JP6756279B2 - Manufacturing method of positive electrode active material - Google Patents

Description

The present invention relates to a method for producing a positive electrode active material, and more particularly to a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery.

In recent years, small electronic devices such as home video cameras, notebook computers, and smartphones have become widespread, and there is a demand for higher battery capacity and longer life. Non-aqueous electrolyte secondary batteries are often used in these small electronic devices.

Non-aqueous electrolyte secondary batteries are also used in electric vehicles. The electric vehicle is, for example, a hybrid vehicle, a plug-in hybrid vehicle, and an electric vehicle. In these electric vehicles, the non-aqueous electrolyte secondary battery is also required to have a high energy density.

As a non-aqueous electrolyte secondary battery having a high energy density, there is a lithium ion secondary battery. The lithium ion secondary battery contains a positive electrode, a negative electrode and an electrolytic solution. As an active material material for the positive electrode of a lithium ion secondary battery (hereinafter referred to as a positive electrode active material), there is a lithium transition metal composite oxide. The lithium transition metal composite oxide is, for example, lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ) and lithium nickelate (LiNiO ₂ ), and one of the transition metals in these lithium transition metal composite oxides. The part is replaced with another transition metal or the like. LiCoO ₂ tends to have a high density, but has a high Co cost and low thermal stability. LiMn ₂ O ₄ has a low cost but a small capacity. LiNiO ₂ has the highest capacity but is difficult to synthesize. Therefore, from the viewpoint of the balance between performance and cost, recently, an increasing number of use of _{Li a Ni 1-xy Co x} Mn y O 2 of the ternary system. However, the conventional positive electrode active material may have low durability characteristics at high temperature (hereinafter referred to as high temperature durability characteristics). The high temperature referred to here is a temperature higher than room temperature, for example, 45 to 60 ° C.

Techniques for enhancing high temperature durability characteristics include JP-A-2006-73482 (Patent Document 1), JP-A-2011-82133 (Patent Document 2), JP-A-2015-144119 (Patent Document 3), and JP-A-2014. -216186 (Patent Document 4). In these documents, mother particles made of a lithium transition metal composite oxide are coated to enhance high temperature durability characteristics (hereinafter, also referred to as high temperature storage characteristics) when stored at a high temperature.

Specifically, Patent Document 1 describes a positive electrode obtained by mixing a lithium-containing composite oxide containing Ni, Co and Al or a lithium-containing composite oxide containing Ni, Co and Mn with a phosphorus compound and then heat-treating. Disclose the active material.

In Patent Document 2, at least one of sulfur (S), phosphorus (P) and fluorine (F) is aggregated on the surface of the composite oxide particles containing the transition metal and the metal element M. Disclosed is a positive electrode active material that exists in the form and has a concentration gradient in which the metal element M becomes thicker from the center of the composite oxide particles toward the surface. In such a positive electrode active material, a compound containing lithium, a compound containing a transition metal, and a compound containing a metal element M are mixed in advance and fired, and sulfur (S), phosphorus (P), and fluorine (F) are generated. It is obtained by adhering a compound containing at least one of the above to the surface of the composite oxide particles and firing again.

Patent Document 3 describes the composition formula Li _a Ni _1−x−y Co _x M ¹ _y M ² _z O ₂ (1.00 ≦ a ≦ 1.50, 0.00 ≦ x ≦ 0.50, 0.00 ≦. y ≦ 0.50, 0.00 ≦ z ≦ 0.02, 0.00 ≦ x + y ≦ 0.70, M ¹ is at least one element selected from the group consisting of Mn and Al, and M ² is Zr. , W, Ti, Mg, Ta, Nb and Mo), and at least a part of the surface of the core particles containing the lithium transition metal composite oxide represented by the group. Discloses a positive electrode active material for a non-aqueous electrolyte secondary battery that is present in the region and contains a coating layer containing magnesium, phosphorus and oxygen. Such a coating layer of the positive electrode active material is obtained by supplying a first solution containing a magnesium salt of an organic acid and a second solution containing phosphorus and oxygen to the surface of the core particles and heat-treating them. ..

Patent Document 4 discloses a method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery. The method for producing a positive electrode active material in this document includes a step of bringing at least a part of the surface of the active material into contact with phosphoric acid before being fixed to the positive electrode current collector. After the contacting step, the step of heating includes the step of heating both the active material and the phosphoric acid, and the step of heating is performed at 200 to 400 ° C. In the contacting step, the entire surface of the particulate active material is brought into contact with phosphoric acid.

Japanese Unexamined Patent Publication No. 2006-73482 Japanese Unexamined Patent Publication No. 2011-82133 Japanese Unexamined Patent Publication No. 2015-144119 Japanese Unexamined Patent Publication No. 2014-216186

The positive electrode active material is required to further improve the charge / discharge efficiency in order to increase the energy density of the battery. The positive electrode active material is also required to have a higher capacity and a higher voltage. Therefore, increasing the capacity and voltage by increasing the charging voltage (increasing the amount of Li drawn out) is being studied. Specifically, it is being studied to raise the charging voltage from the conventional 4.2V to 4.4V or more. Therefore, excellent high-temperature storage characteristics are required even after charging at a high voltage.

Patent Document 1 discloses an example of coating with Li ₃ PO ₄ as a lithium compound. Li ₃ PO ₄ is a powdery solid and has a high melting point. In Patent Document 1, the heating temperature after mixing the lithium-containing composite oxide and Li ₃ PO ₄ is as low as 300 ° C., for example. Therefore, it is presumed that the Li ₃ PO ₄ powder is in a state of point contact with the lithium-containing composite oxide, and the coating is insufficient. As a result, it is considered that the improvement of high temperature storage durability after high voltage charging is insufficient. Further, since Li ₃ PO ₄ has low lithium ion conductivity, it is considered that the battery resistance is high.

In Patent Document 2, since the heat treatment is performed at a high temperature, the reaction between the coating layer and the lithium transition metal composite oxide constituting the mother particles of the positive electrode active material proceeds. In this case, the formation of Li ₃ PO ₄ and the diffusion of the transition metal on the surface of the mother particle of the positive electrode active material proceed. As a result, it is considered that an electron conduction path is formed in a part of the coating layer. As a result, it is considered that the high-temperature storage characteristics after high-voltage charging are insufficient.

In Patent Document 3 and Patent Document 4, since the coating treatment method is a wet method, the positive electrode active material reacts with water. As a result, the surface of the mother particle is deteriorated due to the formation of LiOH or the like on the surface of the mother particle. As a result, the discharge capacity and charge / discharge efficiency are considered to decrease.

An object of the present invention is to provide a method for producing a positive electrode active material, which is used in a non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery and has high charge / discharge efficiency and excellent high temperature durability characteristics. is there.

Method for producing a cathode active material according to an embodiment of the present invention comprises a base particle comprising a lithium transition metal composite oxide represented by the composition formula _{Li a Ni 1-xy Co x} Mn y M z O 2, the mother particles It is a method for producing a positive electrode active material including a phosphorus compound coating layer that covers at least a part of the surface. The method for producing a positive electrode active material according to the present embodiment comprises the step of preparing the above-mentioned mother particles and one or more selected from the group consisting of phosphorus oxide, phosphoric acid and phosphate, and has a melting point of 500 ° C. The following phosphorus compound raw materials and the above-mentioned mother particles are mixed and subjected to mechanochemical treatment to produce a composite particle intermediate, and the above-mentioned composite particle intermediate is heat-treated at 250 to 600 ° C. It has a process.
Here, a, x, y and z in the above composition formula are 0.95 ≦ a ≦ 1.20, 0.09 ≦ x ≦ 0.50, 0.09 ≦ y ≦ 0.50, 0.00. ≦ z ≦ 0.02 and 0.18 ≦ x + y ≦ 0.70, M in the above composition formula is Zr, Nb, W, Ce, Ti, Si, Mo, Al, Ca, Sr, B, Mg, One or more selected from the group consisting of Fe and Zn.

Method for producing a cathode active material according to another embodiment of the present invention comprises a base particle comprising a lithium transition metal composite oxide represented by the composition formula _{Li a Ni 1-xy Co x} Mn y M z O 2, the mother It is a method for producing a positive electrode active material including a zirconium oxide layer that covers at least a part of the surface of particles. In the method for producing a positive electrode active material according to the present embodiment, the step of preparing the mother particles, the mother particles, the zirconium oxide particles, and carbon black are mixed and subjected to mechanochemical treatment to obtain a composite particle intermediate. It includes a step of manufacturing and a step of performing a heat treatment at 250 to 600 ° C. on the composite particle intermediate.
Here, a, x, y and z in the above composition formula are 0.95 ≦ a ≦ 1.20, 0.09 ≦ x ≦ 0.50, 0.09 ≦ y ≦ 0.50, 0.00. ≦ z ≦ 0.02 and 0.18 ≦ x + y ≦ 0.70, M in the above composition formula is Zr, Nb, W, Ce, Ti, Si, Mo, Al, Ca, Sr, B, Mg, One or more selected from the group consisting of Fe and Zn.

The method for producing a positive electrode active material according to the present embodiment can provide a positive electrode active material having high charge / discharge efficiency and high temperature durability characteristics.

First, the present inventor investigated in detail a method for improving the charge / discharge efficiency of the positive electrode active material and the high-temperature storage characteristics after high-voltage charging. As a result, the present inventor obtained the following findings.

If the lithium ion secondary battery is charged at a high voltage, the discharge capacity will increase. The normal voltage is 4.2V, but the high voltage is, for example, 4.4V or more. When the battery is charged at such a high voltage and then stored at a high temperature of 45 to 60 ° C., the discharge capacity tends to decrease during the high temperature storage. The reason for this is not clear, but it can be considered as follows.

When a lithium ion secondary battery is charged at a high voltage and then stored at a high temperature, the positive electrode active material and the electrolytic solution may react excessively to generate gas. If gas is generated, the lithium ion secondary battery may expand. If gas is generated, the contact surface between the positive electrode active material and the electrolytic solution is further reduced. As a result, the amount of positive electrode active material that does not contribute to charge / discharge increases, and the discharge capacity decreases.

When the positive electrode active material reacts excessively with the electrolytic solution, a coating layer of decomposition products may be further formed on the surface of the mother particles of the positive electrode active material. In this case, the battery resistance increases and the discharge capacity at the time of discharging at a high voltage decreases.

Therefore, at least a part of the mother particles of the positive electrode active material is coated to reduce the area where the surface of the mother particles of the positive electrode active material comes into direct contact with the electrolytic solution (hereinafter, referred to as uncoated area). The coating is performed using a material having low electron conductivity. This suppresses an excessive reaction between the positive electrode active material and the electrolytic solution.

In a lithium ion secondary battery, when the non-covered area of the positive electrode active material is large, the irreversible reaction with the electrolytic solution increases. As a result, the charge / discharge efficiency decreases. In addition, the high-temperature storage characteristics after high-voltage charging deteriorate.

If at least a part of the surface of the mother particles of the positive electrode active material is covered to reduce the non-covered area of the positive electrode active material, high charge / discharge efficiency and high-temperature storage characteristics after high-voltage charging can be obtained. The material for coating the mother particles of the positive electrode active material is hereinafter referred to as an additive for a coating layer.

Conventionally, a wet coating method (hereinafter referred to as “wet method”) has been used for coating a mother particle made of a lithium transition metal composite oxide. In the wet method, the mother particles are dipped in a liquid additive for a coating layer containing water to coat the particles. The reason for this is that a uniform coating layer can be obtained by the wet method.

However, if the wet method is used, the positive electrode active material reacts with water. When the mother particles of the positive electrode active material contain Ni, if the positive electrode active material reacts with water, the surface deteriorates due to the formation of LiOH or the like on the surface. In this case, it becomes difficult to occlude and release lithium. As a result, the charge / discharge efficiency and the discharge capacity are lowered. That is, the initial characteristics are lowered.

If the dry coating method (hereinafter referred to as "dry method") is used, the above-mentioned reaction between the positive electrode active material and water does not occur. Therefore, the charge / discharge efficiency is increased. However, in the dry method, the coating layer additive used is solid particles. When the additive for the coating layer is a solid particle, an intermolecular force acts and it is easy to aggregate. Due to agglomeration, the particle size of the coating layer additive tends to increase. If the particle size is large, it is difficult for the coating layer additive to adhere uniformly to the surface of the mother particles. As a result, it is difficult to obtain a uniform coating layer.

The mechanochemical treatment, which is a kind of dry method, has an action of unaggregating, so that it is easy to coat uniformly. However, solid particles are also used in the mechanochemical treatment. Therefore, although it depends on the shape of the solid particles, the solid particles are in point contact with the mother particles. Therefore, the non-covered area cannot be sufficiently reduced. Therefore, in order to reduce the non-covered area, a large amount of additives for the coating layer are required, and the discharge capacity is reduced.

Therefore, the present inventor has found a method of using a material having a low melting point as an additive for a coating layer, mixing it with a mother particle, and performing a mechanochemical treatment. Further, the heat treatment is performed at a temperature at which the coating layer of the composite particle intermediate obtained by the mechanochemical treatment can be melted. As a result, the coating layer in the composite particle intermediate wets and spreads on the surface of the mother particles, so that the uncovered area can be reduced.

However, if the coating layer does not have lithium ion conductivity, the entrance and exit of ions will decrease and the battery resistance will increase. Therefore, it is necessary to use a coating layer having ionic conductivity.

Therefore, the present inventor has found that the high temperature storage durability is enhanced by using a phosphorus compound raw material having a low melting point as an additive for a coating layer. When the phosphorus compound raw material and the mother particles are subjected to mechanochemical treatment and the obtained composite particle intermediate is heat-treated at a low temperature of 250 to 600 ° C., the phosphorus compound raw material having excellent ionic conductivity is obtained. A phosphorus compound coating layer composed of is formed. The phosphorus compound coating layer thus obtained can have both excellent initial characteristics and high-temperature storage characteristics after high-voltage charging.

If the heat treatment is carried out at a high temperature of more than 600 ° C., the reaction between the coating layer and the lithium transition metal composite oxide which is a constituent element of the mother particles proceeds. In this case, the diffusion of the transition metal to the surface of the mother particle proceeds. As a result, an electron conduction path is formed in a part of the phosphorus compound coating layer. As a result, the charge / discharge efficiency and the effect of improving the high-temperature storage characteristics after high-voltage charging are reduced.

Since the positive electrode active material of the present embodiment is heat-treated at a low temperature of 250 to 600 ° C., the formation of an electron conduction path can be suppressed. As a result, both excellent initial characteristics and high-temperature storage characteristics after high-voltage charging can be achieved.

The present inventor has further found that the use of zirconium oxide particles as an additive for a coating layer further enhances high temperature durability characteristics. This is because if zirconium oxide particles are used as an additive for the coating layer, the high temperature cycle characteristics are enhanced. The high temperature cycle characteristic means a change in the number of cycles of the charge / discharge characteristic when repeatedly charging / discharging at a high temperature. The reason for this is not clear, but it is thought to be as follows.

At high temperatures, the positive electrode material containing the lithium transition metal composite oxide as the parent particle deteriorates. More specifically, the manganese ion in the positive electrode material is eluted by hydrofluoric acid contained in a trace amount in the electrolytic solution. The eluted manganese ions are consumed on the carbon negative electrode. As a result, the capacity is significantly reduced.

Zirconium oxide reacts with hydrofluoric acid to form fluoride. Therefore, if zirconium oxide particles are used as an additive for the coating layer, hydrofluoric acid in the electrolytic solution can be collected. As a result, deterioration of the positive electrode material is suppressed, and the high temperature cycle characteristics of the positive electrode active material are enhanced.

However, zirconium oxide is an insulator. Therefore, when coated only with zirconium oxide, the high temperature storage characteristics may deteriorate. Therefore, in the prior art, the deterioration of the high temperature storage property has been suppressed by thinning the zirconium oxide layer. In the prior art, the base metal was coated by a wet method in order to thin the zirconium oxide layer. This is because the zirconium oxide layer becomes thicker in the dry method such as the above-mentioned mechanochemical. However, as described above, the wet method deteriorates the initial characteristics.

Therefore, the present inventor further studied and found that if carbon black is used together with the zirconium oxide particles in the mechanochemical treatment as an additive for the coating layer, the high temperature storage property can be maintained even if the zirconium oxide layer is thick. It was. If the zirconium oxide layer may be thick, the base metal can be coated by a dry method instead of a wet method. As a result, the initial characteristics can be improved.

The combustion start temperature of carbon black varies depending on the crystallinity, but is usually 400 to 700 ° C. However, the present inventor has found that when carbon black is mixed with the mother particles and zirconium oxide particles of the present embodiment, the carbon black burns at a temperature lower than the above combustion start temperature.

In the step of producing the composite particle intermediate of the present embodiment, if the mother particles, the zirconium oxide particles, and carbon black are mixed, the mother particles are coated in a state where the zirconium oxide and carbon black are mixed. When the mechanochemical treatment is carried out and the obtained composite particle intermediate is heat-treated at a low temperature of 250 to 600 ° C., only carbon black is burned and disappears. That is, only zirconium oxide remains on the surface of the mother particles of the present embodiment, and the portion where carbon black was present becomes a pore. As a result, a porous zirconium oxide layer is formed on the surface of the mother particles of the present embodiment.

If the coating layer is porous, the entrance and exit of lithium ions will increase. Therefore, even if the zirconium oxide layer is thick, high temperature storage characteristics can be obtained. That is, if the zirconium oxide particles and carbon black are mixed, both the initial characteristics and the high temperature cycle characteristics can be achieved at the same time.

Method for producing a cathode active material according to the present embodiment has been completed based on the above findings, the mother particle made of the lithium transition metal composite oxide represented by the composition formula _{Li a Ni 1-xy Co x} Mn y M z O 2 A method for producing a positive electrode active material, which comprises a phosphorus compound coating layer that covers at least a part of the surface of the mother particles. The method for producing a positive electrode active material according to the present embodiment includes a step of preparing the above-mentioned mother particles, a step of mixing a phosphorus compound raw material and the above-mentioned mother particles and carrying out a mechanochemical treatment to produce a composite particle intermediate. The composite particle intermediate is provided with a step of performing a heat treatment at 250 to 600 ° C. The phosphorus compound raw material comprises one or more selected from the group consisting of phosphorus oxide, phosphoric acid and phosphate, and has a melting point of 500 ° C. or lower.
Here, a, x, y and z in the above composition formula are 0.95 ≦ a ≦ 1.20, 0.09 ≦ x ≦ 0.50, 0.09 ≦ y ≦ 0.50, 0.00. ≦ z ≦ 0.02 and 0.18 ≦ x + y ≦ 0.70, M in the above composition formula is Zr, Nb, W, Ce, Ti, Si, Mo, Al, Ca, Sr, B, Mg, One or more selected from the group consisting of Fe and Zn.

According to the method for producing a positive electrode active material according to the present embodiment, the charge / discharge efficiency of the positive electrode active material and the high temperature durability characteristics are enhanced.

The “positive electrode active material” referred to in the present specification is, for example, a positive electrode active material for a non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery.

The average particle size of the mother particles is, for example, 1 to 30 μm. When the average particle size of the mother particles is 1 μm or more, the uncoated area of the mother particles becomes small. Therefore, the reaction with the electrolytic solution is reduced, and the charge / discharge efficiency and high temperature durability are improved. When the average particle size of the mother particles is 1 μm or more, the bulk density of the positive electrode active material becomes larger and the specific surface area becomes smaller. Therefore, slurry production becomes easy. Further, when the conductive auxiliary agent is also included in the slurry, uniform contact with the conductive auxiliary agent becomes easy, and the battery resistance is lowered. On the other hand, when the average particle size of the mother particles is 30 μm or less, the specific surface area is moderately large, and the entrance / exit of lithium release / occlusion increases. In addition, the diffusion distance inside the particles becomes shorter. Therefore, the battery resistance is lowered. When the average particle size of the mother particles is 30 μm or less, it becomes easier to produce a thin electrode.

In the step of producing a composite particle intermediate by mechanochemical treatment, preferably, when the mother particle is 100 mol%, one or more selected from the group consisting of phosphorus oxide, phosphoric acid and phosphate. The amount of the phosphorus compound raw material added is 0.2 to 2.0 mol% as the phosphorus element. Here, "as a phosphorus element" means as a conversion amount of the phosphorus element in the total amount of substance of the phosphorus compound raw material.

When the amount of the phosphorus compound raw material added as the phosphorus element is 0.2 mol% or more, the mother particles can be sufficiently coated. Therefore, charge / discharge efficiency and high temperature durability characteristics are further improved. When the amount of the phosphorus compound raw material added as a phosphorus element is 2.0 mol% or less, the coating layer that does not contribute to charge / discharge can be reduced. In this case, a decrease in discharge capacity can be suppressed. In addition, an increase in interparticle resistance can be suppressed. As a result, an increase in battery resistance can be suppressed, and a decrease in discharge capacity and charge / discharge efficiency can be suppressed. This is because the coating layer allows lithium ions to pass through, but is an insulator against electrons. The preferable lower limit of the amount of the phosphorus compound raw material added as the phosphorus element is 0.5 mol%. The preferable upper limit of the amount of the phosphorus compound raw material added as the phosphorus element is 1.5 mol%.

In the step of producing the above-mentioned composite particle intermediate, it is preferable to further mix zirconium oxide particles in addition to the above-mentioned mother particles and the above-mentioned phosphorus compound raw material. In this case, since the high temperature cycle characteristic is enhanced, the high temperature durability characteristic is further enhanced.

In the step of producing the above-mentioned composite particle intermediate, it is more preferable to further mix carbon black in addition to the above-mentioned mother particles, the above-mentioned phosphorus compound raw material, and zirconium oxide particles. In this case, in addition to the high temperature cycle characteristics, the high temperature storage characteristics are also enhanced, so that the high temperature durability characteristics are further enhanced.

Method for producing a cathode active material according to another embodiment of the present invention comprises a base particle comprising a lithium transition metal composite oxide represented by the composition formula _{Li a Ni 1-xy Co x} Mn y M z O 2, the mother It is a method for producing a positive electrode active material including a zirconium oxide layer that covers at least a part of the surface of particles. In the method for producing a positive electrode active material according to the present embodiment, the step of preparing the above-mentioned mother particles, the above-mentioned mother particles, zirconium oxide particles, and carbon black are mixed and subjected to mechanochemical treatment to obtain a composite particle intermediate. It includes a step of manufacturing and a step of performing a heat treatment at 250 to 600 ° C. on the composite particle intermediate.
Here, a, x, y and z in the above composition formula are 0.95 ≦ a ≦ 1.20, 0.09 ≦ x ≦ 0.50, 0.09 ≦ y ≦ 0.50, 0.00. ≦ z ≦ 0.02 and 0.18 ≦ x + y ≦ 0.70, M in the above composition formula is Zr, Nb, W, Ce, Ti, Si, Mo, Al, Ca, Sr, B, Mg, One or more selected from the group consisting of Fe and Zn.

In this case, the positive electrode active material can include a zirconium oxide layer that covers at least a part of the surface of the above-mentioned mother particles. In this case, the high temperature cycle characteristics are enhanced. That is, it is possible to achieve both high initial charge / discharge efficiency and excellent high temperature durability characteristics.

Hereinafter, the positive electrode active material that can be produced by the production method of this embodiment will be described in detail.

[Positive electrode active material]
The positive electrode active material produced by the production method of the present embodiment comprises a mother particle and a coating layer for coating the mother particle.

[Mother particle]
The mother particle consists of a lithium transition metal composite oxide. Lithium transition metal composite oxides contain lithium and transition metals, with the balance consisting of impurities. The impurities referred to here are elements that are mixed in from the environment of the manufacturing process of the lithium transition metal composite oxide. Impurities are, for example, Na, S, K, Ca. The transition metal of the lithium transition metal composite oxide of the present embodiment contains one or more selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. In this case, charge / discharge capacity can be obtained. Preferably, the transition metal contains at least one or more selected from the group consisting of Ni, Co and Mn. In this case, the energy density is increased, and higher charge / discharge efficiency and high temperature durability characteristics can be obtained. More preferably, the transition metal is a ternary system containing all of Ni, Co and Mn. In this case, the balance between cost, safety, and energy density is excellent, and higher charge / discharge efficiency and high temperature durability characteristics can be obtained.

Thus, a preferred form of the lithium-transition metal composite oxide of the present embodiment can be represented by the composition formula _{Li a Ni 1-xy Co x} Mn y M z O 2. Here, in the above composition formula, a, x, y and z are 0.95 ≦ a ≦ 1.20, 0.09 ≦ x ≦ 0.50, 0.09 ≦ y ≦ 0.50, 0.00. Satisfy ≦ z ≦ 0.02 and 0.18 ≦ x + y ≦ 0.70. In the above composition formula, M is one or more selected from the group consisting of Zr, Nb, W, Ce, Ti, Si, Mo, Al, Ca, Sr, B, Mg, Fe and Zn. It is an element. When a, x, y and z are in this range, as described above, the balance between cost, safety and energy density is excellent, and higher high temperature durability and charge / discharge efficiency can be obtained.

The reasons for limiting the numerical ranges of a, x, y and z in the above composition formula are as follows.

If the Li content is too low, the number of crystal phases unsuitable for releasing and storing Li increases, and the discharge capacity and charge / discharge efficiency decrease. On the other hand, if the Li content is too high, the lithium transition metal composite oxide tends to aggregate too much, making subsequent handling difficult. If the Li content is too high, excess Li remains as lithium carbonate, and the viscosity of the positive electrode slurry tends to increase. Therefore, a of _{Li a Ni 1-xy Co x} Mn y M z O Li of ₂ in the composition formula is from 0.95 to 1.20. The preferable lower limit of a is 1.00. The preferred upper limit of a is 1.10.

Ni increases the discharge capacity. If the Ni content is too low, this effect cannot be obtained. On the other hand, if the Ni content is too high, the crystal structure becomes unstable. If the Ni content is too high, lithium carbonate or the like is likely to be produced, so that the viscosity of the positive electrode slurry is likely to increase. If the Ni content is too high, it may be more difficult to handle in the atmosphere. Thus, 1-xy of _{Li a Ni 1-xy Co x} Mn y M z O 2 of Ni in the composition formula is 0.30~0.82 (0.18 ≦ x + y ≦ 0.70) ..

Co stabilizes the crystal structure. If the Co content is too low, the crystal structure will not be stable. On the other hand, if the Co content is too high, the material cost increases. Therefore, x of Co in the composition formula of Li _a Ni _1-xy Co _x M n _y M _z O ₂ is 0.09 to 0.50. A preferred lower limit of _{Li a Ni 1-xy Co x} Mn y M z O 2 in the x in the composition formula is 0.10. The preferred upper limit of x is 0.35.

Like Ni, Mn enhances charge / discharge efficiency. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the discharge capacity decreases. Thus _{Li a Ni 1-xy Co x} Mn y M z y of Mn in the composition formula of O ₂ is 0.09 to 0.50. A preferred lower limit of _{Li a Ni 1-xy Co x} Mn y M z O 2 in the y in the composition formula is 0.10. The preferred upper limit of y is 0.35.

M is at least one element or two or more elements selected from the group consisting of Zr, Nb, W, Ce, Ti, Si, Mo, Al, Ca, Sr, B, Mg, Fe and Zn. M is an optional element and may not be contained. When included, M further enhances the properties of the battery. For example, Zr further enhances high temperature endurance properties. Mg further enhances charge / discharge efficiency. If _{Li a Ni 1-xy Co x} Mn y M z O 2 in z of M in the composition formula is 0.02 or less, the balance of various properties is good, further preferred. When comprising M, preferred lower limit of _{Li a Ni 1-xy Co x} Mn y M z O 2 in the z in the composition formula is 0.001. The preferred upper limit of z is 0.01.

[Average particle size of mother particles]
The average particle size of the mother particles is, for example, 1 to 30 μm. When the average particle size of the mother particles is 1 μm or more, the size is appropriate and handling is easy. When the particle size of the mother particles is 1 μm or more, the uncoated area of the mother particles is further small, and the contact area with the electrolytic solution is reduced. Therefore, the reactivity with the electrolytic solution is lowered, and the charge / discharge efficiency and the high temperature durability are improved. It also increases safety. When the average particle size of the mother particles is 30 μm or less, a flat and thin electrode can be produced. When the average particle size of the mother particles is 30 μm or less, the specific surface area is further large, the entrance / exit of lithium release / occlusion is increased, and the diffusion distance inside the particles is shortened, so that the battery resistance is lowered. A more preferable lower limit of the average particle size of the mother particles is 3 μm. A more preferable upper limit of the average particle size of the mother particles is 20 μm.

[Coating layer covering mother particles]
The positive electrode active material of the present embodiment includes the mother particles and a coating layer that covers at least a part of the surface of the mother particles.

The coating layer included in the positive electrode active material of the present embodiment is a phosphorus compound coating layer or a zirconium oxide coating layer.

In the present embodiment, the raw materials of the coating layer are a phosphorus compound raw material, zirconium oxide particles, and carbon black.

[Phosphorus compound raw material]
The phosphorus compound raw material comprises one or more selected from the group consisting of oxidized phosphorus, phosphoric acid and phosphate. Preferably, the phosphorus compound is solid upon mixing and melts during heat treatment after mechanochemical treatment. For that purpose, the melting point of the phosphorus compound raw material is 500 ° C. or lower. In this case, the adhesion between the mother particles after the heat treatment and the coating layer is further enhanced. Phosphorus oxide is, for example, P ₂ O ₅ (melting point 340 ° C.). Phosphoric acid is, for example, H ₃ PO ₄ (melting point 42 ° C.). Phosphates are, for example, NH ₄ H ₂ PO ₄ (melting point 190 ° C.) and (NH ₄ ) ₂ HPO ₄ (melting point 155 ° C.).

In the present embodiment, since the melting point of the phosphorus compound raw material is as low as 500 ° C. or lower, the heat treatment after the mechanochemical treatment described later can be carried out at a low temperature. In this case, the transition metal, which is a constituent element of the mother particle, is suppressed from diffusing to the surface of the mother particle, and the high temperature storage characteristic after high voltage charging is improved.

[Zirconium oxide particles]
Zirconium oxide is an oxide of zirconium, also called zirconium dioxide. The chemical formula is ZrO ₂ . As mentioned above, zirconium oxide reacts with hydrofluoric acid to form fluoride. Therefore, if zirconium oxide particles are used as an additive for the coating layer, hydrofluoric acid in the electrolytic solution can be collected. As a result, deterioration of the positive electrode material is suppressed, the high temperature cycle characteristics of the positive electrode active material are enhanced, and the high temperature durability characteristics are enhanced.

The particle size of zirconium oxide is preferably 1 to 100 nm. When the particle size of zirconium oxide is 1 nm or more, the zirconium oxide particles are sufficiently dispersed in the step of producing the composite particle intermediate. Therefore, a sufficient film of zirconium oxide particles can be secured. As a result, the high temperature cycle characteristics are further enhanced. When the particle size of zirconium oxide is 100 nm or less, a sufficient film of zirconium oxide particles can be secured. As a result, the high temperature cycle characteristics are further enhanced. The particle size of zirconium oxide is more preferably 5 to 50 nm.

The amount of zirconium oxide added is preferably 0.1 to 1.5% by mass with respect to the mother particles. When the amount of zirconium oxide added is 0.1% by mass or more, the effect of collecting hydrofluoric acid can be sufficiently obtained. When the amount of zirconium oxide added is 1.5% by mass or less, sufficient charge / discharge efficiency can be obtained. The amount of zirconium oxide added is more preferably 0.3 to 1.0% by mass.

[Carbon black]
Carbon black is a fine powdered carbon material, and is not particularly limited depending on the raw material. Carbon black is, for example, a fine powder of carbon powder produced by incomplete combustion of heavy oil, natural gas, petroleum, tar and the like. More specifically, the carbon blacks are, for example, acetylene black, oil furnace black, graphitized carbon black, graphite, ketjen black, channel black and furnace black.

The combustion start temperature of carbon black varies depending on the crystallinity, but is usually 400 to 700 ° C. However, when carbon black is mixed with the mother particles and zirconium oxide particles of the present embodiment, the carbon black burns at a temperature lower than the combustion start temperature. Therefore, as described above, if carbon black is used together with the zirconium oxide particles in the mechanical treatment as the additive for the coating layer, only carbon black is burned in the step of performing the heat treatment on the composite particle intermediate of the present embodiment. Then, a porous coating layer is formed. If the coating layer is porous, the entrance and exit of lithium ions will increase. Therefore, the high temperature storage characteristics can be improved. As a result, if carbon black is used together with the zirconium oxide particles as the additive for the coating layer, both charge / discharge efficiency and high temperature durability characteristics can be achieved at the same time.

The particle size of carbon black is preferably 1 to 100 nm. When the particle size of the carbon black is 1 nm or more, the carbon black is sufficiently dispersed in the step of producing the composite particle intermediate. In this case, the above-mentioned effect of carbon black can be sufficiently obtained. As a result, the high temperature cycle characteristics are further enhanced. When the particle size of carbon black is 100 nm or less, the gaps between the zirconium oxide particles become appropriate, and a sufficient film of zirconium oxide can be secured. As a result, the high temperature cycle characteristics are further enhanced. The particle size of zirconium oxide is more preferably 20 to 50 nm.

The amount of carbon black added is preferably 0.5 to 2.0% by mass with respect to the mother particles. When the amount of carbon black added is 0.5% by mass or more, the above-mentioned effect of carbon black can be sufficiently obtained. As a result, the high temperature cycle characteristics are further enhanced. When the amount of carbon black added is 2.0% by mass or less, a sufficient film of zirconium oxide can be secured. As a result, the high temperature cycle characteristics are further enhanced. The amount of carbon black added is more preferably 0.5 to 1.5% by mass.

[Phosphorus compound coating layer]
The phosphorus compound coating layer is composed of a phosphorus compound and impurities. The phosphorus compound in the phosphorus compound coating layer is derived from the above-mentioned phosphorus compound raw material. Impurities referred to here are elements that are mixed in from the environment or the like in the manufacturing process of phosphorus compounds. The mother particles are coated with a phosphorus compound by the production method of the present embodiment. The coating layer covers at least a part of the surface of the mother particles. As a result, the area where the mother particles come into direct contact with the electrolytic solution can be reduced. Therefore, an excessive reaction between the positive electrode active material and the electrolytic solution can be suppressed. As a result, high charge / discharge efficiency and high temperature durability characteristics can be obtained. The phosphorus compound in the phosphorus compound coating layer is not always the same as the phosphorus compound which is a phosphorus compound raw material added at the time of production described later.

The phosphorus compound coating layer may contain zirconium oxide. In this case, as described above, zirconium oxide collects hydrofluoric acid to form zirconium fluoride. This reduces the deterioration of the positive electrode material and enhances the high temperature cycle characteristics. When zirconium oxide is contained, a phosphorus compound coating layer may be present between the zirconium oxide particles. When the zirconium oxide particles are small, the zirconium oxide particles may be present in the phosphorus compound coating layer.

In the step of producing the composite particle intermediate of the present embodiment, when the mother particles, the phosphorus compound raw material, the zirconium oxide particles, and carbon black are mixed, the mixed coating layer may contain the remaining carbon black. Good.

[Zirconium oxide coating layer]
In the step of producing the composite particle intermediate of the present embodiment, when the phosphorus compound raw material is not added, that is, when only the zirconium oxide particles and carbon black are added, the positive electrode active material of the present embodiment has a zirconium oxide coating layer. Be prepared. The zirconium oxide coating layer is composed of zirconium oxide particles. As described above, in the step of producing the composite particle intermediate of the present embodiment, only carbon black is burned, so that the zirconium oxide coating layer is porous.

The zirconium oxide coating layer covers at least a part of the surface of the mother particles. As a result, the area where the mother particles come into direct contact with the electrolytic solution can be reduced. Therefore, an excessive reaction between the positive electrode active material and the electrolytic solution can be suppressed. As a result, high charge / discharge efficiency can be obtained.

As mentioned above, zirconium oxide collects hydrofluoric acid to form zirconium fluoride. As a result, deterioration of the positive electrode material is reduced, high temperature cycle characteristics are enhanced, and high temperature durability is further enhanced.

[Manufacturing method of positive electrode active material]
Next, a method for producing the positive electrode active material will be described. The method for producing the positive electrode active material includes a step of preparing the above-mentioned mother particles, a step of mixing the above-mentioned mother particles and the raw material of the coating layer and carrying out a mechanochemical treatment to produce a composite particle intermediate, and a step of producing a composite particle intermediate. The body is provided with a heat treatment step of performing heat treatment at 250 to 600 ° C.

[Mother particle preparation process]
First, the above-mentioned mother particles are prepared. The mother particles are made of a lithium transition metal composite oxide. The lithium transition metal composite oxide can be synthesized by a known method.

Known methods include, for example, raw material compounds such as carbonates, nitrates, acetates, and sulfates of nickel, manganese, and cobalt (these produce oxides by heat treatment), and the raw material compound and the lithium compound are used. Is mixed and fired according to the target composition.

Further, a ternary hydroxide of nickel, manganese and cobalt is first synthesized by a coprecipitation method or the like (it may be further converted into an oxide by heat treatment), and the obtained ternary hydroxide and a lithium compound are obtained. There is also a method of mixing and firing according to the target composition. Specifically, first, for example, nickel sulfate, manganese sulfate and cobalt sulfate are dissolved in water according to the desired composition. This, by supplying the aqueous NaOH and aqueous ammonia to control the pH and NH ₄ ⁺ concentration, to precipitate the ternary hydroxide. The precipitate is filtered, washed with water and dried to obtain a ternary hydroxide. The obtained ternary hydroxide and a lithium compound raw material (for example, lithium hydroxide or lithium carbonate) are mixed and fired at about 700 to 1100 ° C. to obtain a lithium transition metal composite oxide.

[Mechanochemical treatment process]
The prepared mother particles and the raw materials of the coating layer are mixed and subjected to a mechanochemical treatment by a dry method to produce a composite particle intermediate. Mechanochemical treatment is a method of mixing while applying mechanical energy such as shearing force, collision force or centrifugal force to a mixing target. Examples of the apparatus for performing crushing and mixing by mechanochemical treatment include crushing / dispersing machines such as ball mills, bead mills, and vibration mills.

When a phosphorus compound is used as a raw material, the phosphorus compound coats the surface of the mother particles by mechanochemical treatment. The phosphorus compound is in a solid state when charged into the device. During the mechanochemical treatment, the phosphorus compounds are unaggregated and become small particles. The small particles of the phosphorus compound are dispersed in the mother particles. By applying a load (giving energy) to the charged material, the mother particles and the small particles of the phosphorus compound are compounded. In the mechanochemical treatment, volumetric milling does not occur substantially. That is, the average particle size reduction is 10% or less.

In the step of producing the composite particle intermediate by the mechanochemical treatment, the amount of the phosphorus compound raw material added is preferably 0.2 to 2.0 mol% as the phosphorus element when the lithium transition metal composite oxide is 100 mol%. Is. When the amount of the phosphorus compound raw material added as the phosphorus element is 0.2 mol% or more, the mother particles can be sufficiently coated. Therefore, charge / discharge efficiency and high temperature durability characteristics are further improved. When the amount of the phosphorus compound raw material added as a phosphorus element is 2.0 mol% or less, the coating layer that does not contribute to charge / discharge can be reduced. In this case, the resistance between particles can be suppressed. As a result, the electrical resistance can be suppressed and the capacity retention rate is further increased. This is because the coating layer allows lithium ions to pass through, but is an insulator against electrons. The preferable upper limit of the amount of the phosphorus compound raw material added as the phosphorus element is 1.5 mol%.

The phosphorus compound raw material comprises one or more selected from the group consisting of oxidized phosphorus, phosphoric acid and phosphate. Preferably, the phosphorus compound is solid upon mixing and melts during heat treatment after mechanochemical treatment. For that purpose, the melting point of the phosphorus compound raw material is 500 ° C. or lower. In this case, the adhesion between the mother particles after the heat treatment and the coating layer is further enhanced. Phosphorus pentoxide is, for example, P ₂ O ₅ (melting point 340 ° C.). Phosphoric acid is, for example, H ₃ PO ₄ (melting point 42 ° C.). Phosphates are, for example, NH ₄ H ₂ PO ₄ (melting point 190 ° C.) and (NH ₄ ) ₂ HPO ₄ (melting point 155 ° C.).

Further, since the melting point of the phosphorus compound raw material is as low as 500 ° C. or lower, the heat treatment after the mechanochemical treatment can be performed at a low temperature. In this case, the transition metal, which is a constituent element of the mother particle, is suppressed from diffusing to the surface of the mother particle, and the high temperature storage characteristic after high voltage charging is improved.

By further performing the heat treatment described later after the mechanochemical treatment, the adhesion between the mother particles and the coating layer is enhanced. As a result, charge / discharge efficiency and high temperature durability characteristics are improved.

Although this mechanism is not clear, it is considered that a part of the phosphorus compound raw material is changed to a phosphorus compound having lithium ion conductivity and not electron conductivity. It is considered that at least a part of the phosphorus compound reacts with lithium in the lithium transition metal composite oxide to finally form a low crystallinity Li-PO compound which is not Li ₃ PO ₄ . By carrying out mechanochemical treatment and heat treatment, the phosphorus compound can be unaggregated and the mother particles can be uniformly coated. Therefore, the reactivity of a part of the phosphorus compound with a part of the element such as lithium in the lithium transition metal composite oxide is enhanced, and the Li—PO compound is more formed. Since the bond of the Li-PO compound is strong, it is considered that the adhesion between the mother particle and the coating layer is enhanced. As a result, charge / discharge efficiency and high temperature durability characteristics are improved.

Therefore, the morphology of the composite particle intermediate obtained by the mechanochemical treatment is different from the morphology of the intermediate obtained only by physically mixing the lithium transition metal composite oxide and the phosphorus compound. This difference can be confirmed, for example, in the spectrum of XPS (X-ray photoelectron spectroscopy).

The apparatus used for the mechanochemical treatment is not particularly limited as long as it is an apparatus capable of combining small particles of a mother particle and a phosphorus compound. That is, any device can be used as long as it can repeatedly apply mechanical actions such as compression and shear including the interaction of particles to the particles. The devices used for the mechanochemical treatment are, for example, Hosokawa Micron's trade name Mechanofusion and trade name Nobilta, Nara Kikai Seisakusho's trade name hybridization, Nippon Coke Industries' trade name COMPOSI, and the like, but are not limited thereto.

The mechanochemical processing time is appropriately adjusted according to the type, size, input amount, rotation speed, etc. of the apparatus. The mechanochemical treatment time is, for example, 1 minute to 2 hours. With this treatment time, the effect is stable and the material does not deteriorate. Preferably, the mechanochemical treatment time is 3 minutes to 1 hour.

In the mechanochemical treatment, the load applied to the material (excluding the load during idle operation) is, for example, 0.5 to 7 kW / kg. With a load in this range, the effect is stable and the material is less likely to deteriorate.

Preferably, the mechanochemical treatment is carried out under a dry air flow or an inert gas flow. The inert gas is argon gas, helium gas, nitrogen gas or the like.

In the mechanochemical treatment step, zirconium oxide particles may be added and mixed in addition to the mother particles and the phosphorus compound raw material. The coating layer covers at least a part of the surface of the mother particles. As a result, the area where the mother particles come into direct contact with the electrolytic solution can be reduced. Therefore, an excessive reaction between the positive electrode active material and the electrolytic solution can be suppressed. As a result, high charge / discharge efficiency can be obtained.

As mentioned above, zirconium oxide collects hydrofluoric acid to form zirconium fluoride. As a result, deterioration of the positive electrode material is reduced, high temperature cycle characteristics are enhanced, and high temperature durability characteristics are further enhanced.

In the mechanochemical treatment step, carbon black may be further added and mixed in addition to the mother particles, the phosphorus compound raw material, and the zirconium oxide particles. In this case, in addition to high charge / discharge efficiency, high-temperature storage characteristics and high-temperature cycle characteristics are also enhanced, and high-temperature durability characteristics are further enhanced.

The carbon black burns and disappears during the next heat treatment step. As a result, a porous coating layer is obtained. In this case, the number of ion entrances and exits increases, and the high-temperature storage characteristics after high-voltage charging also improve. Therefore, the high temperature durability characteristic is further enhanced.

The amount of zirconium oxide added is preferably 0.1 to 1.5% by mass with respect to the mother particles. When the amount of zirconium oxide added is 0.1% by mass or more, the effect of collecting hydrofluoric acid can be sufficiently obtained. When the amount of zirconium oxide added is 1.5% by mass or less, sufficient charge / discharge efficiency can be obtained. The amount of zirconium oxide added is more preferably 0.3 to 1.0% by mass.

The amount of carbon black added is preferably 0.5 to 2.0% by mass with respect to the mother particles. When the amount of carbon black added is 0.5% by mass or more, the above-mentioned effect of carbon black can be sufficiently obtained. As a result, the high temperature cycle characteristics are further enhanced. When the amount of carbon black added is 2.0% by mass or less, a sufficient film of zirconium oxide can be secured. As a result, the high temperature cycle characteristics are further enhanced. The amount of carbon black added is more preferably 0.5 to 1.5% by mass.

In the mechanochemical treatment step, only zirconium oxide particles and carbon black may be added and mixed in addition to the mother particles. In this case, the zirconium oxide coating layer covers at least a part of the surface of the mother particles. As a result, the area where the mother particles come into direct contact with the electrolytic solution can be reduced. Therefore, an excessive reaction between the positive electrode active material and the electrolytic solution can be suppressed. As a result, high charge / discharge efficiency can be obtained. In addition, the high temperature cycle characteristics are also enhanced.

[Heat treatment process]
Heat treatment is performed on the composite particle intermediate obtained in the mechanochemical treatment step. As a result, the phosphorus compound constituting the coating layer of the composite particle intermediate wets and spreads on the surface of the mother particle to improve the covering ratio, and stably coats the surface of the mother particle. It is considered that the phosphorus compound after the heat treatment step forms a chemical or physical bond with the element constituting the mother particle and is firmly integrated. As a result, the adhesion between the mother particles and the coating layer is enhanced, and the obtained positive electrode active material is enhanced in charge / discharge efficiency of the uncoated area positive electrode active material and high-temperature storage durability after high-voltage charging. In addition, in the mechanochemical process, strain is introduced into the surface of the mother particle whose crystal structure was originally disturbed, and the strain is released by the subsequent heat treatment process, and at the same time, the crystal structure is rearranged to release ions such as lithium. It is thought that storage becomes easier and charge / discharge efficiency improves.

The heat treatment atmosphere is preferably air, oxygen, carbon dioxide, nitrogen and argon. Considering the cost, air is more preferable. For removing volatile components such as moisture, it is preferably under a dry air flow.

The heat treatment temperature is 250 to 600 ° C. If the heat treatment temperature is too low, the charge / discharge efficiency and the discharge capacity will decrease. The reason for this is that the mother particles deteriorate due to insufficient removal of water from the phosphorus compound raw material and the adsorbed water. If the heat treatment temperature is too low, the reaction between the phosphorus compound raw material and the mother particles becomes insufficient. Furthermore, if the heat treatment temperature is too low, the strain on the surface of the mother particles applied during the mechanochemical treatment cannot be sufficiently removed. As a result, the charge / discharge efficiency and the discharge capacity are lowered. If the heat treatment temperature is too high, the high temperature durability characteristics will deteriorate. The reason for this is that if the heat treatment temperature is too high, the transition metal, which is a constituent element of the mother particle, is diffused to the surface of the mother particle, and an electron conduction path is formed in a part of the coating layer. The preferred lower limit of the heat treatment temperature is 350 ° C. The preferred upper limit of the heat treatment temperature is 550 ° C.

The heat treatment time is preferably 1 minute to 10 hours. The heat treatment time is adjusted according to manufacturing conditions such as the amount of intermediate charged.

[Preparation of positive electrode active material]
[Mother particle preparation process]
In the positive electrodes of Test Nos. 1 to 16 and Test Nos. 20 to 23, 50.00 g of product name CellSeed NMC111 manufactured by Nippon Chemical Industrial Co., Ltd. was used as the mother particles. The composition of the mother particles was LiNi _1/3 Mn _1/3 Co _1/3 O ₂ . The average particle size of the mother particles was 11 μm.

In the positive electrodes of Test Nos. 17 to 19, 40.00 g of NMC532 was used as the mother particles. The manufacturing method of NMC532 was as follows. The ternary hydroxide M (OH) ₂ (M is Ni: Mn: Co = 5: 3: 2 mol ratio, average particle size 6 μm) as a precursor powder and LiOH · H ₂ O are combined with Li: M. The mixture was mixed in a mortar so that = 1.05. The mixture was placed in an alumina container and heat-treated at 900 ° C. for 10 hours under an air flow to obtain mother particles. The composition of the obtained mother particles was Li _1.02 Ni _0.5 Mn _0.3 Co _0.2 O ₂ .

[Mechanical processing process or coating / mixing process]
The additives for the coating layer (phosphorus compound raw material, zirconium oxide particles, and carbon black) are as shown in Table 1. In Table 1, P ₂ O ₅ and NH ₄ H ₂ PO _{4 in the} “Additives for coating layer” column indicate phosphorus compound raw materials, ZrO ₂ indicates zirconium oxide particles, and CB indicates carbon black.

In Test No. 2 to Test No. 9, Test No. 22 and Test No. 23, P ₂ O ₅ manufactured by Wako Pure Chemical Industries was used as an additive for the coating layer. P ₂ O ₅ (melting point 340 ° C.) was 0.29 g, and the amount of phosphorus added was 0.8 mol% with respect to the mother particles in terms of phosphorus element. In Test No. 10, 0.48 g of Li ₃ PO ₄ (melting point 837 ° C.) was used as an additive for the coating layer, and the amount of phosphorus added was 0.8 mol% in terms of phosphorus element. In Test No. 11 and Test No. 12, 0.48 g of NH ₄ H ₂ PO ₄ (melting point 190 ° C.) was used as an additive for the coating layer, and the amount of phosphorus added was 0.8 mol% in terms of phosphorus element. In Test No. 13, 0.037 g of P ₂ O ₅ was used as an additive for the coating layer, and the amount of phosphorus added was 0.1 mol% in terms of phosphorus element. In Test No. 14, 0.074 g of P ₂ O ₅ was used as an additive for the coating layer, and the amount of phosphorus added was 0.2 mol% in terms of phosphorus element. In Test No. 15, 0.74 g of P ₂ O ₅ was used as an additive for the coating layer, and the amount of phosphorus added was 2.0 mol% in terms of phosphorus element. In Test No. 16, 1.10 g of P ₂ O ₅ was used as an additive for the coating layer, and the amount of phosphorus added was 3.0 mol% in terms of phosphorus element. In Test No. 18 and Test No. 19, 0.24 g of P ₂ O ₅ was used as an additive for the coating layer, and the amount of phosphorus added was 0.8 mol% in terms of phosphorus element.

In test numbers 20 to 23, zirconium oxide nanoparticles (10 nm) manufactured by Wako Pure Chemical Industries, Ltd. were used. The amount of zirconium oxide nanoparticles was 0.32 g, and the amount of zirconium oxide nanoparticles (hereinafter, also referred to as zirconium oxide particles) added was 0.64% by mass with respect to the amount of mother particles added.

In test number 21 and test number 23, HS-100 (product name) (10 nm) manufactured by Denka was used as carbon black. The amount of carbon black was 0.50 g, and the amount of carbon black added was 1.00% by mass with respect to the amount of mother particles added. HS-100 (product name) was acetylene black.

In Test No. 1 and Test No. 17, the mother particles were not coated.

The mother particles and additives for the coating layer (phosphorus compound raw material, zirconium oxide particles, and carbon black) were mixed under the conditions shown in Table 1.

In test numbers 2-8, 10, 12-16, 18 and 20-23, mechanochemical treatment was performed to coat the mother particles. As the mechanochemical device, the trade name Nobilta (NOB-MINI) manufactured by Hosokawa Micron was used. The mechanochemical conditions were 4000 rpm x 5 minutes under dry air flow.

In Test No. 9 and Test No. 19, the mother particles were coated by a wet method. In the wet method, 0.8 mol% of P ₂ O ₅ with respect to the mother particles in terms of phosphorus element was dissolved in water at 23 ° C. with respect to the above-mentioned mother particles. The mother particles were put into the obtained aqueous phosphoric acid solution and stirred sufficiently for 5 minutes to obtain a slurry. Then, the obtained slurry was spread on a Teflon (registered trademark) sheet and dried at 100 ° C.

In Test No. 11, the mother particles and the coating layer additive were simply mixed.

[Heat treatment process]
After the mechanochemical treatment or coating / mixing, the heat treatment was performed at the heat treatment temperatures shown in Table 1. The heat treatment was carried out in an alumina container. The heat treatment conditions were 4 hours under air flow.

[Manufacturing of positive electrode]
A positive electrode mixture slurry containing the positive electrode active material of each test number was produced. Specifically, a solution prepared by dissolving polyvinylidene fluoride (PVdF, product name # 1120 manufactured by Kureha) as a binder in N-methylpyrrolidone (NMP) is used as a powdered positive electrode active material and a conductive auxiliary agent. In addition to the carbon black of the above, a positive electrode mixture slurry was produced. The mass ratio was positive electrode active material: carbon black: PVdF = 92: 4: 4.

The produced positive electrode mixture slurry was coated on one side of an aluminum foil having a thickness of 18 μm by the doctor blade method. The aluminum foil coated with the slurry was dried at 100 ° C. for 20 minutes. The dried aluminum foil had a coating film made of a positive electrode active material film on the surface. An aluminum foil having a positive electrode active material film was punched to produce a disk-shaped aluminum foil having a diameter of 13 mm. The aluminum foil after the punching process was pressed to produce a plate-shaped positive electrode. The pressing was adjusted so that the electrode density was 2.6 g / cm ³ . Then, it was dried in vacuum at 100 ° C. for 20 minutes and put into a glove box having an argon atmosphere without being exposed to the atmosphere.

[Manufacturing of negative electrode]
The negative electrodes of test numbers 1 to 23 were prepared by cutting a lithium metal foil to a predetermined size.

[Manufacturing of coin-type non-water test cell]
A coin-shaped non-aqueous test cell was manufactured using the prepared positive electrode, negative electrode, electrolytic solution and separator. A non-aqueous solution was used as the electrolytic solution. The non-aqueous solution was prepared by dissolving LiPF ₆ as a supporting electrolyte in a mixed solvent of ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 1: 3 (volume ratio) so as to have a concentration of 1 M. A polyolefin separator was used as the separator.

[Initial characterization]
The initial discharge capacity and initial charge / discharge efficiency of the coin-type non-aqueous test cell of each test number were evaluated by the following methods.

The coin-type non-aqueous test cell was constantly charged with a current value of 0.2 mA / cm ² until the potential difference was 4.5 V with respect to the counter electrode. After that, while maintaining 4.5 V, charging was continued with respect to the counter electrode at a constant voltage until 0.02 mA / cm ² , and the charge capacity was measured.

Next, discharge was performed at a current value of 0.2 mA / cm ² until the potential difference became 3.0 V, and the discharge capacity was measured. The charge / discharge efficiency was calculated as (discharge capacity) / (charge capacity) × 100.

[High temperature durability characteristic evaluation]
High temperature durability characteristics were evaluated for the coin-type non-water test cells of each test number. As high temperature durability characteristics, high temperature storage characteristics and high temperature cycle characteristics were evaluated.

[High temperature storage characteristic evaluation]
As the high temperature storage characteristics, the capacity retention rate after high temperature storage and the high rate capacity retention rate after high temperature storage were measured. The high rate in this embodiment means charging at a high voltage.

[Capacity retention rate after high temperature storage]
Under the same conditions as the initial charge / discharge efficiency evaluation of the coin-type non-aqueous test cell of each test number, charging / discharging was repeated twice and stopped at the third charging. The coin cell was disassembled in a glove box having an argon atmosphere, and only the positive electrode was taken out. The positive electrode was inserted into an aluminum laminated sheet bag, and an electrolytic solution was newly added dropwise and sealed.

Then, it was left in a 60 ° C. chamber for 10 days. After being left to stand, the positive electrode was taken out from the aluminum laminate bag in the glove box in an argon atmosphere, and a coin-type non-aqueous test cell was prepared again. At this time, a new Li metal foil and an electrolytic solution were used. At a current value of 0.2 mA / cm ^2, the battery was discharged until the potential difference became 3.0 V, and the discharge capacity was measured. The capacity retention rate after high-temperature storage was calculated as (discharge capacity after storage) / (discharge capacity in the second cycle before storage) × 100.

[High rate capacity retention rate after high temperature storage]
Next, the coin-type non-aqueous test cell was charged with a constant current at a current value of 0.2 mA / cm ² until the potential difference became 4.5 V, and 0.02 mA / cm while maintaining 4.5 V. ^It was charged at a constant voltage until it reached ² . Further, it was discharged at a constant current of 0.2 mA / cm ² until it became 3.0 V. Thereafter, until the potential difference 4.5V, constant current charging at a current of 0.2 mA / cm ^2, further while maintaining 4.5V, and charged at a constant voltage until 0.02 mA / cm ^2. Next, the battery was discharged to 3.0 V at a constant current of 10 times the current value of ² mA / cm ² , and the discharge capacity was measured. The high-rate capacity retention rate after high-temperature storage was calculated as (discharge capacity at ² mA / cm ² after storage) / (discharge capacity at 0.2 mA / cm ² in the second cycle before storage).

[High temperature cycle characterization]
As a high temperature cycle characteristic evaluation, the capacity retention rate at the 50th cycle at high temperature was measured for the coin-type non-aqueous test cells of Test No. 1, Test No. 5 and Test No. 20 to Test No. 23.

The coin-type non-aqueous test cell was constantly charged with a current value of 0.2 mA / cm ² until the potential difference was 4.5 V with respect to the counter electrode. Then, while maintaining 4.5 V, charging to the counter electrode was continued at a constant voltage until the voltage reached 0.02 mA / cm ² . Next, discharge was performed with a constant current of 0.2 mA / cm ² until the potential difference became 3.0 V. Next, constant current charging / discharging at a current value of 1 mA / cm ² at 45 ° C. was repeated 50 times in a voltage range of 4.5 V to 3.0 V. The capacity retention rate after the high temperature cycle was calculated as (discharge capacity after 50 cycles at 45 ° C.) / (discharge capacity in the first cycle at 45 ° C.) × 100.

[Measurement result]
The results are shown in Table 2.

With reference to Tables 1 and 2, the methods for producing the coating layer additives and the positive electrode active material of the mother particles of test numbers 4 to 7, 12 to 16, 18, 22 and 23 were appropriate. As a result, the initial discharge capacity was 175 mAh / g or more, and the initial charge / discharge efficiency was 88.5% or more, showing high charge / discharge efficiency characteristics. Further, the capacity retention rate after high temperature storage was 67% or more and the high rate capacity retention rate was 30% or more, showing high high temperature storage characteristics.

Further, in Test No. 22 and Test No. 23, zirconium oxide particles were added in addition to the phosphorus compound raw material. Therefore, as compared with Test No. 5, the capacity retention rate at the 50th cycle was high, and the high temperature cycle characteristics were high.

Further, in Test No. 23, carbon black was added in addition to the phosphorus compound raw material and the zirconium oxide particles. Therefore, as compared with Test No. 5 and Test No. 22, the capacity retention rate at the 50th cycle was higher, and the high temperature cycle characteristics were even higher.

In Test No. 21, since the phosphorus compound raw material was not added, the high rate capacity retention rate after high temperature storage was about the same as the conventional one. However, due to the addition of zirconium oxide particles and carbon black, it exhibited high high temperature cycle characteristics.

On the other hand, Test No. 1 did not coat the mother particles. As a result, the initial charge / discharge efficiency was less than 88.5%, and the high-rate capacity retention rate after high-temperature storage was less than 30%.

In Test No. 2, the additive for the coating layer of the mother particles was correct, and although the mechanochemical treatment was performed, the heat treatment was not performed thereafter. Therefore, the initial charge / discharge efficiency was less than 88.5%.

In Test No. 3, the additive for the coating layer of the mother particles was correct, and although the mechanochemical treatment was also carried out, the heat treatment temperature was low. Therefore, the initial charge / discharge efficiency was less than 88.5%.

In Test No. 8, the additive for the coating layer of the mother particles was correct, and although the mechanochemical treatment was also carried out, the heat treatment temperature was high. Therefore, the initial charge / discharge efficiency was less than 88.5%. Furthermore, the high rate capacity retention rate after high temperature storage was less than 30%.

In Test No. 9, although the additive for the coating layer of the mother particles was correct, a wet treatment was carried out instead of a mechanochemical treatment. Therefore, the initial charge / discharge efficiency was less than 88.5%.

In Test No. 10, Li ₃ PO ₄ was used as an additive for the coating layer of the mother particles. Therefore, the initial charge / discharge efficiency was less than 88.5%. Furthermore, the high rate capacity retention rate after high temperature storage was less than 30%.

In Test No. 11, although the additive for the coating layer of the mother particles was correct, the mechanochemical treatment was not carried out, and the mixture was simply mixed. Therefore, the initial charge / discharge efficiency was less than 88.5%. Furthermore, the high rate capacity retention rate after high temperature storage was less than 30%.

Test number 17 did not coat the mother particles. As a result, the high rate capacity retention rate after high temperature storage was less than 30%.

In Test No. 19, although the additive for the coating layer of the mother particles was correct, a wet treatment was carried out instead of a mechanochemical treatment. Therefore, the capacity retention rate after high-temperature storage was less than 70%, and the high-rate capacity retention rate was less than 30%.

In Test No. 20, although the capacity retention rate was increased due to the addition of zirconium oxide particles, the capacity retention rate after high temperature storage was less than 70% and the high rate capacity retention rate was less than 30% because phosphorus compounds and carbon black were not added. It became.

The embodiments of the present invention have been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

Claims (6)

The mother particles of lithium transition metal composite oxide represented by the composition formula _{Li a Ni 1-xy Co x} Mn y M z O 2, and a phosphorus compound coating layer covering at least a portion of a surface of the mother particle A method for producing a positive electrode active material to be provided.
The process of preparing the mother particles and
A mechanochemical treatment was carried out by mixing a phosphorus compound raw material having a melting point of 500 ° C. or less and one or more selected from the group consisting of phosphorus oxide, phosphoric acid and phosphate, and the mother particles. , The process of producing composite particle intermediates,
A method for producing a positive electrode active material, comprising a step of performing a heat treatment at 250 to 600 ° C. on the composite particle intermediate.
Here, a, x, y and z in the composition formula are 0.95 ≦ a ≦ 1.20, 0.09 ≦ x ≦ 0.50, 0.09 ≦ y ≦ 0.50, 0.00. ≦ z ≦ 0.02 and 0.18 ≦ x + y ≦ 0.70, and M in the composition formula is Zr, Nb, W, Ce, Ti, Si, Mo, Al, Ca, Sr, B, One or more selected from the group consisting of Mg, Fe and Zn. The method for producing a positive electrode active material according to claim 1.
A method for producing a positive electrode active material, wherein the average particle size of the mother particles is 1 to 30 μm. The method for producing a positive electrode active material according to claim 1 or 2.
A method for producing a positive electrode active material, wherein the amount of the phosphorus compound raw material added to the mother particles in the step of producing the composite particle intermediate is 0.2 to 2.0 mol% as a phosphorus element. The method for producing a positive electrode active material according to any one of claims 1 to 3.
A method for producing a positive electrode active material, in which the mother particles, the phosphorus compound raw material, and zirconium oxide particles are mixed in the step of producing the composite particle intermediate. The method for producing a positive electrode active material according to claim 4.
A method for producing a positive electrode active material, in which carbon black is further mixed in the step of producing the composite particle intermediate. A mother particle made of a lithium transition metal composite oxide represented by the composition formula Li _a Ni _1-xy Co _x M n _y M _z O ₂ and a zirconium oxide layer covering at least a part of the surface of the mother particle are provided. A method for producing a positive electrode active material,
The process of preparing the mother particles and
A step of mixing the mother particles, zirconium oxide, and carbon black and performing a mechanochemical treatment to produce a composite particle intermediate.
A method for producing a positive electrode active material, comprising a step of performing a heat treatment at 250 to 600 ° C. on the composite particle intermediate.
Here, a, x, y and z in the composition formula are 0.95 ≦ a ≦ 1.20, 0.09 ≦ x ≦ 0.50, 0.09 ≦ y ≦ 0.50, 0.00. ≦ z ≦ 0.02 and 0.18 ≦ x + y ≦ 0.70, and M in the composition formula is Zr, Nb, W, Ce, Ti, Si, Mo, Al, Ca, Sr, B, One or more selected from the group consisting of Mg, Fe and Zn.