JP7765028B2 - Coated positive electrode active material and method for producing same - Google Patents

Description

The present invention relates to a coated positive electrode active material for lithium ion secondary batteries and a method for manufacturing the same. More specifically, the present invention relates to a coated positive electrode active material for lithium ion secondary batteries that suppresses gas generation during operation at high potentials and a method for manufacturing the same.

Research and development of lithium-ion secondary batteries is being actively conducted for use in portable devices, hybrid vehicles, electric vehicles, and home energy storage. Lithium-ion secondary batteries used in these fields require high safety, long-term cycle stability, and high energy density.

In recent years, lithium-ion secondary batteries using lithium titanate as the negative electrode active material have been proposed from the perspectives of high safety and long-term cycle stability. Because the operating potential of lithium titanate is higher than that of common negative electrode active materials such as graphite, lithium deposition is less likely to occur, improving safety, but this is disadvantageous from the perspective of energy density. Meanwhile, materials have been proposed for the positive electrode active material that operate at a high potential of 4.5 V or more relative to the deposition potential of Li (see, for example, Patent Document 1).

The reduction in energy density caused by the high operating potential of lithium titanate is expected to be improved by combining lithium titanate with a positive electrode active material that operates at a high potential, as shown in the aforementioned Patent Document 1. Meanwhile, even in conventional lithium-ion secondary batteries that use graphite as the negative electrode active material, gas is generated due to the oxidative decomposition of the non-aqueous electrolyte on the surface of the positive electrode active material. However, in secondary batteries with positive electrode active materials that have a higher operating potential than conventional secondary batteries, the problem of gas generation becomes more pronounced.

As such, methods have been reported, such as in Patent Document 2, in which a coating layer is formed on the surface of a positive electrode material that operates at a high potential to suppress gas generation. However, gas generation was still confirmed due to factors such as the large particle size of the coating material, and there was room for improvement.

Japanese Patent Application Laid-Open No. 2001-185148 International Publication No. 2020/049843

The object of the present invention is to provide a positive electrode active material that further suppresses gas generation in a lithium-ion secondary battery that uses a positive electrode active material that operates at a high potential.

In light of the above circumstances, the inventors have continued to investigate means for suppressing the aforementioned gas generation. As a result, they have discovered that gas generation can be effectively suppressed for a coated positive electrode active material in which the positive electrode active material is coated with a lithium ion conductive oxide, if the positive electrode active material has an operating voltage of 4.5 V or higher, which is the standard dissolution and deposition potential of lithium metal, and the lithium ion conductive oxide is amorphous at temperatures below 600°C, and is a transparent sol that has been pulverized to a particle size of 10 nm or less as measured by small-angle X-ray scattering. This discovery led to the completion of the present invention.

The present invention also relates to a transparent sol comprising an alcohol and a compound represented by the following formula (A):
Li _1+p+q (Al, Ga) _p (Ti, Ge) _2-p Si _q P _3-q O ₁₂ ...(A)
(In formula (A), p and q satisfy 0≦p≦1 and 0≦q≦1, respectively.)
and a pulverized product of a lithium ion conductive oxide represented by the formula (I) above, and the content of the pulverized product is preferably 15 to 30 wt % based on the total weight.
Furthermore, the positive electrode active material is a compound represented by the following formula (1):
Li _1+x M _y Mn _2-x-y O ₄ ...(1)
(In formula (1), x and y satisfy 0≦x≦0.2 and 0<y≦0.8, respectively, and M is at least one element selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.)
It is preferable that the lithium manganese compound is a substituted lithium manganese compound represented by the following formula:
In another aspect, the present invention relates to a method for producing a coated positive electrode active material, the method comprising the steps of: pulverizing a lithium ion conductive oxide until the particle size measured by small-angle X-ray scattering is 10 nm or less; adding the pulverized lithium ion conductive oxide powder to alcohol to form a transparent sol; coating a positive electrode active material with the transparent sol to which the lithium ion conductive oxide powder has been added by a mechanochemical method; and heat-treating the positive electrode active material coated with the transparent sol at 100°C or higher and 500°C or lower, wherein in the step of forming the transparent sol, the lithium ion conductive oxide powder is added in an amount of 15 to 30 wt% based on the total weight of the transparent sol.
In the above production method, the lithium ion conductive oxide is represented by the following formula (A):
Li _1+p+q (Al, Ga) _p (Ti, Ge) _2-p Si _q P _3-q O ₁₂ ...(A)
(In formula (A), p and q satisfy 0≦p≦1 and 0≦q≦1, respectively.)
It is preferable that the formula be represented by the following formula:
In addition, the method for producing a positive electrode active material is also preferable, in which the positive electrode active material is a substituted lithium manganese compound represented by the following formula (1).
Li _1+x M _y Mn _2-x-y O ₄ ...(1)
(In formula (1), x and y satisfy 0≦x≦0.2 and 0<y≦0.8, respectively, and M is at least one element selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.)

The present invention provides a positive electrode active material that operates at a high potential and can suppress the generation of gas due to oxidative decomposition of the non-aqueous electrolyte.

One embodiment of the present invention is described below, but the present invention is not limited to this.

The coated positive electrode active material of the present invention is a coated positive electrode active material in which a positive electrode active material is coated with a lithium ion conductive oxide,
The positive electrode active material is a lithium conductive active material having an operating voltage of 4.5 V or more based on the dissolution and deposition potential of lithium metal,
The lithium ion conductive oxide is amorphous at temperatures below 600° C. and is a transparent sol that has been pulverized to a particle size of 10 nm or less as measured by small-angle X-ray scattering.
The coated positive electrode active material may be, for example,
a step of pulverizing the lithium ion conductive oxide until the particle size measured by small angle X-ray scattering is 10 nm or less;
adding the pulverized lithium ion conductive oxide powder to alcohol to form a transparent sol;
a step of coating the transparent sol to which the lithium ion conductive oxide powder has been added onto a positive electrode active material by a mechanochemical method;
and heat-treating the positive electrode active material coated with the transparent sol at 100° C. or higher and 500° C. or lower,
In the step of forming the transparent sol, the powder of the lithium ion conductive oxide is added in an amount of 15 to 30% by weight based on the total weight of the transparent sol.

Lithium-ion secondary batteries generally use non-aqueous electrolytes. (Details will be provided later.) These are liquid non-aqueous electrolytes, consisting of a lithium salt dissolved in a non-aqueous solvent. On the other hand, there are solid electrolytes that combine the functions of both a non-aqueous solvent and a lithium salt. Solid electrolytes have higher oxidation resistance than liquid non-aqueous electrolytes, suppressing oxidative decomposition at high potentials. However, because lithium ion conductivity is lower in solids than in liquids, replacing the entire electrolyte with a solid electrolyte significantly reduces battery performance.

Therefore, by covering only the surface of the high-potential positive electrode active material with a lithium ion conductive oxide, which is a solid electrolyte, gas generation can be suppressed even if the non-aqueous electrolyte remains the same.
The coating method is not particularly limited, but a method capable of achieving uniform coating, such as mechanical coating, is preferred. Mechanical coating is preferred because, in the present invention, a lithium ion conductive oxide in a transparent sol state is used, and the solvent provides adhesiveness and ductility, enabling uniform coating. Furthermore, when volatilizing the solvent by heat treatment, the heat treatment temperature is adjusted, and preferably the particle size of the lithium ion conductive oxide and the mixing ratio with the positive electrode active material are controlled, so that the lithium ion conductive oxide can be coated on the positive electrode active material without increasing the resistance of the positive electrode active material or reducing battery performance.
The lithium ion conductive oxide used in the present invention is in a transparent sol state, but a certain amount of energy is required to coat the solid positive electrode active material. Therefore, a mechanical coating method using a mechanochemical method that can apply shearing force and compressive force is preferred. By coating the positive electrode active material with a solid electrolyte, contact between the conventional nonaqueous electrolyte and the positive electrode active material can be reduced, and gas generation can be suppressed.
Furthermore, as will be described in detail later, by adjusting the heat treatment temperature and preferably controlling the particle size of the lithium ion conductive oxide and the mixing ratio with the positive electrode active material, it is possible to coat the positive electrode active material with the lithium ion conductive oxide without increasing the resistance of the positive electrode active material or reducing the battery performance.

<Mechanical coating method>
Mechanical coating refers to a method of applying at least one type of energy, selected from shear force, compression force, impact force, and centrifugal force, to a base material and/or coating agent while mechanically contacting the base material and coating agent, mixing the base material and coating agent, and coating the surface of the base material with the coating agent. In the present invention, the positive electrode active material corresponds to the base material, and the coating agent corresponds to the lithium ion conductive oxide in a transparent sol state. The apparatus used is not particularly limited, but examples include a grinding mill, such as the Nobilta manufactured by Hosokawa Micron Corporation, and a planetary ball mill (e.g., manufactured by Fritsch). Among these, a grinding mill is preferred from the viewpoints of simple operation and the absence of the need to separate the balls after processing, as in a ball mill.

In the manufacturing method of the present invention, it is preferable to provide a bottomed cylindrical container and a rotor with tip blades, provide a predetermined clearance between the tip blades and the inner periphery of the container, and rotate the rotor to apply compressive and shear forces to the mixture containing the positive electrode active material and the lithium ion conductive oxide in the transparent sol state, thereby performing mechanical coating.

When wet mechanical coating is used, the solvent used is not particularly limited, and water or an organic solvent can be used. Examples of organic solvents that can be used include alcohols such as ethanol. There are no particular limitations on the timing of adding the solvent in wet coating, but a transparent sol of lithium ion conductive oxide can be diluted with a solvent before use in the mechanical coating process. The concentration of the transparent sol of lithium ion conductive oxide in the slurry is not particularly limited, but is, for example, 10 to 30 wt %, preferably 15 to 30 wt %, and more preferably 15 to 25 wt %, based on the total weight.

The mechanical coating treatment temperature is preferably 5 to 100°C, more preferably 8 to 80°C, and even more preferably 10 to 50°C, and the treatment time is preferably 5 to 90 minutes, more preferably 10 to 60 minutes. The treatment atmosphere is not particularly limited, and can be an inert gas atmosphere or an air atmosphere.

While the sample after mechanical coating can be used as is, it is preferable to perform heat treatment. This improves the adhesion between the positive electrode active material and the lithium ion conductive oxide, preventing the lithium ion conductive oxide from peeling off from the positive electrode active material even during repeated charge and discharge, improving the long-term reliability of the battery. If the heat treatment temperature is too high, the crystal structure of the lithium ion conductive oxide may change, reducing Li ion conductivity and preventing normal battery charge and discharge. Therefore, the heat treatment temperature is preferably 500°C or less, and more preferably 100°C or more and 500°C or less. The heat treatment time is preferably 30 minutes or more, more preferably 1 hour or more. There is no upper limit, but it is, for example, 3 hours or less.

<Cathode active material>
The positive electrode active material used in the manufacturing method of the present invention is not particularly limited, but the average potential of lithium deintercalation and intercalation relative to Li ⁺ /Li, i.e., the deposition potential of Li (sometimes expressed as vs. Li ⁺ /Li), is preferably 4.5 V or more, more preferably 4.5 V or more and 5.0 V or less. The potential (hereinafter also referred to as voltage) of the lithium ion intercalation and intercalation reaction (vs. Li ⁺ /Li) can be determined, for example, by measuring the charge/discharge characteristics of a half cell using a working electrode using the positive electrode active material and lithium metal as the counter electrode, and reading the voltage values at the start and end of the plateau. When there are two or more plateaus, it is sufficient that the plateau with the lowest voltage value is 4.5 V (vs. Li ⁺ /Li) or more, and the plateau with the highest voltage value is 5.0 V (vs. Li ⁺ /Li) or less.

There are no particular limitations on the positive electrode active material in which the lithium ion insertion/extraction reaction proceeds at a potential between 4.5 V and 5.0 V relative to the Li deposition potential, but substituted lithium manganese compounds represented by the following formula (1) have been studied and are preferred.

Li _1+x M _y Mn _2-x-y O ₄ ...(1)
In the formula (1), x and y satisfy the conditions 0≦x≦0.2 and 0<y≦0.8, respectively, and M is at least one element selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.

Among the compounds of the formula (1), Ni-substituted lithium manganese compounds in which M is Ni are preferred, and in particular, LiNi _0.5 Mn _1.5 O ₄ in which x=0, y=0.5, and M=Ni is particularly preferred because of its high charge-discharge cycle stability.

There are no particular restrictions on the particle size of the positive electrode active material, but if the particle size is too small, the difference with the particle size of the oxide-based solid electrolyte (described below) will be small, making coating difficult. Therefore, the median diameter d50 is preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more. Furthermore, the median diameter d50 is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 50 μm or less. Taking into account the thickness range when processing into an electrode, the d50 is preferably 10 to 50 μm, and more preferably 20 to 50 μm.

<Lithium ion conductive oxide in a transparent sol state>
The lithium ion conductive oxide in a transparent sol state used in the present invention is not particularly limited, but it is preferable to use an oxide-based solid electrolyte in consideration of chemical stability. Oxide-based solid electrolytes include antifluorite type, NASICON type, perovskite type, garnet type, etc. according to their crystal structure, but are not particularly limited. Examples of oxide-based solid electrolytes include those of the following formula (A):
Li _1+p+q (Al, Ga) _p (Ti, Ge) _2-p Si _q P _3-q O ₁₂ ...(A)
(In formula (A), p and q satisfy 0≦p≦1 and 0≦q≦1, respectively.)
LATP represented by the following formula (A1):
Li _1+p Al _p Ti _2-p P ₃ O ₁₂ ...(A1)
(In formula (A1), p satisfies 0≦p≦1.)
An oxide-based solid electrolyte represented by the following formula is preferred.

The lithium ion conductive oxide in the transparent sol state is pulverized until it becomes amorphous and transparent, so that it inevitably becomes fine particles. Specifically, it is desirable to microparticle the BET specific surface area equivalent diameter (dBET) to preferably 10 nm or less, more preferably 8 nm or less, and even more preferably 6 nm or less.
As a method for microparticulation treatment, known means such as a ball mill, a bead mill, etc. The BET specific surface area equivalent diameter (dBET) is a particle size calculated by determining the nitrogen adsorption BET specific surface area by the nitrogen adsorption single-point method in accordance with the method specified in JIS-Z8830 (2013), and then calculating the particle size from the formula dBET = 6/(density × BET specific surface area).

The ratio of the median diameter d50 of the positive electrode active material to the BET specific surface area equivalent diameter dBET of the lithium ion conductive oxide in the transparent sol state is preferably 10,000:1 to 100:1, more preferably 5,000:1 to 300:1, even more preferably 2,000:1 to 500:1, and particularly preferably 1,000:1 to 500:1.

The ratio of the lithium ion conductive oxide in a transparent sol state (solid content when used as a slurry) to 100 parts by mass of the positive electrode active material is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, even more preferably 2 parts by mass or more, and is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 4 parts by mass or less. The ratio is preferably 1 part by mass or more and 5 parts by mass or less (i.e., the mass ratio of the positive electrode active material to the lithium ion conductive oxide in a transparent sol state is 100:1 to 20:1), and also preferably 2 parts by mass or more and 4 parts by mass or less (i.e., the mass ratio of the positive electrode active material to the lithium ion conductive oxide in a transparent sol state is 50:1 to 25:1).

<Lithium-ion secondary battery>
Lithium-ion secondary batteries are primarily composed of a positive electrode, a negative electrode, and a nonaqueous electrolyte. The positive electrode is fabricated, for example, by applying a positive electrode mixture containing a positive electrode active material, a conductive additive, a binder, etc. to a positive electrode current collector. The negative electrode is fabricated, for example, by applying a negative electrode mixture containing a negative electrode active material, a conductive additive, a binder, etc. to a negative electrode current collector. The coated positive electrode active material obtained by the manufacturing method of the present invention is suitable for use as the positive electrode active material of a lithium-ion secondary battery. Specifically, a positive electrode can be fabricated by applying a positive electrode mixture containing the coated positive electrode active material obtained by the manufacturing method of the present invention to a positive electrode current collector. After applying the positive electrode mixture to the positive electrode current collector and the negative electrode mixture to the negative electrode current collector, they can be dried at approximately 100 to 200°C.

The configuration of a lithium-ion secondary battery using the coated positive electrode active material, the materials used other than the coated positive electrode active material, and the manufacturing equipment and conditions for the lithium-ion secondary battery may be conventionally known and are not particularly limited.

<Negative electrode active material>
As described above, lithium titanate is preferably used as the negative electrode active material because it is less susceptible to lithium deposition and improves safety. Among lithium titanates, lithium titanate with a spinel structure is particularly preferred because it exhibits small expansion and contraction of the active material during the lithium ion insertion and desorption reactions. Lithium titanate may contain trace amounts of elements other than lithium and titanium, such as Nb.

<Conductive additive>
The conductive additive is not particularly limited, but is preferably, for example, a carbon material for both the positive electrode and the negative electrode. Examples of carbon materials include natural graphite, artificial graphite, vapor-grown carbon fiber, carbon nanotubes, acetylene black, ketjen black, and furnace black. These carbon materials may be used alone or in combination. The amount of conductive additive contained in the positive electrode is preferably 1 part by weight to 30 parts by weight, more preferably 2 parts by weight to 15 parts by weight, per 100 parts by weight of the positive electrode active material. Within this range, the conductivity of the positive electrode is ensured. Furthermore, adhesion with the binder described below is maintained, and sufficient adhesion with the current collector can be obtained. The amount of conductive additive contained in the negative electrode is preferably 1 part by weight to 30 parts by weight, more preferably 2 parts by weight to 15 parts by weight, per 100 parts by weight of the negative electrode active material.

<Binder>
The binder is not particularly limited, but for both the positive electrode and the negative electrode, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof can be used. The binder is preferably dissolved or dispersed in a non-aqueous solvent or water for ease of preparation of the positive electrode and the negative electrode. The non-aqueous solvent is not particularly limited, but examples include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate, and tetrahydrofuran. A dispersant or thickener may be added to these. The amount of binder contained in the positive electrode of the present invention is preferably 1 part by weight to 30 parts by weight, more preferably 2 parts by weight to 15 parts by weight, per 100 parts by weight of the positive electrode active material. Within the above range, adhesion between the positive electrode active material and the conductive additive is maintained, and sufficient adhesion to the current collector can be obtained. The amount of binder contained in the negative electrode is preferably 1 part by weight to 30 parts by weight, more preferably 2 parts by weight to 15 parts by weight, per 100 parts by weight of the negative electrode active material.

<Current collector>
The positive electrode current collector and the negative electrode current collector are not particularly limited, but are preferably aluminum or an aluminum alloy. Since aluminum or an aluminum alloy is stable under the positive and negative electrode reaction atmosphere, it is not particularly limited, but high-purity aluminum such as JIS standards 1030, 1050, 1085, 1N90, and 1N99 is preferred. The thickness of the current collector is not particularly limited, but is preferably 10 μm or more and 100 μm or less. Within this range, it is easy to balance the handling during battery production, cost, and obtained battery characteristics. It is also possible to use current collectors in which the surface of a metal other than aluminum (copper, SUS, nickel, titanium, and alloys thereof) is coated with a metal that does not react at the positive and negative electrode potentials.

<Non-aqueous electrolyte>
The non-aqueous electrolyte is not particularly limited, but examples thereof include a non-aqueous electrolytic solution in which a solute is dissolved in a non-aqueous solvent, and a gel electrolyte in which a polymer is impregnated with a non-aqueous electrolytic solution in which a solute is dissolved in a non-aqueous solvent.

The non-aqueous solvent preferably includes, for example, a cyclic aprotic solvent and/or a chain aprotic solvent. Examples of cyclic aprotic solvents include cyclic carbonates, cyclic esters, cyclic sulfones, and cyclic ethers. Chain aprotic solvents may include solvents commonly used as solvents for non-aqueous electrolytes, such as chain carbonates, chain carboxylic acid esters, chain ethers, and acetonitrile. More specifically, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, sulfolane, dioxolane, and methyl propionate. These solvents may be used alone or in combination; however, a mixture of two or more solvents is preferred due to the ease of dissolving the solute (described below) and the high lithium ion conductivity.

When mixing two or more types, due to their high stability at high temperatures and high lithium conductivity at low temperatures, it is preferable to mix one or more chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, and methyl propyl carbonate with one or more cyclic compounds such as ethylene carbonate, propylene carbonate, butylene carbonate, and γ-butyrolactone. It is particularly preferable to mix one or more chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate with one or more cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate.

The solute is not particularly limited, but examples thereof include _LiClO4 , _LiBF4 , _LiPF6 , _LiAsF6 , _LiCF3SO3 , LiBOB (Lithium Bis(Oxalato)Borate), and LiN( _SO2CF3 ) ₂ , which are preferred because they are easily dissolved in the solvent. _The concentration of the solute contained in the non-aqueous _electrolyte is preferably 0.5 mol/L or more and 2.0 mol/L or less. When the concentration is 0.5 mol/L or more, the desired lithium ion conductivity is easily exhibited, while when the concentration is 2.0 mol/L or less, the solute is easily dissolved.

The amount of nonaqueous electrolyte used in the lithium-ion secondary battery of the present invention is not particularly limited, but is preferably 0.1 mL or more per 1 Ah of battery capacity. This amount ensures the conduction of lithium ions involved in the electrode reaction, resulting in the desired battery performance.

The non-aqueous electrolyte may be pre-mixed with the positive electrode, negative electrode, and separator, or may be added after the separator is placed between the positive electrode and negative electrode and then wound or stacked.

In addition to the above components, lithium-ion secondary batteries typically also include a separator and an exterior material.

(separator)
The separator may be disposed between the positive electrode and the negative electrode, and may have any structure as long as it is insulating and capable of containing the non-aqueous electrolyte described below, and examples thereof include woven fabrics, nonwoven fabrics, microporous membranes, etc. made of nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and composites of two or more of these. Nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and nonwoven fabrics made of composites of two or more of these are preferred because they provide excellent stability in cycle characteristics.

The separator may contain various plasticizers, antioxidants, and flame retardants, and may be coated with metal oxides, etc. There are no particular restrictions on the thickness of the separator, but it is preferable that it be between 10 μm and 100 μm. Within this range, it is possible to prevent short-circuiting between the positive and negative electrodes while suppressing an increase in battery resistance. From the standpoints of economy and ease of handling, it is even more preferable that it be between 15 μm and 50 μm.

The porosity of the separator is preferably 30% or more and 90% or less. If it is 30% or more, the diffusibility of lithium ions is less likely to decrease, making it easier to maintain cycle characteristics. On the other hand, if it is 90% or less, the risk of electrode irregularities penetrating the separator and causing a short circuit can be effectively reduced. From the perspective of balancing the diffusibility of lithium ions and the prevention of short circuits, a porosity of 35% or more and 85% or less is more preferable, and a porosity of 40% or more and 80% or less is particularly preferable as this balance is particularly excellent.

(exterior material)
The exterior packaging material is a member that encloses a laminate formed by alternately stacking or winding positive electrodes, negative electrodes, and separators, as well as terminals that electrically connect the laminate. Suitable exterior packaging materials include composite films in which a thermoplastic resin layer for heat sealing is provided on a metal foil, metal layers formed by vapor deposition or sputtering, and metal cans in the shape of a square, oval, cylinder, coin, button, or sheet.

The present invention will be explained in more detail below using examples. The present invention is not limited to the following examples, and it is of course possible to implement the invention with appropriate modifications within the scope of the above-mentioned and below-mentioned aims, and all such modifications are within the technical scope of the present invention.

Lithium-ion secondary batteries fabricated using the coated positive electrodes obtained in the following examples and comparative examples were evaluated using the following methods.

(gas generation amount)
The amount of gas generated from the lithium ion secondary battery before and after the cycle performance evaluation was evaluated using the Archimedes method, that is, the buoyancy of the lithium ion secondary battery. The evaluation was performed as follows.

First, the weight of the lithium ion secondary battery was measured using an electronic balance. Next, the weight in water was measured using a hydrometer (manufactured by Alpha Mirage, product number: MDS-3000), and the buoyancy was calculated by taking the difference between these weights. This buoyancy was calculated based on the density of water (1.0 g/cm
The volume of the lithium ion secondary battery was calculated by dividing the volume by the volume of the battery after aging. The volume after aging and the volume after cycle performance evaluation were compared to calculate the amount of gas generated. A gas generation amount of ^less than 20 ml was considered to be good. A gas generation amount of 15 ml or less is more preferable.

(Evaluation of cycle characteristics of lithium-ion secondary batteries)
The fabricated lithium-ion secondary battery was connected to a charge/discharge device (HJ1005SD8, manufactured by Hokuto Denko Corporation) and cycled. Constant-current charging was performed at a current value equivalent to 1.0 C in a 60°C environment until the battery voltage reached a cut-off voltage of 3.4 V, and charging was then stopped. Subsequently, constant-current discharging was performed at a current value equivalent to 1.0 C, and discharging was stopped when the battery voltage reached 2.5 V. This constituted one cycle, and charging and discharging were repeated. The stability of the cycle characteristics was evaluated as the discharge capacity retention rate (%), where the discharge capacity at the 500th discharge was defined as 100. A discharge capacity retention rate of 80% or more at the 500th discharge was rated as good, and a rate of less than 80% was rated as poor.

Synthesis Example 1 Preparation of Lithium Ion Conductive Oxide Li _1.3 A _10.3 Ti _1.7 (PO ₄ ) ₃ (hereinafter also referred to as LATP) was prepared as a lithium ion conductive oxide. As starting materials, Li ₂ CO ₃ , AlPO ₄ , TiO ₂ , NH ₄ H ₂ PO ₄ , and a solvent of ethanol were mixed in predetermined amounts, and then subjected to a planetary ball mill treatment at 150 G for 1 hour using zirconia balls with a diameter of 3 mm. After removing the zirconia balls from the mixture after the treatment with a sieve, the mixture was dried at 120 ° C to remove the ethanol. Then, the mixture was treated at 800 ° C for 2 hours to obtain LATP powder.

(Preparation of coated positive electrode active material)
Example 1
(i) Pulverization of lithium ion conductive oxide and addition to alcohol. The obtained LATP powder was mixed with a predetermined amount of ethanol as a solvent, and treated in a planetary ball mill using zirconia balls with a diameter of 0.5 mm for 3 to 6 hours. After the treatment, the zirconia balls were removed with a sieve, and ethanol was added to adjust the LATP solid content to 16.4 wt%. This resulted in LATP in a transparent sol state with a dBET of 3 to 10 nm and no peaks derived from LATP in X-ray crystal diffraction measurements.

(ii) Coating Treatment and Heat Treatment As the positive electrode active material, spinel-type lithium nickel manganese oxide (LiNi _0.5 Mn _1.5 O ₄ , hereinafter also referred to as LNMO) having a median diameter of 20 μm was used.

40 g of LNMO was placed in a grinding mill (Hosokawa Micron Corporation, Nobilta) and rotated at 2600 rpm with a clearance of 0.6 mm and a rotor load power of 1.5 kW. 6.1 g of LATP, which had been ground for 3 hours and had been in a transparent sol state, was then added in two batches. The rotor rotation speed was then maintained between 2600 and 3000 rpm, and the mixture was treated at room temperature in an air atmosphere for 10 minutes to obtain LNMO with its surface coated with LATP. The resulting surface-coated LNMO was then heat-treated at 350°C for 1 hour to obtain a coated positive electrode active material.

Example 2
A coated positive electrode active material was produced in the same manner as in Example 1, except that the amount of LATP coated in a transparent sol state was changed to 9.1 g.

Comparative Example 1
In preparing the coated positive electrode active material, the particle size of the LATP was the same, but the surface-coated LNMO obtained was heat-treated at 600°C for 1 hour. The same operation as in Example 1 was carried out to prepare the coated positive electrode active material.

Comparative Example 2
In preparing the coated positive electrode active material, the particle size of the LATP was the same, but the surface-coated LNMO obtained was heat-treated at 50°C for 1 hour. The same operation as in Example 1 was carried out to prepare the coated positive electrode active material.

Comparative Example 3
A coated positive electrode active material was prepared by carrying out the same operation as in Example 1, except that in the grinding of the lithium ion conductive oxide, the grinding time of LATP was set to 1 hour and that particles having a particle diameter of about 20 nm and having a peak derived from LATP in X-ray crystal diffraction measurement were used.

Comparative Example 4
A coated positive electrode active material was prepared by carrying out the same operations as in Example 1, except that in the grinding of the lithium ion conductive oxide, the grinding time of LATP was set to 2 hours, and that the particle diameter of the LATP was about 15 nm, which did not have a peak derived from LATP in the X-ray crystal diffraction measurement but was not in a transparent sol state, was used.

Comparative Example 5
A positive electrode active material was prepared in the same manner as in Example 1, except that LNMO without surface coating was used.

Positive electrode mixtures, positive electrodes, negative electrodes, and lithium-ion secondary batteries were prepared using the positive electrode active materials obtained in Examples 1 and 2 and Comparative Examples 1 to 5 according to the procedures described below. The gas generation rate and cycle characteristics of the lithium-ion secondary batteries were evaluated according to the evaluation methods described above. The evaluation results are shown in Table 1.

(Preparation of Positive Electrode)
A mixture containing the obtained surface-coated LNMO, acetylene black as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder in solid concentrations of 90 parts by weight, 6 parts by weight, and 4 parts by weight, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode mixture. The binder was prepared as an N-methyl-2-pyrrolidone (NMP) solution with a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity to facilitate coating, as described below.

The positive electrode mixture was applied to a 20 μm thick aluminum foil and then dried in an oven at 120°C. This process was repeated on both sides of the aluminum foil, and the foil was then further dried in a vacuum at 170°C to produce a positive electrode.

(Preparation of negative electrode)
Spinel-type lithium titanate (Li ₄ Ti ₅ O ₁₂ , hereinafter also referred to as LTO) was used as the negative electrode active material. A mixture containing the LTO, acetylene black as a conductive additive, and PVdF as a binder in solid concentrations of 100 parts by weight, 5 parts by weight, and 5 parts by weight, respectively, was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode mixture. The binder was prepared as an NMP solution with a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity to facilitate coating, as described below.

The negative electrode mixture was applied to a 20 μm thick aluminum foil and then dried in an oven at 120°C. This process was repeated on both sides of the aluminum foil, and the foil was then further dried in a vacuum at 170°C to produce a negative electrode.

(Fabrication of lithium ion secondary battery)
A battery was fabricated using the positive and negative electrodes fabricated above and a 20 μm polypropylene separator according to the following procedure. First, the positive and negative electrodes were dried under reduced pressure at 80°C for 12 hours. Next, 15 positive electrodes and 16 negative electrodes were stacked in the order of negative electrode/separator/positive electrode. Both outermost layers were separators. Next, aluminum tabs were vibration-welded to both ends of the positive and negative electrodes.

Two aluminum laminate films were prepared as exterior materials, and a depression for the battery section and a depression for the gas collection section were formed by pressing. The electrode laminate was then inserted. The outer periphery, leaving a space for non-aqueous electrolyte injection, was heat-sealed at 180 ° C for 7 seconds. A non-aqueous electrolyte prepared by dissolving _LiPF6 at a concentration of 1 mol/L in a solvent containing ethylene carbonate, propylene carbonate, and ethyl methyl carbonate in a volume ratio of 15/15/70 was then introduced from the unsealed portion. The unsealed portion was then heat-sealed at 180 ° C for 7 seconds while reducing the pressure. The resulting battery was charged at a constant current equivalent to 0.2 C until the battery voltage reached a cut-off voltage of 3.4 V, and the charging was then stopped. After that, the battery was left to stand in an environment of 60°C for 24 hours, and then constant current discharge was performed at a current value equivalent to 0.2 C, and discharge was stopped when the battery voltage reached 2.5 V. After discharging was stopped, the gas accumulated in the gas collection section was removed and the battery was resealed. Through the above operations, a lithium ion secondary battery for evaluation was produced.

The lithium-ion secondary batteries of Examples 1 and 2 generated a small amount of gas during cycle performance evaluation and had a high capacity retention rate.

On the other hand, although the particle size was equivalent to Examples 1 and 2, Comparative Example 1, in which the heat treatment temperature was 600°C, produced a large amount of gas and had a low capacity retention rate. This is thought to be due to recrystallization of LATP.

Furthermore, although the particle size was similar to Examples 1 and 2, Comparative Example 2, in which the heat treatment temperature was set to 50°C, also generated a large amount of gas and had a low capacity retention rate. This is thought to be due to residual solvent remaining during spray coating.

Comparative Example 3, in which the LATP grinding time was 1 hour and the X-ray crystal diffraction measurement showed a peak derived from LATP, also produced a large amount of gas and had a low capacity retention rate.

The LATP was crushed for 2 hours, and although X-ray crystal diffraction measurements showed no peaks attributable to LATP, Comparative Example 4, which was not in a sol state, also produced a large amount of gas and had a low capacity retention rate.

These results demonstrate that lithium-ion secondary batteries using a positive electrode active material that operates at high potential and has its surface coated with a lithium-ion conductive oxide that has been pulverized until it becomes an amorphous, transparent sol have low gas generation even when charged and discharged at high potential, and also have good cycle characteristics.

The coated positive electrode active material of the present invention is suitably used as a positive electrode active material for lithium ion secondary batteries.

Claims (3)

A coated positive electrode active material in which the positive electrode active material is coated with a lithium ion conductive oxide,
The positive electrode active material is a lithium conductive active material having an operating voltage of 4.5 V or more based on the dissolution and deposition potential of lithium metal,
The coated positive electrode active material is characterized in that the lithium ion conductive oxide is amorphous and is in the form of fine particles that have been pulverized to have a particle diameter of 3 nm or less as calculated by BET specific surface area. The fine particles are represented by the following formula (A):
Li _1+p+q (Al, Ga) _p (Ti, Ge) _2-p Si _q P _3-q O ₁₂ ...(A)
(In formula (A), p and q satisfy 0≦p≦1 and 0≦q≦1, respectively.)
The coated positive electrode active material according to claim 1 , comprising a pulverized lithium ion conductive oxide represented by the formula: The positive electrode active material is a compound represented by the following formula (1):
Li _1+x M _y Mn _2-x-y O ₄ ...(1)
(In formula (1), x and y satisfy 0≦x≦0.2 and 0<y≦0.8, respectively, and M is at least one element selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.)
3. The coated positive electrode active material according to claim 1, which is a substituted lithium manganese compound represented by the formula: