JP2020119905A - Method for manufacturing positive electrode active material - Google Patents

Abstract

To provide a method for manufacturing a positive electrode active material with a high discharge capacity.SOLUTION: A method for manufacturing a positive electrode active material according to the present invention comprises the steps of: mixing a hydroxide of which the pore volume per unit mass at a relative pressure (P/P, where P is a pressure (Pa), and Pis a saturated steam pressure (Pa)) of 0.99, which is determined from an adsorption isotherm prepared by using a nitrogen gas as a probe, is 0.055 cm/g or more with a lithium compound in proportions which make the ratio (Li/Me ratio) of a lithium mole quantity of the lithium compound to a total mole quantity (Me) of metal included in the hydroxide 1.1 or higher; and baking the resultant mixture at 800-1100°C.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a positive electrode active material.

A lithium-containing composite oxide is used as a positive electrode active material for a positive electrode of a lithium ion secondary battery. As the lithium-containing composite oxide, a composite oxide containing a transition metal element such as Ni, Co, and Mn and Li as essential components and another metal element that is a doping element as an optional component is generally used. There is. The lithium-containing composite oxide has a layered structure in which the crystal structure has a metal atomic layer, a lithium atomic layer, and an oxygen atomic layer.

In recent years, it has been required to increase the energy density of a lithium ion secondary battery, and as a positive electrode active material, a positive electrode active material capable of increasing the discharge and charge capacities of a lithium ion secondary battery has been required. A lithium-manganese-rich positive electrode active material (hereinafter referred to as a lithium-rich material) is expected as a positive electrode active material capable of obtaining such a high-capacity lithium ion secondary battery. The lithium-rich material includes a solid solution lithium-containing composite oxide having a crystal structure in which the space group is C2/m and a crystal structure in which the space group is R-3m.

Patent Document 1 discloses, as a lithium-rich material of a positive electrode active material for a lithium secondary battery, a composition formula Li _1+α Me _1-α O ₂ (Me is Co, Ni and Mn) having an α-NaFeO ₂ type crystal structure. Containing a transition metal containing α, a lithium transition metal composite oxide represented by α>0), containing 1900 ppm or more and 8000 ppm or less of Na, and having a 50% particle diameter (D ₅₀ ) in particle size distribution measurement of 5 μm or less. Things are listed. The lithium-rich material of Patent Document 1 is manufactured by mixing a hydroxide containing a metal and a lithium compound and firing the mixture.
However, the lithium-rich material described in Patent Document 1 cannot be said to have sufficiently high discharge capacity and energy density when used in a lithium ion secondary battery.

JP, 2014-029828, A

As described above, a lithium ion secondary battery is required to have a high energy density, but a lithium ion secondary battery using a conventional lithium-rich material has not been able to achieve a sufficiently high energy density. In order to further increase the energy density of the lithium ion secondary battery, a material that can further increase the discharge capacity of the lithium ion secondary battery is required as the positive electrode active material.

An object of the present invention is to provide a method for producing a positive electrode active material that can increase the discharge capacity of a lithium ion secondary battery.

The method for producing a positive electrode active material of the present invention has a step of mixing a transition metal-containing hydroxide and a lithium compound and firing the mixture to obtain a lithium-containing composite oxide; the transition metal-containing hydroxide is nitrogen. The relative pressure (P/P _0. P is the pressure (unit: Pa) and P ₀ is the saturated vapor pressure (unit: Pa)) determined from the adsorption isotherm using gas as a probe is at 0.99. The pore volume per unit mass is 0.055 cm ³ /g or more; the transition metal-containing hydroxide and the lithium compound are lithium with respect to the total molar amount (Me) of the metals contained in the transition metal-containing hydroxide. The compounds are mixed at a lithium molar ratio (Li/Me) of 1.1 or more; the calcination is performed at 800 to 1100°C.

According to the method for producing a positive electrode active material of the present invention, a positive electrode active material capable of increasing the initial discharge capacity of a lithium ion secondary battery can be obtained.

The figure which shows the adsorption isotherm which used the nitrogen gas of the transition metal containing hydroxide used in Examples 1-4 of an Example as a probe. The figure which shows the vertical axis differential pore volume with respect to a horizontal axis pore diameter about the pore distribution calculated by BJH method of the transition metal containing hydroxide used in Examples 1-4 of an Example. The figure which shows the adsorption isotherm which used the nitrogen gas of the transition metal containing hydroxide used in Examples 5-7 of an Example as a probe. The figure which shows the vertical axis differential pore volume with respect to a horizontal axis pore diameter about the pore distribution calculated by the BJH method of the transition metal containing hydroxide used in Examples 5-7 of an Example.

In the present specification, the “hydroxide” includes a hydroxide and an oxyhydroxide in which the hydroxide is partially oxidized. That is, the compound described as Me(OH) ₂ (where Me is a metal element other than Li) includes Me(OH) ₂ , MeOOH and a mixture thereof.
In the present specification, the notation "Li" indicates that the element is not the elemental metal but Li element unless otherwise specified. The same applies to other elements such as Ni, Co, and Mn.

(Method for producing positive electrode active material)
The method for producing a positive electrode active material of the present invention (hereinafter referred to as the present production method) has a step of mixing a transition metal-containing hydroxide and a lithium compound and firing the mixture to obtain a lithium-containing composite oxide; The metal-containing hydroxide is a relative pressure (P/P _0. P is a pressure (unit: Pa)) obtained from an adsorption isotherm using nitrogen gas as a probe, and P ₀ is a saturated vapor pressure (unit: Pa). Is 0.95 cm ³ /g or more; and the transition metal-containing hydroxide and the lithium compound are the metals contained in the transition metal-containing hydroxide. The ratio (Li/Me) of the lithium molar amount of the lithium compound to the total molar amount (Me) is 1.1 or more; the calcination is performed at 800 to 1100°C.

(Transition metal-containing hydroxide)
The transition metal-containing hydroxide (hereinafter, abbreviated as hydroxide) used in the present production method is a relative pressure (P/P _{0, where} P is a pressure (unit: Pa), and P ₀ is a saturated vapor pressure (unit: Pa)) and the pore volume per unit mass is 0.99 cm ³ /g or more. By using such a hydroxide, a positive electrode active material having a high tap density and capable of increasing the discharge capacity of a lithium ion secondary battery can be obtained. The reason why the discharge capacity of the lithium ion secondary battery increases is not clear, but it is considered that one reason is that an appropriate pore structure is formed inside the secondary particles of the positive electrode active material after firing.

Pore volume per unit mass is preferably 0.057cm ³ / g or more, 0.060cm ³ / g or more is more preferable. It is considered that the larger the pore volume per unit mass, the more easily the positive electrode active material after firing is impregnated with the electrolytic solution, and the higher the discharge capacity of the lithium ion secondary battery. On the other hand, the pore volume of per unit mass is preferably at most 0.15 cm ³ / g, more preferably at most 0.12 cm ³ / g.
The pore volume per unit mass is confirmed and calculated by the method described in Examples.

The hydroxide has a maximum value of the differential pore volume determined by the BJH method from the adsorption isotherm of the nitrogen gas adsorption method, and the differential pore volume at the maximum value is 0.8 mm ³ /(g·nm). The above is preferable. When such a hydroxide is used, a positive electrode active material capable of further increasing the discharge capacity of the lithium ion secondary battery can be obtained, and the tap density of the positive electrode active material can be further increased. The reason why the discharge capacity of the lithium ion secondary battery increases is not clear, but it is considered that one reason is that an appropriate pore structure is formed inside the secondary particles of the positive electrode active material after firing. Moreover, when the tap density of the positive electrode active material is increased, the volume of the positive electrode active material contained in the positive electrode for the lithium ion secondary battery can be increased. As a result, the discharge capacity per unit volume is increased and the energy density per unit volume is improved.
The maximum value of the differential pore volume and the differential pore volume at the maximum value are confirmed and calculated based on the measurement of the pore distribution by the BJH method from the adsorption isotherm using the nitrogen gas adsorption method. Specifically, it is confirmed and calculated by the method described in the examples.

The maximum value of the differential pore volume of the hydroxide is more preferably 1.8 mm ³ /(g·nm) or less. If the maximum value of the differential pore volume is higher than 1.8 mm ³ /(g·nm), the tap density of the positive electrode active material after firing may be low. When the tap density becomes low, the energy density of the lithium ion secondary battery becomes low. In order to increase the discharge capacity and tap density of the lithium ion secondary battery using the positive electrode active material after firing, the maximum value of the pore volume is 0.9 to 1.7 mm ³ /(g·nm). More preferably, 1.0-1.6 mm ³ /(g·nm) is particularly preferable.

It is preferable that the hydroxide has a maximum value of the differential pore volume within a pore diameter range of 10 to 100 nm. It is preferable that the pore diameter having the maximum value of the differential pore volume is 10 to 100 nm, since a positive electrode active material having a high discharge capacity of the lithium ion secondary battery can be obtained. The maximum value of the differential pore volume is more preferably in the range of 10 to 50 nm in the pore size, and even more preferably in the range of 10 to 30 nm.

The composition of the hydroxide is preferably represented by the following formula 1.
Ni _α Co _β Mn _γ M _δ (OH) ₂ Formula 1
However, M is one or more elements selected from the group consisting of Na, Mg, Ti, Zr, Al, W and Mo, α+β+γ+δ=1, and 0.3≦α≦0.5, 0≦ β≦0.1, 0.5≦γ≦0.7, and 0≦δ≦0.1.

α is the molar ratio of Ni contained in the hydroxide. When α is in the above range, a positive electrode active material capable of increasing the discharge capacity and discharge voltage of the lithium ion secondary battery can be obtained. For the same reason, α is preferably 0.4≦α≦0.5.
β is the molar ratio of Co contained in the hydroxide. When β is within the above range, a positive electrode active material capable of increasing the discharge capacity and discharge voltage of the lithium ion secondary battery can be obtained. For the same reason, β is preferably 0≦β≦0.05.
γ is the molar ratio of Mn contained in the hydroxide. When γ is in the above range and the Li/X ratio described below is 1.1 or more, a lithium-rich material can be obtained by this production method. γ is preferably 0.5≦γ≦0.6 from the viewpoint of obtaining a positive electrode active material capable of increasing the discharge capacity and discharge voltage of a lithium ion secondary battery.

The hydroxide may contain another metal element M, if necessary. From the viewpoint of obtaining a positive electrode active material having a high discharge capacity of a lithium ion secondary battery, the other metal element M is at least one element selected from the group consisting of Na, Mg, Ti, Zr, Al, W and Mo. preferable.
δ is the molar ratio of M contained in the hydroxide. δ is preferably 0≦δ≦0.05.

The specific surface area of the hydroxide is preferably ^{3~60m 2 / g, 5~40m 2 /} g is more preferable. When the specific surface area of the hydroxide is within the above range, a positive electrode active material having a high discharge capacity of the lithium ion secondary battery can be obtained. The specific surface area of the hydroxide is measured by the method described in Examples. The specific surface area of the hydroxide varies greatly depending on the dry state. Therefore, in this specification, the specific surface area of the hydroxide is a value measured after drying the hydroxide at 120° C. for 15 hours.

The average particle diameter (D ₅₀ ) of the hydroxide is preferably 3 to 15.5 μm, more preferably 3.5 to 12.5 μm, and further preferably 4 to 10.5 μm. When the D _{50 of the} hydroxide is within the above range, it is easy to control the D ₅₀ of the positive electrode active material after firing within the preferable range.
In the present specification, the D ₅₀ of the hydroxide is the particle size at the point where it becomes 50% in the cumulative volume distribution curve where the total volume of the particle size distribution determined by volume is 100%, that is, the volume-based cumulative 50% size. is there. Then, the particle size distribution is obtained from a frequency distribution and a cumulative volume distribution curve measured by a laser scattering particle size distribution measuring device (for example, a laser diffraction/scattering type particle size distribution measuring device or the like). The measurement is carried out in a state where the hydroxide is sufficiently dispersed in an aqueous medium by ultrasonic treatment or the like.

The tap density of the hydroxide is preferably from 1 to 2 g / cm ^3, more preferably 1.2~2g / cm ^3, more preferably 1.2~1.5g / cm ^3. When the tap density of the hydroxide is within the above range, it is easy to increase the tap density of the positive electrode active material.
In the present specification, the tap density of the hydroxide is a value measured by, for example, attaching to a tapping device (KYT-4000K manufactured by Seishin Enterprise Co., Ltd.) and tapping 700 times. It was calculated from the ratio (unit: g/cm ³ ) of the mass (unit: g) of the sample added to the container to the volume (unit: cm ³ ) of the sample after tapping.

(Example of hydroxide production)
An example of producing the hydroxide is shown below. In the present invention, the method for producing the hydroxide is not limited to the following method.

The hydroxide is preferably produced by an alkaline coprecipitation method. The alkali coprecipitation method is a method in which an aqueous solution of a metal salt and a pH adjusting solution containing a strong alkali are continuously supplied to a reaction tank and mixed to precipitate a hydroxide while maintaining a constant pH in the mixed solution. It is a method to let. When the hydroxide is produced by the coprecipitation method, a hydroxide having a uniform metal composition as a solid solution can be obtained, which is preferable. In the alkali coprecipitation method, the hydroxide having the above-mentioned differential pore volume can be obtained by controlling the reaction conditions. As described below, the temperature during the reaction, the reaction time, the pH of the mixed solution, and the amount of NH ₃ added as a complexing agent are parameters for controlling the pore volume.

Examples of the metal salt include nitrates, acetates, chlorides, and sulfates of each transition metal element, and the sulfate is preferable from the viewpoint that the material cost is relatively low and excellent battery characteristics can be obtained.
In the present manufacturing method, it is preferable that the hydroxide contains Ni, Co and Mn. Examples of the sulfate containing Ni, Co and Mn include the following.
Examples of the sulfate of Ni include nickel (II) sulfate hexahydrate, nickel (II) sulfate heptahydrate, nickel (II) sulfate ammonium hexahydrate, and the like.
Examples of the sulfate of Co include cobalt (II) sulfate heptahydrate, cobalt (II) sulfate ammonium hexahydrate, and the like.
Examples of the Mn sulfate include manganese(II) sulfate pentahydrate, manganese(II) sulfate ammonium hexahydrate, and the like.

In the production of the hydroxide, the ratio of Ni, Co, Mn and M in the aqueous metal salt solution is set to be the same as the ratio of Ni, Co, Mn and M contained in the hydroxide.
The total concentration of Ni, Co, Mn and M in the aqueous metal salt solution is preferably 0.1 to 3 mol/kg, more preferably 0.5 to 2.5 mol/kg. When the total concentration of Ni, Co, Mn and M is at least the lower limit value described above, the productivity will be excellent. When the total concentration of Ni, Co, Mn and M is not more than the above upper limit, the metal salt can be sufficiently dissolved in water.

The aqueous metal salt solution may contain an aqueous medium other than water.
Examples of aqueous media other than water include methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, polyethylene glycol, butanediol, glycerin and the like. The ratio of the aqueous medium other than water is preferably 0 to 20 parts by mass, more preferably 0 to 10 parts by mass, and 0 to 100 parts by mass of water in terms of safety, environment, handleability, and cost. 1 part by mass is particularly preferred.

As the pH adjusting liquid, an aqueous solution containing a strong alkali is preferable.
The strong alkali is preferably at least one selected from the group consisting of sodium hydroxide, potassium hydroxide and lithium hydroxide.
A complexing agent may be added to the mixed solution in order to adjust the solubilities of Ni, Co, Mn and M ions. Examples of the complexing agent include an aqueous ammonia solution or an aqueous ammonium sulfate solution.

The metal salt aqueous solution and the pH adjusting solution are preferably mixed in a reaction tank while stirring.
Examples of the stirring device include a three-one motor. Examples of the stirring blade include an anchor type, a propeller type, a paddle type and the like.
From the viewpoint of accelerating the reaction, the reaction temperature is preferably 20 to 80°C, more preferably 25 to 60°C.

It is preferable that the metal salt aqueous solution and the pH adjusting liquid are mixed in an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere from the viewpoint of suppressing the oxidation of the hydroxide. From the viewpoint of manufacturing cost, it is particularly preferable to carry out under a nitrogen atmosphere.
During the mixing of the metal salt aqueous solution and the pH adjusting liquid, it is preferable to keep the pH in the reaction tank at the set pH within the range of 10 to 12 from the viewpoint of appropriately promoting the coprecipitation reaction. When the pH of the mixed solution is 10 or more, the coprecipitate is regarded as a hydroxide.

As a method of precipitating hydroxide, a method of extracting the mixed solution in the reaction vessel using a filter material (filter cloth or the like) and conducting a precipitation reaction while concentrating the hydroxide (hereinafter, referred to as a concentrating method) is used. , A method of extracting a mixed solution in a reaction vessel together with a hydroxide without using a filter medium and carrying out a precipitation reaction while keeping the concentration of a hydroxide low (hereinafter, referred to as an overflow method). The concentration method is preferable because the spread of the particle size distribution can be narrowed.

The obtained hydroxide is preferably washed to remove impurity ions. If the impurity ions remain, impurities may exist on the surface and in the crystal of the positive electrode active material obtained by firing, which may adversely affect the battery characteristics.
Examples of the washing method include a method in which pressure filtration and dispersion in distilled water are repeated.
In the case of washing, it is preferable to repeat until the electric conductivity of the supernatant liquid or the filtrate when the hydroxide is dispersed in distilled water becomes 50 mS/m or less, more preferably 20 mS/m or less. preferable.

Further, after washing the hydroxide, it is preferable to dry the hydroxide, if necessary.
60-200 degreeC is preferable and, as for the drying temperature of a hydroxide, 80-130 degreeC is more preferable.
When the drying temperature is at least the lower limit value, the drying time can be shortened. When the drying temperature is at most the above upper limit value, the progress of oxidation of hydroxide can be suppressed.
The drying time may be appropriately set depending on the amount of hydroxide, and is preferably 1 to 300 hours, more preferably 5 to 120 hours.

(Mixing and firing)
In the present production method, the hydroxide and the lithium compound are mixed such that the ratio (Li/Me) of the lithium molar amount of the lithium compound to the total molar amount (Me) of the metals contained in the hydroxide is 1.1 or more. ..

If the Li/Me ratio is mixed within the above range and the mixture is baked, a lithium-rich material is obtained. The Li/Me ratio is preferably 1.1 to 1.5, more preferably 1.1 to 1.3, and even more preferably 1.15 to 1.25. The Li/Me ratio is a factor that determines the composition of the positive electrode active material. That is, the Li/Me ratio is appropriately set within the above range according to the composition of the target positive electrode active material.

The lithium compound is preferably one selected from the group consisting of lithium carbonate, lithium hydroxide and lithium nitrate. Lithium carbonate is more preferable from the viewpoint of easy handling in the manufacturing process.
The method of mixing the hydroxide and the lithium compound is not particularly limited, and for example, a rocking mixer, a Nauta mixer, a spiral mixer, a cutter mill, a V mixer, or the like can be used.

In the present manufacturing method, when the above-mentioned hydroxide and lithium compound are mixed, a compound containing another metal element not contained in the hydroxide may be mixed. As the other metal element, the same metal element as the metal element M in Formula 1 can be used. The compound containing M is preferably one or more selected from the group consisting of oxides, hydroxides, carbonates, nitrates, acetates, chlorides and fluorides containing M. If one or more of these compounds are used, impurities are volatilized in the firing step, and a positive electrode active material with high purity can be obtained.

In the present manufacturing method, the mixture obtained by the above mixing is fired at 800 to 1100° C. to obtain a lithium-containing composite oxide.
The temperature for firing the mixture (hereinafter referred to as firing temperature) is 800 to 1100°C. The firing temperature is preferably 850°C or higher, more preferably 900°C or higher. When the firing temperature is 850° C. or higher, the domain of the crystal structure of the space group C2/m easily grows. Further, the firing temperature is more preferably 1070° C. or lower, further preferably 1050° C. or lower. When the firing temperature is 1100° C. or lower, the volatilization of Li can be suppressed in the firing process, and a lithium-containing composite oxide having a Li/Me ratio close to that at the time of mixing can be obtained.
Examples of the firing device include an electric furnace, a continuous firing furnace, and a rotary kiln.

Since it is necessary to oxidize the hydroxide at the time of firing, it is preferable to perform the firing in the atmosphere, and it is particularly preferable to perform it while supplying air.
The air supply rate is preferably 10 to 200 mL/min, and more preferably 40 to 150 mL/min, per 1 L of the furnace volume of the firing apparatus. By supplying air during firing, the metal element contained in the hydroxide is sufficiently oxidized. As a result, a lithium-containing composite oxide having high crystallinity and having a crystal structure of space group C2/m and a crystal structure of space group R-3m can be obtained.
The firing time is preferably 4 to 40 hours, more preferably 4 to 20 hours.

The calcination may be one-step calcination or two-step calcination in which preliminary calcination is performed before main calcination at 800 to 1100°C. The two-step firing is preferable because Li easily diffuses uniformly in the lithium-containing composite oxide. When performing two-step firing, the main firing temperature is set within the above-mentioned firing temperature range. And the temperature of calcination is preferably 400 to 700°C, more preferably 500 to 650°C.

(Lithium-containing composite oxide)
The lithium-containing composite oxide obtained by this production method is preferably a compound represented by the following formula 2 (hereinafter referred to as composite oxide (2)).
Li _a Ni _b Co _c Mn _{d Me} O ₂ Formula 2
In Formula 2, M is one or more elements selected from the group consisting of Na, Mg, Ti, Zr, Al, W and Mo, a+b+c+d+e=2, and 1.05≦a≦1.20 Yes, 0.25 ≤ b ≤ 0.45, 0 ≤ c ≤ 0.1, 0.45 ≤ d ≤ 0.65, and 0 ≤ e ≤ 0.1.

a is the number of moles of Li contained in the composite oxide (2). a satisfies 1.05≦a≦1.20, preferably 1.05≦a≦1.15, and more preferably 1.07≦a≦1.11. When a is within the above range, a positive electrode active material capable of increasing the discharge capacity of the lithium ion secondary battery can be obtained.
b is the number of moles of Ni contained in the composite oxide (2). b satisfies 0.25≦b≦0.45, and preferably 0.35≦b≦0.45. When b is in the above range, a positive electrode active material capable of increasing the discharge capacity and discharge voltage of the lithium ion secondary battery can be obtained.

c is the number of moles of Co contained in the composite oxide (2). c satisfies 0≦c≦0.10, and preferably 0≦c≦0.05. When c is within the above range, a positive electrode active material capable of increasing the discharge capacity and discharge voltage of the lithium ion secondary battery can be obtained.
d is the number of moles of Mn contained in the composite oxide (2). d satisfies 0.45≦d≦0.65, and preferably 0.45≦d≦0.55. When d is within the above range, a positive electrode active material capable of increasing the discharge capacity and discharge voltage of the lithium ion secondary battery can be obtained.

The composite oxide (2) may contain another metal element M, if necessary. The other metal element M is one or more elements selected from the group consisting of Na, Mg, Ti, Zr, Al, W and Mo. From the viewpoint of easily increasing the discharge capacity of the lithium-ion secondary battery having the positive electrode active material containing the composite oxide (2) and suppressing the decrease in discharge voltage due to repeated charge/discharge cycles, the other metal element M is It is preferably one or more elements selected from the group consisting of, Ti, Zr and Al.
e is the number of moles of M contained in the composite oxide (2). e satisfies 0≦e≦0.1, preferably 0≦e≦0.05, and more preferably 0≦e≦0.03.

The complex oxide (2) is Li(Li _1/3 Mn _2/3 )O ₂ (excess lithium phase) having a layered rock salt type crystal structure of space group C2/m and layered rock salt type crystal of space group R-3m. LiM′O ₂ having a structure (provided that M′ essentially contains at least one element selected from the group consisting of Ni, Co, Mn, and M, and optionally contains another metal element M). Is a solid solution of. It can be confirmed by X-ray diffraction measurement that the composite oxide (2) has these crystal structures.
Typically, in the X-ray diffraction measurement, the peak of the (020) plane of the space group C2/m is found at 2θ=20 to 22 deg. Further, in the X-ray diffraction measurement, the peak of the (003) plane of the space group R-3m is seen at 2θ=18 to 20 deg.

The specific surface area of the lithium-containing composite oxide has a specific surface area is preferably ^{0.5~10m 2 / g, 0.7~5m 2 /} g is more preferable. When the specific surface area of the lithium-containing composite oxide is within the above range, the discharge capacity of the lithium ion secondary battery can be increased. The specific surface area of the lithium-containing composite oxide is measured by the method described in Examples.

The average particle diameter (D ₅₀ ) of the lithium-containing composite oxide is preferably 3 to 15 μm, more preferably 3 to 12 μm, and further preferably 4 to 10 μm. When the D ₅₀ of the lithium-containing composite oxide is within the above range, the discharge capacity of the lithium ion secondary battery can be increased.
In this specification, the D ₅₀ of the lithium-containing composite oxide is measured by the same method as the method for measuring the D ₅₀ of the hydroxide.

The tap density of the lithium-containing composite oxide is preferably from 1.2 to 2.2 grams / cm ^3, more preferably 1.3~2.2g / cm ^3, more preferably 1.5~2.2g / cm ³ .. When the tap density of the lithium-containing composite oxide is within the above range, the energy density of the lithium ion secondary battery can be increased.
In the present specification, the tap density of the lithium-containing composite oxide is a value measured by, for example, attaching to a tapping device (KYT-4000K manufactured by Seishin Enterprise Co., Ltd.) and performing 700 taps. It was calculated from the ratio (unit: g/cm ³ ) of the mass (unit: g) of the sample added to the container to the volume (unit: cm ³ ) of the sample after tapping.

(Coating process)
The method for producing a positive electrode active material of the present invention may include a step of forming a coating layer on the surface of the lithium-containing composite oxide. Here, the term "coating" refers to a state in which a substance having a composition different from that of the lithium-containing composite oxide is chemically or physically adhered or adsorbed on a part or all of the surface of the secondary particles.

The step of forming the coating layer is performed, for example, by spraying a predetermined amount of a liquid (coating liquid) containing a substance having a composition different from that of the lithium-containing composite oxide, and removing the solvent of the coating liquid by baking. Or by immersing the lithium-containing composite oxide in the coating liquid, performing solid-liquid separation by filtration, and removing the solvent by baking.
When the coating layer is provided on the surface of the secondary particles of the lithium-containing composite oxide, the metal component of the lithium-containing composite oxide is less likely to be eluted in the electrolytic solution. As a result, a positive electrode active material having excellent cycle characteristics of a lithium ion secondary battery can be obtained.

Examples of the substance having a composition different from that of the lithium-containing composite oxide include a compound containing a metal. The compound is preferably a compound containing a metal of Groups 3 to 13 in the periodic table or a lithium compound.
The metal of the compound containing a metal of Groups 3 to 13 in the periodic table is at least one metal selected from the group consisting of Al, Y, Ga, In, La, Pr, Nd, Gd, Dy, Er and Yb. Is preferred. Examples of the compound include oxides, halides, phosphorus oxides, sulfates and the like. Al ₂ O ₃ , Y ₂ O ₃ , Gd ₂ O ₃ , Er ₂ O, AlF ₃ , Al ₂ (PO ₄ ) ₃ or Al ₂ (SO _{4 is} preferable in that an electrochemically stable coating layer can be formed. ) ₃ is preferable.
Examples of the lithium compound include compounds containing Li and one or more selected from the group consisting of S, B and F. Specific examples _{include Li 2 SO 4, Li 3 BO} 3, Li 2 B 4 O 7, LiF or their hydrates.

The mass of the coating layer is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.05% by mass or more and 5% by mass or less, and 0.1% by mass or more 3 with respect to the mass of the lithium-containing composite oxide. A mass% or less is particularly preferable.
The presence of the coating layer on the surface of the secondary particles can be confirmed by an electron microscope such as SEM (scanning electron microscope) or TEM (transmission electron microscope). Further, the atoms constituting the coating layer can be confirmed by EDX (energy dispersive X-ray spectroscopy) attached to the electron microscope. The coating amount can be quantified by dissolving the positive electrode active material with an acid and then performing ICP (inductively coupled plasma) measurement.

(Cathode for lithium-ion secondary battery)
The positive electrode for a lithium ion secondary battery contains the positive electrode active material obtained by this manufacturing method. Specifically, the positive electrode active material layer including the positive electrode active material, the conductive material, and the binder obtained by the present manufacturing method is formed on the positive electrode current collector.

Examples of the conductive material include carbon black (acetylene black, Ketjen black, etc.), graphite, vapor grown carbon fiber, carbon nanotube, and the like.
As the binder, a fluorine-based resin (polyvinylidene fluoride, polytetrafluoroethylene, etc.), polyolefin (polyethylene, polypropylene, etc.), a polymer or copolymer having an unsaturated bond (styrene-butadiene rubber, isoprene rubber, butadiene rubber, etc.) ), acrylic acid polymers or copolymers (acrylic acid copolymers, methacrylic acid copolymers, etc.) and the like.
Examples of the positive electrode current collector include aluminum foil and stainless steel foil.

The present positive electrode can be manufactured, for example, by the following method.
The positive electrode active material, the conductive material and the binder obtained by the present manufacturing method are dissolved or dispersed in a medium to obtain a slurry. The obtained slurry is applied to a positive electrode current collector, and the medium is removed by drying or the like to form a positive electrode active material layer. If necessary, the positive electrode active material layer may be formed and then rolled by a roll press or the like. Thereby, the positive electrode is obtained.

In another manufacturing method of the present positive electrode, the positive electrode active material, the conductive material and the binder obtained by the present manufacturing method are kneaded with a medium to obtain a kneaded product. Then, the obtained kneaded product is rolled into a positive electrode current collector to manufacture the present positive electrode.
The present positive electrode contains the positive electrode active material obtained by the present manufacturing method. Therefore, if this positive electrode is used in a lithium ion secondary battery, the discharge capacity can be increased.

(Lithium ion secondary battery)
The lithium ion secondary battery of the present invention (hereinafter referred to as the present battery) has the present positive electrode. Specifically, the positive electrode, the negative electrode, the separator, and the nonaqueous electrolyte are included.

(Negative electrode)
The negative electrode contains a negative electrode active material. Specifically, a negative electrode active material layer containing a negative electrode active material and, if necessary, a conductive material and a binder is formed on a negative electrode current collector.

The negative electrode active material may be any material that can absorb and release lithium ions at a relatively low potential. Examples of the negative electrode active material include lithium metal, lithium alloys, lithium compounds, carbon materials, oxides mainly containing metals of Group 14 of the periodic table, oxides mainly containing metals of Group 15 of the periodic table, carbon compounds, and silicon carbide compounds. , Silicon oxide compounds, titanium sulfide, boron carbide compounds and the like.

Examples of the carbon material for the negative electrode active material include non-graphitizable carbon, artificial graphite, natural graphite, pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glassy carbons, organic high carbons. Examples thereof include calcined molecular compounds (phenolic resins, furan resins, etc., which are calcined at an appropriate temperature to be carbonized), carbon fibers, activated carbon, carbon blacks, and the like.
Examples of the metal of Group 14 of the periodic table used for the negative electrode active material include Si and Sn, and Si is preferable.
Examples of other negative electrode active materials include oxides such as iron oxide, ruthenium oxide, molybdenum oxide, tungsten oxide, titanium oxide and tin oxide, and other nitrides.

As the conductive material and binder for the negative electrode, the same materials as those for the positive electrode can be used.
Examples of the negative electrode current collector include metal foils such as nickel foil and copper foil.

The negative electrode can be manufactured, for example, by the following method.
A negative electrode active material, a conductive material and a binder are dissolved or dispersed in a medium to obtain a slurry.
The medium is removed by applying the obtained slurry to a negative electrode current collector, drying, pressing, etc. to obtain a negative electrode.

(Non-aqueous electrolyte)
Examples of the non-aqueous electrolyte include a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in an organic solvent, an inorganic solid electrolyte, and a solid or gel polymer electrolyte in which an electrolyte salt is mixed or dissolved.

Examples of the organic solvent include those known as organic solvents for non-aqueous electrolyte solutions. Specifically, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, diethyl ether, sulfolane, methylsulfolane, acetonitrile, acetic acid ester, butyric acid. Examples thereof include esters and propionic acid esters. From the viewpoint of voltage stability, cyclic carbonates (propylene carbonate, etc.) and chain carbonates (dimethyl carbonate, diethyl carbonate, etc.) are preferable. The organic solvent may be used alone or in combination of two or more.

The inorganic solid electrolyte may be a material having lithium ion conductivity. Examples of the inorganic solid electrolyte include lithium nitride and lithium iodide.
Examples of the polymer used for the solid polymer electrolyte include ether polymer compounds (polyethylene oxide, crosslinked products thereof, etc.), polymethacrylate ester polymer compounds, acrylate polymer compounds and the like. The polymer compounds may be used alone or in combination of two or more.

Examples of the polymer used in the gel polymer electrolyte include fluorine-based polymer compounds (polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, etc.), polyacrylonitrile, acrylonitrile copolymer, ether-based polymer compound ( Polyethylene oxide, its crosslinked product, etc.) and the like. Examples of the monomer to be copolymerized with the copolymer include polypropylene oxide, methyl methacrylate, butyl methacrylate, methyl acrylate, and butyl acrylate. As the polymer used for the gel polymer electrolyte, a fluorine-based polymer compound is preferable from the viewpoint of stability against redox reaction.
Any electrolyte salt may be used as long as it is used in a lithium ion secondary battery. Examples of the electrolyte salt include LiClO ₄ , LiPF ₆ , LiBF ₄ , CH ₃ SO ₃ Li and the like.

(Separator)
A separator is interposed between the positive electrode and the negative electrode to prevent a short circuit. The separator may be a porous film. The non-aqueous electrolyte is used by impregnating the porous membrane. Further, a gel-like electrolyte obtained by impregnating a porous membrane with a non-aqueous electrolyte to form a gel may be used.

Examples of the material of the battery exterior body include nickel-plated iron, stainless steel, aluminum or an alloy thereof, nickel, titanium, a resin material, a film material, and the like.
Examples of the shape of the present battery include a coin shape, a sheet shape (film shape), a folding shape, a winding type bottomed cylindrical shape, a button shape and the like, and can be appropriately selected depending on the application.
Since the present battery has the present positive electrode, it has a high discharge capacity and energy density.

The present invention will be specifically described with reference to examples. In the following examples, Examples 1 to 4 are working examples, and Examples 5 to 7 are reference examples.

[Measurement of pore volume and pore volume distribution per unit mass]
Regarding the hydroxide, the pore volume per unit mass and the pore volume distribution were measured according to the following conditions and procedures. For the measurement of the pore volume and the pore volume distribution, BELSORP-max manufactured by Bell Japan Ltd. and attached analysis software (BEL analysis software) were used.

1 to 3 g of hydroxide is put into a sample tube for measurement and vacuum dried at 105° C. for 1 h. Next, using a nitrogen gas at a liquid nitrogen temperature (77 K), the adsorption isotherms on the adsorption side and the desorption side were measured at relative pressures (P/P ₀ .P ₀ =about 100 kPa) within the range of 0 to 1. did. From the nitrogen adsorption amount V (unit: mL(STP)/g) when the relative pressure P/P ₀ on the adsorption side is 0.99, assuming that the adsorbed nitrogen molecules fill the pores in the liquid state, The pore volume V _p (unit: cm ³ /g) per unit mass was calculated from the calculation formula.
V _p =(V×M)/(22414×ρ)
Here, M represents the molecular weight (unit: g/mol) of the adsorbate, and ρ (unit: g/cm ³ ) represents the density of the adsorbate in the liquid state. The unit of the coefficient 22414 is mL(STP)/mol.
In nitrogen adsorption, M=28.01348 and ρ=0.807 g/cm ³ were used.

The pore temperature distribution was calculated by the BJH method using the adsorption-side isotherm. Whether or not there is a maximum value of the differential pore volume when the pore diameter dp (unit: nm) is plotted on the horizontal axis and the differential pore volume ΔVp/Δdp (unit: mm ³ /(g·nm)) is plotted on the vertical axis, The magnitude of the maximum value was evaluated.

[Average particle diameter (D ₅₀ )]
The average particle diameter (D ₅₀ ) of the hydroxide and the lithium-containing composite oxide was determined as follows. Each sample was sufficiently dispersed in water by ultrasonic treatment, and measurement was performed with a laser diffraction/scattering particle size distribution measuring device (manufactured by Nikkiso Co., Ltd., MT-3300EX) to obtain a frequency distribution and a cumulative volume distribution curve. A standard particle size distribution was obtained. The average particle size (D ₅₀ ) of each sample was obtained from the obtained cumulative volume distribution curve.

[Specific surface area]
The specific surface area of the hydroxide or the lithium-containing composite oxide was calculated by a nitrogen adsorption BET method using a specific surface area measuring device (HM model-1208 manufactured by Mountech Co., Ltd.). Deaeration was performed under the conditions of 200° C. and 20 minutes.

[Tap density]
The tap density of the hydroxide or the lithium-containing composite oxide was measured by attaching it to a tapping device (KYT-4000K manufactured by Seishin Enterprise Co., Ltd.) and tapping 700 times. It was calculated from the ratio of the mass (unit: g) of the sample added to the container to the volume of the sample after tapping.

[Composition analysis]
The chemical composition of the lithium-containing composite oxide was analyzed by a plasma emission spectrometer (SII3 Nanotechnology, SPS3100H). A, b, c, d and e in Li _a Ni _b Co _c Mn _{d Me} O ₂ were calculated from the molar ratios of Li, Ni, Co and Mn obtained from the composition analysis.

(Example 1)
Hydroxide production:
Nickel (II) sulphate hexahydrate, manganese (II) sulphate hydrate and ammonium sulphate with a Ni and Mn molar ratio of 0.42:0.58 and a total metal concentration of 1.5 mol/kg. And the ammonium sulfate concentration was adjusted to 0.075 mol/kg and dissolved in distilled water to prepare an aqueous solution of a metal salt. Further, sodium hydroxide was dissolved in water to prepare a 3 mol/kg sodium hydroxide aqueous solution.

Distilled water was placed in a reaction tank having a volume of 2 L and heated to 50°C. While stirring the initial solution in the reaction tank with stirring power of 1.66 kw/m ³ , the aqueous solution of the metal salt was added at an addition rate of 5 g/min over 14 hours.
During the addition of the aqueous solution of the metal salt, an aqueous sodium hydroxide solution was added so as to keep the pH in the reaction tank at 10.5, and a hydroxide containing Ni and Mn was precipitated by a coprecipitation method. During the deposition, nitrogen gas was flown into the reaction tank. During the precipitation, only the supernatant liquid was taken out from the overflow outlet of the reaction tank through the filter cloth to leave the coprecipitate in the reaction tank. The growth time was set to 14 hours to obtain a slurry in which the coprecipitate was suspended in the reaction solution. The obtained slurry was filtered, washed with pure water, and then dried at 120° C. for 15 hours to obtain a hydroxide. Table 1 shows the evaluation results of the hydroxide. FIG. 1 shows the adsorption isotherm, and FIG. 2 shows a diagram of the vertical axis differential pore volume with respect to the horizontal axis pore diameter.

Manufacture of positive electrode active material:
The hydroxide and lithium carbonate (manufactured by SQM, MIC grade) have a ratio (Li/Me ratio) of Li molar amount of lithium carbonate to the total molar amount (Me) of metals contained in the hydroxide of 1. Weighed to 0.25 and mixed. The obtained mixture was fired in an electric furnace at 970° C. for 8 hours in an air atmosphere while supplying air to obtain a lithium-containing composite oxide. This lithium-containing composite oxide was used as the positive electrode active material.
Table 2 shows the evaluation results of the positive electrode active material.

(Examples 2 to 7)
Positive electrode active materials of Examples 2 to 7 were obtained in the same manner as in Example 1 except that the hydroxide shown in Table 1 was used as the hydroxide and the Li/Me ratio was 1.20. Table 2 shows the evaluation results of the positive electrode active material. In addition, for each hydroxide of Examples 1 to 7, adsorption isotherms are shown in FIGS. 1 and 3, and diagrams of the ordinate differential pore volume with respect to the abscissa pore diameter are shown in FIGS.

(Battery evaluation)
Using the positive electrode active materials of Examples 1 to 7, lithium secondary batteries 1 to 7 were manufactured and evaluated from the respective positive electrode active materials as follows.

(Manufacture of positive electrode sheet)
A polyvinylidene fluoride solution containing 12.1% by mass of a positive electrode active material, acetylene black (Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo KK), and polyvinylidene fluoride (KFL#1120 manufactured by Kureha) (solvent: N -Methylpyrrolidone), and N-methylpyrrolidone was further added to prepare a slurry. The positive electrode active material, acetylene black, and polyvinylidene fluoride had a mass ratio of 90:5:5.
The slurry was applied to one side of an aluminum foil (E-FOIL, manufactured by Toyo Aluminum Co., Ltd.) having an average thickness of 20 μm using a doctor blade. After drying at 120° C., roll press rolling (0.3 t/cm) was performed twice to prepare a positive electrode.

(Lithium secondary battery)
A 1 mm-thick stainless steel plate and a 500 μm-thick metallic lithium foil (manufactured by Honjo Chemical Co., Ltd., lithium foil) were laminated to form a negative electrode.
As a separator, a 25 μm-thick porous polypropylene (manufactured by Polypore, Celgard (registered trademark) #2500) was prepared.
As a non-aqueous electrolyte, a 1 mol/dm ³ LiPF ₆ solution was prepared. As a solvent for the non-aqueous electrolyte, a mixed solution of ethylene carbonate and diethyl carbonate having a volume ratio of 1:1 was used.
Using a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte, a stainless steel simple closed cell type lithium secondary battery was assembled in an argon glove box.

(Activation process)
Regarding the lithium secondary batteries 1 to 7, after constant-current/constant voltage charging to a positive current of 20 mA per 1 g of the positive electrode active material to constant voltage and constant voltage, low current discharge to 20 V per 1 g of the positive electrode active material to 2 V was performed to activate It was treated as a chemical treatment.

(1C discharge capacity)
Regarding the activated lithium secondary batteries 1 to 7, after constant-current/constant voltage charging to a load current of 200 mA per 1 g of the positive electrode active material up to 4.45 V, the load was reduced to 2 V at a load current of 200 mA per 1 g of the positive electrode active material. The discharge capacity of 1C was measured by performing current discharge.
Table 3 shows the 1 C discharge capacities of the lithium secondary batteries 1 to 7.

(Discussion)
As shown in Tables 1 to ³ , a hydroxide having a pore volume per unit mass of 0.055 cm ³ /g or more and a lithium compound are used with respect to the total molar amount (Me) of metals contained in the hydroxide. A battery having a positive electrode active material obtained by mixing lithium lithium compounds in a molar ratio (Li/Me ratio) of 1.1 or more and firing at 800 to 1100° C. has a 1 C capacity, that is, a discharge capacity. Was high.
Further, the lithium secondary batteries 1 to 4 having the positive electrode active material obtained by using the hydroxide having the maximum value of the differential pore volume of 0.8 mm ³ /g·nm or more had a high discharge capacity. .. Particularly, in Examples 2 to 4, the maximum value of the differential pore volume is 1.8 mm ³ /g·nm or less, and the tap density of the positive electrode active material is high. Therefore, a lithium secondary battery having a higher energy density has been obtained.

Lithium having a positive electrode active material prepared in the same manner as in Examples 1 to 4, using a hydroxide having a pore volume per unit mass of less than 0.055 cm ³ /g as in Examples 5 to 7. The secondary batteries 5 to 7 had a low discharge capacity. Furthermore, in Example 5, the hydroxide has the maximum value of the differential pore volume, but the maximum value is less than 0.8 mm ³ /g·nm, and in Examples 6 and 7, the hydroxide has the differential pore volume. It does not have a maximum volume. Therefore, the lithium secondary batteries 5 to 7 having the positive electrode active material obtained by using the hydroxides of Examples 5 to 7 had a low discharge capacity.

In the method for producing a positive electrode active material of the present invention, the relative pressure (P/P _0. P is the pressure (Pa), which is obtained from the adsorption isotherm using nitrogen gas as a probe, and P ₀ is the saturated vapor pressure (Pa). The total molar amount (Me) of the metal contained in the hydroxide is a hydroxide having a pore volume of 0.055 cm ³ /g or more per unit mass at 0.99 and a lithium compound. The ratio of the lithium molar amount of the lithium compound with respect to (Li/Me ratio) is mixed at a ratio of 1.1 or more, and the mixture is fired at 800 to 1100°C. The positive electrode active material obtained by this method has a high discharge capacity and can be suitably used as a positive electrode active material for a lithium ion secondary battery. Then, a lithium ion secondary battery having a high energy density can be obtained.

Claims (6)

A step of mixing a transition metal-containing hydroxide and a lithium compound and firing to obtain a lithium-containing composite oxide,
The transition metal-containing hydroxide is a relative pressure (P/P _0. P is a pressure (unit: Pa)) obtained from an adsorption isotherm using nitrogen gas as a probe, and P ₀ is a saturated vapor pressure (unit: Pa). ), the pore volume per unit mass at 0.99 is 0.055 cm ³ /g or more,
In the transition metal-containing hydroxide and the lithium compound, the ratio (Li/Me) of the lithium molar amount of the lithium compound to the total molar amount (Me) of the metals contained in the transition metal-containing hydroxide is 1. Mixed by 1 or more,
The method for producing a positive electrode active material, wherein the firing is performed at 800 to 1100°C. In the transition metal-containing hydroxide, the differential pore volume determined by the BJH method from the adsorption isotherm has a maximum value, and the differential pore volume at the maximum value is 0.8 mm ³ /(g·nm) or more. The method for producing a positive electrode active material according to claim 1, wherein The method for producing a positive electrode active material according to claim 2, wherein the transition metal-containing hydroxide has a maximum value of the differential pore volume in a pore size range of 10 to 100 nm. The method for producing a positive electrode active material according to claim 2, wherein the maximum value of the differential pore volume of the transition metal-containing hydroxide is 1.8 mm ³ /(g·nm) or less. The method for producing a positive electrode active material according to claim 1, wherein the transition metal-containing hydroxide is represented by the following formula 1.
Ni _α Co _β Mn _γ M _δ (OH) ₂ Formula 1
However, M is one or more elements selected from the group consisting of Na, Mg, Ti, Zr, Al, W and Mo, α+β+γ+δ=1, and 0.3≦α≦0.5, 0≦β≦0.1, 0.5≦γ≦0.7, and 0≦δ≦0.1. The method for producing a positive electrode active material according to claim 1, wherein the composition of the lithium-containing composite oxide is represented by the following formula 2.
Li _a Ni _b Co _c Mn _{d Me} O ₂ Formula 2
However, M is one or more elements selected from the group consisting of Na, Mg, Ti, Zr, Al, W and Mo, a+b+c+d+e=2, 1.05≦a≦1.20, 0.25≦b≦0.45, 0≦c≦0.1, 0.45≦d≦0.65, and 0≦e≦0.1.

Images