode active material of the present invention, the surface of the lithium metal composite oxide particles is coated with tungsten at a very high coverage rate, and a lithium ion secondary battery using this positive electrode active material for the positive electrode has a high battery capacity. is maintained, the positive electrode resistance (reaction resistance) can be reduced, and the output characteristics can be improved. Moreover, according to the method for producing a positive electrode active material of the present invention, the positive electrode active material as described above can be produced easily on an industrial scale with high productivity.

FIG. 1 is a diagram schematically showing an example of the positive electrode active material according to this embodiment. FIG. 2 is a diagram showing an example of a method for producing a positive electrode active material according to this embodiment. FIG. 3 is a graph showing the tungsten coverage (mol %) and the total carbon content (mass %) of the base material used in the example, the positive electrode active materials obtained in Comparative Examples 8 and 9. is. 4 is a graph showing the tungsten coverage (mol %) of the positive electrode active materials obtained in Comparative Examples 6, 7, 1, and 3. FIG. FIG. 5 is a graph showing the tungsten coverage (mol %) of the positive electrode active materials obtained in Comparative Example 9 and Example 1, and the initial discharge capacity (relative value). 6A and 6B are SEM backscattered electron images (COMPO images) of the positive electrode active materials obtained in Comparative Example 9 and Example 1, respectively. FIG. 7 is a schematic cross-sectional view of a coin-type battery used for battery evaluation. FIG. 8 is a schematic explanatory diagram of an example of impedance evaluation measurement and an equivalent circuit used for analysis. FIG. 9 is a graph showing the relationship between the tungsten content (mass %) of the positive electrode active materials obtained in Examples and Comparative Examples and the tungsten coverage (mol %). 10A and 10B are SEM backscattered electron images (COMPO images) of the positive electrode active material (surface and cross section) obtained in Example 1, in place of photographs.

Hereinafter, a positive electrode active material for a lithium ion secondary battery (hereinafter also referred to as a "secondary battery"), a manufacturing method thereof, and a lithium ion secondary battery according to embodiments will be described with reference to the drawings. It should be noted that the present invention is not limited to the embodiments described below. In addition, in the drawings, in order to make each configuration easy to understand, some parts are emphasized or some parts are simplified, and the actual structure, shape, scale, etc. may differ.

1. 1. Positive Electrode Active Material FIG. 1A is a schematic diagram showing an example of the positive electrode active material of the present embodiment. The positive electrode active material 100 has lithium metal composite oxide particles 10 and a layer WL containing tungsten. The layer WL containing tungsten covers at least part of the surface of the lithium metal composite oxide particles 10 . The positive electrode active material according to this embodiment will be described below with reference to FIG.

(1) Layer Containing Tungsten In the positive electrode active material 100, the presence of the layer WL containing tungsten W on the surface of the lithium metal composite oxide particles 10 improves lithium ion conductivity. Positive electrode resistance can be reduced. In addition, since the cathode active material 100 has a high coverage of tungsten (W) on the surface of the lithium metal composite oxide particles 10 (the entire surface of the powder), tungsten is more uniformly distributed throughout the particles (powder). When coated and used for the positive electrode of a secondary battery, the positive electrode resistance can be reduced, and the decrease in battery capacity due to the coating of the dissimilar element can be suppressed.

[Content of tungsten on particle surface]
The content of tungsten on the surface of the lithium metal composite oxide particles 10 measured by X-ray photoelectron spectroscopy (XPS) is the same as that of all metal elements other than lithium on the surface of the particles 10. content is more than 50 mol %, preferably 55 mol % or more, more preferably 60 mol % or more, and still more preferably 65 mol % or more. When the content of tungsten is within the above range, the surface of the lithium metal composite oxide particles 10 (the surface of the entire powder) can be more uniformly coated with tungsten (W).

The upper limit of the content of tungsten in the surface of the lithium metal composite oxide particles 10 is not particularly limited, and may be, for example, 90 mol% or less, 80 mol% or less, or 75 mol. % or less.

As a result of extensive studies by the present inventors, it was found that the positive electrode active material having a layer containing tungsten obtained by a conventional manufacturing method does not have a sufficient tungsten coverage. , Even if the amount of tungsten added is increased, the content (mol%) of tungsten on the surface of the lithium metal composite oxide particles 10 is about 30 to 35 mol%, and there is a tendency not to increase beyond that. was found (see Comparative Examples 4 to 6, 8, and 10 in FIG. 9). On the other hand, in the positive electrode active material 100 according to the present embodiment, for example, by using the manufacturing method described later, the content (mol%) of tungsten on the surface of the lithium metal composite oxide particles 10 can be set within the above range. can.

In addition, usually, when the surface of the lithium metal composite oxide particles 10 is coated with a different element, in a secondary battery using this, battery characteristics such as battery capacity (eg, initial charge/discharge capacity) may decrease. However, in the positive electrode active material 100, by setting the tungsten content (mol%) on the surface of the lithium metal composite oxide particles 10 to the above range, a high battery capacity is maintained while further positive electrode resistance (reaction resistance ) can be reduced. Therefore, when the positive electrode active material 100 according to the present embodiment is used for the positive electrode of a lithium ion secondary battery (including a non-aqueous electrolyte secondary battery and an all-solid secondary battery), battery characteristics can be improved. can.

The content (mol %) of tungsten described above is one of indices indicating the coverage of the layer WL containing tungsten on the surface of the lithium metal composite oxide particle 10 . Therefore, in this specification, the tungsten content (mol %) on the surface of the lithium metal composite oxide particles 10 is also simply referred to as "tungsten coverage (mol %)".

In the present specification, the tungsten content (mol%) on the surface of the lithium metal composite oxide particles 10 is defined by an X-ray photoelectron spectrometer (ESCALAB220i-XL, manufactured by VG-Scientific) with an Al and the X-ray source is Al-Kα ray, the voltage is 10 kV, the current is 15 mA, and the lithium metal composite oxide particles 10 are measured by X-ray photoelectron spectroscopy (XPS). It is a value obtained from the intensity (area) ratio of peaks derived from each metal element.

[Content of tungsten in whole positive electrode active material]
The content of tungsten in the entire positive electrode active material 100 is not particularly limited, and can be appropriately adjusted depending on the characteristics (particle size, particle structure, etc.) of the lithium metal composite oxide particles 10. For example, 1% by mass. more than 4% by mass, preferably more than 1% by mass and 2% by mass or less, more preferably 1.2% by mass or more and 2% by mass or less. When the content of tungsten with respect to the entire positive electrode active material 100 is within the above range, both a high tungsten coverage (mol%) and a reduction in battery capacity when the positive electrode active material 100 is used in a secondary battery are to be achieved. can be done.

When the content of tungsten is within the above range, the surface of the lithium metal composite oxide particles 10 (the surface of the entire powder) can be more uniformly coated with tungsten, and the battery capacity (e.g., initial charge/discharge capacity etc.) can be suppressed. The content of tungsten (W) in the positive electrode active material (whole) can be measured by ICP emission spectrometry (ICP method).

[Thickness of layer containing tungsten]
At least part of the layer WL containing tungsten may have a thickness of 10 nm or less, may have a thickness of 5 nm or less, or may have a thickness of 1 nm or less. When the thickness of the layer WL containing tungsten is within the above range, the entire surface of the lithium metal composite oxide particles 10 can be more uniformly coated with tungsten, and the coating of the layer WL containing tungsten improves the battery capacity, etc. deterioration of the battery characteristics can be further suppressed (see, for example, FIG. 10B).

The thickness of the layer WL containing tungsten can be obtained from a backscattered electron image (COMPO image) obtained by a scanning electron microscope (SEM), STEM, SEM-EDS, or the like. It can be obtained by the method described in the example.

Note that the layer WL containing tungsten does not have to cover the entire surface of the lithium metal composite oxide particles 10 as long as the tungsten coverage (mol%) is within the range described above. Part of the surface of the particle 10 may be covered. Note that when the cross section of 10 or more randomly selected lithium metal composite oxide particles 10 is observed with a backscattered electron image (COMPO image) by SEM, the average thickness of the layer WL containing tungsten is 10 nm or less. It may be 5 nm or less.

Further, the form of existence of tungsten in the layer WL containing tungsten is not particularly limited, and may be, for example, a compound containing lithium and tungsten, or lithium tungstate. For example, when using the method for manufacturing the positive electrode active material 100 described later, a compound containing tungsten reacts with lithium present on the surface of the particles (base material) of the lithium metal composite oxide to form lithium tungstate. As a result, the amount of surplus lithium on the surface of the obtained lithium metal composite oxide particles 10 can be reduced. In addition, the existence form of tungsten can be confirmed by, for example, X-ray diffraction (XRD).

[Particles of Lithium Metal Composite Oxide]
FIG. 1B is a schematic diagram showing an example of particles 10 of lithium metal composite oxide. Particles 10 of lithium metal composite oxide are composed of secondary particles 2 formed by aggregation of a plurality of primary particles 1 . The shape of the secondary particles 2 is not particularly limited, and may have, for example, a solid structure, a hollow structure, a porous structure, or the like. The lithium metal composite oxide particles 10 may include not only the secondary particles 2 but also a small number of individual primary particles 1 . Also, the lithium-metal composite oxide particles 10 may be an oxide containing lithium and a metal other than lithium, and preferably have a hexagonal layered crystal structure. The layer WL containing tungsten described above may be formed on the surface of the secondary particle 2 or may be formed on the surface of the primary particle 1 inside the secondary particle 2 .

[Composition of positive electrode active material]
The positive electrode active material 100 includes, for example, lithium (Li), nickel (Ni), cobalt (Co), and an element M, and the material amount ratio (molar ratio) of these elements is Li:Ni:Co:M= s: (1-xy): x: y (where 0.95 ≤ s ≤ 1.3, 0 ≤ x ≤ 0.35, 0 ≤ y ≤ 0.35, M is Mn, Mo, V , Mg, Ca, Al, Ti, Cr, Zr, La, Nb, Hf, and Ta). When the positive electrode active material 100 has the above composition, high battery capacity and excellent output characteristics can be obtained when used for the positive electrode of a secondary battery. The content of each element can be measured by the ICP method.

Nickel (Ni) is an element that contributes to increasing the potential and capacity of secondary batteries. In the above molar ratio, the value of (1-xy) indicating the Ni content is, for example, 0.3 or more, preferably 0.6 or more, more preferably 0.65 or more and 0.99 or less, It is more preferably 0.8 or more and 0.95 or less. When the value of (1−x−y) exceeds 0.6, it can have a high battery capacity.

Cobalt (Co) is an element that contributes to the improvement of charge-discharge cycle characteristics. In the above molar ratio, the value of x indicating the Co content is 0 or more and 0.35 or less, preferably 0.05 or more and 0.35 or less, more preferably 0.1 or more and 0.3 or less. When the value of x is 0.05 or more, the charge/discharge cycle characteristics are improved.

M representing an additive element is at least one element selected from Mn, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, Nb, Hf, and Ta. In the above molar ratio, the value of y indicating the content of M is 0 or more and 0.35 or less. Among them, when y is greater than 0, thermal stability, output characteristics, cycle characteristics, etc. can be improved. . Also, M preferably contains Al. When M contains Al and the content of Al is y1 and the content of M other than Al is y2 (where y1 + y2 = y) in the above molar ratio, the value of y1 is preferably 0.01 or more and 0 .1 or less. Also, the value of y2 may be 0 or more and 0.1 or less, or may be 0.

In the above molar ratio, the value of s, which indicates the content of lithium, is preferably 0.95 or more and 1.3 or less, more preferably 1.0 or more and 1.10 or less, and exceeds 1.0. It may be 1.10 or less. If the value of s is less than 0.95, the sites that should be occupied by lithium in the crystals of the lithium metal composite oxide particles 10 are occupied by other elements, and the charge/discharge capacity of the secondary battery may decrease. . On the other hand, when the value of s exceeds 1.3, there is a surplus lithium compound that does not contribute to charging and discharging in addition to the lithium-nickel composite oxide particles 10 and the compound containing lithium and tungsten. The positive electrode resistance (reaction resistance) may increase, or the charge/discharge capacity may decrease.

In addition, the positive electrode active material 100 has a composition excluding tungsten, represented by the general formula (1): Li _s Ni _1-xy Co _x M _y O _2+α (where 0.95≦s≦1 .30, 0≤x≤0.35, 0≤y≤0.35, 0≤α≤0.5, M is Mn, Mo, V, Mg, Ca, Al, Ti, Cr, Zr, La, one or more elements selected from Nb, Hf, and Ta). In addition, in the general formula (1), the preferable range of each element can be the same range as the molar ratio described above.

[Volume average particle size]
The size of the positive electrode active material 100 is not particularly limited, but the volume average particle diameter (MV) is preferably about 1 μm or more and 30 μm or less, more preferably 5 μm or more and 20 μm or less. The volume average particle diameter (MV) can be obtained, for example, from the volume integrated value measured with a laser beam diffraction/scattering particle size distribution analyzer.

[Particle size variation index]
The positive electrode active material 100 may have a particle size variation index [(D90−D10)/MV] of, for example, 0.65 or more, or 0.70 or more. When the variation index of the positive electrode active material 100 is within the above range, the positive electrode active material 100 in the positive electrode is prevented from being deteriorated in the cycle characteristics and output characteristics of the obtained secondary battery by moderately mixing fine particles and coarse particles. of particles can be improved. The upper limit of [(D90-D10)/MV] is not particularly limited, but from the viewpoint of suppressing excessive contamination of fine particles or coarse particles into the positive electrode active material 100, the dispersion index of the composite hydroxide is 1. It is preferably 2 or less, more preferably 1.0 or less.

In the above [(D90-D10) / MV], D10 is the particle size where the number of particles in each particle size is accumulated from the smaller particle size side, and the cumulative volume is 10% of the total volume of all particles. Similarly, by accumulating the number of particles, each means a particle size whose cumulative volume is 90% of the total volume of all particles. MV, D90 and D10 can be measured using a laser light diffraction scattering particle size analyzer.

The method for producing the positive electrode active material 100 is not particularly limited as long as the above characteristics can be obtained, but it is preferable to use the method for producing the positive electrode active material described later.

2. Method for Manufacturing Positive Electrode Active Material FIG. 2 is a diagram showing an example of a method for manufacturing a positive electrode active material for a lithium-ion secondary battery according to the present embodiment (hereinafter also referred to as a “method for manufacturing a positive electrode active material”). . As shown in FIG. 2, the positive electrode active material 100 can be easily manufactured on an industrial scale with high productivity by the method for manufacturing a positive electrode active material according to the present embodiment. Note that FIG. 2 is an example of a method for producing a positive electrode active material, and the method is not limited to this method.

The method for producing the positive electrode active material 100 includes, in a decarboxylation atmosphere, mixing the lithium metal composite oxide particles (base material) while spraying water to obtain a first mixture (step S10); mixing the mixture of 1 and particles of a compound containing tungsten to obtain a second mixture (step S20), and drying the second mixture to remove the surface of the lithium metal composite oxide particles 10 covering at least a portion with a layer WL containing tungsten (step S30). Each step will be described below with reference to FIG.

[Step S10]
First, in a decarboxylation atmosphere, water is sprayed onto and mixed with lithium metal composite oxide particles (base material) to obtain a first mixture (step S10). The lithium metal composite oxide particles used as the base material are hereinafter also referred to as "lithium metal composite oxide particles 10a". As will be described later, lithium present on the surface of the lithium metal composite oxide particles 10a reacts with a compound containing tungsten to obtain the lithium metal composite oxide particles 10 forming the layers WL containing tungsten. is considered possible.

(Atmosphere of water spray)
In the manufacturing method according to the present embodiment, the coverage of tungsten can be greatly improved by creating a decarbonated atmosphere when water is sprayed and mixed. Hereinafter, the influence of the atmosphere in which water is sprayed on the tungsten coverage (mol %) on the surface of the lithium metal composite oxide particles 10 will be described with reference to FIG. Each example in FIG. 3 shows a base material used in Examples and cathode active materials obtained in Comparative Examples 8 and 9, which will be described later.

The bar graph in FIG. 3 is a graph showing the tungsten coverage (mol%) in each example, and the line graph in FIG. 3 is the total carbon amount (TC amount ) is a graph.

As shown in FIG. 3, the positive electrode active material obtained by coating tungsten under the same conditions except for the atmosphere when water is sprayed and mixed, and the positive electrode obtained by spraying water in a decarbonated atmosphere. The active material (Comparative Example 9) has a significantly higher tungsten coverage (mol %) than the positive electrode active material (Comparative Example 8) obtained by spraying water in an air atmosphere.

In addition, in the positive electrode active material (Comparative Example 8) obtained by spraying water in an air atmosphere, the amount of TC contained in the positive electrode active material 100 was lower than that of the lithium metal composite oxide used as the base material (Fig. 3 left side of the graph) and the positive electrode active material obtained by spraying water in a decarbonated atmosphere (Comparative Example 9).

Although the details of these reasons are unknown, for example, when water is sprayed in an air atmosphere, carbon dioxide gas (CO ₂ ) contained in the air and the surface of the lithium metal composite oxide particles (base material) The existing lithium reacts to form a carbonate, which inhibits the formation of the tungsten-containing layer WL in the entire lithium metal composite oxide particle (whole powder), thereby reducing the tungsten coverage (molar %) and an increase in the amount of TC.

The decarboxylation atmosphere refers to an atmosphere in which the content of carbon dioxide (CO ₂ ) is lower than that of the atmospheric atmosphere. For example, the content of carbon dioxide (CO ₂ ) may be less than 100 volume ppm, It may be 50 volume ppm or less, 20 volume ppm or less, or 10 volume ppm or less. The lower limit of the content of carbon dioxide gas (CO ₂ ) in the decarbonated atmosphere is not particularly limited, and may be 0 ppm by volume or more, and may be 0 ppm by volume.

The content of components other than carbon dioxide is not particularly limited in the decarboxylation atmosphere, and any atmosphere can be used. For example, air with a reduced carbon dioxide content (decarboxylation air), an inert gas atmosphere, an oxygen atmosphere, etc. may be used, and it is preferable to use decarbonated air.

(Amount of water to be sprayed)
The amount of water to be sprayed onto the lithium metal composite oxide particles 10a is not particularly limited. % by mass or more, more preferably 3% by mass or more, and even more preferably 4% by mass or more.

FIG. 4 is a graph showing the tungsten coverage (mol %) in each example in which the amount of sprayed water was changed. Each example in FIG. 4 shows positive electrode active materials obtained in Comparative Examples 6 and 7, and Examples 1 and 3, which will be described later.

As shown in FIG. 4, in the positive electrode active material obtained by coating tungsten under the same conditions except that the amount of sprayed water was changed, the amount of sprayed water was changed to 8% by mass. (Comparative Example 7, Example 3) compared with the positive electrode active material (Comparative Example 6, Example 1) obtained by setting the water spray amount to 4% by mass, and the tungsten coverage (mol% ) tends to rise.

The upper limit of the amount of water to be sprayed is not particularly limited, and can be appropriately adjusted depending on the particle shape of the lithium metal composite oxide particles 10a used as the base material. % by mass or less.

(Other Conditions for Spraying and Mixing Water)
The water to be sprayed may be, for example, in a fog state, or may be in a fog state with an average of 100 μm or less. In addition, as a device for spraying and mixing with water, any device can be used as long as a uniform mixture can be obtained. may be used.

(Particles of lithium metal composite oxide)
The composition of the particles 10a of the lithium metal composite oxide used as the base material is not particularly limited, and may be, for example, the same range as the composition of the positive electrode active material 100 described above. Since the preferable range of each element contained in the particles 10a of the lithium metal composite oxide is the same as the composition of the positive electrode active material 100 described above, description thereof is omitted here.

The method for producing the lithium metal composite oxide particles 10a is not particularly limited, but for example, they may be produced by crystallization. When produced by crystallization, lithium metal composite oxide particles 10a having a relatively uniform composition can be obtained. In addition, for example, when the particles 10a of the lithium metal composite oxide having a high nickel content are used as the positive electrode active material 100, the positive electrode active material according to the present embodiment may be washed with water before use. In the method, even the lithium metal composite oxide particles 10a having a high nickel content can be used as a base material without being washed with water.

In addition, since the particle properties of the lithium metal composite oxide particles 10a used as the base material are also inherited in the positive electrode active material 100, the particle size of the lithium metal composite oxide particles 10a, the particle size variation index, etc. , the desired particle size of the positive electrode active material 100 and the particle size variation index.

[Step S20]
Next, the first mixture and particles of a compound containing tungsten are mixed to obtain a second mixture (step S20). In the production method according to the present embodiment, the lithium metal composite oxide particles 10a and water are mixed (step S10), and then a compound containing tungsten is mixed (step S20) to obtain a tungsten coverage (molar %) and can have a high battery capacity when used in a secondary battery.

(order of mixing)
The bar graph in FIG. 5 is a graph showing the tungsten coverage (mol %) in each example, and the line graph in FIG. 5 is a graph showing the initial discharge capacity (relative value) in each example. Hereinafter, with reference to FIG. 5, the effect on the tungsten coverage (mol %) when changing the order of mixing the lithium metal composite oxide particles 10a with water and the compound containing tungsten will be described. do. In addition, each sample in FIG. 5 uses the positive electrode active material 100 obtained in Comparative Example 9 and Example 1, which will be described later.

As shown in FIG. 5, when tungsten (1.5% by mass) was coated under the same conditions except that the order of mixing water and the tungsten-containing compound was changed, after mixing water, tungsten was added. When a compound containing tungsten is mixed (Example 1), the tungsten coverage (mol%) is higher than when water is mixed after mixing a compound containing tungsten (Comparative Example 6), and The initial discharge capacity is also increased.

6A and 6B are SEM backscattered electron images of the positive electrode active materials obtained in the above examples (Comparative Example 9 and Example 1). . In FIGS. 6(A) and 6(B) , it is considered that white portions (bright portions) indicate tungsten (heavy element). As shown in FIGS. 6A and 6B, after mixing the compound containing tungsten (Example 1), after mixing the compound containing tungsten, , Although the content of tungsten contained in the positive electrode active material is about the same as when water is mixed (Comparative Example 9), there are fewer parts that look white, indicating that the tungsten is more uniformly coated. .

Although the details of this reason are unknown, for example, by mixing water before the compound containing tungsten, the reactivity between the lithium metal composite oxide particles 10a and the compound containing tungsten is improved, It is believed that the entire surface (entire powder) of the lithium metal composite oxide particles 10 is uniformly coated with tungsten.

(Particles of compounds containing tungsten)
The particles of tungsten-containing compound are preferably solid-state compounds capable of reacting with lithium. Further, the particles of the compound containing tungsten preferably do not contain lithium.

Compounds containing tungsten include, for example, tungsten oxide, tungstic acid, and ammonium tungstate. Among these, at least one of tungsten oxide and tungstic acid is preferred, and tungsten oxide is more preferred. In addition, the compound containing tungsten may be used individually by 1 type, and may be used in mixture of 2 or more types.

The amount of the mixture of particles of the compound containing tungsten is not particularly limited, and can be appropriately adjusted so as to obtain the positive electrode active material 100 described above. , it may be more than 1% by mass and 4% by mass or less, preferably more than 1% by mass and 2% by mass or less, more preferably 1.2% by mass or more and 2% by mass or less. Note that the mixed amount of the particles of the compound containing tungsten is set by, for example, performing a preliminary test in advance and confirming the mixed amount that allows the positive electrode active material 100 to be obtained, thereby setting the mixed amount of the compound containing tungsten. may

[Step S30]
Next, the second mixture is dried to cover at least part of the surface of the lithium metal composite oxide particles 10a with a layer WL containing tungsten (step S30). In step S30, while moisture is removed, the lithium present on the surface of the lithium metal composite oxide particles 10a reacts with the tungsten-containing compound to form the tungsten-containing layer WL.

The drying temperature is not particularly limited as long as it is a temperature at which at least part of the moisture can be removed and the layer WL containing tungsten can be formed. ° C. or less, more preferably 90° C. or higher and 200° C. or lower. When the drying temperature is within the above range, the reaction between the lithium present on the surfaces of the lithium metal composite oxide particles 10a and the tungsten-containing compound is allowed to sufficiently proceed, and the obtained lithium metal composite oxide particles 10 are dried. Tungsten can be well dispersed on the surface.

The drying time is not particularly limited, but is, for example, 5 hours or more and 72 hours or less, preferably 10 hours or more and 48 hours or less, more preferably 15 hours or more and 25 hours or less.

In addition, from the viewpoint of dispersing tungsten more sufficiently on the surface of the obtained lithium metal composite oxide particles 10, the drying may be performed in at least two steps. For example, as the first drying step, After drying for 5 hours or more and 24 hours or less at a temperature of 50 ° C. or more and 200 ° C. or less, a second drying step is performed at a temperature of 50 ° C. or more and 250 ° C. or less, and more than the first drying step. Drying may be performed at a high temperature for a time of 5 hours or more and 24 hours or less.

In addition, the atmosphere during drying is dehydrated from the viewpoint of suppressing the reaction between the moisture and carbon dioxide gas (CO ₂ ) in the atmosphere and the unreacted lithium compound remaining on the surface of the lithium metal composite oxide particles 10a. A carbonic acid atmosphere (eg, decarbonated air), an inert gas atmosphere, or a vacuum atmosphere is preferred, and a vacuum atmosphere is more preferred.

Moreover, the pressure of the atmosphere during drying is not particularly limited, but it is preferably lower than the atmospheric pressure, and preferably 1 atm or less. If the atmospheric pressure is higher than 1 atmospheric pressure, the water content in the positive electrode active material 100 may not be sufficiently reduced.

[Characteristics of positive electrode active material]
In the method for producing a positive electrode active material according to this embodiment, the obtained positive electrode active material 100 preferably has the following properties.

(Total carbon content)
The total carbon content (TC content) contained in the positive electrode active material 100 may be 0.07% by mass or less, or may be 0.065% by mass or less with respect to the entire positive electrode active material 100. . When the total carbon content (TC content) is within the above range, formation of carbonate on the surface of the lithium metal composite oxide particles 10 is suppressed. In addition, the amount of TC contained in the positive electrode active material 100 is 0.9 times or more and within 1.1 times the amount of TC contained in the particles 10a of the lithium metal composite oxide used as the base material. Preferably.

(initial discharge capacity)
For the positive electrode active material 100, for example, the initial discharge capacity when using the lithium metal composite oxide particles 10a (that is, the base material) before spraying water as the positive electrode active material of the 2032 type coin battery, The initial discharge capacity (initial The discharge capacity/initial discharge capacity of the base material) is preferably 0.9 times or more, more preferably 0.95 times or more. The upper limit of (initial discharge capacity of positive electrode active material 100/initial discharge capacity of base material) is not particularly limited, but may be, for example, 1.1 times or less, or 1.05 times or less. .

The 2032-type coin-type battery can be manufactured by the method described in Examples below, and the initial discharge capacity (mAh/g) can be evaluated by the method described in Examples below.

(reaction resistance)
The positive electrode active material 100 is, for example, the positive electrode resistance (SOC 20%) when the lithium metal composite oxide particles 10a (that is, the base material) before spraying water is used as the positive electrode active material of the 2032 type coin battery. On the other hand, the positive electrode resistance (SOC 20%) ( The positive electrode resistance of the positive electrode active material 100/the positive electrode resistance of the base material) is preferably 0.7 times or less, more preferably 0.65 times or less, and still more preferably 0.6 times or less. The lower limit of (the positive electrode resistance of the positive electrode active material 100/the positive electrode resistance of the base material) is not particularly limited, but may be 0.3 times or more, or may be 0.4 times or more.

The 2032-type coin-type battery can be manufactured by the method described in Examples below, and the positive electrode resistance (reaction resistance, Ω) can be evaluated by the method described in Examples below.

3. Lithium Ion Secondary Battery A lithium ion secondary battery (hereinafter also referred to as a “secondary battery”) according to the present embodiment includes a positive electrode containing the positive electrode active material 100 described above, a negative electrode, and an electrolyte. Lithium ion secondary batteries can be composed of the same components as conventionally known lithium ion secondary batteries. may be Also, the secondary battery may be, for example, an all-solid secondary battery comprising a positive electrode, a negative electrode, and a solid electrolyte. Each component will be described below.

Note that the embodiments described below are merely examples, and the lithium ion secondary battery according to the present embodiment can be variously modified based on the knowledge of those skilled in the art based on the embodiments described herein. , can be implemented in modified form. Moreover, the lithium ion secondary battery according to the present embodiment is not particularly limited in its application.

(positive electrode)
The manufacturing method of the positive electrode is not particularly limited, but for example, it may be manufactured as follows.

First, the positive electrode active material 100, a conductive material, and a binder are mixed, and if necessary, activated carbon and a solvent for viscosity adjustment are added, and the mixture is kneaded to prepare a positive electrode mixture paste. . At that time, the mixing ratio of each component in the positive electrode mixture paste can be appropriately adjusted according to the intended performance of the secondary battery. For example, when the solid content of the positive electrode mixture excluding the solvent is 100 parts by mass, the content of the positive electrode active material 100 is 60 parts by mass or more and 95 parts by mass or less, and the content of the conductive material is 1 part by mass or more and 20 parts by mass. The content of the binder may be 1 part by mass or more and 20 parts by mass or less.

The conductive material is not particularly limited, and for example, graphite (natural graphite, artificial graphite, expanded graphite, etc.) and carbon black-based materials such as acetylene black and ketjen black can be used.

The binder plays a role of binding the active material particles, and is not particularly limited. system resin, polyacrylic acid, etc. can be used.

If necessary, the positive electrode active material, the conductive material, and the activated carbon may be dispersed, and a solvent that dissolves the binder may be added to the positive electrode mixture. As the solvent, specifically, an organic solvent such as N-methyl-2-pyrrolidone can be used. In addition, activated carbon may be added to the positive electrode mixture in order to increase the electric double layer capacity.

The obtained positive electrode mixture paste is applied, for example, to the surface of a current collector made of aluminum foil, and dried to scatter the solvent. If necessary, pressure may be applied by a roll press or the like in order to increase the electrode density. Thus, a sheet-like positive electrode can be produced. The sheet-like positive electrode can be cut into an appropriate size depending on the intended battery, and used for manufacturing the battery. The method for producing the positive electrode is not limited to the method described above, and other methods may be used.

(negative electrode)
For the negative electrode, metal lithium, lithium alloy, etc. may be used. The material may be applied to the surface of a metal foil current collector such as copper, dried, and optionally compressed to increase electrode density.

As the negative electrode active material, for example, natural graphite, artificial graphite, sintered organic compounds such as phenol resin, and powdery carbon substances such as coke can be used. In this case, a fluorine-containing resin such as PVDF can be used as the binder for the negative electrode in the same manner as the binder for the positive electrode. Solvents can be used. In addition to the above organic binder such as PVDF, a water-based binder such as styrene-butanediene rubber may be used as the binder. Carboxymethyl cellulose (CMC) may be used as a viscosity modifier for the water-based binder. For example, with a water-based binder containing styrene-butadiene latex as a main additive and CMC as a viscosity modifier, the binding strength can be enhanced with a small amount.

(separator)
Between the positive electrode and the negative electrode, a separator is interposed and arranged as necessary. The separator separates the positive electrode from the negative electrode and holds the electrolyte, and may be a thin film of polyethylene, polypropylene, or the like, having a large number of minute pores.

(Non-aqueous electrolyte)
A non-aqueous electrolyte can be used as the non-aqueous electrolyte. The non-aqueous electrolytic solution may be, for example, one obtained by dissolving a lithium salt as a supporting salt in an organic solvent. Further, as the non-aqueous electrolytic solution, an ionic liquid in which a lithium salt is dissolved may be used. The ionic liquid refers to a salt composed of cations and anions other than lithium ions and exhibiting a liquid state even at room temperature.

Organic solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate and trifluoropropylene carbonate; chain carbonates such as diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate and dipropyl carbonate; One selected from ether compounds such as methyltetrahydrofuran and dimethoxyethane, sulfur compounds such as ethylmethylsulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate may be used alone, or two or more may be mixed. can be used as

As the supporting salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiN(CF ₃ SO ₂ ) ₂ , composite salts thereof, and the like can be used. Furthermore, the non-aqueous electrolytic solution may contain radical scavengers, surfactants, flame retardants, and the like.

Moreover, when the lithium ion secondary battery is an all-solid secondary battery, a solid electrolyte may be used as the non-aqueous electrolyte. Solid electrolytes have the property of withstanding high voltages. Solid electrolytes include inorganic solid electrolytes and organic solid electrolytes.

As the inorganic solid electrolyte, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, or the like is used.

The oxide-based solid electrolyte is not particularly limited, and can be used as long as it contains oxygen (O) and has lithium ion conductivity and electronic insulation. Examples of oxide-based solid electrolytes include lithium phosphate (Li ₃ PO ₄ ), Li ₃ PO ₄ N _x , LiBO ₂ N _x , LiNbO ₃ , LiTaO ₃ , Li ₂ SiO ₃ , Li ₄ SiO ₄ -Li _{3 PO4} , _Li4SiO4 - _Li3VO4 , _{Li2O - B2O3 - P2O5} , _{Li2O - SiO2 , Li2O} - _{B2O3 -} ZnO, Li1 _{+X AlXTi 2-X} (PO ₄ ) ₃ (0≦X≦1), Li _1+X Al _X Ge _2-X (PO ₄ ) ₃ (0≦X≦1), LiTi ₂ (PO ₄ ) ₃ , Li _3X La _{2/ 3-X} TiO ₃ (0≦X≦2/3), Li ₅ La ₃ Ta ₂ O ₁₂ , Li ₇ La ₃ Zr ₂ O ₁₂ , Li ₆ BaLa ₂ Ta ₂ O ₁₂ , Li _3.6 Si _0.6 P _0.4 O ₄ and the like.

The sulfide-based solid electrolyte is not particularly limited, and any solid electrolyte containing sulfur (S) and having lithium ion conductivity and electronic insulation can be used. Examples of sulfide solid electrolytes include Li ₂ SP ₂ S ₅ , Li ₂ S—SiS ₂ , LiI—Li ₂ S—SiS ₂ , LiI—Li ₂ SP ₂ S ₅ , LiI—Li ₂ S—B ₂ S ₃ , Li ₃ PO ₄ —Li ₂ S—Si ₂ S, Li ₃ PO ₄ —Li ₂ S—SiS ₂ , LiPO ₄ —Li ₂ S—SiS, LiI—Li ₂ S—P ₂ O ₅ , LiI—Li ₃ PO ₄ —P ₂ S ₅ and the like.

Inorganic solid electrolytes other than those described above may be used, such as Li ₃ N, LiI, Li ₃ N--LiI--LiOH, and the like.

The organic solid electrolyte is not particularly limited as long as it is a polymer compound exhibiting ionic conductivity. For example, polyethylene oxide, polypropylene oxide, copolymers thereof, and the like can be used. Moreover, the organic solid electrolyte may contain a supporting salt (lithium salt). When a solid electrolyte is used, the solid electrolyte may also be mixed in the positive electrode material in order to ensure contact between the electrolyte and the positive electrode active material 100 .

(Battery shape and configuration)
The lithium ion secondary battery according to the present embodiment, which is composed of the positive electrode, the negative electrode, the separator, the non-aqueous electrolyte solution, the positive electrode, the negative electrode, and the solid electrolyte described above, has a cylindrical shape, a laminated shape, and the like. Various shapes are possible.

When a non-aqueous electrolytic solution is used as the non-aqueous electrolyte, the positive electrode and the negative electrode are laminated with a separator interposed to form an electrode body, and the obtained electrode body is impregnated with the non-aqueous electrolytic solution. A lead for current collection is used to connect between the connected positive electrode terminal and between the negative electrode current collector and the negative electrode terminal connected to the outside, and the battery case is sealed to complete the lithium ion secondary battery. .

EXAMPLES The present invention will be specifically described below using Examples, but the present invention is not limited to these Examples. The characteristics of the positive electrode active material and the secondary battery obtained according to this embodiment were evaluated by the following methods. In addition, each sample of special grade reagent manufactured by Wako Pure Chemical Industries, Ltd. was used for the production of the positive electrode active material and the secondary battery in the present example.

[composition]
The composition of the base material and the positive electrode active material and the content of tungsten were measured with an ICP emission spectrometer (ICPS-8100 manufactured by Shimadzu Corporation) after dissolving the base material and the positive electrode active material with acid.

[Total carbon content]
The total carbon content (TC content) of the base material and positive electrode active material was measured with a carbon-sulfur analyzer (CS-600 manufactured by LECO).

[Tungsten coverage (mol%)]
Using an X-ray photoelectron spectrometer (VG-Scientific, ESCALAB220i-XL), target: Al, X-ray source: Al-Kα ray, voltage: 10 kV, current: 15 mA, on the surface of the lithium metal composite oxide particles, XPS spectra of tungsten, nickel, and cobalt were measured, and the coverage of tungsten (content of tungsten on the surface of the lithium metal composite oxide particles) was determined from the ratio of their peak intensities.

[Thickness (nm) of layer WL containing tungsten]
A Schottky field emission type scanning electron microscope SEM-EDS, JSM-7001F (manufactured by JEOL Ltd.), was used to observe the particle surface and cross section of the positive electrode active material. In addition, the thickness of the layer WL containing tungsten is determined by observing a processed cross-sectional sample with a cross-section polisher (CP) [manufactured by JEOL Ltd., product number: SM-09010], and measuring each lithium metal composite oxide particle. Draw two straight lines that are perpendicular to each other at approximately the center of the cross section, measure the thickness of the tungsten-containing layer WL at four locations that intersect these two straight lines, and calculate the value for 10 particles (n = 40) , the average thickness of the layer WL containing tungsten was obtained.

[Manufacture and Evaluation of Secondary Battery]
A method for manufacturing a secondary battery and a method for evaluating battery characteristics will be described below.

(Manufacture of secondary batteries)
Positive electrode active material: 52.5 mg, acetylene black: 15 mg, and polytetrafluoroethylene resin (PTFE): 7.5 mg were mixed, press-molded at a pressure of 100 MPa to a diameter of 11 mm and a thickness of 100 μm, and placed in a vacuum dryer. It was dried at 120° C. for 12 hours to prepare a positive electrode PE (evaluation electrode).

Next, using this positive electrode PE, a 2032-type coin-type battery CBA (see FIG. 7) was produced in an Ar atmosphere glove box with a controlled dew point of -80.degree. Lithium metal with a diameter of 17 mm and a thickness of 1 mm was used for the negative electrode NE of this coin-type battery CBA, and the electrolyte was ethylene carbonate (EC) and diethyl carbonate (DEC) with 1 M LiClO ₄ as the supporting electrolyte. A mixed solution (manufactured by Toyama Pharmaceutical Co., Ltd.) was used. A polyethylene porous film having a film thickness of 25 μm was used as the separator SE. As shown in FIG. 7, the coin-type battery CBA has a gasket GA and is assembled into a coin-shaped battery with a positive electrode can PC and a negative electrode can NC.

(initial discharge capacity)
The coin-type battery CBA was left for about 24 hours after it was manufactured, and after the open circuit voltage OCV (Open Circuit Voltage) was stabilized, the current density to the positive electrode was set to 0.1 mA/cm ² and charged to a cutoff voltage of 4.3 V. The initial charge capacity was defined as the capacity at that time, and the initial discharge capacity was defined as the capacity when the battery was discharged to a cutoff voltage of 3.0 V after resting for 1 hour. A value obtained by dividing the initial discharge capacity by the initial charge capacity was defined as the initial charge/discharge efficiency (initial discharge capacity/initial charge capacity).

(positive electrode resistance)
The coin-type battery CBA was left for about 24 hours after it was produced, and after the open circuit voltage OCV (Open Circuit Voltage) was stabilized, the current density for the positive electrode was 0.1 mA / cm ² and the cutoff voltage was 4.3 V. , and after resting for 1 hour, it was discharged until the cutoff voltage reached 3.0V. After that, the coin-type battery CBA was charged to potentials corresponding to SOC 20% and SOC 80%, and the positive electrode resistance (reaction resistance ) was calculated.

A method for calculating the positive electrode resistance (reaction resistance) will be described below. First, when the coin cell CBA is measured by the AC impedance method using a frequency response analyzer and a potentiogalvanostat, a Nyquist plot as shown in FIG. 8 is obtained. Since this Nyquist plot is expressed as the sum of characteristic curves showing the solution resistance, the negative electrode resistance and its capacity, and the positive electrode resistance and its capacity, a fitting calculation is performed using an equivalent circuit based on this Nyquist plot, and the positive electrode resistance (reaction resistance) was calculated.

(Example 1)
Lithium metal composite oxide (Li _1.025 Ni _0.82 Co _0.15 Al _0.03 O ₂ , average particle size (MV): 12.3 μm, [(D90-D10 )/MV]: 0.82, not washed with water) was used as the base material. The total carbon content (TC content) of the base material was 0.064% by mass.

In a decarbonated atmosphere (decarbonated air), while stirring and mixing the lithium metal composite oxide (base material), 4% by mass of water was sprayed on the entire particles of the lithium metal composite oxide (base material). (Step S10). After that, tungsten oxide (WO ₃ ) was added and mixed in such an amount that the tungsten in the tungsten oxide was 1.5% by mass with respect to the whole particles of the lithium metal composite oxide (step S20).

After that, it was dried at 100° C. for 12 hours and then at 190° C. for 10 hours in a vacuum atmosphere to obtain a positive electrode active material. The tungsten (W) content, tungsten coverage (mol %), etc. of the obtained positive electrode active material were evaluated by the above methods. Further, a coin-type battery CBA was produced using the obtained positive electrode active material, and battery characteristics were evaluated. These evaluation results are shown in Table 1. Further, the total carbon content (TC content) of the positive electrode active material obtained in Example 1 was 0.064% by mass.

(Example 2)
In the same manner as in Example 1, a positive electrode active material was obtained and evaluated. The amount of tungsten contained in the obtained positive electrode active material was 1.3% by mass with respect to the whole positive electrode active material. Table 1 shows the evaluation results.

(Example 3)
A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the spray amount of water was 8% by mass with respect to the entire lithium metal composite oxide particles. Table 1 shows the evaluation results.

(Comparative example 1)
The amount of water sprayed is 8% by mass with respect to the entire lithium metal composite oxide particles, and the mixed amount of tungsten oxide is such that the tungsten in the tungsten oxide is 1% by mass with respect to the entire lithium metal composite oxide particles. A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that Table 1 shows the evaluation results.

(Comparative example 2)
The spray amount of water is set to 10% by mass with respect to the entire lithium metal composite oxide particles, and the mixed amount of tungsten oxide is such that the amount of tungsten in the tungsten oxide is 0.5 with respect to the entire lithium metal composite oxide particles. A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the amount was adjusted to the mass %. Table 1 shows the evaluation results.

(Comparative Example 3)
The amount of water sprayed is 8% by mass with respect to the entire lithium metal composite oxide particles, and the amount of tungsten oxide added is such that the tungsten in the tungsten oxide is 1% by mass with respect to the entire lithium metal composite oxide particles. A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that the amount was set to . Table 1 shows the evaluation results.

(Comparative Example 4)
After mixing the lithium metal composite oxide (base material) and tungsten oxide, the resulting mixture is sprayed with water (steps S10 and S20 are performed in the reverse order of Example 1), and water is sprayed. is performed in an air atmosphere, and the amount of tungsten oxide added is set to an amount such that the amount of tungsten in the tungsten oxide is 0.2% by mass with respect to the entire lithium metal composite oxide particles. Similarly, a positive electrode active material was obtained and evaluated. Table 1 shows the evaluation results.

(Comparative Example 5)
A positive electrode active material is obtained in the same manner as in Comparative Example 4, except that the amount of tungsten oxide added is such that the amount of tungsten in the tungsten oxide is 0.5% by mass with respect to the entire lithium metal composite oxide particles. evaluated with. Table 1 shows the evaluation results.

(Comparative Example 6)
A positive electrode active material was obtained and evaluated in the same manner as in Comparative Example 4, except that the amount of tungsten oxide added was such that the amount of tungsten in the tungsten oxide was 1% by mass with respect to the entire lithium metal composite oxide particles. bottom. Table 1 shows the evaluation results.

(Comparative Example 7)
The amount of water sprayed is 8% by mass with respect to the entire lithium metal composite oxide particles, and the amount of tungsten oxide added is such that the tungsten in the tungsten oxide is 1% by mass with respect to the entire lithium metal composite oxide particles. A positive electrode active material was obtained and evaluated in the same manner as in Comparative Example 4 except that the amount was set to . Table 1 shows the evaluation results.

(Comparative Example 8)
A positive electrode active material is obtained in the same manner as in Comparative Example 4, except that the amount of tungsten oxide added is such that the amount of tungsten in the tungsten oxide is 1.5% by mass with respect to the entire lithium metal composite oxide particles. evaluated with. Table 1 shows the evaluation results.

(Comparative Example 9)
After the lithium metal composite oxide (base material) and tungsten oxide were mixed, the resulting mixture was sprayed with water (Steps S10 and S20 were performed in the reverse order of Example 1). A positive electrode active material was obtained and evaluated in the same manner as in 1. Table 1 shows the evaluation results.

(Comparative Example 10)
A positive electrode active material was obtained and evaluated in the same manner as in Comparative Example 4, except that the amount of tungsten oxide added was such that the amount of tungsten in the tungsten oxide was 2% by mass with respect to the entire lithium metal composite oxide particles. bottom. Table 1 shows the evaluation results.

(Evaluation results)
As shown in Table 1, the positive electrode active materials obtained in Examples have a tungsten coverage (mol%) of more than 50 mol%, and the entire positive electrode active material is coated with tungsten more uniformly. was shown. In addition, the secondary battery using the positive electrode active material of the example has the same initial charge/discharge capacity and initial charge/discharge efficiency as compared to the lithium metal composite oxide used as the base material, and has a low It was shown to have positive electrode resistance (reaction resistance).

As shown in FIGS. 10A and 10B, when the positive electrode active material obtained in Example 1 was observed with a scanning electron microscope (SEM) backscattered electron image, it was found that the lithium metal composite oxide It was confirmed that a layer WL containing tungsten with a thickness of 10 nm or less was formed on the surface of the particles.

FIG. 9 is a graph showing the relationship between the tungsten content (mass%) and the tungsten coverage (mol%) of the positive electrode active materials obtained in Examples and Comparative Examples (in FIG. 9, Except for the tungsten content (% by mass) in the positive electrode active material, only examples and comparative examples produced under the same conditions are shown). As shown in FIG. 9, in the positive electrode active materials obtained in Comparative Examples 1 to 3, the tungsten content was 0.5 to 1 mass %, and the tungsten coverage was 50 mol % or less. The positive electrode resistance also tended to be higher than in the examples.

Further, in Comparative Examples 4 to 8 and 10, after mixing the lithium metal composite oxide (base material) and tungsten oxide (0.2% by mass to 2% by mass), the resulting mixture was sprayed with water. (Steps S10 and S20 are performed in the reverse order of Example 1), and water is sprayed in an air atmosphere, and in the positive electrode active materials obtained in these comparative examples, the coverage of tungsten is It was as low as 32 mol % or less. Further, as shown in FIG. 9, even in the positive electrode active material of Comparative Example 10 in which the amount of tungsten oxide added (2% by mass) was increased, the tungsten coverage was as low as 31 mol%, and the initial charge/discharge capacity was very high. Decreased.

In addition, as shown in FIG. 4, Comparative Example 6 in which the amount of water sprayed is 4% by mass with respect to the lithium metal composite oxide, and the amount of water sprayed with respect to the lithium metal composite oxide is 8% by mass. Comparing with Comparative Example 7, the positive electrode active material obtained in Comparative Example 7 had a larger tungsten coverage (mol %).

In addition, the positive electrode active material obtained in Comparative Example 9 was manufactured in the same manner as in Example 1 ( Water is sprayed in a decarbonated atmosphere), and as shown in FIG. rice field.

Since the positive electrode active material according to the present invention has a very high tungsten coverage (mol%), when it is used as a positive electrode material for a lithium ion secondary battery, it has a high battery capacity and a positive electrode resistance (reaction The output characteristics can be improved by reducing the resistance). Therefore, the positive electrode active material according to the present invention is particularly suitable as a positive electrode active material for lithium ion secondary batteries (nonaqueous electrolyte secondary batteries, all-solid secondary batteries, etc.) used as power sources for hybrid vehicles and electric vehicles. can be used.

DESCRIPTION OF SYMBOLS 100... Positive electrode active material 1... Primary particle 2... Secondary particle 10... Particles of lithium metal composite oxide 10a... Particles of lithium metal composite oxide (base material)
WL... Layer containing tungsten CBA... Coin type battery PE... Positive electrode NE... Negative electrode GA... Gasket PE... Positive electrode NE... Negative electrode SE... Separator

Claims (7)

A positive electrode active material obtained by mixing particles of a lithium metal composite oxide and particles of a compound containing tungsten,
Lithium metal composite oxide particles, and a layer containing tungsten covering at least part of the surface of the lithium metal composite oxide particles,
The content of tungsten on the surface of the particles of the lithium metal composite oxide measured by X-ray photoelectron spectroscopy (XPS) is more than 50 mol%,
The content of tungsten with respect to the entire positive electrode active material is more than 1% by mass and 4% by mass or less.
Positive electrode active material for lithium ion secondary batteries. 2. The cathode active material for a lithium ion secondary battery according to claim 1, wherein at least part of said layer containing tungsten has a thickness of 10 nm or less. Lithium (Li), nickel (Ni), cobalt (Co), and an element M are included, and the material amount ratio (molar ratio) of these elements is Li:Ni:Co:M=s:(1-xy ): x: y (where 0.95 ≤ s ≤ 1.30, 0 ≤ x ≤ 0.35, 0 ≤ y ≤ 0.35, M is Mn, Mo, V, Mg, Ca, Al, Ti , Cr, Zr, La, Nb, Hf, and Ta). 4. The positive electrode active material for a lithium ion secondary battery according to claim 3, wherein (1-xy) is 0.6 or more. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 4, wherein at least part of said layer containing tungsten has a thickness of 1 nm or less. The positive electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the layer containing tungsten has an average thickness of 10 nm or less. A lithium ion secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises the positive electrode active material according to any one of claims 1 to 6.

Images