阅读说明：本技术 非水电解质二次电池用负极活性物质及非水电解质二次电池、以及非水电解质二次电池用负极材料的制备方法 (Negative electrode active material for nonaqueous electrolyte secondary battery, and method for producing negative electrode material for nonaqueous electrolyte secondary battery ) 是由 高桥广太 广濑贵一 松野拓史 于 2018-05-18 设计创作，主要内容包括：本发明为一种非水电解质二次电池用负极活性物质,其包含负极活性物质颗粒,该非水电解质二次电池用负极活性物质的特征在于：所述负极活性物质颗粒含有包含硅化合物(SiO<Sub>x</Sub>：0.5≤x≤1.6)的硅化合物颗粒,所述硅化合物颗粒含有Li化合物,所述硅化合物颗粒的至少一部分被碳材料所覆盖,所述硅化合物颗粒的表面或所述碳材料的表面或该两个表面的至少一部分被含有具备甲硅烷基的化合物的层所覆盖。由此,提供一种对水系浆料的稳定性高且为高容量,且同时循环特性及初次效率良好的非水电解质二次电池用负极活性物质。(The present invention is a negative electrode active material for a nonaqueous electrolyte secondary battery, which contains negative electrode active material particles, characterized in that: the negative electrode active material particles contain a silicon compound (SiO) x : 0.5. ltoreq. x.ltoreq.1.6), the silicon compound particles containing a Li compound, at least a part of the silicon compound particles being covered with a carbon material, and a layer containing a compound having a silyl group being formed on the surface of the silicon compound particles or on the surface of the carbon material or on at least a part of both surfacesAnd (4) covering. Thus, a negative electrode active material for a nonaqueous electrolyte secondary battery is provided which has high stability to an aqueous slurry, has a high capacity, and has excellent cycle characteristics and initial efficiency.)

1. A negative electrode active material for a nonaqueous electrolyte secondary battery, comprising negative electrode active material particles, characterized in that:

the negative electrode active material particles contain a silicon compound (SiO)_x: x is more than or equal to 0.5 and less than or equal to 1.6),

the silicon compound particles contain a Li compound,

at least a part of the silicon compound particles is covered with a carbon material,

at least a part of the surface of the silicon compound particles or the surface of the carbon material or both surfaces thereof is covered with a layer containing a compound having a silyl group.

2. The negative electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein the silyl group is a silyl group having an organic group having 1 to 5 carbon atoms.

3. The negative electrode active material for a nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein at least a part of the silicon compound particles contain Li₄SiO₄、Li₂SiO₃、Li₂Si₂O₅ToMore than one.

4. The negative electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein a coverage amount of the carbon material is 0.5 mass% or more and 15 mass% or less with respect to a total amount of the silicon compound particles and the carbon material.

5. The negative electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the negative electrode active material particles have a volume resistivity of 0.01 Ω · cm or more and less than 100 Ω · cm when applied at 38.2MPa, as measured by a four-probe method according to JIS K7194.

6. The negative electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein a half-value width (2 θ) of a diffraction peak due to a Si (111) crystal plane obtained by X-ray diffraction of the silicon compound particles is 1.2 ° or more, and a crystallite size corresponding to the crystal plane is 7.5nm or less.

7. The negative electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the silicon compound particles have a median diameter of 0.5 μm or more and 20 μm or less.

8. A nonaqueous electrolyte secondary battery characterized in that: the negative electrode active material for a nonaqueous electrolyte secondary battery, which comprises the negative electrode active material according to any one of claims 1 to 7.

9. A method for producing a negative electrode material for a nonaqueous electrolyte secondary battery, the negative electrode material comprising negative electrode active material particles containing silicon compound particles, the method comprising:

preparation of a silicon-containing Compound (SiO)_x: x is more than or equal to 0.5 and less than or equal to 1.6)A step of granulating the compound;

covering at least a part of the silicon compound particles with a carbon material;

modifying the silicon compound particles by absorbing lithium into the silicon compound particles; and

forming a layer containing a silyl group functionalized compound on at least a part of the surface of the modified silicon compound particle or the surface of the carbon material or both surfaces;

the negative electrode material for a nonaqueous electrolyte secondary battery is produced by using the silicon compound particles having the layer containing the compound having a silyl group as negative electrode active material particles.

Technical Field

The present invention relates to a negative electrode active material for a nonaqueous electrolyte secondary battery, and a method for producing a negative electrode material for a nonaqueous electrolyte secondary battery.

Background

In recent years, small-sized electronic devices typified by mobile terminals and the like have been widely spread, and further miniaturization, weight reduction, and long life have been strongly demanded. In response to such market demands, development of a secondary battery which is particularly small and lightweight and can achieve high energy density has been advanced. The application of the secondary battery is not limited to small-sized electronic devices, and its application to large-sized electronic devices such as automobiles and power storage systems such as houses is also being studied.

Among them, lithium ion secondary batteries are expected to be able to achieve a higher energy density than lead batteries and nickel-cadmium batteries because they are easy to be downsized and have a higher capacity.

The lithium ion secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte, and the negative electrode contains a negative electrode active material involved in charge and discharge reactions.

As the negative electrode active material, carbon materials are widely used, and further improvement of battery capacity is demanded in accordance with recent market demand. In order to increase the battery capacity, the use of silicon as a negative electrode active material is being studied. This is because the theoretical capacity of silicon (4199mAh/g) is 10 times or more greater than the theoretical capacity of graphite (372mAh/g), and therefore, a significant increase in battery capacity can be expected. Development of a silicon material as a negative electrode active material has been studied not only for a simple substance of silicon but also for compounds represented by alloys and oxides. In addition, regarding the shape of the active material, studies have been made on carbon materials ranging from a standard coating type to an integral type directly deposited on a current collector.

However, when silicon is used as a main material for the negative electrode active material, the negative electrode active material expands and contracts during charge and discharge, and thus, mainly in the vicinity of the surface layer of the negative electrode active material, the negative electrode active material is likely to be cracked. Further, an ionic material is generated in the active material, and the negative electrode active material is easily broken. If the surface layer of the negative electrode active material is broken, a new surface is generated, and the reaction area of the active material increases. In this case, the decomposition reaction of the electrolyte occurs on the fresh surface, and at the same time, a film of the decomposition product of the electrolyte is formed on the fresh surface, so that the electrolyte is consumed. And thus the cycle characteristics of the battery become easily degraded.

Heretofore, various studies have been made on negative electrode materials and electrode configurations for lithium ion secondary batteries, which mainly comprise silicon materials, in order to improve the initial efficiency and cycle characteristics of the batteries.

Specifically, silicon and amorphous silica are simultaneously deposited using a vapor phase method for the purpose of obtaining good cycle characteristics and high safety (for example, refer to patent document 1). Further, in order to obtain high battery capacity and safety, a carbon material (conductive material) is provided on the surface layer of the silicon oxide particles (for example, refer to patent document 2). Further, in order to improve cycle characteristics and obtain high input-output characteristics, an active material containing silicon and oxygen is prepared, and an active material layer having a high oxygen ratio in the vicinity of a current collector is formed (for example, refer to patent document 3). In order to improve cycle characteristics, the silicon active material is formed so as to contain oxygen and have an average oxygen content of 40 at% or less and a large oxygen content near the current collector (see, for example, patent literature).

In addition, in order to improve the initial charge-discharge efficiencyUsing Si-containing phase, SiO₂、M_yA nanocomposite of an O metal oxide (for example, refer to patent document 5). Further, in order to improve cycle characteristics, SiO is added_x(0.8. ltoreq. x.ltoreq.1.5, particle size range of 1 to 50 μm) is mixed with a carbon material and subjected to high-temperature calcination (for example, refer to patent document 6). In order to improve cycle characteristics, the molar ratio of oxygen to silicon in the negative electrode active material is set to 0.1 to 1.2, and the active material is controlled so that the difference between the maximum value and the minimum value of the molar ratio in the vicinity of the interface between the active material and the current collector is 0.4 or less (see, for example, patent document 7). In addition, in order to improve the load characteristics of the battery, a metal oxide containing lithium is used (for example, refer to patent document 8).

Further, as a method for suppressing particle aggregation, it is reported that the particle surface is treated with a silane compound or the like (for example, refer to patent document 9). Further, in order to suppress the amount of moisture entering the battery, the surface of the inorganic particles is subjected to a hydrophobic treatment (for example, refer to patent document 10). In order to improve cycle characteristics, a hydrophobic layer such as a silane compound is formed on a surface layer of a silicon material (see, for example, patent document 11).

Disclosure of Invention

Technical problem to be solved by the invention

As described above, in recent years, high performance and multi-functionalization of small electronic devices such as mobile terminals have been progressing, and there is a demand for an increase in battery capacity of lithium ion secondary batteries as their main power sources. As one method for solving this problem, development of a lithium ion secondary battery including a negative electrode using a silicon material as a main material has been desired.

Further, it is desired that the battery characteristics of the lithium ion secondary battery using the silicon material be equal to or similar to those of the lithium ion secondary battery using the carbon material. Therefore, silicon oxide modified by Li absorption and partial desorption is used as the negative electrode active material, thereby improving the cycle maintenance rate and the initial efficiency of the battery. However, the modified silicon oxide is modified by using Li, and therefore has low water resistance. Therefore, the slurry containing the modified silicon oxide produced in the production of the negative electrode may not be sufficiently stabilized, and the slurry may generate gas with time, and when the carbon-based active material is applied, there is a problem that a conventionally used apparatus or the like cannot be used or cannot be easily used. Further, there is a problem that the silicon compound particles modified with Li have a strong cohesive property and thus cannot be coated with a slurry. In this way, in the case of using a silicon oxide modified with Li to improve the initial efficiency and cycle maintenance rate, the stability of the aqueous slurry becomes insufficient, and therefore, a negative electrode active material for a nonaqueous electrolyte secondary battery, which is advantageous in industrial production of a secondary battery, has not been proposed yet.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a negative electrode active material for a nonaqueous electrolyte secondary battery, which has high stability to an aqueous slurry and high capacity, and which has excellent cycle characteristics and initial efficiency.

Means for solving the problems

In order to achieve the above object, the present invention provides a negative electrode active material for a nonaqueous electrolyte secondary battery, including negative electrode active material particles, characterized in that:

the negative electrode active material particles contain a silicon compound (SiO)_x: x is more than or equal to 0.5 and less than or equal to 1.6),

the silicon compound particles contain a Li compound,

at least a part of the silicon compound particles is covered with a carbon material,

at least a part of the surface of the silicon compound particles or the surface of the carbon material or both surfaces is covered with a layer containing a compound having a silyl group.

In the negative electrode active material of the present invention, the negative electrode active material particles containing the silicon compound particles have a compound having a silyl group on the outermost surface thereof, and therefore, when an electrode is produced, aggregation of the active materials is suppressed, and water resistance to an aqueous slurry can be improved. In the present invention, at least a part of the surface of the silicon compound particle is covered with a carbon material (hereinafter, also referred to as a carbon coating), and therefore, the conductivity is excellent. Therefore, when the negative electrode active material of the present invention is used, a nonaqueous electrolyte secondary battery can be produced advantageously in industrial production, which utilizes the original characteristics of silicon oxide modified with Li, and has a high battery capacity and a good cycle maintenance rate. Hereinafter, the negative electrode active material particles containing the silicon compound particles are also referred to as silicon-based active material particles. The negative electrode active material containing the silicon-based active material particles is also referred to as a silicon-based active material.

The silyl group is preferably a silyl group having an organic group having 1 to 5 carbon atoms.

Such a silyl group can impart appropriate hydrophobicity to the negative electrode active material particles.

Further, it is preferable that at least a part of the silicon compound particles contain Li₄SiO₄、Li₂SiO₃、Li₂Si₂O₅At least one of them.

Like Li₄SiO₄、Li₂SiO₃、Li₂Si₂O₅Such lithium silicate is relatively stable as a lithium compound, and therefore, more favorable battery characteristics can be obtained.

Further, the coverage amount of the carbon material is preferably 0.5 mass% or more and 15 mass% or less with respect to the total of the silicon compound particles and the carbon material.

When the amount of the carbon material is such a covering amount, negative electrode active material particles having a high capacity and excellent conductivity can be obtained.

It is preferable that the volume resistivity of the negative electrode active material particles when applied at 38.2MPa, as measured by the four-probe method according to JIS K7194, is 0.01 Ω · cm or more and less than 100 Ω · cm.

When the volume resistivity is 0.01 Ω · cm or more and less than 100 Ω · cm, the conductivity can be sufficiently ensured, and therefore, more favorable battery characteristics can be obtained.

Further, it is preferable that the silicon compound particles have a half-value width (2 θ) of a diffraction peak due to an Si (111) crystal plane obtained by X-ray diffraction of 1.2 ° or more, and a crystallite size corresponding to the crystal plane of 7.5nm or less.

The silicon compound particles having such a crystallite size have low crystallinity and a small amount of Si crystals are present, and therefore, battery characteristics can be improved.

Further, the silicon compound particles preferably have a median particle diameter of 0.5 μm or more and 20 μm or less.

If the median particle diameter is 0.5 μm or more, the area where side reactions occur on the surface of the silicon compound particles is small, and therefore, additional Li is not consumed, and a high cycle maintenance rate of the battery can be maintained. When the median diameter is 20 μm or less, the expansion at the time of Li absorption is small, and cracking is not easily caused and cracks are not easily generated. Further, since the silicon compound particles have small expansion, for example, a negative electrode active material layer in which a carbon-based active material is mixed with a silicon-based active material that is generally used is not easily broken.

Further, the present invention provides a nonaqueous electrolyte secondary battery characterized in that: the negative electrode active material for a nonaqueous electrolyte secondary battery of the present invention is contained.

Such a secondary battery has high cycle maintenance rate and initial efficiency, and can be industrially advantageously manufactured.

Further, the present invention provides a method for producing a negative electrode material for a nonaqueous electrolyte secondary battery, the negative electrode material including negative electrode active material particles containing silicon compound particles, the method comprising:

preparation of a silicon-containing Compound (SiO)_x0.5. ltoreq. x. ltoreq.1.6);

covering at least a part of the silicon compound particles with a carbon material;

modifying the silicon compound particles by absorbing lithium into the silicon compound particles; and

forming a layer containing a silyl group functionalized compound on at least a part of the surface of the modified silicon compound particle or the surface of the carbon material or both surfaces;

the negative electrode material for a nonaqueous electrolyte secondary battery is produced by using the silicon compound particles having the layer containing the compound having a silyl group as negative electrode active material particles.

In the method for producing the negative electrode material for a nonaqueous electrolyte secondary battery, a nonaqueous negative electrode material having a high battery capacity and a good cycle maintenance rate can be obtained by utilizing the original characteristics of silicon oxide modified by using Li. Further, since the negative electrode material prepared in this manner contains the silicon-based active material particles having the layer containing the compound having the silyl group as described above, the slurry prepared at the time of manufacturing the negative electrode becomes stable. Namely, a negative electrode material which can industrially advantageously produce a secondary battery can be obtained.

Effects of the invention

The negative electrode active material of the present invention can improve the stability of a slurry prepared in the production of a secondary battery, and can form an industrially usable coating film by using the slurry, so that the battery capacity, cycle characteristics, and initial charge-discharge characteristics can be substantially improved. The secondary battery of the present invention containing the negative electrode active material can be industrially advantageously produced, and is excellent in battery capacity, cycle characteristics, and initial charge-discharge characteristics. The same effects can be obtained also in electronic devices, electric tools, electric vehicles, power storage systems, and the like, in which the secondary battery of the present invention is used.

In addition, the method for preparing the negative electrode material of the present invention can prepare a negative electrode material capable of improving the stability of a slurry prepared in the manufacture of a secondary battery, and improving the battery capacity, cycle characteristics, and initial charge and discharge characteristics.

Drawings

Fig. 1 is a schematic diagram showing the structure of silicon-based active material particles contained in the negative electrode active material of the present invention in the vicinity of a layer containing a compound having a silyl group.

Fig. 2 is a sectional view showing the structure of a negative electrode containing the negative electrode active material of the present invention.

Fig. 3 is a schematic view showing an example of an in-body reforming apparatus that can be used in producing the negative electrode active material of the present invention.

Fig. 4 is an exploded view showing an example of the structure (laminate film type) of a lithium ion secondary battery including the negative electrode active material of the present invention.

Detailed Description

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the embodiments.

As described above, as one method of increasing the battery capacity of a lithium ion secondary battery, it has been studied to use, as a negative electrode of the lithium ion secondary battery, a negative electrode using a silicon-based active material as a main material. Although it is expected that the cycle characteristics and initial efficiency of a lithium ion secondary battery using a silicon-based active material as a main material are equivalent to and close to those of a lithium ion secondary battery using a carbon material, it is difficult to produce a stable slurry and to produce a negative electrode having good quality by modifying a silicon-based active material using Li in order to obtain cycle characteristics and initial efficiency equivalent to and close to those of a lithium ion secondary battery using a carbon material.

Accordingly, the present inventors have repeatedly and earnestly studied to obtain a negative electrode active material for a nonaqueous electrolyte secondary battery which can easily produce a high battery capacity and is excellent in cycle characteristics and initial efficiency, and have completed the present invention.

The anode active material of the present invention contains anode active material particles. The negative electrode active material particles contain a silicon-containing compound (SiO)_x: x is not less than 0.5 and not more than 1.6). The silicon compound particles contain a Li compound. Further, at least a part of the silicon compound particles is covered with the carbon material. Further, at least a part of the surface of the silicon compound particles or the surface of the carbon material or both surfaces is covered with a layer containing a compound having a silyl group.

Here, fig. 1 shows an outline of the vicinity of the surface layer portion of the silicon compound particle 1. As shown in fig. 1, a carbon coating film 2 is formed on the surface of the silicon compound particle 1. In the case of fig. 1, the carbon coating is formed on a part of the surface of the silicon compound particle, but the carbon coating may be formed on the entire surface of the silicon compound particle. Further, a layer (silyl region) 3 containing a compound having a silyl group is formed on the surface of the silicon compound particle 1 and the surface of the carbon coating film 2. In fig. 1, the case where the silyl group in the layer 3 containing a compound having a silyl group is a trimethylsilyl group is exemplified, but the present invention is not particularly limited thereto.

In such a negative electrode active material, since the silicon-based active material particles have a silyl group such as an alkylsilyl group on the outermost surface thereof, aggregation of the silicon-based active material particles is suppressed, and further, the water resistance to the aqueous slurry is increased. Conventionally, silicon oxide modified by absorption and desorption of Li is easily aggregated in an aqueous slurry, and further generates gas with time, and therefore is not suitable for mass production. However, in the present invention, by providing the silicon-based active material particles with the layer containing the compound having a silyl group, aggregation in the slurry is suppressed, and the slurry is less likely to generate gas with time change, and a stable coating film can be obtained, and sufficient adhesion can be ensured. In addition, since at least a part of the surface of the silicon compound particle of the present invention is covered with a carbon coating film, the conductivity is excellent. Therefore, when the negative electrode active material of the present invention is used, a nonaqueous electrolyte secondary battery can be produced advantageously in industrial production, which utilizes the original characteristics of silicon oxide modified with Li, and has a high battery capacity and a good cycle maintenance rate. The coating amount of the layer containing the compound having a silyl group is not particularly limited, and may be, for example, 10 mass% or less with respect to the negative electrode active material particles. The lower limit of the coating amount may be a coating amount of 0.01 mass% or more with respect to the negative electrode active material particles, for example, as long as a layer containing a compound having a silyl group can be formed on the surface of the negative electrode active material particles.

[ constitution of negative electrode ]

Next, the structure of the negative electrode of the secondary battery containing the negative electrode active material of the present invention will be described.

Fig. 2 is a sectional view of a negative electrode containing the negative electrode active material of the present invention. As shown in fig. 2, the negative electrode 10 has a structure in which a negative electrode active material layer 12 is provided on a negative electrode current collector 11. The negative electrode active material layer 12 may be provided on both surfaces of the negative electrode collector 11 or only on one surface. Further, the negative electrode of the nonaqueous electrolyte secondary battery of the present invention may not have the negative electrode current collector 11.

[ negative electrode Current collector ]

The negative electrode current collector 11 is an excellent conductive material and is made of a material having excellent mechanical strength. Examples of the conductive material that can be used for the negative electrode current collector 11 include copper (Cu) and nickel (Ni). The conductive material is preferably a material that does not form an intermetallic compound with lithium (Li).

It is preferable that the negative electrode current collector 11 contains carbon (C) and sulfur (S) in addition to the main elements. This is because the physical strength of the negative electrode current collector is improved. This is because, particularly in the case of an active material layer which swells during charging, if the current collector contains the above-described elements, the effect of suppressing deformation of the electrode including the current collector is obtained. The content of the element is not particularly limited, but is preferably 100ppm or less. This is because a higher effect of suppressing deformation can be obtained.

The surface of the anode current collector 11 may or may not be roughened. The roughened negative electrode current collector is, for example, a metal foil subjected to electrolytic treatment, embossing (embossing) treatment, or chemical etching treatment. The non-roughened negative electrode current collector is, for example, a rolled metal foil.

[ negative electrode active material layer ]

The anode active material layer 12 may contain a plurality of anode active materials such as a carbon active material in addition to the silicon active material particles. Further, other materials such as a thickener (also referred to as "binder" ") and a conductive aid may be further included in consideration of the battery design. In addition, the shape of the negative electrode active material may be a particle shape.

As described above, the negative electrode active material of the present invention contains silicon compound particles, and the silicon compound particles contain a silicon compound (SiO)_x: 0.5. ltoreq. x.ltoreq.1.6), x being preferably close to 1 as its composition. This is because high cycle characteristics can be obtained. The composition of the silicon oxide material in the present invention does not necessarily mean that the purity is 100%, and may contain a trace amount of impurity elements, Li, or the like.

In the present invention, the lower the crystallinity of the silicon compound, the better. Specifically, it is desirable that the half-value width (2 θ) of the diffraction peak of the silicon compound particles due to the Si (111) crystal plane obtained by X-ray diffraction is 1.2 ° or more, and the crystallite size corresponding to the crystal plane is 7.5nm or less. The crystallite size can be calculated from the half-value width of the diffraction peak due to the Si (111) crystal plane obtained by X-ray diffraction. In this way, particularly, since the crystallinity is low and the amount of Si crystals present is small, not only can the battery characteristics be improved, but also a stable lithium compound can be produced.

The median particle diameter of the silicon compound particles is not particularly limited, but is preferably 0.5 μm or more and 20 μm or less. This is because, when the amount is within this range, the lithium ions are easily absorbed and released during charge and discharge, and the silicon-based active material particles are less likely to be broken. When the median particle diameter is 0.5 μm or more, the surface area is not excessively large, so that a side reaction is less likely to occur during charge and discharge, and the irreversible capacity of the battery can be reduced. On the other hand, if the median diameter is 20 μm or less, the silicon-based active material particles are less likely to be broken and to have a fresh surface, and therefore, this is preferable.

Further, in the present invention, it is preferable that Li is present in the silicon-based active material₄SiO₄、Li₂SiO₃、Li₂Si₂O₅At least one kind of (2) as a lithium compound contained in at least a part of the silicon compound particles. In contrast to other lithium compounds, Li₄SiO₄、Li₂SiO₃、Li₂Si₂O₅Since such lithium silicate is relatively stable, a silicon-based active material containing such a lithium compound can obtain more stable battery characteristics. These lithium compounds can be formed by forming SiO inside the silicon compound particles₂A part of the component is selectively replaced by a lithium compound to modify the silicon compound particles.

The lithium compound inside the silicon compound particle can be quantified by NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy). The XPS and NMR can be measured, for example, under the following conditions.

XPS

An apparatus: x-ray photoelectron spectrometer

X-ray source: monochromatized Al Ka ray

X-ray focal spot diameter: 100 μm

Ar ion gun sputtering conditions: 0.5kV 2mm

²⁹Si MAS NMR (magic angle rotating nuclear magnetic resonance)

An apparatus: 700NMR spectrometer manufactured by Bruker

Probe: 4mmHR-MAS rotor 50 μ L

Sample rotation speed: 10kHz

Measurement of ambient temperature: 25 deg.C

In the present invention, when modifying the silicon compound particles, electrochemical methods, modification by redox reactions, thermal doping as a physical method, and other methods can be used. In particular, when the silicon compound particles are modified by an electrochemical method or by redox modification, the battery characteristics of the negative electrode active material are improved. In addition, the modification can be performed not only by taking in lithium into the silicon compound particles, but also by performing stabilization of the lithium compound by heat treatment and desorption of lithium from the silicon compound particles together. This further improves the stability of the slurry, such as the water resistance of the negative electrode active material.

In the anode active material of the present invention, it is preferable that the silicon compound particles have a structure of²⁹Derived from SiO at-95 to-150 ppm as chemical shift values obtained by Si-MAS-NMR spectroscopy₂The peak of the region. Thus, SiO in the silicon compound particles is not modified₂The whole region was replaced with a lithium compound, and a certain amount of SiO remained₂In this region, the stability of the slurry is further improved.

As described above, the silicon-based active material particles have a layer containing a compound having a silyl group on the surface of the silicon compound particles or the surface of the carbon coating film.

Examples of the silyl group include silyl groups having an organic group such as an alkyl group. In this case, the silyl group preferably has an organic group having 1 to 5 carbon atoms. Further preferably a silyl group having an alkyl group having 1 to 5 carbon atoms. In this case, the alkyl group is preferably a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, a tert-butyl group, or an isobutyl group. When the alkyl chain has 5 or less carbon atoms, the hydrophobicity is not excessively increased, and the dispersibility in the aqueous slurry is not easily deteriorated. Examples of the silyl group having such an alkyl group include trialkylsilyl groups such as a trimethylsilyl group and a triethylsilyl group. In this case, the three alkyl groups may be the same as or different from those described above. In addition, the organic group with 1-5 carbon atoms can contain unsaturated bonds. Further, a fluorine atom may be contained in the alkyl group.

By covering the surface of the silicon compound particles with a layer containing a compound having a silyl group such as a trialkylsilyl group, aggregation of the silicon compound particles is suppressed during the production of the electrode, and the water resistance is further improved, thereby further improving the stability of the slurry.

Silyl groups such as alkylsilyl groups on the surface can be identified by TOF-SIMS. The detection of the fragment of the silyl group functionalized compound such as the alkylsilyl group-based compound in the outermost layer can be performed, for example, under the following conditions.

PHI TRIFT 2 manufactured by ULVAC-PHI, INCORPORATED

Primary ion source: gallium (Ga)

Temperature of the sample: 25 deg.C

Acceleration voltage: 5kV

Focal spot size: 100 μm × 100 μm

Sputtering: ga. 100 μm × 100 μm, 10s

Cationic mass spectrometry

Sample: indium metal powder compaction

In the negative electrode active material of the present invention, the coverage amount of the carbon material is preferably 0.5 mass% or more and 15 mass% or less with respect to the total of the silicon compound particles and the carbon material. When the amount of the carbon material is the coating amount, the negative electrode active material particles having a high capacity and excellent conductivity can be obtained.

In the negative electrode active material of the present invention, it is preferable that the volume resistivity of the negative electrode active material particles when applied at 38.2MPa, measured by the four-probe method according to JIS K7194, is 0.01 Ω · cm or more and less than 100 Ω · cm. If the volume resistivity is 0.01 Ω · cm or more and less than 100 Ω · cm, sufficient conductivity can be secured, and therefore, more favorable battery characteristics can be obtained.

[ method for producing negative electrode ]

Next, an example of a method for producing a negative electrode of a nonaqueous electrolyte secondary battery according to the present invention will be described.

A method for producing the anode material included in the anode is initially described. First, a silicon-containing compound (SiO) is prepared_x: x is not less than 0.5 and not more than 1.6). Next, at least a part of the silicon compound particles is covered with a carbon material. Next, the silicon compound particles are modified by taking lithium into the silicon compound particles. In this case, the lithium compound can be simultaneously generated in the interior and on the surface of the silicon compound particle. Further, at this time, a part of Li among Li that has been taken in may be taken out from the silicon compound particles. Next, a layer containing a silyl group functionalized compound is formed on at least a part of the surface of the modified silicon compound particle or the surface of the carbon material or both surfaces. Then, the silicon compound particles having the layer containing the silyl group functionalized compound are used as negative electrode active material particles, and mixed with a conductive assistant and a binder, thereby producing a negative electrode material and a negative electrode.

More specifically, the negative electrode material can be produced, for example, by the following steps.

First, a raw material for generating a silicon oxide gas is heated at a temperature ranging from 900 to 1600 ℃ in the presence of an inert gas or under reduced pressure to generate a silicon oxide gas. In this case, the raw material is a mixture of the metal silicon powder and the silica powder, and considering the presence of oxygen on the surface of the metal silicon powder and a trace amount of oxygen in the reaction furnace, the mixing molar ratio is desirably in the range of 0.8< metal silicon powder/silica powder < 1.3. The Si crystallites in the particles are controlled by varying the feed range and vaporization temperature, as well as the post-production heat treatment. The generated gas is deposited on the adsorption plate. The deposit is taken out in a state where the temperature in the reaction furnace is lowered to 100 ℃ or lower, and is pulverized and powdered by using a ball mill, a jet mill, or the like.

Next, a carbon coating film (carbon material) is formed on the surface layer of the obtained powder material (silicon compound particles). The carbon coating is effective for further improving the battery characteristics of the negative electrode active material.

As a watch on powder materialsThe method for forming a carbon coating film on the layer is desirably a thermal decomposition CVD method. Pyrolytic CVD is to place silicon oxide powder in a furnace, fill the furnace with hydrocarbon gas, and raise the temperature in the furnace. The decomposition temperature is not particularly limited, and is preferably 1200 ℃ or lower. More desirably, 950 ℃ or lower, the unexpected disproportionation of the silicon oxide can be suppressed. The hydrocarbon gas is not particularly limited, and is desirably C_nH_mThe composition of (a) is a hydrocarbon gas of 3. gtoreq.n. This is because the production cost can be reduced and the physical properties of the decomposition product can be improved.

Next, the modification in the bulk of the powder material is performed. Examples of the method for in vivo modification include an electrochemical method, a redox method, and a thermal doping method. The electrochemical method is desirably performed using a device capable of electrochemically absorbing and desorbing Li. The apparatus structure is not particularly limited, and the in-vivo modification can be performed using, for example, the in-vivo modification apparatus 20 shown in fig. 3. The in-body reforming apparatus 20 includes: a bath 27 filled with the organic solvent 23; an anode electrode (lithium source, reforming source) 21 disposed in the bath 27 and connected to one side of the power source 26; a powder container 25 disposed in the bath 27 and connected to the other side of the power source 26; and a separator 24 provided between the anode 21 and the powder container 25. In the powder container 25, silicon oxide particles (silicon-based active material particles) 22 are contained. In this manner, the powder container contains the silicon oxide particles, and the power supply applies a voltage to the anode electrode (lithium source) and the powder container containing the silicon oxide particles. This makes it possible to absorb lithium into the silicon compound particles or to release lithium from the silicon compound particles, thereby modifying the silica particles 22. The obtained silica particles are heat-treated at 400 to 800 ℃ to stabilize the Li compound. After the modification, the resin composition can be washed with ethanol, alkaline water, weak acid, pure water, or the like.

As the organic solvent 23 in the bath 27, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, or the like can be used. As the electrolyte salt contained in the organic solvent 23, lithium hexafluorophosphate (LiPF) can be used₆) Lithium tetrafluoroborate (LiBF)₄) And the like.

A lithium foil may be used for the anode electrode 21, and a compound containing Li may be used. Examples of the Li-containing compound include lithium carbonate, lithium oxide, lithium cobaltate, olivine lithium iron, lithium nickelate, lithium vanadium phosphate, and the like.

In the modification by the redox method, for example, first, lithium can be absorbed by immersing silicon-based active material particles in a solution a in which lithium is dissolved in an ether-based solvent. The solution a may further contain a polycyclic aromatic compound or a linear polyphenyl compound. The Li compound can be stabilized by heat-treating the obtained silicon-based active material particles at 400 to 800 ℃. In addition, after the absorption of lithium, the silicon-based active material particles may be immersed in a solution B containing a polycyclic aromatic compound or a derivative thereof to release active lithium from the silicon-based active material particles. As the solvent of the solution B, for example, an ether solvent, a ketone solvent, an ester solvent, an alcohol solvent, an amine solvent, or a mixed solvent of these solvents can be used. After the modification, the resin composition can be washed with ethanol, alkaline water, weak acid, pure water, or the like.

As the ether solvent used in the solution a, diethyl ether, tert-butyl methyl ether, tetrahydrofuran, dioxane, 1, 2-dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, or a mixed solvent of these solvents can be used. Among them, tetrahydrofuran, dioxane, 1, 2-dimethoxyethane, and diethylene glycol dimethyl ether are particularly preferably used. Preferably these solvents are dehydrated, more preferably deoxygenated.

Further, naphthalene, anthracene, phenanthrene, naphthacene, pentacene, pyrene, triphenylene, coronene, or the like can be used as the polycyclic aromatic compound contained in the solution A,

And one or more derivatives of these polycyclic aromatic compounds, and as the linear polyphenyl compounds, one or more of biphenyl, terphenyl, and derivatives of these linear polyphenyl compounds can be used.

AsThe polycyclic aromatic compound contained in the solution B may be naphthalene, anthracene, phenanthrene, condensed tetraphene, condensed pentacene, pyrene, triphenylene, coronene, or the like,And one or more derivatives of these polycyclic aromatic compounds.

As the ether solvent of the solution B, diethyl ether, tert-butyl methyl ether, tetrahydrofuran, dioxane, 1, 2-dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, or a mixed solvent of these ether solvents, and the like can be used.

As the ketone solvent, acetone, acetophenone, or the like can be used.

As the ester solvent, methyl formate, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, and the like can be used.

As the alcohol solvent, methanol, ethanol, propanol, isopropanol, or the like can be used.

As the amine solvent, methylamine, ethylamine, ethylenediamine, and the like can be used.

In the case of modification by the thermal doping method, for example, modification can be performed by mixing a powder material (silicon-based active material particles) with LiH powder or Li powder and then heating the mixture in a non-oxidizing atmosphere. As the non-oxidizing atmosphere, for example, an Ar atmosphere or the like can be used. More specifically, first, LiH powder or Li powder and silicon oxide powder are sufficiently mixed in an Ar atmosphere, sealed, and stirred together with a sealed container to be homogenized. Then, the mixture is heated at a temperature ranging from 700 ℃ to 750 ℃ to modify the mixture. In this case, for the purpose of removing Li from the silicon compound particles, a method of sufficiently cooling the heated powder and then washing the powder with ethanol, alkaline water, weak acid, or pure water, or the like can be used.

Next, a layer containing a silyl group functionalized compound is formed on at least a part of the surface of the modified silicon compound particle or the surface of the carbon material or both surfaces. The layer containing a compound having a silyl group is preferably formed by treatment with an alkylsilazane. In this way, a layer containing a compound having a silyl group can be efficiently formed by performing a treatment on the surface of the material. More specifically, for example, a layer containing a compound having a silyl group can be formed by the following procedure. Further, it is considered that OH groups are present on the surface of the silicon compound particles or the carbon material, and the OH groups react with the decomposition product of the alkylsilazane, whereby trialkylsilyl groups are introduced onto the surface of the silicon compound particles or the carbon material.

First, dehydrated toluene, one-fourth part by mass of modified silicon compound particles of dehydrated toluene, and HMDS (hexamethyldisilazane) corresponding to 3% by mass of the modified silicon compound particles were put in a vessel and stirred for 1 hour. Then, the silicon compound particles in which the layer containing the silyl group functionalized compound is formed are filtered out as negative electrode active material particles.

Next, if necessary, a silicon-based active material containing the silicon compound particles having the layer is mixed with a carbon-based active material, and the negative electrode active material is mixed with other materials such as a binder and a conductive assistant to prepare a negative electrode mixture, and then an organic solvent, water, or the like is added to prepare a slurry.

Next, as shown in fig. 2, the slurry of the negative electrode mixture is applied to the surface of the negative electrode current collector 11 and dried to form the negative electrode active material layer 12. In this case, heating and pressurizing may be performed as necessary. In the above manner, the negative electrode of the nonaqueous electrolyte secondary battery of the present invention can be manufactured.

< lithium ion Secondary Battery >

Next, the nonaqueous electrolyte secondary battery of the present invention will be explained. The nonaqueous electrolyte secondary battery of the present invention contains the negative electrode active material of the present invention. Here, a laminate film type lithium ion secondary battery will be described as a specific example of the nonaqueous electrolyte secondary battery of the present invention.

[ Structure of laminated film type Secondary Battery ]

The laminated film type lithium ion secondary battery 30 shown in fig. 4 mainly contains a wound electrode assembly 31 inside a sheet-shaped exterior member 35. The wound electrode body 31 is wound with a separator between the positive electrode and the negative electrode. Further, a laminate may be housed with a separator between the positive electrode and the negative electrode. In any of the electrode bodies, a cathode lead 32 is attached to the cathode and an anode lead 33 is attached to the anode. The outermost peripheral portion of the electrode body is protected by a protective tape.

The positive and negative electrode leads 32 and 33 are led out in one direction from the inside of the exterior member 35 toward the outside, for example. The positive electrode lead 32 is formed of a conductive material such as aluminum, and the negative electrode lead 33 is formed of a conductive material such as nickel or copper.

The exterior member 35 is, for example, a laminated film in which a fusion-bonded layer, a metal layer, and a surface protective layer are laminated in this order, and in the laminated film, outer peripheral edge portions of the fusion-bonded layers of the two films are fusion-bonded to each other so that the fusion-bonded layer faces the wound electrode body 31, or are bonded with an adhesive or the like. The fusion-bonded portion is a film of polyethylene, polypropylene, or the like, for example, and the metal portion is an aluminum foil or the like. The protective layer is, for example, nylon or the like.

An adhesion film 34 for preventing intrusion of external air is inserted between the exterior member 35 and the positive and negative electrode leads. The material is, for example, polyethylene, polypropylene, polyolefin resin.

The positive electrode has a positive electrode active material layer on both surfaces or one surface of a positive electrode current collector, for example, as in the negative electrode 10 of fig. 2.

The positive electrode current collector is formed of a conductive material such as aluminum.

The positive electrode active material layer contains one or more of positive electrode materials capable of occluding and releasing lithium ions, and may contain other materials such as a positive electrode binder, a positive electrode conductive auxiliary agent, and a dispersant according to design. In this case, the details of the positive electrode binder and the positive electrode conductive auxiliary are the same as those of the negative electrode binder and the negative electrode conductive auxiliary described above, for example.

The positive electrode material is desirably a lithium-containing compound. Examples of the lithium-containing compound include a composite oxide composed of lithium and a transition metal element, and a phosphoric acid compound having lithium and a transition metal element. Among these positive electrode materials, those having at least one or more of nickel, iron, manganese, and cobalt are preferableA compound (I) is provided. The chemical formula of these positive electrode materials is, for example, Li_xM₁O₂Or Li_yM₂PO₄And (4) showing. In the formula, M₁、M₂Represents at least one transition metal element. The values of x and y are different depending on the charge/discharge state of the battery, but are usually 0.05. ltoreq. x.ltoreq.1.10 and 0.05. ltoreq. y.ltoreq.1.10.

Examples of the composite oxide having lithium and a transition metal element include lithium cobalt composite oxide (Li)_xCoO₂) Lithium nickel composite oxide (Li)_xNiO₂) And lithium nickel cobalt composite oxides. Examples of the lithium nickel cobalt composite oxide include lithium nickel cobalt aluminum composite oxide (NCA) and lithium nickel cobalt manganese composite oxide (NCM).

Examples of the phosphate compound having lithium and a transition metal element include a lithium iron phosphate compound (LiFePO)₄) Or lithium iron manganese phosphate compounds (LiFe)_1-uMn_uPO₄，(0<u<1) Etc.). When these positive electrode materials are used, a high battery capacity can be obtained and excellent cycle characteristics can be obtained at the same time.

[ negative electrode ]

The negative electrode has the same structure as the negative electrode 10 for a lithium ion secondary battery shown in fig. 2, and has, for example, negative electrode active material layers on both surfaces of a current collector. It is preferable that the negative electrode charge capacity of the negative electrode becomes larger with respect to the electric capacity obtained by the positive electrode active material (as the charge capacity of the battery). This can suppress precipitation of lithium metal on the negative electrode.

The positive electrode active material layer is provided on a part of both surfaces of the positive electrode current collector, and similarly, the negative electrode active material layer is also provided on a part of both surfaces of the negative electrode current collector. In this case, for example, the following regions are provided: the negative electrode active material layer provided on the negative electrode current collector has no region of the positive electrode active material layer opposed thereto. This is for stable cell design.

The region where the negative electrode active material layer and the positive electrode active material layer do not face each other is hardly affected by charge and discharge. Therefore, the state of the negative electrode active material layer can be maintained after formation, and the composition of the negative electrode active material and the like can be detected with good reproducibility and accuracy without depending on whether or not charging and discharging are performed.

[ separator ]

The separator separates the positive electrode from the negative electrode, prevents short-circuiting of current accompanying contact of the two electrodes, and allows lithium ions to pass therethrough. The separator may be formed of a porous film made of, for example, a synthetic resin or ceramic, or may have a laminated structure in which two or more porous films are laminated. Examples of the synthetic resin include polytetrafluoroethylene, polypropylene, and polyethylene.

[ electrolyte ]

A liquid electrolyte (electrolytic solution) is impregnated into at least a part of the active material layer or the separator. The electrolyte solution contains an electrolyte salt dissolved in a solvent, and may contain other materials such as additives.

The solvent can be, for example, a nonaqueous solvent. Examples of the nonaqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, methylpropyl carbonate, 1, 2-dimethoxyethane, tetrahydrofuran, and the like. Among them, it is desirable to use at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. This is because more favorable characteristics can be obtained. In this case, by using a high-viscosity solvent such as ethylene carbonate or propylene carbonate in combination with a low-viscosity solvent such as dimethyl carbonate, methylethyl carbonate, or diethyl carbonate, more advantageous characteristics can be obtained. This is because the dissociation property and ion mobility of the electrolyte salt are improved.

The solvent additive is preferably a cyclic carbonate containing an unsaturated carbon bond. This is because a stable film is formed on the surface of the negative electrode during charge and discharge, and the decomposition reaction of the electrolyte can be suppressed. Examples of the unsaturated carbon-bonded cyclic carbonate include vinylene carbonate and vinylethylene carbonate.

The solvent additive preferably contains sultone (cyclic sulfonate). The reason for this is that chemical stability of the battery is improved. Examples of the sultone include propane sultone and propylene sultone.

Further, the solvent preferably contains an acid anhydride. The reason for this is that the chemical stability of the electrolyte is improved. Examples of the acid anhydride include propane disulfonic acid anhydride (propane disulfonic acid anhydride).

The electrolyte salt may contain any one or more of light metal salts such as lithium salts. The lithium salt includes, for example, lithium hexafluorophosphate (LiPF)₆) Lithium tetrafluoroborate (LiBF)₄) And the like.

The content of the electrolyte salt is preferably 0.5mol/kg or more and 2.5mol/kg or less with respect to the solvent. This is because high ion conductivity can be obtained.

[ method for producing laminated film type Secondary Battery ]

The positive electrode is first produced using the positive electrode material. First, a positive electrode active material is mixed with a positive electrode binder, a positive electrode conductive additive, and the like as needed to prepare a positive electrode mixture, and then the mixture is dispersed in an organic solvent to prepare a positive electrode mixture slurry. Next, the mixture slurry is applied to the positive electrode current collector by a coating device such as a die coater having a knife roll (knife roll) or a die head (die head), and the mixture slurry is hot-air dried to obtain a positive electrode active material layer. Finally, the positive electrode active material layer is compression-molded by a roll press or the like. At this time, heating may be performed, and compression may be repeated several times.

Subsequently, a negative electrode is produced by forming a negative electrode active material layer on a negative electrode current collector by the same procedure as that for producing the negative electrode 10 for a lithium ion secondary battery.

When manufacturing the positive electrode and the negative electrode, respective active material layers are formed on both surfaces of the positive electrode current collector and the negative electrode current collector. At this time, in either electrode, the active material application lengths of both face portions may not be uniform (refer to fig. 2).

Next, an electrolytic solution was prepared. Next, the cathode lead 32 is attached to the cathode current collector by ultrasonic welding or the like, and the anode lead 33 is attached to the anode current collector at the same time. Next, the positive electrode and the negative electrode are laminated or wound with a separator interposed therebetween to produce a wound electrode assembly 31, and a protective tape is bonded to the outermost portion of the wound electrode assembly. Subsequently, the wound body is molded in a flat shape. Next, the wound electrode assembly is sandwiched between the folded film-shaped exterior members 35, and then the insulating portions of the exterior members are bonded to each other by a heat welding method, and the wound electrode assembly is sealed in a state of being opened only in one direction. Next, a sealing film is inserted between the positive electrode lead and the negative electrode lead and the exterior member. Subsequently, a predetermined amount of the electrolyte solution prepared above was added through the open part, and vacuum impregnation was performed. After impregnation, the open portions were bonded by a vacuum heat fusion method. Thus, the laminate film type secondary battery 30 can be manufactured.

In the nonaqueous electrolyte secondary battery of the present invention such as the multilayer film type secondary battery 30 manufactured as described above, the negative electrode utilization rate at the time of charge and discharge is preferably 93% to 99%. If the negative electrode utilization rate is set in the range of 93% or more, the battery capacity can be greatly increased without lowering the initial charging efficiency. When the negative electrode utilization rate is set to 99% or less, Li is not precipitated, and safety can be ensured.