JP7215260B2 - Cathode active material showing layered rock salt structure and containing lithium, nickel, cobalt, tungsten, aluminum, zirconium and oxygen, and method for producing the same - Google Patents

Description

The present invention relates to a positive electrode active material exhibiting a layered rock salt structure and containing lithium, nickel, cobalt, tungsten, aluminum, zirconium and oxygen, and a method for producing the same.

Various materials are known to be used as positive electrode active materials for lithium ion secondary batteries. Among them, lithium nickel oxide represented by LiNiO ₂ was widely used as a positive electrode active material at the beginning of development of lithium ion secondary batteries, as described in Patent Document 1.

In addition, a lithium-nickel metal oxide in which a part of the nickel in LiNiO ₂ is replaced with another metal has been developed, and research on lithium-ion secondary batteries using the lithium-nickel metal oxide as a positive electrode active material has been vigorously conducted. has been done. Especially in recent years, there have been many reports of lithium-ion secondary batteries that employ, as a positive electrode active material, lithium-nickel-cobalt-aluminum oxide having a layered rock salt structure in which part of the nickel in LiNiO ₂ is replaced with cobalt and aluminum.

Patent Document 2 specifically describes a lithium ion secondary battery that employs LiNi _0.81 Co _0.15 Al _0.04 O ₂ as a positive electrode active material.

_{In Patent Document 3 , a lithium ion _} A secondary battery is specifically described.

Patent Document 4 specifically describes a lithium ion secondary battery that employs LiNi _0.8 Co _0.15 Al _0.05 O ₂ as a positive electrode active material.

_{Patent Document 5 discloses Li1.013Ni0.831Co0.119Al0.050O2 , Li1.013Ni0.858Co0.123Al0.020O2 or Li1} as _a positive electrode active _{material .} A lithium ion secondary battery employing _.013 Ni _0.867 Co _0.098 Al _0.035 O ₂ is specifically described.

JP-A-63-121260 JP 2006-128119 A JP 2006-278341 A JP 2014-139926 A JP 2017-195020 A

However, demand for positive electrode active materials for lithium ion secondary batteries is increasing, and there is a strong desire to provide a new positive electrode active material and a method for producing the same.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a new positive electrode active material and a method for producing the same.

As a result of intensive studies, the present inventors have found that a layered rock salt structure of lithium nickel cobalt tungsten oxide in which part of nickel in LiNiO ₂ is replaced with cobalt and tungsten is a suitable positive electrode active material. Furthermore, the present inventors have found that a lithium mixed metal oxide having a layered rock salt structure in which a portion of nickel in LiNiO ₂ is replaced with cobalt, tungsten, aluminum and zirconium is a suitable positive electrode active material.

As a result of intensive studies by the present inventors on the method for producing the positive electrode active material, the solubility of the metal in water is improved by adding a chelate compound to a metal aqueous solution in which nickel, cobalt, tungsten, aluminum and zirconium are dissolved. After that, we developed a technique to coprecipitate these metals by controlling the pH of the metal aqueous solution. Using this technique, they succeeded in uniformly distributing nickel, cobalt, tungsten, aluminum and zirconium in the particles of the positive electrode active material.
They also found that the performance of the positive electrode active material is improved by coating the precursor of the positive electrode active material with a metal compound.
The present invention has been completed based on such findings.

The first manufacturing method of the present invention is
A precipitation step of mixing an aqueous solution in which nickel, cobalt, tungsten, aluminum, zirconium and a chelate compound are dissolved with a basic substance to precipitate a transition metal hydroxide containing nickel, cobalt, tungsten, aluminum and zirconium;
A precursor forming step of heating the transition metal hydroxide to form a precursor from which adhering water is removed or a transition metal oxide is formed;
a firing step of mixing and firing the precursor and the lithium salt;
A method for producing a positive electrode active material having a layered rock salt structure and containing lithium, nickel, cobalt, tungsten, aluminum, zirconium and oxygen, characterized by having

The second manufacturing method of the present invention is
A precipitation step of mixing an aqueous solution in which nickel, cobalt, tungsten, aluminum, zirconium and a chelate compound are dissolved with a basic substance to precipitate a transition metal hydroxide containing nickel, cobalt, tungsten, aluminum and zirconium;
A precursor forming step of heating the transition metal hydroxide to form a precursor from which adhering water is removed or a transition metal oxide is formed;
a coating step of coating the precursor with a metal compound;
a firing step of mixing and firing the precursor coated with the metal compound and a lithium salt;
A method for producing a positive electrode active material having a layered rock salt structure and containing lithium, nickel, cobalt, tungsten, aluminum, zirconium and oxygen, characterized by having

ADVANTAGE OF THE INVENTION By this invention, a new positive electrode active material and its manufacturing method can be provided.

4 is an SEM image of the positive electrode active material of Example 1. FIG. 4 is an SEM image of the positive electrode active material of Example 2. FIG. 9 is a graph showing the results of Evaluation Example 3;

BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention will be described below. Unless otherwise specified, the numerical range "x to y" described herein includes the lower limit x and the upper limit y. Further, a numerical range can be configured by arbitrarily combining these upper and lower limits and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from within the numerical range can be used as upper and lower numerical values.

The first manufacturing method of the present invention is
A precipitation step of mixing an aqueous solution in which nickel, cobalt, tungsten, aluminum, zirconium and a chelate compound are dissolved with a basic substance to precipitate a transition metal hydroxide containing nickel, cobalt, tungsten, aluminum and zirconium;
A precursor forming step of heating the transition metal hydroxide to form a precursor from which adhering water is removed or a transition metal oxide is formed;
a firing step of mixing and firing the precursor and the lithium salt;
A method for producing a positive electrode active material having a layered rock salt structure and containing lithium, nickel, cobalt, tungsten, aluminum, zirconium and oxygen, characterized by having

The second manufacturing method of the present invention differs from the first manufacturing method of the present invention in that it has a coating step of coating the precursor in the first manufacturing method of the present invention with a metal compound.
Hereinafter, the first manufacturing method of the present invention and the second manufacturing method of the present invention are collectively referred to as the manufacturing method of the present invention. The positive electrode active material manufactured by the manufacturing method of the present invention is called the positive electrode active material of the present invention.

In the production method of the present invention, coprecipitation of nickel, cobalt, tungsten, aluminum and zirconium in the deposition step is a technical key.

A precipitation process is demonstrated. Unless otherwise specified, the pH defined in this specification refers to the value measured at 25°C.

In order to prepare an aqueous solution in which nickel, cobalt, tungsten, aluminum, zirconium and a chelate compound are dissolved (hereinafter sometimes referred to as a transition metal-containing aqueous solution), the metal composition ratio in the transition metal hydroxide to be deposited and A nickel compound, a cobalt compound, a tungsten compound, an aluminum compound, and a zirconium compound are added to water in an appropriate ratio, and a chelate compound is added to dissolve these compounds in water.

Examples of nickel compounds include nickel sulfate, nickel carbonate, nickel nitrate, nickel acetate, and nickel chloride. Examples of cobalt compounds include cobalt sulfate, cobalt carbonate, cobalt nitrate, cobalt acetate, and cobalt chloride. Examples of tungsten compounds include tungstates such as zirconium tungstate, Li ₂ WO ₄ , Na ₂ WO ₄ , K ₂ WO ₄ and (NH ₄ ) ₂ WO ₄ . Examples of aluminum compounds include aluminum sulfate, aluminum carbonate, aluminum nitrate, and aluminum chloride. Examples of zirconium compounds include zirconium tungstate, zirconium sulfate, zirconium chloride, and zirconium iodide.

Chelate compounds are amino groups, amide groups, imide groups, imino groups, cyano groups, azo groups, hydroxyl groups, alkoxy groups, carboxyl groups, ester groups, ether groups, carbonyl groups, and phosphoric acid groups that can coordinate to metal ions. , phosphate ester group, phosphonic acid group, phosphonate ester group, phosphinate group, phosphinate ester group, phosphenate group, phosphinate ester group, phosphenite group, phosphite ester group, thiol group, sulfide group, It is a compound having a structure that has a plurality of sulfinyl groups, sulfonyl groups, sulfonic acid groups, thiocarboxyl groups, thioester groups, or thiocarbonyl groups, and that these groups can coordinate to metal ions.

Specific examples of chelate compounds include polyamine compounds such as ethylenediamine and diethylenetriamine, amino acids such as glycine, alanine, cysteine, glutamine, arginine, asparagine, aspartic acid, serine, and ethylenediaminetetraacetic acid, malonic acid, succinic acid, glutaric acid, and maleic acid. Mention may be made of acids, dicarboxylic acids such as phthalic acid, and hydroxycarboxylic acids.

Hydroxycarboxylic acids are particularly preferred as chelate compounds. Examples of hydroxycarboxylic acids having a hydroxyl group and a carboxylic acid group in the molecule include aliphatic hydroxycarboxylic acids and aromatic hydroxycarboxylic acids.

Aliphatic hydroxycarboxylic acids include glycolic acid, lactic acid, tartronic acid, glyceric acid, 2-hydroxybutyric acid, 3-hydroxybutyric acid, γ-hydroxybutyric acid, malic acid, tartaric acid, citramaric acid, citric acid, isocitric acid, and leucic acid. , mevalonic acid, pantoic acid, quinic acid, and shikimic acid.

Examples of aromatic hydroxycarboxylic acids include o-hydroxybenzoic acid derivatives such as salicylic acid, gentisic acid and orceric acid, mandelic acid, benzilic acid and 2-hydroxy-2-phenylpropionic acid.

All of the above specific hydroxycarboxylic acids are capable of forming conformations in which OH and CO ₂ H groups can coordinate to the same metal ion.

It is generally known that metal hydroxides are deposited when the pH of the aqueous metal solution is made basic. The pH at which the metal hydroxide is deposited differs depending on the type of metal. Therefore, when a basic substance is added to an aqueous metal solution in which multiple types of metal ions are dissolved to make the pH of the aqueous metal solution basic, the type of metal hydroxide that precipitates differs for each pH. It will be. As a result, it is assumed that the composition of the metal hydroxide will vary.

However, in the production method of the present invention, the chelate compound is dissolved in the transition metal-containing aqueous solution. Here, since the chelate compound coordinates with the metal ion to form a stable complex, the solubility of the metal ion in water is improved. Therefore, even if the metal is deposited at a relatively low pH under conditions in which no chelate compound is present, the deposition pH becomes high in the transition metal-containing aqueous solution in which the chelate compound is dissolved. As a result, nickel, cobalt, tungsten, aluminum and zirconium co-precipitate as hydroxides or derivatives thereof in similar pH ranges.

When comparing the solubility of nickel, cobalt, tungsten, aluminum and zirconium in basic water, aluminum and zirconium are poorly soluble. Therefore, it can be said that the chelate compound is added to the transition metal-containing aqueous solution in order to improve the solubility of aluminum and zirconium.

The molar ratio of the chelate compound to the total moles of aluminum and zirconium in the transition metal-containing aqueous solution is preferably 1 or more, preferably in the range of 1 to 10, more preferably in the range of 1.5 to 8, and 2 to 6. is more preferably within the range of

For preparing the transition metal-containing aqueous solution, it is preferable to use a reaction vessel equipped with a stirring device, and more preferably a reaction vessel equipped with a device capable of introducing an inert gas such as nitrogen or argon. Moreover, it is more preferable to use a reaction vessel equipped with a device that provides constant temperature conditions.

The transition metal-containing aqueous solution is preferably heated to 40 to 90°C, more preferably 40 to 80°C.

In order to mix the transition metal-containing aqueous solution and the basic substance while suitably controlling the pH, it is rational to use the basic aqueous solution as the basic substance.
To prepare a basic aqueous solution, a basic compound may be dissolved in water.

For preparing the basic aqueous solution, it is preferable to use a reaction vessel equipped with a stirring device, and more preferably a reaction vessel equipped with a device capable of introducing an inert gas such as nitrogen or argon. Moreover, it is more preferable to use a reaction vessel equipped with a device that provides constant temperature conditions.

The pH of the basic aqueous solution is preferably in the range of 11-14, more preferably in the range of 11-13, even more preferably in the range of 11-12. The basic compound that can be used may be one that dissolves in water and exhibits basicity. Examples include ammonia, alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, and carbonate alkali metal carbonates such as potassium and lithium carbonate; alkali metal phosphates such as trisodium phosphate, tripotassium phosphate and trilithium phosphate; and alkali metal acetates such as sodium acetate, potassium acetate and lithium acetate. can be done. A basic compound may be used alone or in combination.
Since the pH of the reaction solution is preferably kept within a suitable range in the precipitation step, the basic aqueous solution preferably contains at least a basic compound having a buffering capacity. Examples of basic compounds having a buffering capacity include ammonia, alkali metal carbonates, alkali metal phosphates, and alkali metal acetates.

The basic aqueous solution is preferably heated to 40 to 90°C, more preferably 40 to 80°C.

In the precipitation step, by mixing the transition metal-containing aqueous solution and the basic substance, metal ions and hydroxide ions react to contain nickel, cobalt, tungsten, aluminum and zirconium, which have low solubility in water. A transition metal hydroxide is formed and precipitates. When a tungstate is used, tungsten is believed to be deposited as tungstic acid O ₂ W(OH) ₂ or a salt thereof together with other metals. are collectively called transition metal hydroxides. The precipitated transition metal hydroxide particles form the basis of the primary particles of the positive electrode active material of the present invention. Therefore, if the precipitation process is carried out under conditions in which the precipitation rate of the transition metal hydroxide is extremely high, i.e., under conditions in which transition metal hydroxide nuclei are generated everywhere, disordered transition metal hydroxide particles are formed. As a result, the primary particles of the positive electrode active material of the present invention may have an unfavorable crystal habit. Therefore, in the precipitation step, it is preferable to precipitate the transition metal hydroxide particles under as mild conditions as possible.

In the precipitation step, it is preferable to keep the reaction solution in which the transition metal-containing aqueous solution and the basic substance are mixed at a constant pH. In addition, the pH value here means the numerical value itself which measured the reaction solution with the pH meter. The pH is preferably in the range of 10-14, more preferably in the range of 10.5-12, and particularly preferably in the range of 10.6-11.

The deposition step is preferably carried out in a reaction vessel equipped with a stirring device, and more preferably carried out in a reaction vessel equipped with a device capable of introducing an inert gas such as nitrogen or argon. Further, a reaction vessel equipped with a device that provides constant temperature conditions is more preferable.

In the precipitation step, it is preferable that the amount of dissolved oxygen present in the reaction system is small. If the amount of dissolved oxygen present in the reaction system is large, there is a possibility that an undesirable oxidation reaction may occur, and suitable crystallization of the transition metal hydroxide accompanying precipitation of the transition metal hydroxide may be inhibited.

In order to reduce the amount of dissolved oxygen present in the reaction system, the precipitation step should be carried out under heating, while introducing an inert gas into the reaction system, deoxidizing agent, reducing agent, antioxidant It is preferable to carry out in the presence of an agent or the like.

As for heating, ranges of 40 to 90°C and 60 to 80°C can be exemplified.
Examples of inert gases include nitrogen, argon, and helium.
Oxygen scavengers, reducing agents, and antioxidants include ascorbic acid and its salts, glyoxylic acid and its salts, hydrazine, dimethylhydrazine, hydroquinone, dimethylamine borane, NaBH ₄ , NaBH ₃ CN, KBH ₄ , sulfurous acid and its salts. , thiosulfuric acid and its salts, pyrosulfite and its salts, phosphorous acid and its salts, and hypophosphorous acid and its salts.

After the precipitation step, the transition metal hydroxide is separated by filtration or the like. A transition metal hydroxide can be obtained by the above method.

A compound containing a doping element other than lithium, nickel, cobalt, tungsten, aluminum and zirconium is added to the transition metal-containing aqueous solution in the precipitation step to produce a transition metal hydroxide containing the doping element. good too.

Next, the precursor formation process will be described. The precursor forming step is a step of heating a transition metal hydroxide to form a transition metal hydroxide from which adhering water has been removed, or heating a transition metal hydroxide to form a transition metal oxide. is. Both the transition metal hydroxide from which the adhering water has been removed or the transition metal oxide are precursors of the positive electrode active material.

The heating temperature is preferably in the range of 100 to 800°C, more preferably in the range of 200 to 700°C, and particularly preferably in the range of 300 to 600°C. The precursor formation step may be performed under normal pressure or under reduced pressure.

In the first manufacturing method of the present invention, the firing step is performed following the precursor forming step.
In the second manufacturing method of the present invention, following the precursor formation step, the coating step of coating the precursor with the metal compound is performed, and then the baking step is performed.

The coating process will be explained. The coating step is a step of coating the precursor "transition metal hydroxide from which adhering water has been removed" or "transition metal oxide" with a metal compound to form a coated body.
A case of coating a "transition metal oxide" with a metal compound will be described below. In the case of coating the "transition metal hydroxide from which adhering water has been removed" with a metal compound, in the following description, "transition metal oxide" is appropriately replaced with "transition metal hydroxide from which adhering water has been removed". should be read as .

Specific examples of metal compounds include _ZrO2 , _CaVO3 , _MnO2 , _La2CuO4 , _La2NiO4 , _SnO2 , _{Tl2Mn2O7 , EuO} , _{Fe2O3 , CaMnO3 , SrMnO3 , (} Sr,La) _TiO3 , _LaTiO3 , _SrFeO3 , _BaMoO3 , _CaMoO3 , _{Ln2Os2O7 ( Ln} is an element selected from Y and rare earth _elements ), _Tl2Ir2O7 , _Cd2Re2O7 , _{Lu2Ir2O7 , Bi2Rh2O7 , Bi2Ir2O7 , Ti2O3} , _{WO2 , VO , V2O3 , LaMnO3 , CaCrO3 , LaCoO 3} , (ZnO) ₅ , SrCrO ₃ , In _0.97 Y _0.03 O ₃ , Zn _x Al _y O (x+y=1), LiV ₂ O ₄ , Na _1-x CoO ₂ (0<x<1) _{, LiTi2O4 , SrMoO3 , BaPbO3 , Tl2Os2O7 , Pb2Os2O7 , Pb2Ir2O7 , Lu2Ru2O7 , Bi2Ru2O7 , SrRuO3 , _ CaRuO3} , CrO2, _MoO2 , _ReO2 , TiO, _LaO , _SmO , _LaNiO3 , _SrVO3 , _ReO3 , IrO2, _RuO2 , _RhO2 , _OsO2 , _{NdO ,} NbO, La2O3, NiO, Metal oxides selected from _LaSrxCoyO3 ( _x + _y = ₁ ), _NaCoO3 , _NaNiO3 , _LiCoO3 , LiNiO3 or metal hydroxides of their precursors can be exemplified.
Among metal compounds, metal oxides preferably exhibit a crystal structure such as a perovskite type.

In order to coat the transition metal oxide with the metal compound, a method of spraying the precursor of each metal compound or an aqueous solution in which each metal is dissolved onto the transition metal oxide and/or simultaneously drying may be adopted. . Alternatively, the transition metal oxide is immersed in an aqueous solution in which the precursor of each metal compound or each metal is dissolved, so that the precursor of each metal compound or the hydroxide of each metal is attached to the surface of the transition metal oxide. After that, a method of drying by heating may be adopted. In particular, a transition metal oxide dispersion is mixed with a precursor of each metal compound and an aqueous solution in which each metal is dissolved to deposit the hydroxide of each metal on the surface of the transition metal oxide, followed by drying. It is preferable to employ a method (hereinafter sometimes referred to as a precipitation method).

A suitable deposition method when the metal of the metal compound is Zr will be described in detail below. The precipitation method has the following coat-1) step, coat-2) step and coat-3) step. When the metal of the metal compound is a metal other than Zr, zirconium in the coat-1) step, the coat-2) step, and the coat-3) step may be replaced with the metal. Further, when the metal compound contains a plurality of metals, an aqueous solution containing a plurality of metals may be used in the coat-2) step, or while changing the metal species of the metal aqueous solution, coat-1) The steps coat-2) and coat-3) may be repeated.

coat-1) a dispersion preparation step of dispersing the transition metal oxide in water;
coat-2) A zirconium deposition step of mixing an aqueous zirconium solution containing a chelate compound with the dispersion to deposit zirconium hydroxide on the surface of the transition metal oxide;
coat-3) A step of drying a transition metal oxide having zirconium hydroxide deposited on its surface to form a coated body

The transition metal oxide is preferably pulverized before the coat-1) step. Further, it is preferable to adjust the pH so that the pH of the dispersion is in the range of about 9 to 12.

Next, the coat-2) step will be described.
A zirconium aqueous solution containing a chelate compound is produced by dissolving a zirconium salt and a chelate compound in water. A zirconium aqueous solution containing a chelate compound is usually an acidic solution. The molar ratio of the chelate compound to zirconium in the zirconium aqueous solution is preferably 1 or more, preferably in the range of 1 to 10, more preferably in the range of 1.5 to 8, and even more preferably in the range of 2 to 6.

Zirconium salts include, for example, zirconium oxide, zirconium hydroxide, zirconium sulfate, zirconium nitrate, zirconium phosphate, and zirconium halides.

For the explanation of the chelate compound, the explanation in the precipitation step is used.

In the coat-2) step, it is preferable to control the pH of the mixed solution in the coat-2) step in order to deposit zirconium efficiently. Here, it is assumed that zirconium hydroxide with low solubility is deposited on the surface of the transition metal oxide by adjusting the pH of the mixed solution to the alkaline side. For example, it is preferable to add a basic aqueous solution so that the pH of the solution in step coat-2) is within the range of 9-13, 11-13, 12-13. As the basic aqueous solution, the one described in the precipitation step may be adopted.

The transition metal oxide that has undergone the coat-2) step is separated by a method such as filtration and subjected to the coat-3) step.

Drying in the coat-3) step is preferably performed under heating and/or under reduced pressure. As the heating temperature, ranges of 100 to 500°C and 200 to 400°C can be exemplified.
The main purpose of the drying in the coat-3) step is to remove water adhering to the transition metal oxide on which zirconium hydroxide is deposited on the surface. However, by increasing the heating temperature, zirconium hydroxide present on the surface of the transition metal oxide may be dehydrated and changed to zirconium oxide. That is, the coated body may be a transition metal oxide coated with zirconium hydroxide or a transition metal oxide coated with zirconium oxide.

Next, the firing process will be described. The firing step is a step of mixing and firing a precursor and a lithium salt, or a step of mixing and firing a precursor coated with a metal compound and a lithium salt.

Examples of lithium salts include lithium carbonate, lithium hydroxide, lithium nitrate, lithium acetate, lithium oxalate, and lithium halide. The amount of the lithium salt to be blended may be appropriately determined so as to obtain a positive electrode active material with a desired lithium composition.

Examples of the mixing device include a mortar and pestle, a stirring mixer, a V-type mixer, a W-type mixer, a ribbon-type mixer, a drum mixer, and a ball mill.

In the firing step, compounds other than the lithium salt may be mixed. In particular, it is preferable to mix compounds selected from Na compounds, F compounds and P compounds. Due to the presence of Na, F, and P, it can be expected that the rate characteristics and/or the capacity retention rate of the lithium ion secondary battery comprising the positive electrode active material of the present invention will be improved.

Examples of Na compounds include sodium salts such as NaF, NaCl, NaBr, NaI, Na ₃ PO ₄ , Na ₂ HPO ₄ , NaH ₂ PO ₄ , Na ₂ SO ₄ , NaHSO ₄ , NaNO ₃ and CH ₃ CO ₂ Na. can.
Examples of F compounds include metal fluorides such as LiF, NaF, KF, MgF ₂ , CaF ₂ , BaF ₂ and AlF ₃ .
_P compounds _{include H3PO4} , _LiH2PO4 , _{Li2HPO4 , Li3PO4 , NaH2PO4 , Na2HPO4 , Na3PO4 , KH2PO4 , K2HPO4 ,} Phosphoric acid and phosphates such as K ₃ PO ₄ can be exemplified.

Firing may be performed in an air atmosphere or an oxygen gas atmosphere, or may be performed in the presence of an inert gas such as helium or argon. The heating temperature in the firing step can be exemplified in the ranges of 400 to 1100°C, 500 to 1000°C and 600 to 800°C. The heating time in the firing step can be exemplified from 1 to 50 hours.

The firing may be performed under a single temperature condition, may be performed by combining a plurality of firing steps with different temperature conditions, or may be performed by setting a specific temperature raising program.

As a method of combining a plurality of firing steps with different temperature conditions, a first firing step of heating at 400 to 800 ° C. to form a first fired body, and a second firing step of heating the first fired body at 550 to 1000 ° C. A baking process can be mentioned. A suitable positive electrode active material can be produced by combining a plurality of firing steps.

As the temperature of the first firing step, ranges of 400 to 800°C and 650 to 750°C can be exemplified. As the heating time of the first firing step, ranges of 3 to 30 hours, 5 to 20 hours, and 5 to 15 hours can be exemplified.

The second firing step is a step of heating the first fired body at 550 to 1000°C.
Here, from the point of view of crystal formation of the positive electrode active material, heating at a temperature as low as possible facilitates formation of crystals having a uniform composition and a uniform shape. Therefore, as the temperature of the second firing step, ranges of 550 to 950°C, 550 to 900°C, 550 to 850°C, and 550 to 800°C can be exemplified.

As the heating time of the second firing step, ranges of 3 to 30 hours, 5 to 20 hours, and 5 to 15 hours can be exemplified.

In the second production method of the present invention, since the transition metal oxide particles are coated with the metal compound in the coating step, the coated metal compound acts as a barrier in the first firing step and the second firing step, It is considered that nickel is suppressed from moving to the lithium site of the layered rock salt structure.

The sintered body obtained in the sintering step is preferably subjected to a pulverization step and a classification step to obtain a positive electrode active material having a constant particle size distribution. As for the range of particle size distribution, the average particle size (D ₅₀ ) is preferably 50 μm or less, more preferably 1 μm or more and 30 μm or less, and even more preferably 1 μm or more and 20 μm or less, as measured by a general laser scattering diffraction type particle size distribution meter. , 2 μm or more and 10 μm or less are particularly preferable.

The positive electrode active material of the present invention includes primary particles and secondary particles that are agglomeration of the primary particles.

The size of the primary particles of the positive electrode active material of the present invention is preferably within the range of 50 nm to 1000 nm, more preferably within the range of 100 nm to 500 nm, and more preferably within the range of 150 nm to 500 nm when observed with a microscope. More preferred. In addition, a primary particle means a particle recognized as one particle during SEM observation.

The primary particles of the positive electrode active material of the present invention have a layered rock salt structure and contain lithium, nickel, cobalt, tungsten, aluminum, zirconium and oxygen inside.
Further, in a preferred aspect of the positive electrode active material of the present invention, the Zr concentration is higher in the surface layer of the secondary particles where the primary particles are aggregated than in the interior of the primary particles. Alternatively, in a preferred embodiment of the positive electrode active material of the present invention, the metal concentration derived from the metal compound in the coating step is higher in the surface layer of the secondary particles where the primary particles are aggregated than in the interior of the primary particles.

The positive electrode active material of the present invention exhibits a layered rock salt structure. The layered rock salt structure can be confirmed by X-ray diffraction measurement.

In the positive electrode active material of the present invention, lithium is the charge carrier. Lithium is released from the lithium sites of the layered rock salt structure during charge, and is inserted into the lithium sites of the layered rock salt structure during discharge.
Nickel is believed to preferentially contribute to oxidation-reduction reactions during charging and discharging, and cobalt, tungsten, aluminum and zirconium are believed to contribute to maintaining the layered rock salt structure.

A compound represented by the following general formula can be exemplified as a preferred embodiment of the positive electrode active material of the present invention. The compound represented by the following general formula is a positive electrode active material with excellent capacity because of its high nickel content.

_{General formula : LiaNibCocWdAleZrfDgOhFi _ _ _ _ _}
In the general formula, a, b, c, d, e, f, g, h, and i are 0.5≦a≦2, 0.85≦b<1, 0<c<0.15, 0<d <0.15, 0<e<0.15, 0<f<0.15, 0≤g<0.15, b+c+d+e+f+g=1, 1.8≤h≤2.2, 0≤i≤0.2 satisfy. D is a doping element.

The value of b in the general formula is a value that greatly affects the capacity of the positive electrode active material.
b in the general formula preferably satisfies 0.85 ≤ b ≤ 0.99, more preferably satisfies 0.9 ≤ b ≤ 0.98, and satisfies 0.93 ≤ b ≤ 0.97 more preferably.

c in the general formula preferably satisfies 0.001 ≤ c ≤ 0.1, more preferably satisfies 0.005 ≤ c ≤ 0.08, and satisfies 0.01 ≤ c ≤ 0.07 It is more preferable to satisfy 0.03≤c≤0.06.

In the general formula, d preferably satisfies 0.001 ≤ d ≤ 0.1, more preferably 0.002 ≤ d ≤ 0.05, and 0.003 ≤ d ≤ 0.03. satisfying 0.004≤d≤0.01 is particularly preferable.

In the general formula, e preferably satisfies 0.001 ≤ e ≤ 0.1, more preferably 0.001 ≤ e ≤ 0.05, and 0.001 ≤ e ≤ 0.01. satisfying 0.002≤e≤0.008 is particularly preferable.

In the general formula, f preferably satisfies 0.001 ≤ f ≤ 0.1, more preferably 0.001 ≤ f ≤ 0.05, and 0.001 ≤ f ≤ 0.01. satisfying 0.002≤f≤0.008 is particularly preferable.

Values of a, g, h, and i may be within the ranges defined by the general formula, preferably 0.5≤a≤1.5, 0≤g≤0.1, 1.8≤h≤2. .1, 0<i≤0.15, more preferably 0.8≤a≤1.3, 0<g≤0.01, 1.9≤h≤2.1, 0<i≤0.1 can be exemplified.

D in the general formula is a doping element that can improve the properties of the positive electrode active material. F in the general formula is also an element capable of improving the properties of the positive electrode active material.

D includes Na, Ca, V, Cu, Sn, Tl, Fe, Sr, Ti, Ba, Y, rare earth elements, Os, Ir, Cd, Re, Bi, Rh, Cr, Zn, In, Pb, Ru , Nb, P, and S can be exemplified.

The positive electrode active material of the present invention is used as a positive electrode active material for lithium ion secondary batteries. A lithium ion secondary battery comprising the positive electrode active material of the present invention (hereinafter sometimes referred to as the lithium ion secondary battery of the present invention) will be described below. Specifically, the lithium ion secondary battery of the present invention comprises a positive electrode, a negative electrode, a solid electrolyte or electrolytic solution, and a separator, each comprising the positive electrode active material of the present invention.

The positive electrode has a current collector and a positive electrode active material layer bonded to the surface of the current collector.

A current collector refers to a chemically inert electronic conductor for continuing current flow to an electrode during discharge or charge of a lithium ion secondary battery. As the current collector, at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, stainless steel, etc. A metal material can be exemplified. The current collector may be covered with a known protective layer. A current collector whose surface has been treated by a known method may be used as the current collector.

The current collector can take the form of foil, sheet, film, wire, rod, mesh, and the like. Therefore, metal foils such as copper foil, nickel foil, aluminum foil, and stainless steel foil can be preferably used as the current collector. When the current collector is in the form of foil, sheet or film, the thickness is preferably in the range of 1 μm to 100 μm.

The positive electrode active material layer contains a positive electrode active material and, if necessary, a conductive aid and/or a binder.

As the positive electrode active material, only the positive electrode active material of the present invention may be employed, or the positive electrode active material of the present invention and a known positive electrode active material may be used in combination.

A conductive aid is added to increase the conductivity of the electrode. Therefore, the conductive aid may be added arbitrarily when the conductivity of the electrode is insufficient, and may not be added when the conductivity of the electrode is sufficiently excellent. The conductive aid may be a chemically inert high electron conductor, and examples include carbon black, graphite, vapor grown carbon fiber, and various metal particles, which are carbonaceous fine particles. be. Examples of carbon black include acetylene black, Ketjenblack (registered trademark), furnace black, and channel black. These conductive aids can be added to the active material layer singly or in combination of two or more.

The mixing ratio of the conductive aid in the active material layer is preferably active material: conductive aid=1:0.005 to 1:0.5, more preferably 1:0.01 to 1:0. .2 is more preferred, and 1:0.03 to 1:0.1 is even more preferred. This is because if the amount of the conductive aid is too small, an efficient conductive path cannot be formed, and if the amount of the conductive aid is too large, the formability of the active material layer is deteriorated and the energy density of the electrode is lowered.

The binder serves to bind the active material and conductive aid to the surface of the current collector and maintain the conductive network in the electrode. Binders include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber; thermoplastic resins such as polypropylene and polyethylene; imide resins such as polyimide and polyamideimide; alkoxysilyl group-containing resins; Examples include acrylic resins such as meth)acrylic acid, styrene-butadiene rubber (SBR), and carboxymethyl cellulose. These binders may be used singly or in combination.

The mixing ratio of the binder in the active material layer is preferably active material:binder=1:0.001 to 1:0.3, more preferably 1:0.005 to 1:0. .2 is more preferred, and 1:0.01 to 1:0.15 is even more preferred. This is because if the amount of the binder is too small, the formability of the electrode will deteriorate, and if the amount of the binder is too large, the energy density of the electrode will be low.

The negative electrode has a current collector and a negative electrode active material layer bonded to the surface of the current collector. As for the current collector, the one described for the positive electrode may be appropriately adopted. The negative electrode active material layer contains a negative electrode active material and, if necessary, a conductive aid and/or a binder.

As the negative electrode active material, a known material may be adopted, and examples include a carbonaceous material capable of intercalating and deintercalating lithium, an element capable of being alloyed with lithium, and a compound containing an element capable of being alloyed with lithium. .

Examples of carbon-based materials include non-graphitizable carbon, graphite, cokes, graphites, vitreous carbons, sintered organic polymer compounds, carbon fibers, activated carbon, and carbon blacks. Here, the calcined organic polymer compound refers to a carbonized material obtained by calcining a polymer material such as phenols and furans at an appropriate temperature.

Specific examples of elements that can be alloyed with lithium include Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si , Ge, Sn, Pb, Sb, and Bi, and Si or Sn is particularly preferred.

Specific examples of compounds having an element capable of being alloyed with lithium include _ZnLiAl , AlSb, _SiB4 , _SiB6 , Mg2Si, _Mg2Sn , _Ni2Si , _TiSi2 , _MoSi2 , _CoSi2 , _NiSi2 , CaSi2, _CrSi2 , _Cu5Si , FeSi2, _MnSi2 , _NbSi2 , _TaSi2 , _VSi2 , _WSi2 , _ZnSi2 , SiC _{, Si3N4} , _Si2N2O , _{SiOv ( 0} < _v ≦2), SnO _w (0<w≦2), SnSiO ₃ , LiSiO or LiSnO, especially SiO _x (0.3≦x≦1.6, or 0.5≦x≦1.5) is preferred.

As the negative electrode active material, one containing a Si-based material containing Si is preferred. The Si-based material is preferably made of silicon or/and a silicon compound capable of intercalating and deintercalating lithium ions, and is preferably SiO _x (0.5≦x≦1.5), for example. Although silicon has a large theoretical charge/discharge capacity, silicon undergoes a large change in volume during charge/discharge. Therefore, by using SiO _x containing silicon as the negative electrode active material, the volume change of silicon can be alleviated.

As the negative electrode active material, a Si material obtained by heating a layered polysilane obtained by treating CaSi ₂ with an acid such as hydrochloric acid or hydrofluoric acid at 300 to 1000° C. may be used. Furthermore, the above Si material may be heated together with a carbon source and coated with carbon, which may be employed as the negative electrode active material.

As the negative electrode active material, one or more of the above can be used.

As for the conductive aid and the binder used for the negative electrode, those described for the positive electrode may be suitably adopted in the same mixing ratio.

In order to form an active material layer on the surface of the current collector, a conventionally known method such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, a curtain coating method, etc. is used to form a current collector. The active material may be applied to the surface of the body. Specifically, an active material, a solvent, and optionally a binder and/or a conductive aid are mixed to prepare a slurry. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. After coating the slurry on the surface of the current collector, it is dried. In order to increase the electrode density, it may be compressed after drying.

As the solid electrolyte, one that can be used as a solid electrolyte for lithium ion secondary batteries may be appropriately adopted.

The electrolytic solution contains a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent.

Cyclic carbonates, cyclic esters, chain carbonates, chain esters, ethers and the like can be used as non-aqueous solvents. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, and examples of cyclic esters include gamma-butyrolactone, 2-methyl-gamma-butyrolactone, acetyl-gamma-butyrolactone, and gamma-valerolactone. Examples of chain carbonates include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate and ethyl methyl carbonate, and examples of chain esters include alkyl propionate, dialkyl malonate and alkyl acetate. Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. As the non-aqueous solvent, a compound in which some or all of the hydrogen atoms in the chemical structure of the above specific solvent are substituted with fluorine may be employed.

Examples of electrolytes include lithium salts such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN(FSO ₂ ) ₂ and LiN(CF ₃ SO ₂ ) ₂ .

As the electrolytic solution, a solution obtained by dissolving a lithium salt in a nonaqueous solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, or diethyl carbonate at a concentration of about 0.5 mol/L to 1.7 mol/L can be exemplified.

The separator separates the positive electrode from the negative electrode and allows lithium ions to pass through while preventing short circuits due to contact between the two electrodes. Separators include synthetic resins such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (aromatic polyamide), polyester, and polyacrylonitrile, polysaccharides such as cellulose and amylose, and natural materials such as fibroin, keratin, lignin, and suberin. Porous bodies, non-woven fabrics, woven fabrics, etc., using one or a plurality of electrically insulating materials such as polymers and ceramics can be mentioned. Also, the separator may have a multilayer structure.

Next, an example of a method for manufacturing a lithium ion secondary battery will be described.

An electrode body is formed by interposing a separator between the positive electrode and the negative electrode, if necessary. The electrode body may be of either a laminate type in which the positive electrode, the separator and the negative electrode are stacked, or a wound type in which the positive electrode, the separator and the negative electrode are wound. After connecting the current collector of the positive electrode and the current collector of the negative electrode to the positive electrode terminal and the negative electrode terminal leading to the outside using a current collecting lead or the like, an electrolyte solution is added to the electrode body to form a lithium ion secondary A battery may be used. Also, the lithium ion secondary battery of the present invention may be charged and discharged in a voltage range suitable for the type of active material contained in the electrode.

The shape of the lithium-ion secondary battery of the present invention is not particularly limited, and various shapes such as cylindrical, square, coin-shaped, and laminate-shaped can be employed.

The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be any vehicle that uses electrical energy from a lithium-ion secondary battery as a power source in whole or in part, and may be, for example, an electric vehicle or a hybrid vehicle. When a lithium ion secondary battery is mounted on a vehicle, it is preferable to connect a plurality of lithium ion secondary batteries in series to form an assembled battery. Devices equipped with lithium ion secondary batteries include, in addition to vehicles, personal computers, mobile communication devices, various home electric appliances driven by batteries, office equipment, industrial equipment, and the like. Furthermore, the lithium ion secondary battery of the present invention is used for wind power generation, solar power generation, hydraulic power generation, and other power storage devices and power smoothing devices for power systems, power sources for ships and/or auxiliary equipment, aircraft, power source for spacecraft and/or auxiliary equipment, auxiliary power source for vehicles that do not use electricity as a power source, power source for mobile home robots, power source for system backup, power source for uninterruptible power supply, It may be used as a power storage device that temporarily stores electric power required for charging in a charging station for an electric vehicle.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. Various modifications, improvements, etc. that can be made by those skilled in the art can be implemented without departing from the scope of the present invention.

EXAMPLES The present invention will be described more specifically below with reference to examples, comparative examples, and the like. It should be noted that the present invention is not limited by these examples.

(Example 1)
A cathode active material of Example 1 was produced as follows.

94 g (357.6 mmol) nickel sulfate hexahydrate, 4.2 g (14.94 mmol) cobalt sulfate heptahydrate, 0.62 g (1.88 mmol) sodium tungstate dihydrate, 0.35 g (0.93 mmol) of aluminum nitrate nonahydrate, 0.26 g (0.73 mmol) of zirconium sulfate tetrahydrate and 0.49 g (6.44 mmol) of glycolic acid as a chelate compound in 400 mL of pure A transition metal-containing aqueous solution was prepared by dissolving in water.
The molar ratio of nickel, cobalt, tungsten, aluminum and zirconium in the transition metal-containing aqueous solution is 95.5:4:0.5:0.25:0.20.

A basic aqueous solution was prepared by mixing sodium hydroxide, aqueous ammonia and pure water.

Precipitation step In a constant temperature bath maintained at 60°C, a basic aqueous solution was supplied to a transition metal-containing aqueous solution under nitrogen gas introduction and stirring conditions to obtain a reaction solution. The pH of the reaction solution was maintained within the range of 10.8 to 10.85 to deposit nickel, cobalt, tungsten, aluminum and zirconium as transition metal hydroxides. In addition, the pH value here means the numerical value itself which measured the reaction solution with the pH meter.

Transition metal hydroxides were separated by filtration. The transition metal hydroxide was washed with pure water using an ultrasonic cleaner, and then filtered to isolate the transition metal hydroxide.

Precursor forming step A transition metal hydroxide was heated at 300°C for 5 hours in the atmosphere to form a transition metal oxide as a precursor.

Calcination step 10 g of precursor, 3.0 g (125 mmol) of anhydrous lithium hydroxide, 0.475 g (1.25 mmol) of Na ₃ PO ₄ dodecahydrate, 0.032 g (1.25 mmol) of LiF They were mixed in a mortar to form a mixture. Then, the mixture was heated at 600° C. for 10 hours in an air atmosphere to obtain a first fired body.

The first fired body was pulverized in a mortar into powder. The powdery first fired body was heated at 725° C. for 15 hours in an oxygen gas atmosphere to obtain a second fired body. The second fired body was pulverized in a mortar to obtain the positive electrode active material of Example 1.
The theoretical composition of the positive electrode active material of Example 1 is Li ₁ Ni _0.955 Co _0.04 W _0.005 Al _0.0025 Zr _0.002 Na _0.03 P _0.01 O ₂ F _0.01 is.

A positive electrode and a lithium ion secondary battery of Example 1 were manufactured as follows.

An aluminum foil having a thickness of 20 μm was prepared as a current collector for positive electrode. 94 parts by mass of the positive electrode active material of Example 1 as a positive electrode active material, 3 parts by mass of acetylene black as a conductive aid, and 3 parts by mass of polyvinylidene fluoride as a binder were mixed. This mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone to prepare a slurry. The slurry was put on the surface of the aluminum foil and coated with a doctor blade so as to form a film. By heating and drying the aluminum foil coated with the slurry, N-methyl-2-pyrrolidone was removed by volatilization, and a positive electrode active material layer was formed on the surface of the aluminum foil. The aluminum foil having the positive electrode active material layer formed on the surface thereof was compressed using a roll press machine, and the aluminum foil and the positive electrode active material layer were tightly bonded together to form a bonded product. The bonded product was heated using a vacuum dryer and cut into a predetermined shape to obtain a positive electrode.

A negative electrode was prepared as follows.
98.3 parts by mass of graphite was mixed with 1 part by mass of styrene-butadiene rubber and 0.7 parts by mass of carboxymethyl cellulose as binders, and the mixture was dispersed in an appropriate amount of ion-exchanged water to prepare a slurry. This slurry was applied to a copper foil having a thickness of 20 μm as a current collector for the negative electrode using a doctor blade so as to form a film, and the slurry-coated current collector was dried and then pressed to form a bonded product. The bonded product was heated using a vacuum dryer and cut into a predetermined shape to obtain a negative electrode.

A laminate type lithium ion secondary battery was manufactured using the above positive electrode and negative electrode. Specifically, a rectangular sheet having a thickness of 25 μm and made of a resin film having a three-layer structure of polypropylene/polyethylene/polypropylene was interposed as a separator between the positive electrode and the negative electrode to form an electrode plate assembly. This electrode plate assembly was covered with a pair of laminate films, and after three sides were sealed, an electrolytic solution was injected into the bag-shaped laminate film. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ to 1 mol/L in a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 3:3:4 was used. After that, the remaining one side was sealed to obtain a laminate type lithium ion secondary battery of Example 1 in which the four sides were airtightly sealed and the electrode plate group and the electrolytic solution were sealed.

(Example 2)
In the same manner as in Example 1, except that the following coating step was performed between the precursor formation step and the baking step, and the precursor in the baking step was used after the coating step, the sample of Example 2 was obtained. A positive electrode active material, a positive electrode, and a lithium ion secondary battery were manufactured.
The theoretical composition of the positive electrode active material of Example 2 is Li ₁ Ni _0.955 Co _0.04 W _0.005 Al _0.0025 Zr _0.004 Na _0.03 P _0.01 O ₂ F _0.01 is.

- Coating step 30 g of transition metal oxide was added to 400 mL of pure water to prepare a transition metal oxide dispersion.
A coating solution was prepared by dissolving 0.26 g (0.73 mmol) of zirconium sulfate tetrahydrate and 0.21 g (2.76 mmol) of glycolic acid as a chelate compound in water.
The transition metal oxide dispersion liquid and the coating solution were mixed to prepare a mixed liquid. Next, an aqueous sodium hydroxide solution was added until the pH of the mixed solution reached 12.5 to obtain a coated body in which zirconium hydroxide was deposited on the surface of the transition metal oxide. After the coated body was separated by filtration, it was dried and subjected to a firing process.

(Reference example 1)
Nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and sodium tungstate dihydrate are added to pure water in an amount such that the molar ratio of nickel, cobalt, and tungsten is 95.5:4:0.5. to prepare 400 mL of a transition metal-containing aqueous solution.
30 g of 25% aqueous ammonia was mixed with pure water to prepare 400 mL of the first basic aqueous solution.
Sodium hydroxide, aqueous ammonia and pure water were mixed to prepare a second basic aqueous solution with a pH of 10.75.

・Precipitation step In a constant temperature bath maintained at 60 ° C., a transition metal-containing aqueous solution is supplied to the second basic aqueous solution under nitrogen gas introduction and stirring conditions, and nickel, cobalt and tungsten are used as transition metal hydroxides. Precipitated. At this time, in order to maintain the pH of the reaction solution within the range of 10.75 to 10.80, the first basic aqueous solution and the 48 wt % sodium hydroxide aqueous solution were appropriately added dropwise. In addition, the pH value here means the numerical value itself which measured the reaction solution with the pH meter.
Transition metal hydroxides were separated by filtration. The transition metal hydroxide was washed with pure water using an ultrasonic cleaner, and then filtered to isolate the transition metal hydroxide.

Thereafter, a precursor forming step, a coating step, and a baking step were carried out in the same manner as in Example 2 to produce a positive electrode active material of Reference Example 1. A positive electrode and a lithium ion secondary battery of Reference Example 1 were produced in the same manner as in Example 1, except that the positive electrode active material of Reference Example 1 was used.
The theoretical composition of the positive electrode active material of Reference Example 1 is Li ₁ Ni _0.955 Co _0.04 W _0.005 Zr _0.002 Na _0.03 P _0.01 O ₂ F _0.01 .

(Comparative example 1)
A lithium ion secondary battery of Comparative Example 1 was manufactured in the same manner as in Example 1, except that LiNi _0.85 Co _0.11 Al _0.04 O ₂ having a layered rock salt structure was used as the positive electrode active material. .

(Evaluation example 1)
The surfaces of the positive electrode active materials of Examples 1 and 2 were observed with a scanning electron microscope (SEM). FIG. 1 shows an SEM image of the positive electrode active material of Example 1, and FIG. 2 shows an SEM image of the positive electrode active material of Example 2. As shown in FIG.
From both SEM images, it can be seen that the positive electrode active materials of Examples 1 and 2 both contain secondary particles that are aggregated primary particles.

In addition, when the secondary particles of the positive electrode active materials of Examples 1 and 2 were analyzed by SEM-EDX, which is a combination of scanning electron microscope and energy dispersive X-ray spectroscopy, nickel, cobalt, tungsten, aluminum , zirconium and oxygen were uniformly distributed.
A higher concentration of Zr was observed on the surfaces of the primary particles and secondary particles of the positive electrode active material of Example 2 than on the surfaces of the primary particles and secondary particles of the positive electrode active material of Example 1. In view of this phenomenon, it is presumed that the metal used in the coating step is present at a high concentration on the surface of the positive electrode active material.

(Evaluation example 2)
The crystal structures of the positive electrode active materials of Examples 1, 2, and Reference Example 1 were analyzed with a powder X-ray diffractometer using Cu-κα.
It was confirmed that all positive electrode active materials exhibited a diffraction pattern of a layered rock salt structure.

(Evaluation example 3)
For the lithium ion secondary batteries of Examples 1, 2, Reference Example 1, and Comparative Example 1, a charge-discharge cycle of charging to 4.4 V and discharging to 2.5 V was repeatedly performed at a rate of 0.1 C. rice field.
Table 1 shows the discharge capacity in the first charge/discharge cycle and the discharge capacity after repeating the charge/discharge cycle 20 times, together with the metal composition ratio in the deposition step in the production of each positive electrode active material. FIG. 3 is a graph showing the relationship between the number of charge/discharge cycles and the discharge capacity of each lithium ion secondary battery.

From the results of Table 1 and FIG. 3, the lithium ion secondary batteries of Examples 1, 2, and Reference Example 1, which include the positive electrode active material containing tungsten, are the same as those of Comparative Example 1, which includes the positive electrode active material that does not contain tungsten. It can be seen that the discharge capacity is larger than that of the lithium ion secondary battery.

Moreover, it can be seen that the lithium ion secondary batteries of Examples 1 and 2, which include the positive electrode active material containing tungsten, aluminum and zirconium, are excellent in maintaining discharge capacity. In particular, it can be seen that the lithium ion secondary battery of Example 2 is remarkably excellent in maintaining the discharge capacity. It can be said that the presence or absence of the coating process affected the capacity retention of the positive electrode active material.

Claims (5)

A positive electrode active material having a layered rock salt structure and represented by the following general formula.
_{General formula : LiaNibCocWdAleZrfDgOhFi _ _ _ _ _}
In the general formula, a, b, c, d, e, f, g, h, and i are 0.5≦a≦2, 0.85≦b<1, 0<c≦0.07, 0<d <0.15, 0<e<0.15, 0<f<0.15, 0≤g<0.15, b+c+d+e+f+g=1, 1.8≤h≤2.2, 0≤i≤0.2 satisfy. D is a doping element (except those containing Mn in D). 2. The cathode active material of claim 1 , comprising primary particles exhibiting a layered rock salt structure and containing therein lithium, nickel, cobalt, tungsten, aluminum, zirconium and oxygen. 3. The positive electrode active material according to claim 2 , wherein the Zr concentration is higher in the surface layer of the secondary particles in which the primary particles are aggregated than in the interior of the primary particles. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 3 . A lithium ion secondary battery comprising the positive electrode according to claim 4 .

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