JP7271944B2 - Method for producing positive electrode active material for lithium ion secondary battery - Google Patents

Description

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

In recent years, with the spread of mobile electronic devices such as mobile phones and laptop computers, there is a strong demand for the development of small, lightweight non-aqueous electrolyte secondary batteries with high energy density. In addition, there is a strong demand for development of a high-output secondary battery as a battery for electric vehicles including hybrid vehicles.

As a secondary battery that satisfies such requirements, there is a lithium ion secondary battery. A 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 for the negative electrode and the positive electrode.

Lithium-ion secondary batteries are currently being actively researched and developed. Among them, lithium-ion secondary batteries using a layered or spinel-type lithium metal composite oxide as a positive electrode active material are 4V class. Since high voltage can be obtained, it is being put to practical use as a battery with high energy density.

As positive electrode active materials mainly proposed so far, 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 (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), lithium-manganese composite oxide (LiMn ₂ O ₄ ) using manganese, and the like can be mentioned.

Of these, lithium-nickel composite oxides are attracting attention as materials from which high battery capacity can be obtained. Furthermore, in recent years, emphasis has been placed on reducing the resistance necessary for increasing the output. Addition of different elements is used as a method for realizing the above-mentioned low resistance, and transition metals such as W, Mo, Nb, Ta, and Re, which can take high numbers, are particularly useful.

Further, in recent years, there has been a demand for higher output, and various studies have been made.

For example, in Patent Document 1, the general formula Li _z Ni _1-xy Co _x M _y O ₂ (where 0.10≦x≦0.35, 0≦y≦0.35, 0.97≦z≦ 1.20, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al) from primary particles and secondary particles formed by agglomeration of the primary particles A lithium metal composite oxide comprising fine particles containing lithium tungstate represented by any one of Li ₂ WO ₄ , Li ₄ WO ₅ and Li ₆ W ₂ O ₉ on the surface of the lithium metal composite oxide A positive electrode active material for a non-aqueous electrolyte secondary battery is disclosed, which is characterized by having

Patent Documents 2 to 4 also disclose examples in which lithium tungstate is formed on the surfaces of primary particles of lithium metal composite oxide.

In the positive electrode active materials for non-aqueous electrolyte secondary batteries disclosed in Patent Documents 1 to 4, by forming fine particles containing the lithium tungstate on the surface of the lithium metal composite oxide powder, at the interface with the electrolyte solution Since it forms a conductive path for lithium, it is said that the reaction resistance of the active material can be reduced and the output characteristics can be improved.

JP 2013-125732 A JP 2016-110999 A Japanese Unexamined Patent Application Publication No. 2016-111000 JP 2016-127004 A

However, there is a demand for a positive electrode active material for a lithium ion secondary battery that can achieve higher output when used as a lithium ion secondary battery.

Therefore, in view of the above problems of the prior art, it is an object of one aspect of the present invention to provide a positive electrode active material for a lithium ion secondary battery that can achieve high output when used as a lithium ion secondary battery.

In order to solve the above problems, according to one aspect of the present invention, there is provided a positive electrode active material for a lithium ion secondary battery containing a lithium metal composite oxide powder,
The lithium metal composite oxide powder is
General formula: Li _z Ni _1-x-y Co _x My _{O 2+α} (where 0 < x ≤ 0.35, 0 ≤ y ≤ 0.35, 0.95 ≤ z ≤ 1.30, 0 ≤ α ≤ 0.15, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al), and a lithium metal composite oxide particle having a layered crystal structure; a coating of lithium tungstate disposed on the surface of the lithium metal composite oxide particles;
By SEM observation under the observation conditions of an acceleration voltage of 5 kV and an observation magnification of 0.1 k, five observation images are obtained by changing the observation location, and the five observation images are subjected to a binarization process, and the lithium metal containing the coating is obtained. The surfaces of the composite oxide particles are divided into the lithium tungstate enriched portion where the lithium tungstate particle size is larger than 200 nm and the film thickness is thicker than 200 nm, and the portion other than the lithium tungstate enriched portion. a positive electrode active material for a lithium ion secondary battery, wherein the surface area ratio of the lithium tungstate-enriched portion is 50% or less in the surface of the coating-containing lithium metal composite oxide particles when the particles are separated into two.

According to one aspect of the present invention, it is possible to provide a positive electrode active material for a lithium ion secondary battery that can achieve high output when used as a lithium ion secondary battery.

It is a schematic explanatory drawing of the equivalent circuit used for the measurement example of impedance evaluation, and an analysis. FIG. 2 is an explanatory diagram of a 2032-type coin battery used for battery evaluation; 1 is an example showing the results of surface observation by a scanning electron microscope of the positive electrode active material obtained in Example 1. FIG. It is an example which shows the result of binarizing the observation image of FIG.

Hereinafter, the embodiments for carrying out the present invention will be described with reference to the drawings. Various modifications and substitutions can be made to.
(1) Positive Electrode Active Material for Lithium Ion Secondary Batteries Hereinafter, one structural example of the positive electrode active material for lithium ion secondary batteries according to the present embodiment will be described.

The positive electrode active material for a lithium ion secondary battery of the present embodiment (hereinafter also simply referred to as "positive electrode active material") contains a lithium metal composite oxide powder.
The lithium metal composite oxide powder is a coating-containing lithium metal composite oxide particle having a lithium metal composite oxide particle and a lithium tungstate coating disposed on the surface of the lithium metal composite oxide particle. contains.
The lithium metal composite oxide has the general formula: Li _z Ni _1-x-y Co _x M _y O _2+α (where 0 < x ≤ 0.35, 0 ≤ y ≤ 0.35, 0.95 ≤ z ≤1.30, 0≤α≤0.15, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al), and the crystal structure is a layered structure.
The area ratio of the lithium tungstate-enriched portion in the surfaces of the coating-containing lithium metal composite oxide particles can be set to 50% or less.

The inventors of the present invention have extensively studied a positive electrode active material capable of increasing the output of a lithium ion secondary battery. As a result, when the lithium tungstate coating is arranged on the surface of the lithium metal composite oxide particles, the state of the lithium tungstate coating is made uniform within the lithium metal composite oxide powder, thereby increasing the output. The present invention was completed by discovering that it can be achieved.

The positive electrode active material of this embodiment contains the lithium metal composite oxide powder as described above. In addition, the positive electrode active material of the present embodiment can also be composed of a lithium metal composite oxide powder.

The lithium metal composite oxide powder can have lithium metal composite oxide particles as a base material and a coating of lithium tungstate disposed on the surface of the base material.

As the base material, the general formula: Li _z Ni _1-x-y Co _x M _y O _2+α (where 0 < x ≤ 0.35, 0 ≤ y ≤ 0.35, 0.95 ≤ z ≤ 1.30 , 0 ≤ α ≤ 0.15, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al), a lithium metal composite oxide with a layered crystal structure Particles can be used. A high charge/discharge capacity can be obtained by using such lithium metal composite oxide particles.

In order to obtain a higher charge/discharge capacity, it is preferable to satisfy x+y≦0.2 and 0.95≦z≦1.10 in the above general formula. If high thermal stability is required, x+y>0.2 is preferred.

The particles of the lithium metal composite oxide preferably have a form composed of primary particles and secondary particles formed by aggregation of the primary particles. The coated lithium metal composite oxide particles preferably have a coating of lithium tungstate on part or all of the surfaces of the primary particles of the lithium metal composite oxide particles.

In general, if the surface of the positive electrode active material is completely covered with a dissimilar compound, it is considered that the movement (intercalation) of lithium ions is greatly restricted.

However, lithium tungstate has high lithium ion conductivity and has the effect of promoting movement of lithium ions. Therefore, by arranging lithium tungstate on the surface of the primary particles of the lithium metal composite oxide particles, it is possible to form a lithium conduction path at the interface with the electrolyte. In addition, the reaction resistance of the positive electrode active material, that is, the positive electrode resistance, can be reduced to improve the output characteristics of the battery.

By reducing the positive electrode resistance, the voltage lost in the battery is reduced, and the voltage actually applied to the load side becomes relatively high, resulting in high output. In addition, since the lithium is sufficiently inserted and removed from the positive electrode by increasing the voltage applied to the load side, the charge/discharge capacity of the battery, that is, the battery capacity can also be improved.

Lithium tungstate has many forms such as Li ₂ WO ₄ , Li ₄ WO ₅ and Li ₆ W ₂ O ₉ , and the coating may contain lithium tungstate in any state. However, Li ₂ WO ₄ has high lithium ion conductivity, and has the property of being less likely to be dissociated by a solvent such as water compared to other lithium tungstates.

Therefore, by increasing the content of Li ₂ WO ₄ in the coating, the resistance of the positive electrode can be greatly reduced, and the effect of improving the output characteristics can be obtained. In addition, it is possible to improve the battery capacity by reducing the positive electrode resistance. Furthermore, by increasing the ratio of Li ₂ WO ₄ contained in the lithium tungstate of the coating, it is possible to suppress the amount of gas generated when the battery is stored at high temperatures, so it is possible to particularly improve safety. is.

For this reason, the lithium tungstate of the coating preferably has a high content of Li ₂ WO ₄ , for example, higher than 50 mol %.

Although the upper limit of the ratio of Li ₂ WO ₄ in the lithium tungstate of the coating is not particularly limited, it is preferably 90% mol or less, more preferably 80 mol% or less. This is because the presence of a small amount of Li ₄ WO ₅ , which has a high lithium ion conductivity and a positive electrode resistance reduction effect greater than that of Li ₂ WO ₄ , can further reduce the positive electrode resistance, which is preferable.

The existence form of lithium tungstate can be evaluated by instrumental analysis using, for example, X-rays or electron beams. Alternatively, it may be calculated by pH titration analysis with hydrochloric acid.

Since contact between the electrolyte and the positive electrode active material occurs on the surface of the primary particles of the lithium metal composite oxide contained in the positive electrode active material, lithium tungstate is preferably arranged on the surface of the primary particles of the lithium metal composite oxide. . Here, the primary particle surface refers to the primary particle surface exposed on the outer surface of the secondary particle, and the voids near the surface and inside of the secondary particle where the electrolyte can contact (penetrate) through the secondary particle exterior. It includes exposed primary particle surfaces, as well as the surfaces of primary particles that exist alone. In addition, even grain boundaries between primary particles are included in the surfaces of primary particles if the bonding of the primary particles is incomplete and the electrolyte can contact (permeate) the surfaces of the grain boundaries.

The contact between the positive electrode active material and the electrolyte occurs not only on the outer surface of the secondary particles formed by aggregation of the primary particles of the lithium metal composite oxide contained in the positive electrode active material, but also on the vicinity of the surface of the secondary particles and the inner primary particles. Voids between grains and even imperfect grain boundaries occur. For this reason, it is preferable to arrange a coating of lithium tungstate on the surfaces of the primary particles to promote movement of lithium ions. In this way, by arranging the coating of lithium tungstate on most of the surfaces of the primary particles of the lithium metal composite oxide that can be in contact with the electrolyte, the positive electrode resistance can be further reduced, which is preferable.

Although the form of the lithium tungstate forming the coating is not particularly limited, it is preferably in the form of fine particles.

Lithium tungstate is preferably particles having a particle diameter of, for example, 1 nm or more and 200 nm or less. This is because lithium tungstate having a particle size of 1 nm or more can exhibit sufficient lithium ion conductivity. Also, by setting the particle size of the lithium tungstate to 200 nm or less, a coating can be formed particularly uniformly on the surface of the lithium metal composite oxide particles, and the positive electrode resistance can be particularly suppressed.

Lithium tungstate does not need all particles to be distributed in the range of 1 nm or more and 200 nm or less in particle diameter in order to express its properties. , 50% or more in number preferably have a size of 1 nm or more and 200 nm or less.

Lithium tungstate does not need to be arranged on the entire surface of the lithium metal composite oxide particles.

In addition, lithium tungstate can exhibit its properties even when it is present in the form of a thin film on the surface of the lithium metal composite oxide particles. By coating the surface of the lithium metal composite oxide particles with a thin film, it is possible to form a lithium conduction path at the interface with the electrolyte while suppressing the decrease in the specific surface area, further improving the battery capacity and reducing the positive electrode resistance. effect is obtained.

When the lithium tungstate is arranged in the form of a thin film on the surface of the lithium metal composite oxide particles, the thin film preferably has a thickness of 1 nm or more and 200 nm or less.

This is because when lithium tungstate exists in the form of a thin film, the lithium tungstate thin film can exhibit sufficient lithium ion conductivity by setting the thickness of the film to 1 nm or more and 200 nm or less.

Even when the lithium tungstate exists in the form of a thin film, it does not need to be arranged on the entire surface of the lithium metal composite oxide particles, and the properties can be exhibited even if it exists only partially.

Further, when the lithium tungstate exists in the form of a thin film, the thickness of the lithium tungstate thin film as a whole does not need to be 1 nm or more and 200 nm or less. characteristics can be expressed even if

The coating of lithium tungstate can exhibit characteristics even when a plurality of states, for example, the fine particle state and the thin film state described above are mixed.

The properties of lithium tungstate on the surface of such lithium metal composite oxide particles can be determined, for example, by observation with a scanning electron microscope (hereinafter abbreviated as SEM).

As a specific observation method, for example, by setting the acceleration voltage to an appropriate value and using a backscattered electron detector as a detector, fine particles and thin film portions of lithium tungstate can be observed prominently, and tungsten It is possible to grasp the state of distribution of lithium oxide.

Since lithium tungstate contains tungsten, it has a higher average atomic number than a lithium metal composite oxide, and thus has a high reflected electron generation efficiency during electron beam irradiation. Therefore, by using a backscattered electron detector as a detector to take an image, the covering portion such as the fine particles or thin film of lithium tungstate distributed on the particle surface of the lithium metal composite oxide is more likely than the lithium metal composite oxide portion. A contrast SEM image that appears white can be obtained.

Although the specific conditions for observing the distribution of the lithium tungstate coating by SEM are not particularly limited, the accelerating voltage for SEM observation in this case is desirably 5 kV or less. When the acceleration voltage is higher than 5 kV, the depth of information during SEM observation becomes deeper, and the ratio of the information of the covering portion of lithium tungstate distributed on the particle surface of the lithium metal composite oxide becomes lower, resulting in the aforementioned contrast. This is because it becomes difficult to stick.

In selecting such an appropriate acceleration voltage, the simulation result of the electron beam penetration depth by the Monte Carlo method can be referred to, and an appropriate value can be selected according to the existence form of lithium tungstate. can.

Other conditions for SEM observation are not particularly limited as long as the contrast between the lithium metal composite oxide and lithium tungstate can be clearly observed, but the observation magnification is preferably 100 times or more, for example. This is because if the observation magnification is lower than 100 times, the enriched portion in the coating may not be clearly observed. Moreover, it is desirable that the working distance is 12 mm or less. This is because the difference in generation efficiency of backscattered electrons is caused by the channeling effect (difference in crystal orientation) in addition to the difference in generation efficiency caused by the average atomic number of the substance. This is because when the thickness is 12 mm or less, the difference in generation efficiency due to the average atomic number of the substance becomes more conspicuous, and the contrast between the lithium metal composite oxide and the lithium tungstate becomes clear. In addition, it is preferable that the number of pixels during observation is 1000 pixels or more in order to clearly observe the distribution of lithium tungstate. In addition, it is desirable that the accumulated time per pixel during observation is 10 μs or more in order to clearly observe the distribution of lithium tungstate.

Furthermore, the inventors of the present invention quantitatively grasp the distribution state on the particle surface of the lithium metal composite oxide of the lithium tungstate coating portion by image analysis of the observation image observed by the above method, and the distribution It was found that the battery characteristics can be improved by further improving the state.

According to the studies of the inventors of the present invention, when a coating of lithium tungstate is arranged on the particles of the lithium metal composite oxide, the enriched portion in the coating of lithium tungstate on the surface of the primary particle of the lithium metal composite oxide can be made uniform in the distribution state of the lithium tungstate coating in the lithium metal composite oxide powder. As a result, the positive electrode resistance of the lithium ion secondary battery using the positive electrode active material containing the particles of the lithium metal composite oxide coated with the lithium tungstate can be suppressed, the output can be further increased, and the capacity can be increased. , can be highly durable.

Here, the concentrated portion of lithium tungstate means coarse grains or a thick film portion in which lithium tungstate is concentrated and grown coarsely in the lithium tungstate coating. Specifically, the enriched portion of lithium tungstate is, for example, a portion having a particle size of approximately larger than 200 nm, among the particles contained in the coating of lithium tungstate arranged on the surface of the primary particles of the lithium metal composite oxide, Of the film contained in the lithium tungstate coating, the portion having a film thickness of approximately 200 nm or more is shown. The enriched portion itself does not have any adverse effect on the battery characteristics, but a large amount of the enriched portion indirectly indicates that the area where lithium tungstate is not arranged on the surface or inside the particles of the lithium metal composite oxide increases. Therefore, by reducing the concentrated portion, the uniformity of lithium tungstate distribution can be improved.

The inventors of the present invention quantitatively grasp the ratio of the concentrated portion of lithium tungstate on the surface of the coating-containing lithium metal composite oxide particles using the binarization method from the contrast of the SEM backscattered electron image. found a method for evaluating the uniformity of lithium tungstate distribution.

The image analysis tool is not particularly limited as long as it can separate the coating-containing lithium metal composite oxide region (non-lithium tungstate enriched region) other than the target enriched portion from the lithium tungstate enriched portion. Such image analysis software can be used. In the binarization, the lithium tungstate enriched portion and the coating-containing lithium metal composite oxide portion (non-lithium tungstate enriched region) other than the enriched portion are separated from the SEM image based on the value of the Gray Scale, which indicates the strength of the contrast. , to carry out the separation. The gray scale threshold can be determined by referring to a histogram obtained by separating the SEM image for each pixel and plotting the gray scale on the horizontal axis and the frequency on the vertical axis.

From the separation results, the area ratio of the lithium tungstate-enriched portion to the entire image (hereinafter referred to as "area ratio A") and the coating-containing lithium metal composite oxide portion other than the enriched portion (non-lithium tungstate-enriched portion). to the entire image (hereinafter referred to as "area ratio B"). Then, using the area ratio A and the area ratio B, the area ratio (hereinafter, " area ratio C”) can be calculated.

(Area ratio C (%)) = Area ratio A/(Area ratio A + Area ratio B) x 100 (1)
In the evaluation, it is also possible to perform observation and binarization processing in a plurality of fields of view, and to calculate the area ratio C using the area ratio A and the area ratio B for the plurality of fields of view observed.

According to the studies of the inventors of the present invention, the area ratio C, which is the area ratio of the concentrated portion, is preferably 50% or less, more preferably 30% or less, and further preferably 10% or less. preferable.

This is because, according to the studies of the inventors of the present invention, the uniformity of the distribution of lithium tungstate can be improved by setting the area ratio C to 50% or less.

Since it is preferable that the concentrated portion does not exist, the lower limit of the area ratio C of the concentrated portion contained in the coating-containing lithium metal composite oxide particles can be 0%. That is, the area ratio C is preferably 0% or more.

Although the amount of tungsten contained in the positive electrode active material of the present embodiment is not particularly limited, it is preferably 0.05 atomic % or more with respect to the total number of atoms of Ni, Co and M contained in the positive electrode active material.

This is because the amount of tungsten with respect to the total number of atoms of Ni, Co and M contained in the positive electrode active material is 0.05 atomic % or more, so that the amount of lithium tungstate formed is reduced to reduce the positive electrode resistance. This is because a sufficient amount can be obtained. Therefore, by setting the amount of tungsten to 0.05 atomic % or more, the output characteristics of the lithium ion secondary battery using the positive electrode active material can be sufficiently improved.

Note that the upper limit of the amount of tungsten contained in the positive electrode active material of this embodiment is not particularly limited. However, the amount of tungsten contained in the positive electrode active material of the present embodiment is more preferably 1.5 atomic % or less with respect to the total number of atoms of Ni, Co and M contained in the positive electrode active material. 0 atomic % or less is more preferable.

This is because the amount of tungsten with respect to the total number of atoms of Ni, Co, and M contained in the positive electrode active material is 1.5 atomic % or less, so that the amount of lithium tungstate formed can be suppressed from becoming excessively large. is. When lithium tungstate is excessively formed, lithium conduction between the lithium metal composite oxide and the electrolyte may be inhibited, and the charge-discharge capacity may decrease. Such a decrease in charge/discharge capacity can be prevented more reliably. Moreover, by setting the amount of tungsten to 1.5 atomic % or less with respect to the total number of atoms of Ni, Co, and M contained in the positive electrode active material, it is possible to particularly suppress the formation of concentrated portions of lithium tungstate.

In addition, the amount of lithium in the entire positive electrode active material of the present embodiment is increased by the amount of lithium contained in lithium tungstate as compared with the amount of lithium in the lithium composite oxide particles that are the base material. Li/Me, which is the ratio of the sum of the number of Ni, Co, and M atoms (Me) to the number of Li atoms in the positive electrode active material of the present embodiment, is not particularly limited, but is 0.95 or more and 1.30 or less. It is preferably 0.97 or more and 1.25 or less, more preferably 0.97 or more and 1.20 or less, and particularly preferably 0.97 or more and 1.15 or less.

By setting the Li/Me of the entire positive electrode active material of the present embodiment within the above range, the Li/Me of the lithium composite oxide particles that are the base material is sufficiently increased, high battery capacity is obtained, and lithium tungstate is formed. Sufficient amounts of lithium can be reserved for

The Li/Me of the lithium composite oxide particles as the base material is also not particularly limited, but it is preferably 0.95 or more and 1.25 or less, more preferably 0.95 or more and 1.20 or less. , 0.95 or more and 1.10 or less.

Here, the base material is a lithium metal composite oxide particle that does not contain lithium tungstate as a coating. By forming a coating of lithium tungstate on the surface of the primary particles of the lithium metal composite oxide particles, the coated lithium metal composite oxide particles contained in the positive electrode active material of the present embodiment can be obtained.

As described above, by setting the Li/Me ratio of the lithium composite oxide particles as the base material to 0.95 or more, the positive electrode resistance of the lithium ion secondary battery using the positive electrode active material containing the lithium composite oxide particles is increased. In particular, it can be suppressed and the output characteristics can be improved. In addition, by setting the Li/Me ratio of the lithium composite oxide particles as the base material to 1.25 or less, the initial discharge capacity of the lithium ion secondary battery using the positive electrode active material containing the lithium composite oxide particles is improved. and the positive electrode resistance can be particularly suppressed.

According to the positive electrode active material of the present embodiment described above, the state of the coating of lithium tungstate arranged on the surface of the particles of the lithium metal composite oxide is uniform for the contained lithium metal composite oxide powder. . Therefore, when a lithium-ion secondary battery is formed, high output can be achieved.
(2) Method for manufacturing positive electrode active material Next, 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 "method for manufacturing a positive electrode active material") will be described. In addition, according to the manufacturing method of the positive electrode active material for lithium ion secondary batteries of this embodiment, the above-mentioned positive electrode active material for lithium ion secondary batteries can be manufactured. Therefore, some of the items already explained will be omitted.

The manufacturing method of the positive electrode active material of this embodiment can have the following steps.

General formula: Li _z Ni _1-x-y Co _x My _{O 2+α} (where 0 < x ≤ 0.35, 0 ≤ y ≤ 0.35, 0.95 ≤ z ≤ 1.30, 0 ≤ α ≤ 0.15, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al) Lithium containing particles of a lithium metal composite oxide having a layered crystal structure A water washing step in which the metal composite oxide powder is washed with water, and after washing with water, solid-liquid separation is performed to obtain a lithium metal composite oxide cake.
A mixing step of mixing a lithium metal composite oxide cake with a tungsten compound or a mixture of a tungsten compound and a lithium compound to prepare a raw material mixture.

A heat treatment process for heat-treating the raw material mixture.
Then, the moisture content in the raw material mixture can be 2% by mass or more and 8% by mass or less.
Further, the amount of tungsten contained in the raw material mixture can be 0.05 atomic % or more and 1.5 atomic % or less with respect to the total number of atoms of Ni, Co and M.
Furthermore, in the heat treatment step, until the temperature is raised to 80° C., the atmosphere has a degree of vacuum of 5.0×10 ² Pa or more and 5.0×10 ⁴ Pa or less, and the temperature rise rate is 0.4° C./min or more. It can be 4.0° C./min or less.

Each step will be described below.
[Washing process]
A lithium metal composite oxide obtained by firing a mixture of a metal composite hydroxide or metal composite oxide, which is a precursor of the lithium metal composite oxide, and a lithium compound, is formed on the surfaces of the secondary particles and the primary particles. Unreacted lithium compounds are present. Therefore, it is possible to have a water washing step of washing the lithium metal composite oxide powder with water in order to remove excess lithium.

The metal composite oxide is represented by, for example, Ni _1-xy Co _x M _y O _1+β . Also, the metal composite hydroxide is represented by, for example, Ni _1-xy Co _x M _y (OH) _2+γ . Since x, y, and the element M in the formula are the same as in the case of the lithium metal composite oxide described above, their description is omitted here. In the above formula, β can be 0≦β≦0.15, and γ can be 0≦γ≦0.15. These metal composite oxides and metal composite hydroxides can be prepared by a crystallization method or the like.

As the lithium compound, for example, one or more selected from lithium hydroxide and lithium carbonate can be preferably used. When lithium hydroxide is used, it is preferable to roast it in advance to reduce hydration water. As the lithium hydroxide, anhydrous lithium hydroxide that has been dehydrated can be preferably used.

In the water washing step, for example, the lithium metal composite oxide powder and water are mixed to form a slurry, whereby water washing can be performed. After washing with water, the water content of the resulting lithium metal composite oxide cake can be adjusted by solid-liquid separation.

The water washing conditions in the water washing step are not particularly limited, and the unreacted lithium compound is sufficiently reduced. More preferably, it should be reduced to 0.05% by mass or less. When slurrying, it is preferable to add the lithium metal composite oxide powder to the slurry so that the concentration thereof is, for example, 200 g or more and 5000 g or less per 1 L of water.

By setting the concentration of the lithium metal composite oxide powder within the above range, the amount of unreacted lithium compounds can be sufficiently reduced while suppressing deterioration due to elution of lithium from the lithium metal composite oxide particles.

The water washing time and water washing temperature may be set within a range that can sufficiently reduce the amount of unreacted lithium compounds. preferable. It is preferable to stir the slurry during the washing time.

Solid-liquid separation can be performed after the water washing step. The solid-liquid separation method is not particularly limited, but solid-liquid separation can be carried out by pressing with a pressing means such as a filter press or belt filter. Solid-liquid separation can also be performed by suction filtration using a Nutsche or the like.
[Mixing process]
In the mixing step, a raw material mixture can be prepared by mixing the lithium metal composite oxide cake with a tungsten compound or a mixture of a tungsten compound and a lithium compound.

The tungsten compound is preferably water-soluble and dissolves in water contained in the raw material mixture, in order to allow the tungsten compound to permeate to the surface of the primary particles inside the secondary particles of the lithium metal composite oxide. However, even if the tungsten compound is difficult to dissolve in water at room temperature, any compound that dissolves in water upon heating during heat treatment may be used. Furthermore, since the water in the raw material mixture becomes alkaline due to the contained lithium, it may be a compound that is soluble in alkali.

The state of the tungsten compound is not particularly limited, and may be, for example, a solid state or an aqueous solution state.

As described above, the tungsten compound is not limited as long as it is soluble in water, but preferably one or more selected from tungsten oxide, lithium tungstate, ammonium tungstate, and sodium tungstate. In particular, one or more selected from tungsten oxide, lithium tungstate, and ammonium tungstate, which are less likely to be contaminated with impurities, is more preferable, and one or more selected from tungsten oxide and lithium tungstate is even more preferable.

The amount of the tungsten compound added in the mixing step is not particularly limited, but the amount of tungsten contained in the raw material mixture is 0.05 atomic % or more and 1.5 atomic % with respect to the total number of atoms of Ni, Co and M. or less, more preferably 0.05 atomic % or more and 1.0 atomic % or less.

By setting the amount of tungsten to 0.05 atomic % or more, a sufficient coating can be formed, and output characteristics and durability can be particularly improved. However, if the amount of tungsten exceeds 1.5 atomic %, the amount of lithium tungstate formed is too large, hindering lithium conduction between the lithium metal composite oxide and the electrolyte, and reducing the charge-discharge capacity. There is Therefore, it is preferable to set the amount of tungsten to 1.5 atomic % or less.

Also, by setting the amount of tungsten to 1.5 atomic % or less, it is possible to suppress the generation of concentrated portions of lithium tungstate.

When adding a lithium compound in the mixing step, the lithium compound to be added is not particularly limited, but one or more selected from, for example, lithium hydroxide and lithium carbonate can be suitably used. When the lithium compound is added in the mixing step, the amount of the lithium compound added can be adjusted so that, for example, the Li/Me in the positive electrode active material obtained after the heat treatment step has a desired value.

Mixing of the lithium metal composite oxide cake with the tungsten compound or the mixture of the tungsten compound and the lithium compound is not limited as long as it is a device capable of uniform mixing, and a general mixer can be used. For example, a shaker mixer, a Loedige mixer, a Julia mixer, a V blender, or the like may be used to sufficiently mix the lithium metal composite oxide particles with the tungsten compound or the like to such an extent that the shape of the lithium metal composite oxide particles is not destroyed.

According to the study of the inventor of the present invention, the moisture content in the raw material mixture is preferably 2% by mass or more and 8% by mass or less, more preferably 2% by mass or more and 6% by mass or less, and 3% by mass. It is more preferable to make the content not less than 6% by mass.

By setting the moisture content of the raw material mixture to 2% by mass or more, the diffusion phenomenon in which the lithium tungstate generated by addition or reaction diffuses through the moisture on the surface of the secondary particles of the lithium metal composite oxide, and the secondary particle exterior It is possible to sufficiently generate a percolation phenomenon in which voids between primary particles communicating with and imperfect grain boundaries are penetrated. Therefore, lithium tungstate can be distributed particularly uniformly on the surface of the lithium metal composite oxide.

Further, according to the study of the inventors of the present invention, the above-mentioned concentrated portion is produced by concentration of the lithium tungstate aqueous solution on the surface of specific particles during the heat treatment process after the mixing step. Therefore, by reducing the moisture content of the raw material mixture, it is possible to suppress the concentration of lithium tungstate on the surface of specific particles. As described above, the moisture content of the raw material mixture is preferably 8% by mass or less, more preferably 6% by mass or less, and even more preferably 5% by mass or less.

Incidentally, the moisture content of the raw material mixture can be adjusted by adjusting the dehydration conditions of the lithium metal composite oxide cake after washing with water. Also, the moisture content can be measured using a heat drying moisture meter or the like.

The washing step and the mixing step can be carried out simultaneously. In this case, the lithium metal composite oxide powder as the base material is added to an aqueous solution containing a tungsten compound or a mixture of a tungsten compound and a lithium compound, immersed, stirred, and solid-liquid separated, thereby performing a water washing step and mixing. can be performed simultaneously.
[Heat treatment process]
The heat treatment step is a step of heat-treating the produced raw material mixture.

As a result, lithium tungstate is formed on the surface of the lithium metal composite oxide particles from the lithium and tungsten contained in the water content of the raw material mixture, and a positive electrode active material for non-aqueous electrolyte secondary batteries is obtained.

The atmosphere of the heat treatment step is preferably a vacuum atmosphere in order to avoid deterioration due to reaction of moisture or carbonic acid in the atmosphere with the lithium metal composite oxide.

Although the heat treatment temperature in the heat treatment step is not particularly limited, it is preferable that the maximum temperature reached during the heat treatment is, for example, 100° C. or more and 600° C. or less. If the highest temperature reached during the heat treatment is less than 100°C, there is a possibility that the moisture will not be sufficiently removed. On the other hand, when the maximum temperature reached during the heat treatment exceeds 600° C., the primary particles of the lithium metal composite oxide contained in the positive electrode active material are sintered and some tungsten is fixed to the layered structure of the lithium metal composite oxide. It may melt and the charge/discharge capacity of the battery may decrease. For this reason, it is preferable that the highest temperature is 100° C. or higher and 600° C. or lower.

Furthermore, the inventors of the present invention have found that it is possible to suppress the concentration of lithium tungstate on the surface of specific particles by simultaneously precisely controlling the degree of vacuum and the rate of temperature increase during heat treatment in a vacuum atmosphere. rice field.

That is, in the raw material mixture, lithium tungstate produced by addition or reaction diffuses and permeates into the surfaces of the secondary particles and internal voids via moisture in the raw material mixture. At this time, if the moisture evaporates quickly, diffusion and permeation of lithium tungstate become insufficient, and lithium tungstate concentrates on the surfaces of specific particles. Therefore, it is possible to suppress the concentration of lithium tungstate by sufficiently slowing down the evaporation rate until the moisture evaporates.

Specifically, in the heat treatment step, it is preferable to create an atmosphere with a degree of vacuum of 5.0×10 ² Pa or more and 5.0×10 ⁴ Pa or less until the temperature is raised to 80° C., and is 1.0×10 ³ . It is more preferable to set the atmosphere to Pa or more and 1.0×10 ⁴ Pa or less.

The degree of vacuum in the vacuum atmosphere after exceeding 80° C. is not particularly limited. For example, it can be less than 5.0×10 ² Pa.

Moreover, in addition to the above-mentioned vacuum atmosphere, it is preferable to set the heating rate within a predetermined range as described above. Specifically, the rate of temperature increase is preferably 0.4°C/min or more and 4.0°C/min or less, and is preferably 0.6°C/min or more and 2.0°C until the temperature is raised to 80°C. /min or less, more preferably 0.6°C/min or more and 1.0°C/min or less.

By setting the temperature up to 80° C. in the heat treatment step and the rate of temperature increase within the above ranges, the concentration of lithium tungstate can be particularly suppressed.
(3) Lithium Ion Secondary Battery Next, one configuration example of the lithium ion secondary battery of the present embodiment will be described.

The lithium ion secondary battery (hereinafter also referred to as “secondary battery”) of the present embodiment can have a positive electrode using the positive electrode active material described above as a positive electrode material.

Hereinafter, one configuration example of the secondary battery of the present embodiment will be described for each component. The secondary battery of this embodiment includes, for example, a positive electrode, a negative electrode, and a non-aqueous electrolyte, and is composed of components similar to those of a general lithium ion secondary battery. It should be noted that the embodiments described below are merely examples, and the lithium ion secondary battery of the present embodiment is implemented in a form in which various changes and improvements are made based on the knowledge of those skilled in the art, including the following embodiments. can do. Moreover, the secondary battery is not particularly limited in its application.
(positive electrode)
The positive electrode included in the secondary battery of the present embodiment can contain the above-described positive electrode active material.

An example of the method for producing the positive electrode will be described below. First, a positive electrode active material (powder), a conductive material, and a binder are mixed to form a positive electrode mixture, and if necessary, activated carbon and a solvent for purposes such as viscosity adjustment are added, and this is kneaded. Then, a positive electrode mixture paste can be produced.

Since the mixing ratio of each material in the positive electrode mixture is a factor that determines the performance of the lithium ion secondary battery, it can be adjusted according to the application. The mixture ratio of the materials can be the same as that of the positive electrode of a known lithium ion secondary battery. It can contain 60% by mass or more and 95% by mass or less, 1% by mass or more and 20% by mass or less of the conductive material, and 1% by mass or more and 20% by mass or less of the binder.

The obtained positive electrode mixture paste is applied, for example, to the surface of a current collector made of aluminum foil, dried to scatter the solvent, and a sheet-like positive electrode is produced. If necessary, pressure may be applied by a roll press or the like in order to increase the electrode density. The sheet-shaped positive electrode thus obtained can be cut into a size suitable for the intended battery, and used for the manufacture of the battery.

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

The binding agent (binder) plays a role of binding the active material particles, and examples thereof include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, ethylene propylene diene rubber, styrene butadiene, and cellulose. Resin, polyacrylic acid, and the like can be used.

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

The method for producing the positive electrode is not limited to the above-described examples, and other methods may be used. For example, it can be produced by press-molding a positive electrode mixture and then drying it in a vacuum atmosphere.
(negative electrode)
Metallic lithium, a lithium alloy, or the like can be used for the negative electrode. In addition, the negative electrode is prepared by mixing a negative electrode active material capable of intercalating and deintercalating lithium ions with a binder, adding an appropriate solvent to make a paste, and applying the negative electrode mixture to the surface of a metal foil current collector such as copper. It may be applied, dried, and compressed to increase the electrode density as necessary.

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.
(separator)
A separator can be interposed between the positive electrode and the negative electrode, if necessary. The separator separates the positive electrode from the negative electrode and holds the electrolyte, and a known one can be used. For example, a thin film such as polyethylene or polypropylene having a large number of fine pores can be used. can.
(Non-aqueous electrolyte)
As the non-aqueous electrolyte, for example, a non-aqueous electrolytic solution can be used.

As the non-aqueous electrolytic solution, for example, a solution obtained by dissolving a lithium salt as a supporting salt in an organic solvent can be used. Further, as the non-aqueous electrolytic solution, an ionic liquid in which a lithium salt is dissolved may be used. The ionic liquid is a salt that is composed of cations and anions other than lithium ions and that is liquid even at room temperature.

Examples of 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; and ether compounds such as dimethoxyethane, sulfur compounds such as ethyl methyl sulfone and butane sultone, and phosphorus compounds such as triethyl phosphate and trioctyl phosphate. can also be used.

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.

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.

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

The oxide-based solid electrolyte is not particularly limited, and for example, one containing oxygen (O) and having lithium ion conductivity and electronic insulation can be preferably used. 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 One or more selected from P _0.4 O ₄ and the like can be used.

The sulfide-based solid electrolyte is not particularly limited, and for example, one containing sulfur (S) and having lithium ion conductivity and electronic insulation can be preferably 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 can be used.

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 ion 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).
(Shape and configuration of secondary battery)
The lithium-ion secondary battery of this embodiment described above can have various shapes such as a cylindrical shape and a laminated shape. Regardless of which shape is adopted, if the secondary battery of the present embodiment uses a non-aqueous electrolyte as a non-aqueous electrolyte, the positive electrode and the negative electrode are laminated with a separator interposed to form an electrode body, The obtained electrode body is impregnated with a non-aqueous electrolytic solution, and a current collecting lead is formed between the positive electrode current collector and the positive electrode terminal connected to the outside, and between the negative electrode current collector and the negative electrode terminal connected to the outside. etc., and sealed in the battery case.

As described above, the secondary battery of the present embodiment is not limited to the form using a non-aqueous electrolyte as a non-aqueous electrolyte. For example, a secondary battery using a solid non-aqueous electrolyte, that is, a It can also be a solid state battery. In the case of an all-solid-state battery, the configuration other than the positive electrode active material can be changed as necessary.
(Characteristic)
A lithium-ion secondary battery using the above-described positive electrode active material can suppress the positive electrode resistance, increase the discharge capacity, and also improve the durability.

The lithium-ion secondary battery of the present embodiment using the above-described positive electrode active material, for example, when used as a 2032-type coin battery, has a high initial discharge capacity of 200 mAh/g or more and a low positive electrode resistance. High capacity and high output. Moreover, it can be said that the thermal stability is high and the safety is also excellent.

As for the method for measuring the positive electrode resistance, when the frequency dependence of the battery reaction is measured by the AC impedance method, which is a general electrochemical evaluation method, solution resistance, negative electrode resistance and negative electrode capacity, and positive electrode resistance and positive electrode capacity is obtained as shown in FIG. 1(A). Since the Nyquist diagram appears as the sum of the characteristic curves showing the solution resistance, the negative electrode resistance and capacity, and the positive electrode resistance and capacity, fitting calculations were performed using the equivalent circuit of FIG. value can be estimated. The positive electrode resistance is equal to the diameter of the semicircle on the low frequency side of the resulting Nyquist diagram.

From the above, it is possible to estimate the positive electrode resistance by performing AC impedance measurement on the positive electrode to be manufactured and performing fitting calculation with an equivalent circuit to the obtained Nyquist diagram.

The lithium-ion secondary battery of this embodiment described above includes a positive electrode using the positive electrode active material described above as a positive electrode material. Therefore, it is possible to obtain a lithium ion secondary battery capable of suppressing the positive electrode resistance, increasing the discharge capacity, and improving the durability.

The lithium ion secondary battery of the present embodiment can be suitably used as a power source for small portable electronic devices (notebook personal computers, mobile phone terminals, etc.) that always require high capacity, and high output and durability are required. It can also be suitably used for electric vehicle batteries.

In addition, the lithium-ion secondary battery of the present embodiment has excellent safety, can be made smaller and has higher output, and is highly durable. It is suitable as In addition, the lithium ion secondary battery of the present embodiment is used not only as a power source for electric vehicles that are driven purely by electric energy, but also as a power source for so-called hybrid vehicles that are used in combination with combustion engines such as gasoline engines and diesel engines. be able to.

EXAMPLES The present invention will be specifically described below using Examples, but the present invention is not limited to these Examples.

Performances (initial discharge capacity, positive electrode resistance, cycle characteristics) of lithium ion secondary batteries having positive electrodes using the positive electrode active materials obtained in the following Examples and Comparative Examples were evaluated. Therefore, first, a method for manufacturing a lithium ion secondary battery and an evaluation method will be described.
(Manufacturing and evaluation of lithium-ion secondary batteries)
A 2032-type coin battery 20 (hereinafter referred to as a coin battery) shown in FIG. 2 was used to evaluate the initial discharge capacity and positive electrode resistance of the positive electrode active material.

As shown in FIG. 2, the coin-type battery 20 is composed of a case 21 and an electrode 22 housed in the case 21 . The case 21 has a hollow positive electrode can 21a with one end open, and a negative electrode can 21b arranged in the opening of the positive electrode can 21a. , a space for accommodating the electrode 22 is formed between the negative electrode can 21b and the positive electrode can 21a. The electrode 22 consists of a positive electrode 22a, a separator 22c, and a negative electrode 22b, which are stacked in this order, with the positive electrode 22a contacting the inner surface of the positive electrode can 21a and the negative electrode 22b contacting the inner surface of the negative electrode can 21b. It is housed in the case 21 as shown.

The case 21 is provided with a gasket 21c, and the gasket 21c fixes relative movement between the positive electrode can 21a and the negative electrode can 21b so as to maintain a non-contact state. The gasket 21c also has the function of sealing the gap between the positive electrode can 21a and the negative electrode can 21b to isolate the inside of the case 21 from the outside in an air-tight and liquid-tight manner. The coin-type battery 20 shown in FIG. 2 was manufactured as follows. First, 52.5 mg of the positive electrode active material for lithium secondary batteries prepared in each example and comparative example, 15 mg of acetylene black, and 7.5 mg of polytetrafluoroethylene (PTFE) resin were mixed, and a pressure of 100 MPa was applied to obtain a mixture having a diameter of 11 mm and a thickness of 11 mm. The positive electrode 22a was produced by press-molding to 100 μm. The produced positive electrode 22a was dried in a vacuum dryer at 120° C. for 12 hours.

Using the positive electrode 22a, the negative electrode 22b, the separator 22c, and the electrolytic solution, the coin-type battery 20 shown in FIG. 2 was produced in an Ar atmosphere glove box with a controlled dew point of -80.degree. For the negative electrode 22b, a negative electrode sheet in which graphite powder having an average particle size of about 20 μm and polyvinylidene fluoride were applied to a copper foil was used. A polyethylene porous film having a film thickness of 25 μm was used for the separator 22c. As the electrolytic solution, an equal mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with 1M LiClO ₄ as the supporting electrolyte (manufactured by Toyama Pharmaceutical Co., Ltd.) was used.

The initial discharge capacity, positive electrode resistance, and cycle characteristics indicating the performance of the manufactured coin-type battery 20 were evaluated as follows. For the initial discharge capacity, the coin-type battery 20 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 for the positive electrode was 0.1 mA / cm ² and the cutoff voltage was 4. The battery was charged to 3 V, rested for 1 hour, and then discharged to a cut-off voltage of 3.0 V. The capacity was taken as the initial discharge capacity.

The positive electrode resistance is measured by the AC impedance method using a frequency response analyzer and a potentiogalvanostat (manufactured by Solartron, 1255B) after charging the coin-type battery 20 at a charging potential of 4.1 V. The Nyquist plot shown 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, the equivalent circuit shown in FIG. A fitting calculation was performed to calculate the value of the positive electrode resistance. The positive electrode resistance is shown as a relative value, with the positive electrode resistance in Example 1 being 100.

In addition, the cycle characteristics were evaluated by measuring the capacity retention rate after 500 cycles of charging and discharging. Specifically, the prepared coin-type battery 20 was charged to a cutoff voltage of 4.9 V at a current density of 0.3 mA/cm ² in a constant temperature bath maintained at 25° C. After resting for 1 hour, After performing conditioning by repeating 5 cycles of discharging to a cutoff voltage of 3.5 V, the battery was charged to a cutoff voltage of 4.9 V at a current density of 2.0 mA/cm ² in a thermostat maintained at 60°C. After resting for 1 hour, a cycle of discharging to a cut-off voltage of 3.5 V was repeated 500 times, and the discharge capacity of each cycle was measured for evaluation. Then, the capacity retention ratio was calculated by dividing the discharge capacity obtained in the 500th cycle after conditioning by the discharge capacity obtained in the first cycle after conditioning of the coin-type battery.
[Example 1]
(Conditions for manufacturing positive electrode active material)
Represented by Li _1.030 Ni _0.82 Co _0.15 Al _0.03 O ₂ obtained by a known technique of mixing and firing a metal composite oxide powder containing Ni as a main component and lithium hydroxide Particles of lithium metal composite oxide powder were used as a base material (core material). This lithium metal composite oxide powder had an average particle size of 12.4 μm and a specific surface area of 0.3 m ² /g. In addition, the average particle size was evaluated using the volume integrated average value in the laser diffraction scattering method, and the specific surface area was evaluated using the BET method by nitrogen gas adsorption.

On the other hand, 15.6 g of tungsten oxide (WO ₃ ) is added to an aqueous solution of 5.6 g of lithium hydroxide (LiOH) dissolved in 100 mL of pure water having an electric conductivity of 0.8 μS/cm and stirred. to obtain an aqueous solution of a tungsten compound.

Next, 75 g of the lithium metal composite oxide powder as the base material is immersed in the aqueous solution of the tungsten compound whose temperature is adjusted to 25 ° C., and stirred for 10 minutes to sufficiently mix the lithium metal composite oxide powder. was washed with water. Thereafter, solid-liquid separation was performed by suction filtration using a Nutsche to obtain a tungsten mixture (raw material mixture) composed of lithium metal composite oxide particles, a liquid component, and a tungsten compound. By adjusting the suction filtration time, the moisture content of the raw material mixture was adjusted to fall within the target range. The moisture content measured using a heat drying moisture meter was 5.5% by mass. Further, it was confirmed that the tungsten content of the raw material mixture was 0.30 atomic % with respect to the total number of atoms of Ni, Co and Al (washing process, mixing process).

The obtained tungsten mixture was placed in a stainless steel (SUS) baking container and heat-treated in a vacuum atmosphere using a vacuum dryer.

As the heat treatment conditions, the temperature was raised to 80°C at a temperature elevation rate of 1.0°C/min, and after reaching 80°C, the temperature was raised to 100°C and dried for 12 hours, and the temperature was further raised to 190°C for 3 hours. Heat treated and then furnace cooled to room temperature. The pressure in the vacuum dryer was controlled so as to maintain a vacuum atmosphere of 1.0×10 ³ Pa or more and 1.0×10 ⁴ Pa or less up to 80° C.

Finally, the powder was pulverized through a sieve with an opening of 38 μm to obtain a positive electrode active material composed of a lithium metal composite oxide powder composed of lithium metal composite oxide particles containing a coating. The coated lithium metal composite oxide particles have a coating of lithium tungstate on their primary particle surfaces.

The composition, tungsten content, and Li/Me of the obtained positive electrode active material were analyzed by ICP emission spectroscopy, and the atomic ratio of Ni:Co:Al was 82:15:3, and the tungsten content was Ni. , was confirmed to have a composition of 0.30 atomic % with respect to the total number of atoms of Co and Al, the Li/Me ratio was 0.987, and the base material Li/Me ratio was 0.985. . That is, the composition of the lithium metal composite oxide as the base material is Li _0.985 Ni _0.82 Co _0.15 Al _0.03 O ₂ , and the X-ray diffraction pattern of the obtained positive electrode active material is As a result of measurement, it was also confirmed that the crystal structure of the base material was a layered structure.

It was also confirmed that the lithium metal composite oxide used as the base material had a layered crystal structure in the following other examples as well.

The Li/Me of the base material is analyzed by ICP emission spectroscopy of the lithium metal composite oxide powder used as the raw material of the base material washed with water under the same conditions using a lithium hydroxide solution containing Li at the same concentration as when washed with water. I asked for it.
(Form and distribution evaluation of lithium tungstate)
The obtained positive electrode active material was evaluated.

SEM observation was carried out in order to calculate the ratio (area ratio C) of the concentrated lithium tungstate portion on the surface of the lithium metal composite oxide particles containing the coating of the obtained positive electrode active material. The sample was fixed on a conductive double-sided tape, and SEM observation was performed as it was. A backscattered electron image was acquired using a backscattered electron detector under the conditions of SEM observation conditions of an acceleration voltage of 5 kV, a working distance of 12 mm, an observation magnification of 0.1 k, a pixel count of 1280×1024 pixels, and an integration time of 10 μs per pixel. Five observation images were obtained by changing the observation position.

An example of the observed image obtained is shown in FIG.

The obtained five observation images were subjected to binarization processing using image analysis software Image J, and the lithium metal composite oxide portion excluding the lithium tungstate enriched portion and the lithium tungstate enriched portion. separated. FIG. 4 shows an example of an image after the binarization processing of the observed image shown in FIG.

Then, the area ratio (area ratio A) of the lithium tungstate concentrated portion to the entire image and the area ratio (area ratio B) of the coated lithium metal composite oxide portion other than the concentrated portion to the entire image were determined. Next, the proportion of the concentrated portion of lithium tungstate in the surface of the coating-containing lithium metal composite oxide particles (area ratio C) was calculated according to the previously described formula (1). As a result, as shown in Table 1, the area ratio was 3.3%. In Table 1, the area ratio of the lithium tungstate-enriched portion of the coating-containing lithium metal composite oxide particles is described as the area ratio of the enriched portion.

Furthermore, the coating-containing lithium metal composite oxide particles contained in the obtained positive electrode active material were embedded in a resin and processed with a cross-section polisher to prepare a sample for cross-sectional observation. Using the sample for cross-sectional observation, cross-sectional observation was performed by SEM at a magnification of 5000 times. It was confirmed that the particles consisted of secondary particles and that fine particles were formed on the surfaces of the primary particles. The primary particle surface includes the surface of the primary particles forming the surface of the secondary particles and the surface of the primary particles existing inside the secondary particles. The particle size of the fine particles was 20 nm or more and 200 nm or less. In addition, secondary particles having fine particles formed on the surfaces of the primary particles accounted for 90% of the observed number of secondary particles, and it was confirmed that fine particles were uniformly formed among the secondary particles. Also, from the difference in contrast between the primary particles and secondary particles and the fine particles, it was found that the primary particles and secondary particles and the fine particles are different substances.

Furthermore, when the vicinity of the surface of the obtained primary particles of the positive electrode active material was observed with a transmission electron microscope (TEM), it was found that a coating with a thickness of 2 nm or more and 80 nm or less was formed on the surface of the primary particles, and electron energy loss Analysis by spectroscopy (EELS) confirmed that the coating was lithium tungstate.

It was also confirmed that the coating-containing lithium metal composite oxide particles had the same particle structure and the coating of the same form in other examples below.
[Example 2]
Regarding the heat treatment conditions of the heat treatment step, the temperature was raised to 80°C at a temperature elevation rate of 0.4°C/min, and the inside of the vacuum dryer was vacuumed to 80°C at a vacuum of 1.0 × 10 ⁴ Pa or more and 5.0 × 10 ⁴ Pa or less. The pressure range was controlled so as to create an atmosphere. A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except for the above points. The raw material mixture obtained in the mixing step had a moisture content of 5.6 mass % and a tungsten content of 0.31 atomic % relative to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the surface of the lithium metal composite oxide particles containing the coating of the positive electrode active material was 1.5%. The Li/Me composition of the entire positive electrode active material and the composition of the base material were the same as in Example 1. Also, the content of tungsten in the positive electrode active material was 0.31 atomic % with respect to the total number of atoms of Ni, Co and Al.
[Example 3]
Particles of the same lithium metal composite oxide powder as used in Example 1 were used as the base material.

Then, 10.8 g of tungsten oxide (WO ₃ ) is added to an aqueous solution of 3.9 g of lithium hydroxide (LiOH) dissolved in 100 mL of pure water having an electrical conductivity of 0.8 μS/cm and stirred. to obtain an aqueous solution of a tungsten compound.

Next, 75 g of the lithium metal composite oxide powder as the base material is immersed in the aqueous solution of the tungsten compound whose temperature is adjusted to 25 ° C., and stirred for 10 minutes to sufficiently mix the lithium metal composite oxide powder. was washed with water. Thereafter, solid-liquid separation was performed by suction filtration using a Nutsche to obtain a tungsten mixture (raw material mixture) composed of lithium metal composite oxide particles, a liquid component, and a tungsten compound. By adjusting the suction filtration time, the moisture content of the raw material mixture was adjusted to fall within the target range. The moisture content of the raw material mixture measured using a heat drying moisture meter was 7.5% by mass. Further, it was confirmed that the tungsten content of the raw material mixture was 0.30 atomic % with respect to the total number of atoms of Ni, Co and Al (washing process, mixing process).

After that, using the obtained raw material ^mixture , the temperature was raised to 80° C. at a rate of 4.0° C./min with respect to the heat treatment conditions of the heat treatment step. The pressure range was controlled so that the vacuum atmosphere was 1.0×10 ³ Pa or less. A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except for the above points.
The composition, tungsten content, and Li/Me of the obtained positive electrode active material were analyzed by ICP emission spectroscopy. 0.986, and the Li/Me of the matrix was 0.984. That is, it was confirmed that the composition of the lithium metal composite oxide, which is the base material, was Li _0.984 Ni _0.82 Co _0.15 Al _0.03 O ₂ .
Also, the content of tungsten in the positive electrode active material was 0.30 atomic % with respect to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the surface of the lithium metal composite oxide particles containing the coating of the positive electrode active material was 29.8%.
[Example 4]
2.2 g of tungsten oxide in an aqueous solution of 0.8 g of lithium hydroxide (LiOH) dissolved in 100 mL of pure water having an electric conductivity of 0.8 μS/cm. A positive electrode active material was obtained and evaluated in the same manner as in Example 1, except that (WO ₃ ) was added and stirred.

The raw material mixture had a moisture content of 5.4% by mass and a content of tungsten of 0.05% by atom with respect to the total number of atoms of Ni, Co and Al.

The composition, tungsten content, and Li/Me of the obtained positive electrode active material were analyzed by ICP emission spectroscopy. 0.984, and the Li/Me of the matrix was 0.983. That is, it was confirmed that the composition of the lithium metal composite oxide, which is the base material, was Li _0.983 Ni _0.82 Co _0.15 Al _0.03 O ₂ .

Also, the content of tungsten in the positive electrode active material was 0.05 atomic % with respect to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the surface of the lithium metal composite oxide particles containing the coating of the positive electrode active material was 0.005%.
[Example 5]
Regarding the heat treatment conditions of the heat treatment step, the temperature was raised to 80°C at a temperature elevation rate of 0.4°C/min, and the inside of the vacuum dryer was a vacuum of 1.0 × 10 ⁴ Pa or more and 5.0 × 10 ⁴ Pa or less. The pressure range was controlled so as to create an atmosphere. A positive electrode active material was obtained and evaluated in the same manner as in Example 4 except for the above points. The raw material mixture had a moisture content of 5.7% by mass and a tungsten content of 0.05 atomic% relative to the total number of atoms of Ni, Co and Al.

The content of tungsten in the positive electrode active material was 0.05 atomic % with respect to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the surface of the coating-containing lithium metal composite oxide particles of the positive electrode active material was 0.003%.

The Li/Me of the entire positive electrode active material and the composition of the base material were the same as in Example 4.
[Example 6]
Particles of the same lithium metal composite oxide powder as used in Example 1 were used as the base material.

Then, 100 mL of pure water having a temperature of 25° C. and an electrical conductivity of 0.8 μS/cm was added to 150 g of the base material to prepare a slurry, which was washed with water for 15 minutes. After washing with water, solid-liquid separation was performed by filtering using Nutsche. The obtained lithium metal composite oxide cake had a moisture content of 5.4% by mass (water washing step).

Next, tungsten oxide (WO ₃ ) is added to the lithium metal composite oxide cake so that the amount of W is 1.5 atomic % with respect to the total number of atoms of Ni, Co and Al contained in the lithium nickel composite oxide. 5.29 g was added and sufficiently mixed using a shaker mixer device (TURBULA Type T2C manufactured by Willie & Bacchofen (WAB)) to obtain a raw material mixture. Incidentally, the moisture content of the raw material mixture was 5.2% by mass.

A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the obtained raw material mixture was used.

The composition, tungsten content, and Li/Me of the obtained positive electrode active material were analyzed by ICP emission spectroscopy. 0.982, and the Li/Me of the matrix was 0.978. That is, it was confirmed that the composition of the lithium metal composite oxide, which is the base material, was Li _0.978 Ni _0.82 Co _0.15 Al _0.03 O ₂ .
The content of tungsten in the positive electrode active material was 1.50 atomic % with respect to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the coated lithium metal composite oxide particles of the positive electrode active material was 25.3%.
[Example 7]
In the mixing step, the moisture content of the raw material mixture is adjusted to 7.5% by mass, and with respect to the heat treatment conditions in the heat treatment step, the temperature is raised to 80 ° C. at a temperature elevation rate of 4.0 ° C./min, and the vacuum dryer is heated to 80 ° C. The pressure range was controlled so that the vacuum atmosphere was 5.0×10 ² Pa or more and 1.0×10 ³ Pa or less. A positive electrode active material was obtained and evaluated in the same manner as in Example 6 except for the above points.

The composition, tungsten content, and Li/Me of the obtained positive electrode active material were analyzed by ICP emission spectroscopy. 0.983, and the Li/Me of the matrix was 0.980. That is, it was confirmed that the composition of the lithium metal composite oxide, which is the base material, was Li _0.980 Ni _0.82 Co _0.15 Al _0.03 O ₂ .
Also, the content of tungsten in the positive electrode active material was 1.50 atomic % with respect to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the coated lithium metal composite oxide particles of the positive electrode active material was 45.5%.
[Comparative Example 1]
Particles of the same lithium metal composite oxide powder as used in Example 1 were used as the base material.

Then, 8.3 g of tungsten oxide (WO ₃ ) is added to an aqueous solution of 3.0 g of lithium hydroxide (LiOH) dissolved in 100 mL of pure water having an electric conductivity of 0.8 μS/cm and stirred. to obtain an aqueous solution of a tungsten compound.

Next, 75 g of the lithium metal composite oxide powder as the base material is immersed in the aqueous solution of the tungsten compound whose temperature is adjusted to 25 ° C., and stirred for 10 minutes to sufficiently mix the lithium metal composite oxide powder. was washed with water. Thereafter, solid-liquid separation was performed by suction filtration using a Nutsche to obtain a tungsten mixture (raw material mixture) composed of lithium metal composite oxide particles, a liquid component, and a tungsten compound. By adjusting the suction filtration time, the moisture content of the raw material mixture was adjusted to fall within the target range. The moisture content measured using a heat drying moisture meter was 9.5% by mass. Further, it was confirmed that the tungsten content of the raw material mixture was 0.30 atomic % with respect to the total number of atoms of Ni, Co and Al (washing process, mixing process).

A positive electrode active material was obtained and evaluated in the same manner as in Example 1 except that the obtained raw material mixture was used.

The content of tungsten in the positive electrode active material was 0.30 atomic % with respect to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the coated lithium metal composite oxide particles of the positive electrode active material was 60.3%.
[Comparative Example 2]
Example except that in the water washing step, the water content of the lithium metal composite oxide cake after water washing was adjusted to 7.3% by mass, and the weight of tungsten oxide added to the lithium metal composite oxide cake was 7.05 g. A positive electrode active material was obtained and evaluated in the same manner as in 6. The moisture content of the raw material mixture was 7.0% by mass. The content of tungsten in the raw material mixture was 2.00 atomic % with respect to the total number of atoms of Ni, Co and Al.

The content of tungsten in the positive electrode active material was 2.0 atomic % with respect to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the coated lithium metal composite oxide particles of the positive electrode active material was 58.8%.
[Comparative Example 3]
Regarding the heat treatment conditions of the heat treatment step, the temperature rising rate up to 80° C. is set to 5.0° C./min, and the inside of the vacuum dryer up to 80° C. has a vacuum atmosphere of 1.0×10 ² Pa or more and 5.0×10 ² Pa or less. was controlled within the pressure range. A positive electrode active material was obtained and evaluated in the same manner as in Example 3 except for the above points.

The content of tungsten in the positive electrode active material was 0.30 atomic % with respect to the total number of atoms of Ni, Co and Al. In addition, the area ratio of the lithium tungstate-enriched portion in the coated lithium metal composite oxide particles of the positive electrode active material was 57.9%.

表１に示した結果によると、濃縮部分の面積比が５０％以下である実施例１～実施例７では、濃縮部分の面積比が５０％を超える比較例１～比較例３と比較して、正極抵抗が抑制できることを確認できた。実施例１～実施例７の正極活物質では、上述のように正極抵抗が低減されることから、電池内で損失される電圧が減少し、実際に負荷側に印加される電圧が高くなるため、リチウムイオン二次電池に適用した場合に高出力が得られる。

According to the results shown in Table 1, in Examples 1 to 7 in which the area ratio of the concentrated portion is 50% or less, compared with Comparative Examples 1 to 3 in which the area ratio of the concentrated portion exceeds 50% , it was confirmed that the positive electrode resistance can be suppressed. In the positive electrode active materials of Examples 1 to 7, since the positive electrode resistance is reduced as described above, the voltage lost in the battery is reduced, and the voltage actually applied to the load side is increased. , a high output can be obtained when applied to a lithium-ion secondary battery.

Further, in Examples 1 to 7, it was confirmed that the initial discharge capacity was 200 mAh/g or more and the capacity retention rate after 500 cycles was 80% or more. It is considered that this is because by suppressing the positive electrode resistance as described above, insertion and extraction of lithium in the positive electrode is sufficiently performed, and thus the battery capacity and cycle characteristics can be improved.

Claims (1)

General formula: Li _z Ni _1-x-y Co _x My _{O 2+α} (where 0 < x ≤ 0.35, 0 ≤ y ≤ 0.35, 0.95 ≤ z ≤ 1.30, 0 ≤ α ≤ 0.15, M is at least one element selected from Mn, V, Mg, Mo, Nb, Ti and Al) Lithium containing particles of a lithium metal composite oxide having a layered crystal structure a water washing step of obtaining a lithium metal composite oxide cake by washing the metal composite oxide powder with water and performing solid-liquid separation after washing with water;
a mixing step of mixing the lithium metal composite oxide cake with a tungsten compound or a mixture of a tungsten compound and a lithium compound to prepare a raw material mixture;
and a heat treatment step of heat-treating the raw material mixture,
Moisture content in the raw material mixture is 2% by mass or more and 8% by mass or less,
The amount of tungsten contained in the raw material mixture is 0.05 atomic % or more and 1.5 atomic % or less with respect to the total number of atoms of Ni, Co and M,
In the heat treatment step, until the temperature is raised to 80° C., the atmosphere has a degree of vacuum of 5.0×10 ² Pa or more and 5.0×10 ⁴ Pa or less, and the temperature rise rate is 0.4° C./min or more. A method for producing a positive electrode active material for a lithium ion secondary battery at a temperature of 0° C./min or less.

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