JP2022154600A - Anode active material for lithium ion secondary battery, and manufacturing method thereof - Google Patents

Abstract

To provide an anode active material capable of suppressing gas generation owing to reductive decomposition of a nonaqueous electrolyte solution in a lithium ion secondary battery arranged by use of a cathode active material operating at a high electric potential, and a manufacturing method thereof.SOLUTION: An anode active material for a lithium ion secondary battery is to be used in combination with a cathode active material operating at a high electric potential, of which the standard electrode potential of lithium metal is 4.5 V or higher. The anode active material is coated with a lithium ion-conducting solid electrolyte. The lithium ion-conducting oxide is amorphous below 600°C. The anode active material is a transparent sol comminuted to a particle size of 10 nm or less according to a small angle X-ray scattering method.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a negative electrode active material for lithium ion secondary batteries and a method for producing the same, and more particularly to a negative electrode active material coated with a solid electrolyte and a method for producing the same.

Research and development of lithium-ion secondary batteries are actively carried out for mobile devices, hybrid vehicles, electric vehicles, and household power storage applications. Lithium ion secondary batteries used in these fields are required to have high safety, long-term cycle stability, high energy density, and the like.

In recent years, lithium ion secondary batteries using lithium titanate as a negative electrode active material have been proposed from the viewpoint of high safety and long-term cycle stability. Since the operating potential of lithium titanate is higher than that of graphite, which is a general negative electrode active material, lithium deposition is less likely to occur and safety is improved, but it is disadvantageous from the viewpoint of energy density. On the other hand, as a positive electrode active material, a material that operates at a high potential of 4.5 V or more relative to the deposition potential of Li has been proposed (for example, Patent Document 1).

The reduction in energy density due to the high operating potential of lithium titanate can be improved by combining lithium titanate with a positive electrode active material that operates at a high potential as shown in Patent Document 1 described above. There is expected. On the other hand, in a conventional lithium ion secondary battery using graphite as a negative electrode active material, gas is generated by oxidative decomposition of the non-aqueous electrolyte on the surface of the positive electrode active material, but the operation of the positive electrode active material is more difficult than in the conventional secondary battery. In the case of a secondary battery with a higher potential, the above-described problem of gas generation becomes more pronounced.

Therefore, as in Patent Document 2, a method for suppressing gas generation by forming a coating layer on the surface of a positive electrode material that operates at a high potential has also been reported. However, gas generation is still confirmed due to the large particle size of the coating substance, etc., and there is room for improvement.

Japanese Patent Application Laid-Open No. 2001-185148 WO2020/049843

An object of the present invention is to provide a negative electrode active material capable of further suppressing gas generation in a lithium ion secondary battery using a positive electrode active material that operates at a high potential.

In view of the above circumstances, the inventors of the present invention have investigated means for suppressing the gas generation described above, and as a result, it is used in combination with a positive electrode active material having an operating voltage of 4.5 V or higher, which is the dissolution and deposition potential standard for lithium metal. In lithium ion secondary batteries, when using a negative electrode active material made of an oxide, it was found that gas generation can be effectively suppressed by coating the negative electrode active material with a lithium ion conductive oxide. That is, the present invention provides a negative electrode active material for a lithium ion secondary battery, which is used in combination with a positive electrode active material that operates at a high potential of lithium metal with a standard electrode potential of 4.5 V or more, wherein the negative electrode active material is is coated with a lithium ion conductive solid electrolyte, the lithium ion conductive oxide is amorphous at less than 600 ° C., and is pulverized to a particle size of 10 nm or less by X-ray small angle scattering method A negative electrode active material for a lithium ion secondary battery, characterized by being a transparent sol.

The method for producing a negative electrode active material of the present invention includes the steps of pulverizing a lithium ion conductive oxide to a particle size of 10 nm or less by a small angle X-ray scattering method, and adding to alcohol to make a transparent sol state; coating the transparent sol to which the powder of the lithium ion conductive oxide is added to the negative electrode active material by a mechanochemical method; and coating with the transparent sol. and heat-treating the obtained negative electrode active material at 100° C. or more and 500° C. or less, and in the step of making the transparent sol state, the powder of the lithium ion conductive oxide is added to the total weight of the transparent sol. It is characterized in that it is added at a rate of 15 to 30% by weight.

In the negative electrode active material and the method for producing the same of the present invention, the negative electrode active material has the following formula (1):
Li ₄ Ti ₅ O ₁₂ (1) is preferably a lithium-titanium compound.

Further, the above solid electrolyte is represented by the following formula (2): Li1 _{+p+q (} Al,Ga) _p (Ti,Ge) _2- pSiqP3 _{- qO12} (0≤p≤1, 0≤ q≦1) It is preferably coated with an oxide-based solid electrolyte represented by (2).

The positive electrode active material has the following formula (3): Li1 _{+ xMyMn2 -xyO4} ( ₃ )
(In formula (3), x and y satisfy 0 ≤ x ≤ 0.2 and 0 < y ≤ 0.8, and M is Al, Mg, Zn, Ni, Co, Fe, Ti, Cu and is at least one selected from the group consisting of Cr.).

According to the present invention, when used as a lithium ion secondary battery together with a positive electrode active material that easily generates gas and operates at a high potential, generation of gas due to oxidative decomposition of the non-aqueous electrolyte can be suppressed.

An embodiment of the invention will be described below, but the invention is not limited thereto.

The negative electrode active material of the present invention is coated with a solid electrolyte as described below, and is used in combination with a positive electrode active material that operates at a high potential of lithium metal with a standard electrode potential of 4.5 V or higher. The negative electrode active material is coated with a lithium ion conductive solid electrolyte, and the lithium ion conductive oxide is amorphous at a temperature of less than 600° C. and has a particle size of 10 nm or less according to a small angle X-ray scattering method. It is a transparent sol that has been ground to

The coated negative electrode active material can be obtained, for example, by pulverizing a lithium ion conductive oxide to a particle size of 10 nm or less by a small angle X-ray scattering method, and adding the pulverized lithium ion conductive oxide powder to alcohol. forming a transparent sol state; coating the negative electrode active material with the transparent sol to which the lithium ion conductive oxide powder is added by a mechanochemical method; and the negative electrode active material coated with the transparent sol. and heat-treating at 100° C. or higher and 500° C. or lower, and in the step of making the transparent sol state, 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. It is obtained by a method of adding at a ratio of

Lithium-ion secondary batteries generally use a non-aqueous electrolyte, which will be described later in detail, and which is a liquid non-aqueous electrolyte obtained by dissolving a lithium salt in a non-aqueous solvent. On the other hand, there are solid electrolytes in a solid state that have both the functions of a non-aqueous solvent and a lithium salt. A solid electrolyte has higher oxidation resistance than a non-aqueous electrolyte in a liquid state, so oxidative decomposition at a high potential is suppressed. However, since a solid has a lower lithium ion conductivity than a liquid, if all the electrolytes are replaced with solid electrolytes, the performance as a battery will be greatly reduced.

When a positive electrode active material that operates at a high potential is used, gas generation becomes more pronounced. Gas can be suppressed even with a conventional water electrolyte.

Although the coating method is not particularly limited, a method capable of uniform coating such as mechanical coating is preferable. Mechanical coating is preferable because the present invention uses a lithium ion conductive oxide in a transparent sol state, so that the solvent imparts adhesiveness and spreadability, enabling uniform coating. In addition, when volatilizing the solvent by heat treatment, the heat treatment temperature is adjusted, preferably the particle size of the lithium ion conductive oxide and the mixing ratio with the negative electrode active material are controlled, so that the resistance of the negative electrode active material is not increased. The lithium ion conductive oxide can be coated on the negative electrode active material without deteriorating the battery performance.

Although the lithium ion conductive oxide used in the present invention is in a transparent sol state, a certain amount of energy is required to coat the solid negative electrode active material. Therefore, a mechanical coating method using a mechanochemical method capable of imparting shear force and compressive force is preferable. Gas generation can be suppressed.

Furthermore, although details will be described later, the heat treatment temperature is adjusted, and preferably the particle size of the lithium ion conductive oxide and the mixing ratio with the negative electrode active material are controlled, so that the battery performance can be improved without increasing the resistance of the negative electrode active material. The lithium ion conductive oxide can be coated on the negative electrode active material without lowering the .

<Mechanical coating method>
Mechanical coating refers to imparting at least one type of energy of shear force, compression force, impact force and centrifugal force to the base material and / or coating agent (preferably shear force and compression force can be imparted, shear force, compression force and It is more preferable that a collision force can be applied), and the base material and the coating agent are brought into mechanical contact with each other to mix the base material and the coating agent and coat the surface of the base material with the coating material. In the present invention, the negative 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 to be used is not particularly limited, but for example, a grinding mill represented by Nobilta manufactured by Hosokawa Micron Corporation and a planetary ball mill (eg manufactured by Fritsch) can be preferably used. Among these, a grinding mill is preferable from the viewpoint of simple operation and the fact that there is no need to separate the balls after treatment as in the case of a ball mill.

In the manufacturing method of the present invention, a bottomed cylindrical container and a rotor having tip blades are provided, a predetermined clearance is provided between the tip blades and the inner circumference of the container, and the rotor is rotated to obtain the negative electrode. It is preferable to perform mechanical coating by applying compressive force and shear force to the mixture containing the active material and the lithium ion conductive oxide in the transparent sol state.

When the mechanical coating method is a wet process, the solvent used is not particularly limited, and water and an organic solvent can be used. Alcohol such as ethanol can be used as the organic solvent. The timing of adding the solvent in the wet process is not particularly limited, but the lithium ion conductive oxide in the transparent sol state may be diluted with a solvent and used in the mechanical coating method. The concentration of the oxide is not particularly limited, but is, for example, 10 to 30% by weight, preferably 15 to 30% by weight, more preferably 15 to 25% by weight, relative to the total weight.

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

Although it is possible to use the sample after mechanical coating as it is, it is preferable to perform a heat treatment. As a result, the adhesion between the negative electrode active material and the lithium ion conductive oxide is improved, and the lithium ion conductive oxide is prevented from peeling off from the negative electrode active material even after repeated charging and discharging, thereby improving the battery performance. Long-term reliability is improved. If the heat treatment temperature is too high, the crystal structure of the lithium ion conductive oxide changes, the Li ion conductivity decreases, and charging and discharging of the battery may not be performed normally, so the heat treatment temperature is 500 ° C. or less. and more preferably 100° C. or higher and 500° C. or lower. The heat treatment time is preferably 30 minutes or longer, more preferably 1 hour or longer, and the upper limit is not particularly limited, but is, for example, 3 hours or shorter.

<Negative electrode active material>
As described above, it is preferable to use lithium titanate as the oxide negative electrode active material used in the production method of the present invention from the viewpoint that lithium deposition is less likely to occur and safety is improved. Among the lithium titanates, lithium titanate having a spinel structure is particularly preferable because the expansion and contraction of the active material in the lithium ion insertion/extraction reaction is small. Lithium titanate may contain a small amount of elements other than lithium and titanium, such as Nb.

<Positive electrode active material>
The ^positive electrode active material is not particularly limited ^. 0.5V or more, more preferably 4.5V or more and 5.0V or less. The potential of lithium ion insertion/extraction reaction (hereinafter also referred to as voltage) (vs. Li ⁺ /Li) is, for example, the charge-discharge characteristics of a half-cell with a working electrode using a positive electrode active material and a lithium metal as a counter electrode. It can be determined by measuring and reading the voltage values at the start and end of the plateau. When there are two or more plateaus, the lowest voltage plateau may be 4.5 V (vs. Li ⁺ /Li) or more, and the highest voltage plateau is 5.0 V (vs. Li ⁺ / Li) or less.

The positive electrode active material in which the lithium ion insertion/extraction reaction proceeds at 4.5 V or more and 5.0 V or less with respect to the deposition potential of Li is not particularly limited, but substituted lithium manganese represented by the following formula (3) Compounds have been considered in the past and are preferred.

Li1 _{+ xMyMn2 -xyO4} ( ₃ )
In the above formula (3), x and y satisfy 0 ≤ x ≤ 0.2 and 0 < y ≤ 0.8, and M is Al, Mg, Zn, Ni, Co, Fe, Ti, Cu and Cr At least one selected from the group consisting of

Among the above formula (3), a Ni-substituted lithium manganese compound in which M is Ni is preferable, and in particular x = 0, y = 0.5, and M = Ni, that is, LiNi _0.5 Mn _1.5 O ₄ is effective in the charge-discharge cycle. It is particularly preferred because of its high stabilizing effect.

The particle size of the negative electrode active material is not particularly limited, but if the particle size is too small, the difference from the particle size of the oxide-based solid electrolyte described later becomes small, making coating difficult. It is preferably 5 μm or more, and still more preferably 10 μm or more. Also, the median diameter d50 is preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 50 μm or less. Considering the thickness range when processing into electrodes, the above d50 is preferably 5 to 50 μm, more preferably 10 to 50 μm.

<Lithium ion conductive oxide in transparent sol state>
Although the transparent sol-state lithium ion conductive oxide used in the present invention is not particularly limited, it is preferable to use an oxide-based solid electrolyte in consideration of chemical stability. The oxide-based solid electrolyte has a reverse fluorite type, a NASICON type, a perovskite type, a garnet type, and the like depending on the crystal structure, but is not particularly limited. As an oxide-based solid electrolyte, for example, the following formula (2):
Li1 _{+p+q (} Al,Ga) _p (Ti,Ge) _2- pSiqP3 _{-qO12 (} 2)
(In formula (2), p and q satisfy 0≤p≤1 and 0≤q≤1, respectively.)
LATP represented by can be used, especially the following formula (2-1):
Li _1+p Al _p Ti _2-p P ₃ O ₁₂ (2-1)
(In formula (2-1), p satisfies 0≦p≦1.)
An oxide-based solid electrolyte represented by is preferable.

The particle size of the lithium ion conductive oxide in the transparent sol state is inevitably fine because the pulverization is carried out until it becomes an amorphous and transparent sol state. Specifically, it is desired that the BET specific surface area equivalent diameter (dBET) is preferably 10 nm or less, more preferably 8 nm or less, and still more preferably 6 nm or less.

Known means such as a ball mill and a bead mill can be used as a method of microparticulation treatment. In addition, the BET specific surface area conversion diameter (dBET) is determined by the nitrogen adsorption BET specific surface area according to the method specified in JIS-Z8830 (2013), dBET = 6 / (density × BET specific surface area ) is the particle size obtained by the formula.

The ratio of the median diameter d50 of the negative electrode active material to the BET specific surface area conversion diameter dBET of the lithium ion conductive oxide in the transparent sol state is preferably 10000:1 to 100:1, more preferably 5000:1. to 300:1, more preferably 2000:1 to 500:1, particularly preferably 1000:1 to 500:1.

The ratio of the lithium ion conductive oxide in the transparent sol state (solid content when used in slurry) to 100 parts by mass of the negative electrode active material is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more. more preferably 2 parts by mass or more, 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 (that is, the mass ratio of the negative electrode active material and the lithium ion conductive oxide in the transparent sol state is 100: 1 to 20: 1), and 2 parts by mass. It is also preferable that the content is 4 parts by mass or less (that is, the mass ratio of the negative electrode active material and the transparent sol-state lithium ion conductive oxide is 50:1 to 25:1).

<Lithium ion secondary battery>
A lithium ion secondary battery is mainly composed of a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode is produced, for example, by applying a positive electrode mixture containing a positive electrode active material, a conductive aid, a binder, and the like to a positive electrode current collector, and the negative electrode includes, for example, a negative electrode active material, a conductive aid, a binder, and the like. It is produced by applying a negative electrode mixture to a negative electrode current collector. The coated negative electrode active material obtained by the production method of the present invention is suitably used as a negative electrode active material for lithium ion secondary batteries. A negative electrode can be produced by applying the agent to a negative electrode current collector. After applying the positive electrode mixture to the positive electrode current collector and after applying the negative electrode mixture to the negative electrode current collector, they may be dried at about 100 to 200°C.

The structure of the lithium ion secondary battery using the coated negative electrode active material, the materials used other than the coated negative electrode active material, and the manufacturing apparatus and conditions of the lithium ion secondary battery are conventionally known, and are not particularly limited. .

<Conductivity aid>
The conductive aid is not particularly limited, but for both the positive electrode and the negative electrode, for example, a carbon material is preferable. Carbon materials include, for example, natural graphite, artificial graphite, vapor-grown carbon fiber, carbon nanotube, acetylene black, ketjen black, and furnace black. One type of these carbon materials may be used, or two or more types may be used. The amount of the conductive aid contained in the positive electrode is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less with respect to 100 parts by weight of the positive electrode active material. If it is the said range, the conductivity of a positive electrode will be ensured. In addition, the adhesiveness to the binder, which will be described later, is maintained, and sufficient adhesiveness to the current collector can be obtained. The amount of the conductive aid contained in the negative electrode is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less with respect to 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, for example, it is selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof. At least one type can be used. The binder is preferably dissolved or dispersed in a non-aqueous solvent or water in order to facilitate the production of the positive electrode and the negative electrode. Non-aqueous solvents include, but are not limited to, N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate, and tetrahydrofuran. A dispersant and a thickener may be added to these. The amount of the binder contained in the positive electrode of the present invention is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less with respect to 100 parts by weight of the positive electrode active material. If it is the said range, the adhesiveness of a positive electrode active material and a conductive support material will be maintained, and adhesiveness with a collector can fully be obtained. The amount of the binder contained in the negative electrode is preferably 1 part by weight or more and 30 parts by weight or less, more preferably 2 parts by weight or more and 15 parts by weight or less with respect to 100 parts by weight of the negative electrode active material.

<Current collector>
Although neither the positive electrode current collector nor the negative electrode current collector is particularly limited, aluminum or an aluminum alloy is preferable. Aluminum or aluminum alloys are not particularly limited because they are stable in the positive and negative electrode reaction atmospheres, but high-purity aluminum represented by JIS standards 1030, 1050, 1085, 1N90, 1N99, etc. is preferable. Although the thickness of the current collector is not particularly limited, it is preferably 10 μm or more and 100 μm or less. Within this range, it is easy to achieve a good balance in terms of handleability in battery production, cost, and obtained battery characteristics. As the current collector, a metal other than aluminum (copper, SUS, nickel, titanium, and alloys thereof) coated with a metal that does not react with the potentials of the positive and negative electrodes can also be used.

<Non-aqueous electrolyte>
The non-aqueous electrolyte is not particularly limited, but includes, for example, a non-aqueous electrolytic solution in which a solute is dissolved in a non-aqueous solvent, 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, and the like. can be used.

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. As the chain aprotic solvent, a chain carbonate, a chain carboxylic acid ester, a chain ether, a solvent generally used as a solvent for non-aqueous electrolytes, such as acetonitrile, may be used. More specifically, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyl lactone, 1,2-dimethoxyethane, sulfolane, dioxolane, propion Methyl acid and the like can be used. These solvents may be used alone, or two or more of them may be used in combination. is preferably used.

Chains exemplified by dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, and methyl propyl carbonate, when two or more types are mixed, have high stability at high temperatures and high lithium conductivity at low temperatures. It is preferable to mix one or more of cyclic carbonates with one or more of cyclic compounds exemplified by ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyl lactone, and dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate. It is particularly preferable to mix one or more of the linear carbonates exemplified in (1) with one or more of the cyclic carbonates exemplified by ethylene carbonate, propylene carbonate and butylene carbonate.

_{The solute is not particularly limited . _ _} It is preferable because it dissolves easily. 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 it is 0.5 mol/L or more, the desired lithium ion conductivity is easily exhibited, while when it is 2.0 mol/L or less, the solute is easily dissolved.

Although the amount of the non-aqueous electrolyte used in the lithium ion secondary battery of the present invention is not particularly limited, it is preferably 0.1 mL or more per 1 Ah of battery capacity. With this amount, the conduction of lithium ions accompanying the electrode reaction can be ensured, and the desired battery performance is exhibited.

The non-aqueous electrolyte may be included in the positive electrode, the negative electrode and the separator in advance, or may be added after winding or laminating the separator arranged between the positive electrode side and the negative electrode side.

A lithium-ion secondary battery usually further includes a separator and an exterior material in addition to the above-described structure.

(separator)
The separator is placed between the positive electrode and the negative electrode, and may have any structure as long as it is insulating and can contain a non-aqueous electrolyte described later. Examples include nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, and polyimide. , polyamide, polyethylene terephthalate, and woven fabrics, non-woven fabrics, and microporous membranes of composites of two or more thereof. Nonwoven fabrics of nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and composites of two or more of these are preferred because of their excellent stability in cycle characteristics.

The separator may contain various plasticizers, antioxidants, and flame retardants, and may be coated with metal oxide or the like. Although the thickness of the separator is not particularly limited, it is preferably 10 μm or more and 100 μm or less. Within this range, it is possible to prevent a short circuit between the positive electrode and the negative electrode while suppressing an increase in battery resistance. From the viewpoints of economic efficiency and handling, the thickness is more preferably 15 μm or more and 50 μm or less.

The porosity of the separator is preferably 30% or more and 90% or less. When it is 30% or more, the diffusibility of lithium ions is less likely to decrease, so that the cycle characteristics can be easily maintained. can be done. From the viewpoint of ensuring the diffusibility of lithium ions and preventing short circuits, it is more preferably 35% or more and 85% or less.

(Exterior material)
The exterior material is a member that encloses a laminate obtained by alternately laminating or winding a positive electrode, a negative electrode, and a separator, and a terminal that electrically connects the laminate. Exterior materials include composite films in which a heat-sealing thermoplastic resin layer is provided on a metal foil, metal layers formed by vapor deposition or sputtering, or rectangular, elliptical, cylindrical, coin-shaped, button-shaped, or sheet-shaped films. A metal can is preferably used.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples. The present invention is not limited by the following examples, and it is of course possible to make appropriate modifications within the scope that can be adapted to the spirit of the above and the following. Included in scope.

Lithium-ion secondary batteries produced based on the coated negative electrodes obtained in the following examples and comparative examples were evaluated by the following methods.

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

First, the weight of the lithium ion secondary battery was measured with an electronic balance. Next, the weight in water was measured using a hydrometer (manufactured by Alpha Mirage Co., Ltd., product number: MDS-3000), and the buoyancy was calculated by taking the difference between these weights. The volume of the lithium ion secondary battery was calculated by dividing this buoyancy by the density of water (1.0 g/cm ³ ). The amount of generated gas was calculated by comparing the volume after aging and the volume after cycle characteristic evaluation. A gas generation amount of less than 20 ml was judged to be good. More preferably, the amount of gas generated is 15 ml or less.

(Evaluation of cycle characteristics of lithium-ion secondary battery)
The produced lithium ion secondary battery was connected to a charging/discharging device (HJ1005SD8, manufactured by Hokuto Denko Co., Ltd.) and subjected to cycle operation. Under an environment of 60° C., constant current charging was performed at a current value corresponding to 1.0 C until the battery voltage reached the final voltage of 3.4 V, and charging was stopped. Subsequently, constant current discharge was performed at a current value corresponding to 1.0 C, and discharge was stopped when the battery voltage reached 2.5V. This cycle was defined as one cycle, and charging and discharging were repeated. The stability of the cycle characteristics was evaluated by taking the discharge capacity at the 500th time assuming that the discharge capacity at the first time was 100, as the discharge capacity retention rate (%). A discharge capacity retention rate of 80% or more at the 500th cycle was judged as good, and a discharge capacity retention rate of less than 80% was judged as unsatisfactory.

Synthesis Example 1 Preparation of Lithium Ion Conductive Oxide As a lithium ion conductive oxide, _{Li1.3A10.3Ti1.7} ( _PO4 ) _{3 (} hereinafter also referred to as _LATP ) was prepared. A predetermined amount of Li ₂ CO ₃ , AlPO ₄ , TiO ₂ , NH ₄ H ₂ PO ₄ and ethanol as a solvent were mixed as starting materials, and subjected to a planetary ball mill treatment at 150 G for 1 hour using zirconia balls having a diameter of 3 mm. . After removing the zirconia balls from the treated mixture with a sieve, the mixture was dried at 120° C. to remove the ethanol. Thereafter, treatment was performed at 800° C. for 2 hours to obtain LATP powder.

(Preparation of coated negative electrode active material)
Example 1
(i) Pulverization of Lithium Ion Conductive Oxide and Addition to Alcohol A predetermined amount of ethanol as a solvent is mixed with the obtained LATP powder, and planetary ball mill treatment is performed for 3 to 6 hours using zirconia balls having a diameter of 0.5 mm. gone. After removing the treated zirconia balls with a sieve, ethanol was mixed to adjust the LATP solid content to 16.4% by weight. As a result, LATP in a transparent sol state with a dBET of 3 to 10 nm and having no LATP-derived peak in X-ray crystal diffraction measurement was obtained.

(ii) Coating Treatment, Heat Treatment Spinel-type lithium titanate (Li ₄ Ti ₅ O ₁₂ , hereinafter also referred to as LTO) having a median diameter of 10 μm was used as the negative electrode active material.

40 g of LTO was placed in a grinding mill (Nobilta, manufactured by Hosokawa Micron Corporation) and pulverized for 3 hours at a clearance of 0.6 mm, a rotor load power of 1.5 kW, and 2600 rpm to obtain 6.1 g of LATP in a transparent sol state. was added in two batches. After that, the rotation speed of the rotor was kept in the range of 2600 to 3000 rpm, and the treatment was performed at room temperature for 10 minutes in an air atmosphere to obtain LTO whose surface was coated with LATP. The obtained surface-coated LTO was heat-treated at 350° C. for 1 hour to obtain a coated negative electrode active material.

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

Comparative example 1
In the preparation of the coated negative electrode active material, the same operation as in Example 1 was performed except that the LATP particle size was the same, but the obtained surface-coated LTO was heat-treated at 600° C. for 1 hour. , to prepare a coated negative electrode active material.

Comparative example 2
In the preparation of the coated negative electrode active material, the same operation as in Example 1 was performed except that the LATP particle size was the same, but the obtained surface-coated LTO was heat-treated at 50°C for 1 hour. , to prepare a coated negative electrode active material.

Comparative example 3
In the pulverization of the lithium ion conductive oxide, the LATP pulverization time is set to 1 hour, and the particle size having a LATP-derived peak in the X-ray crystal diffraction measurement is about 20 nm. The operation was carried out to produce a coated negative electrode active material.

Comparative example 4
In the pulverization of the lithium ion conductive oxide, the LATP pulverization time is set to 2 hours, and the particle diameter of about 15 nm that does not have a LATP-derived peak in the X-ray crystal diffraction measurement but is not in a transparent sol state is used. performed the same operation as in Example 1 to prepare a coated negative electrode active material.

Comparative example 5
A negative electrode active material was produced in the same manner as in Example 1, except that LTO with no surface coating was used.

Using the negative electrode active materials obtained in Examples 1 and 2 and Comparative Examples 1 and 5, a negative electrode mixture, a positive electrode, a negative electrode, and a lithium ion secondary battery were produced by the following operations. The amount of gas generated and the cycle characteristics of the ion secondary battery were evaluated. The evaluation results are shown in Table 1 below.

(Preparation of negative electrode)
A mixture containing 90 parts by weight, 5 parts by weight, and 5 parts by weight of the obtained surface-coated LTO, acetylene black as a conductive aid, and polyvinylidene fluoride (PVdF) as a binder, respectively, in terms of solid concentration, was added to N- A negative electrode mixture was prepared by dispersing it in methyl-2-pyrrolidone (NMP). The binder used was prepared in an N-methyl-2-pyrrolidone (NMP) solution having a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity so as to facilitate the later-described coating.

After coating the negative electrode mixture on a 20 μm aluminum foil, it was dried in an oven at 120°C. After this operation was performed on both sides of the aluminum foil, the negative electrode was produced by further vacuum-drying at 170°C.

(Preparation of positive electrode)
Spinel-type lithium nickel manganate (LiNi _0.5 Mn _1.5 O ₄ hereinafter, also referred to as LNMO) was used as the positive electrode active material. A mixture containing the above LNMO, acetylene black as a conductive agent, and PVdF as a binder, each at a solid content concentration of 90 parts by weight, 6 parts by weight, and 4 parts by weight, is added to N-methyl-2-pyrrolidone (NMP). A dispersed negative electrode mixture was produced. The above binder was prepared as an NMP solution having a solid concentration of 5% by weight, and NMP was further added to adjust the viscosity so as to facilitate the later-described coating.

After coating the positive electrode mixture on a 20 μm aluminum foil, it was dried in an oven at 120°C. After this operation was performed on both sides of the aluminum foil, the positive electrode was produced by further vacuum-drying at 170°C.

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

Two sheets of aluminum laminate film serving as exterior materials were prepared, and after forming a depression serving as a battery portion and a depression serving as a gas collecting portion by pressing, the electrode laminate was put therein. The outer periphery leaving a space for non-aqueous electrolyte injection was heat-sealed at 180 ° C. for 7 seconds, and ethylene carbonate, propylene carbonate and ethyl methyl carbonate were added from the unsealed portion on a volume basis to ethylene carbonate/propylene carbonate/ethyl. After adding a non-aqueous electrolyte prepared by dissolving LiPF ₆ at a ratio of 1 mol/L in a solvent mixed with methyl carbonate at a ratio of 15/15/70, the unsealed portion was heated at 180° C. for 7 seconds while reducing the pressure. heat sealed. The obtained battery was subjected to constant current charging at a current value corresponding to 0.2 C until the battery voltage reached the final voltage of 3.4 V, and charging was stopped. Then, after standing in an environment of 60° C. for 24 hours, constant current discharge was performed at a current value corresponding to 0.2 C, and discharge was stopped when the battery voltage reached 2.5V. After the discharge was stopped, the gas collected in the gas collecting portion was extracted, and the sealing was performed again. A lithium ion secondary battery for evaluation was produced by the above operation.

In the lithium ion secondary batteries of Examples 1 and 2, the amount of gas generated was small and the capacity retention rate was high as a result of cycle characteristic evaluation.

On the other hand, Comparative Example 1, in which the particle diameter was the same as that of Examples 1 and 2 but the heat treatment temperature was 600° C., resulted in a large amount of generated gas and a low capacity retention rate. This is considered to be caused by recrystallization of LATP.

Furthermore, Comparative Example 2, which had the same particle size as Examples 1 and 2 but had a heat treatment temperature of 50° C., also produced a large amount of generated gas and a low capacity retention rate. This is considered to be caused by residual solvent during spray coating.

Comparative Example 3, in which the LATP pulverization time was set to 1 hour and had a LATP-derived peak in the X-ray crystal diffraction measurement, also resulted in a large amount of generated gas and a low capacity retention rate.

Although the LATP pulverization time was set to 2 hours and the LATP-derived peak was not observed in the X-ray crystal diffraction measurement, Comparative Example 4, which was not in a sol state, also resulted in a large amount of gas generated and a low capacity retention rate.

From the above results, it can be concluded that for a positive electrode active material that operates at a high potential, lithium using a negative electrode active material in which the surface of the negative electrode active material is coated with a lithium ion conductive oxide pulverized to an amorphous and transparent sol state It was found that the ion secondary battery generates less gas even when charged and discharged at a high potential and has good cycle characteristics.

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

Claims (7)

A negative electrode active material for a lithium ion secondary battery used in combination with a positive electrode active material that operates at a high potential of lithium metal with a standard electrode potential of 4.5 V or more,
The negative electrode active material is coated with a lithium ion conductive solid electrolyte, and the lithium ion conductive oxide is amorphous at a temperature of less than 600° C. and has a particle size of 10 nm or less according to a small angle X-ray scattering method. A negative electrode active material for a lithium ion secondary battery, which is a transparent sol pulverized into The negative electrode active material has the following formula (1):
_{Li4Ti5O12 ( 1} )
The negative electrode active material for a lithium ion secondary battery according to claim 1, which is a lithium titanium compound represented by: The solid electrolyte has the following formula (2):
Li1 _{+p+q (} Al, Ga) _p (Ti, Ge) _2- pSiqP3 _{- qO12} (0≤p≤1, 0≤q≤1) (2)
The negative electrode active material for a lithium ion secondary battery according to claim 1 or 2, which is coated with an oxide-based solid electrolyte represented by. The positive electrode active material has the following formula (3):
Li1 _{+ xMyMn2 -xyO4} ( ₃ )
(In formula (3), x and y satisfy 0 ≤ x ≤ 0.2 and 0 < y ≤ 0.8, and M is Al, Mg, Zn, Ni, Co, Fe, Ti, Cu and It is at least one selected from the group consisting of Cr.)
The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, which is a substituted lithium manganese compound represented by. pulverizing the lithium ion conductive oxide to a particle size of 10 nm or less by a small angle X-ray scattering method;
adding the pulverized lithium ion conductive oxide powder to alcohol to form a transparent sol;
a step of coating the negative electrode active material with the transparent sol to which the powder of the lithium ion conductive oxide is added by a mechanochemical method;
and heat-treating the negative electrode active material coated with the transparent sol at 100° C. or higher and 500° C. or lower;
In the step of making the transparent sol state, the powder of the lithium ion conductive oxide is added at a rate of 15 to 30% by weight with respect to the total weight of the transparent sol. A method for producing a negative electrode active material for a battery. 6. The method for producing a coated negative electrode active material according to claim 5, wherein the negative electrode active material is a lithium titanium compound represented by the following formula (1).
_{Li4Ti5O12 ( 1} ) The lithium ion conductive oxide has the following formula (2)
Li1 _{+p+q (} Al,Ga) _p (Ti,Ge) _2- pSiqP3 _{-qO12 (} 2)
7. The negative electrode active material for a lithium ion secondary battery according to claim 5 or 6, represented by (in formula (A), p and q satisfy 0≤p≤1 and 0≤q≤1, respectively). manufacturing method.