JP6995488B2 - Negative electrode active material, mixed negative electrode active material, and method for manufacturing negative electrode active material - Google Patents

Description

The present invention relates to a negative electrode active material, a mixed negative electrode active material material, and a method for producing a negative electrode active material.

In recent years, small electronic devices such as mobile terminals have become widespread, and further miniaturization, weight reduction, and long life are strongly required. In response to such market demands, the development of a secondary battery that is particularly compact, lightweight, and capable of obtaining a high energy density is underway. This secondary battery is being considered for application not only to small electronic devices but also to large electronic devices such as automobiles and power storage systems such as houses.

Among them, lithium-ion secondary batteries are highly expected because they are compact and easy to increase in capacity, and can obtain higher energy density than lead-acid batteries and nickel-cadmium batteries.

The above-mentioned lithium ion secondary battery includes an electrolytic solution together with a positive electrode, a negative electrode, and a separator, and the negative electrode contains a negative electrode active material involved in a charge / discharge reaction.

While carbon-based active materials are widely used as the negative electrode active material, further improvement in battery capacity is required due to recent market demands. In order to improve the battery capacity, it is being studied to use silicon as a negative electrode active material. This is because the theoretical capacity of silicon (4199 mAh / g) is more than 10 times larger than the theoretical capacity of graphite (372 mAh / g), so that a significant improvement in battery capacity can be expected. The development of Kay material as a negative electrode active material is being studied not only for silicon alone, but also for alloys and compounds typified by oxides. In addition, the shape of the active material has been studied from the standard coating type for carbon-based active materials to the integrated type that deposits directly on the current collector.

However, when silicon is used as the main raw material as the negative electrode active material, the negative electrode active material expands and contracts during charging and discharging, so that it is easily cracked mainly in the vicinity of the surface layer of the negative electrode active material. In addition, an ionic substance is generated inside the active material, and the negative electrode active material becomes a fragile substance. When the surface layer of the negative electrode active material is cracked, a new surface is created, which increases the reaction area of the active material. At this time, the decomposition reaction of the electrolytic solution occurs on the new surface, and the electrolytic solution is consumed because a film which is a decomposition product of the electrolytic solution is formed on the new surface. Therefore, the cycle characteristics tend to deteriorate.

So far, in order to improve the initial efficiency of the battery and the cycle characteristics, various studies have been made on the negative electrode material for lithium ion secondary batteries and the electrode configuration, which are mainly made of Kay material.

Specifically, silicon and amorphous silicon dioxide are simultaneously deposited using the vapor phase method for the purpose of obtaining good cycle characteristics and high safety (see, for example, Patent Document 1). Further, in order to obtain high battery capacity and safety, a carbon material (electron conductive material) is provided on the surface layer of the silicon oxide particles (see, for example, Patent Document 2). Further, in order to improve the 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 is formed in the vicinity of the current collector (). For example, see Patent Document 3). Further, in order to improve the cycle characteristics, oxygen is contained in the silicon active material, and the average oxygen content is 40 at% or less, and the oxygen content is increased in a place close to the current collector. (See, for example, Patent Document 4).

Further, in order to improve the initial charge / discharge efficiency, a _{nanocomposite} containing a Si phase, SiO ₂ , and My O metal oxide is used (see, for example, Patent Document 5). Further, in order to improve the cycle characteristics, SiO _x (0.8 ≦ x ≦ 1.5, particle size range = 1 μm to 50 μm) and a carbon material are mixed and fired at a high temperature (see, for example, Patent Document 6). In addition, in order to improve the 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 difference between the maximum and minimum molar ratios near the interface between the active material and the current collector is set. The active material is controlled within the range of 0.4 or less (see, for example, Patent Document 7). Further, in order to improve the battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 8). Further, in order to improve the cycle characteristics, a hydrophobic layer such as a silane compound is formed on the surface layer of the Kay material (see, for example, Patent Document 9). Further, in order to improve the cycle characteristics, silicon oxide is used and a graphite film is formed on the surface layer thereof to impart conductivity (see, for example, Patent Document 10). In Patent Document 10, broad peaks appear at 1330 cm ^-1 and 1580 cm ^-1 with respect to the shift value obtained from the RAMAN spectrum for the graphite coating, and their intensity ratios I ₁₃₃₀ / I ₁₅₈₀ are 1.5 <I ₁₃₃₀ / I. ₁₅₈₀ <3. Further, in order to improve the battery capacity and cycle characteristics, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used (see, for example, Patent Document 11). Further, in order to improve the overcharge and overdischarge characteristics, a silicon oxide in which the atomic number ratio of silicon and oxygen is controlled to 1: y (0 <y <2) is used (see, for example, Patent Document 12).

Japanese Unexamined Patent Publication No. 2001-185127 Japanese Unexamined Patent Publication No. 2002-042806 Japanese Unexamined Patent Publication No. 2006-164954 Japanese Unexamined Patent Publication No. 2006-114454 Japanese Unexamined Patent Publication No. 2009-070825 Japanese Unexamined Patent Publication No. 2008-282819 Japanese Unexamined Patent Publication No. 2008-251369 Japanese Unexamined Patent Publication No. 2008-177346 JP-A-2007-234255 Japanese Unexamined Patent Publication No. 2009-21204 Japanese Unexamined Patent Publication No. 2009-205950 Japanese Patent No. 2997741

As mentioned above, in recent years, small electronic devices such as mobile terminals have been promoted to have higher performance and higher functionality, and the lithium ion secondary battery, which is the main power source thereof, is required to have an increased battery capacity. ing. As one method for solving this problem, it is desired to develop a lithium ion secondary battery made of a negative electrode using a Kay material as a main material. Further, a lithium ion secondary battery using a Kay material is desired to have initial efficiency and cycle characteristics close to those of a lithium ion secondary battery using a carbon material, and stability of an aqueous slurry produced at the time of producing a negative electrode. There is. However, it has not been possible to propose a negative electrode active material exhibiting initial efficiency, cycle stability, and slurry stability equivalent to those of a lithium ion secondary battery using a carbon material.

The present invention has been made in view of the above-mentioned problems, and can stabilize the slurry produced at the time of producing the negative electrode of the secondary battery, and when used as the negative electrode active material of the secondary battery, the initial charge / discharge is performed. It is an object of the present invention to provide a negative electrode active material capable of improving characteristics and cycle characteristics, and a mixed negative electrode active material material containing the negative electrode active material. Another object of the present invention is to provide a method for producing a negative electrode active material, which can stabilize the slurry produced at the time of producing the negative electrode and can improve the initial charge / discharge characteristics and the cycle characteristics.

In order to achieve the above object, the present invention is a negative electrode active material containing negative negative active material particles, and the negative negative active material particles contain a silicon compound (SiO _x : 0.5 ≦ x ≦ 1.6). The silicon compound particles contain at least one of Li ₂ SiO ₃ and Li ₄ SiO ₄ , and among the primary particles of the silicon compound particles, the primary particles having a particle diameter of 1 μm or less. Provided is a negative electrode active material characterized in that the ratio is 15% or less on a volume basis.

Since the negative electrode active material of the present invention contains negative electrode active material particles (also referred to as silicon-based active material particles) containing silicon compound particles, the battery capacity can be improved. Further, when the silicon compound particles contain the above-mentioned lithium silicate, the irreversible capacity generated during charging can be reduced. This makes it possible to improve the initial efficiency and cycle characteristics of the battery. Further, among the primary particles of the silicon compound particles, the ratio of the primary particles having a particle diameter of 1 μm or less is 15% or less on a volume basis, and the amount of fine particles having a large surface area per volume in which lithium ions are easily eluted is small. Lithium ions are difficult to elute from compound particles. As a result, when producing a slurry (water-based negative electrode slurry) in which the negative electrode active material or the like is dispersed in an aqueous solvent, the elution of Li ions from the negative electrode active material particles is suppressed, the stability of the water-based negative electrode slurry is improved, and the first time. Improves efficiency and cycle characteristics.

At this time, it is preferable that the proportion of the primary particles having a particle diameter of 1 μm or less among the primary particles of the silicon compound particles is 8% or less on a volume basis.

In such a case, since the amount of fine powder having a large surface area per volume contained in the primary particles of the silicon compound particles is small, the stability of the aqueous negative electrode slurry is further improved, and the initial efficiency and the cycle characteristics are further improved. do.

Further, it is preferable that the BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more and 6 m ² / g or less. Further, it is particularly preferable that the BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more and 4 m ² / g or less.

When the BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more, a sufficient contact area between the particles can be sufficiently secured, so that the conductivity of the negative electrode active material is improved. As a result, the cycle characteristics and initial efficiency of the battery are further improved. When the BET specific surface area of the primary particles of the silicon compound particles is 6 m ² / g or less, particularly 4 m ² / g or less, the specific surface area of the silicon compound particles is within a range that is more appropriately smaller, and the elution of lithium ions is further suppressed. can. As a result, the stability of the water-based negative electrode slurry is further improved, and the initial efficiency is further improved.

Further, the mode diameter in the volume-based particle size distribution of the silicon compound particles is 5 μm or more and 10 μm or less, and the distributed cumulative value in the volume-based particle size distribution is 99.9%. It is preferable that _99.9 is 20 μm or less.

When the mode diameter is 5 μm or more and 10 μm or less, the silicon compound particles have an appropriate size, so that a sufficient contact area between the particles can be secured and the conductivity of the negative electrode active material is improved. As a result, the cycle characteristics and initial efficiency of the battery are improved. Further, when D _99.9 is 20 μm or less, the local variation in the diameter of the negative electrode active material in the electrode can be reduced. As a result, the cycle characteristics are improved.

Further, the silicon compound particles have a peak width at half maximum (2θ) of 1.2 ° or more due to the Si (111) crystal plane in the X-ray diffraction spectrum using Cu—Kα rays, and the crystal plane has a peak width at half maximum (2θ). The corresponding crystallite size is preferably 7.5 nm or less.

If the negative electrode active material in which the silicon compound particles have the above-mentioned silicon crystallinity is used as the negative electrode active material of the lithium ion secondary battery, better cycle characteristics and initial charge / discharge characteristics can be obtained.

Further, the negative electrode active material of the present invention is the maximum peak intensity value A of the Si and Li silicate regions given at a chemical shift value of -60 to -95 ppm obtained from the ²⁹ Si-MAS-NMR spectrum in the silicon compound particles. It is preferable that the peak intensity value B in the SiO ₂ region given as a chemical shift value of −96 to −150 ppm satisfies the relationship of A> B.

If the silicon compound particles have a larger amount of Si and Li ₂ SiO ₃ with respect to the SiO ₂ component, the negative electrode active material can sufficiently obtain the effect of improving the battery characteristics by inserting Li.

Further, a test cell composed of a negative electrode containing a mixture of the negative electrode active material and a carbon-based active material and counterpolar lithium is prepared, and in the test cell, charging is performed by passing a current so as to insert lithium into the negative electrode active material. , A charge / discharge consisting of a discharge in which a current is passed so as to desorb lithium from the negative electrode active material is carried out 30 times, and the discharge capacity Q in each charge / discharge is differentiated by the potential V of the negative electrode electrode with respect to the counter electrode lithium. When a graph showing the relationship between the differential value dQ / dV and the potential V is drawn, the potential V of the negative electrode during the Xth and subsequent discharges (1 ≦ X ≦ 30) is 0.40 V to 0. It is preferable that the peak is in the range of 55V.

The above-mentioned peak in the V-dQ / dV curve is similar to the peak of the Kay material, and the discharge curve on the higher potential side rises sharply, so that the capacity is easily expressed when designing the battery. Further, if the peak is expressed by charging / discharging within 30 times, it becomes a negative electrode active material in which a stable bulk is formed.

Further, it is preferable that the median diameter of the primary particles of the silicon compound particles, which is the particle diameter at which the cumulative distribution value in the volume-based particle size distribution is 50%, is 1.0 μm or more and 15 μm or less.

When the median diameter of the primary particles of the silicon compound particles is 1.0 μm or more, it is possible to suppress an increase in the irreversible capacity of the battery due to an increase in the surface area per mass. On the other hand, when the median diameter is 15 μm or less, the particles are less likely to crack and a new surface is less likely to appear.

Further, it is preferable that the negative electrode active material particles contain a carbon material in the surface layer portion.

As described above, when the negative electrode active material particles contain a carbon material in the surface layer portion thereof, improvement in conductivity can be obtained.

Further, the average thickness of the carbon material is preferably 10 nm or more and 5000 nm or less.

If the average thickness of the carbon material is 10 nm or more, the conductivity can be improved. Further, if the average thickness of the carbon material to be coated is 5000 nm or less, a sufficient amount of silicon compound particles can be secured by using the negative electrode active material containing such negative electrode active material particles in the lithium ion secondary battery. , It is possible to suppress a decrease in battery capacity.

Provided is a mixed negative electrode active material material characterized by containing the above-mentioned negative electrode active material and a carbon-based active material.

As described above, by including the carbon-based active material together with the negative electrode active material (silicon-based negative electrode active material) of the present invention as the material for forming the negative electrode active material layer, the conductivity of the negative electrode active material layer can be improved. At the same time, it becomes possible to relieve the expansion stress associated with charging. Further, the battery capacity can be increased by mixing the silicon-based negative electrode active material with the carbon-based active material.

Further, in order to achieve the above object, the present invention is a method for producing a negative electrode active material containing negative electrode active material particles containing silicon compound particles, wherein the silicon compound (SiO _x : 0.5 ≦ x ≦ 1). Negative electrode active material particles by a step of producing silicon compound particles containing .6) and a step of inserting lithium into the silicon compound particles to contain at least one of Li ₂ SiO ₃ and Li ₄ SiO ₄ . The selection comprises the step of selecting from the negative electrode active material particles that the ratio of the primary particles having a particle diameter of 1 μm or less is 15% or less on a volume basis among the primary particles of the silicon compound. Provided is a method for producing a negative electrode active material, which comprises producing a negative electrode active material using the negative electrode active material particles.

By selecting the negative electrode active material particles in this way to produce the negative electrode active material, the aqueous slurry produced at the time of producing the negative electrode can be stabilized, and the negative electrode active material is used as the negative electrode active material of the lithium ion secondary battery. It is possible to produce a negative electrode active material having a high capacity and good cycle characteristics and initial charge / discharge characteristics.

The negative electrode active material of the present invention can stabilize the aqueous slurry produced at the time of producing the negative electrode, and when used as the negative electrode active material of the secondary battery, it has a high capacity and good cycle characteristics and initial charge / discharge characteristics. Is obtained. Further, the same effect can be obtained with the mixed negative electrode active material containing the negative electrode active material. Further, according to the method for producing a negative electrode active material of the present invention, the aqueous slurry produced at the time of producing the negative electrode can be stabilized, and a good cycle is obtained when used as the negative electrode active material of the lithium ion secondary battery. A negative electrode active material having characteristics and initial charge / discharge characteristics can be produced.

It is sectional drawing which shows an example of the structure of the negative electrode for a non-aqueous electrolyte secondary battery containing the negative electrode active material of this invention. This is an example of a ²⁹ Si-MAS-NMR spectrum measured from silicon compound particles when modified by a redox method. This is an example of the ²⁹ Si-MAS-NMR spectrum measured from silicon compound particles when modified by the heat doping method. It is a figure which shows the structural example (laminate film type) of the lithium secondary battery which contains the negative electrode active material of this invention. It is a graph which shows the relationship between the ratio of silicon-based active material particles with respect to the total amount of a negative electrode active material, and the rate of increase of the battery capacity of a secondary battery.

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

As described above, as one method for increasing the battery capacity of a lithium ion secondary battery, it is considered to use a negative electrode using a Kay material as a main material as a negative electrode of a lithium ion secondary battery. Lithium-ion secondary batteries using this Kay material are desired to have slurry stability, initial charge / discharge characteristics, and cycle characteristics that are close to those of lithium-ion secondary batteries using carbon-based active materials. It has not been possible to propose a negative electrode active material having slurry stability, initial charge / discharge characteristics, and cycle characteristics equivalent to those of a lithium ion secondary battery using an active material.

Therefore, the present inventors have made extensive studies in order to obtain a negative electrode active material having a high battery capacity and good slurry stability, cycle characteristics, and initial efficiency when used in a secondary battery. It led to the invention.

The negative electrode active material of the present invention contains negative electrode active material particles. The negative electrode active material particles contain silicon compound particles containing a silicon compound (SiO _x : 0.5 ≦ x ≦ 1.6). The silicon compound particles contain at least one lithium silicate of Li ₂ SiO ₃ and Li ₄ SiO ₄ . The silicon compound particles satisfy the condition that the proportion of the primary particles having a particle diameter of 1 μm or less among the primary particles is 15% or less on a volume basis.

Since such a negative electrode active material contains negative electrode active material particles including silicon compound particles, the battery capacity can be improved. Further, since the silicon compound particles contain the above-mentioned lithium silicate, the irreversible capacity generated during charging can be reduced. Further, when the ratio of the primary particles having a particle diameter of 1 μm or less among the primary particles of the silicon compound particles is 15% or less on a volume basis, the abundance of fine particles of the silicon compound particles in which lithium is easily eluted is small, so that the aqueous negative electrode is used. It is possible to suppress the elution of lithium ions from the negative electrode active material when preparing a slurry. As a result, the stability of the water-based negative electrode slurry at the time of producing the negative electrode is improved, and the initial efficiency and the cycle characteristics are improved.

Further, it is particularly preferable that the ratio of the primary particles having a particle diameter of 1 μm or less among the primary particles of the silicon compound particles is 8% or less on a volume basis. In such a case, the abundance of fine powder in which lithium is easily eluted is smaller in the primary particles of the silicon compound particles, so that the stability of the aqueous negative electrode slurry at the time of producing the negative electrode is further improved. The proportion of the primary particles having a particle diameter of 1 μm or less in the primary particles of the silicon compound particles is preferably smaller, more preferably 4% or less, and further preferably 1% or less on a volume basis. The most preferable ratio of the primary particles having a particle size of 1 μm or less is 0%, but for example, since there is a measurement limit in the particle size distribution measurement of the existing method as described later, the primary particles having a particle size of 1 μm or less are completely used. It is difficult to conclude that it is not included. However, based on the measured particle size distribution, it is most preferable that the proportion of primary particles having a particle diameter of 1 μm or less can be determined to be 0% on a volume basis.

Regarding the particle size distribution of the primary particles of the silicon compound particles, when the silicon compound particles are coated with a carbon material, the primary particles of 1 μm or less may not be detected due to the aggregation of the primary particles or the like. Therefore, in this case, the carbon material coated as described below can be removed, the primary particles can be dispersed, and then the particle size distribution of the primary particles can be measured. For example, a sample consisting of negative electrode active material particles for measurement is heat-treated to remove the carbon material coated on the silicon compound particles. The heat treatment may be performed, for example, at 600 ° C. in the atmosphere for about 72 hours. After that, the primary particles are dispersed by applying ultrasonic waves, and the particle size distribution is measured. An ultrasonic irradiation time of about 5 minutes is sufficient. Further, when the silicon compound particles are not coated with the carbon material, the above treatment may not be performed.

For the measurement of the particle size distribution, for example, a laser diffraction type particle size distribution measuring device SALD-3100 (manufactured by Shimadzu Corporation) may be used. For example, dispersed water in which silicon compound particles (SiOx material) are dispersed by a surface activator can be dropped onto a laser diffraction type particle size distribution measuring device to measure the particle size.

The present inventors have found that lithium ions are easily eluted from the primary particles of silicon compound particles having a particle diameter of 1 μm or less, and that the fine powder of the primary particles of such silicon compound particles has a large effect on the stability of the aqueous negative electrode slurry. I found out. However, in the prior art, when measuring the particle size of the negative electrode active material particles, the particle size distribution is usually measured in a state where the negative electrode active material particles include the aggregated secondary particles, so that the measurement result can be obtained. The particle size distribution was not the particle size distribution of the primary particles of the silicon compound particles, but the particle size distribution of the negative electrode active material particles including the secondary particles. Further, even if the silicon compound particles contain primary particles having a particle diameter of 1 μm or less, when the silicon compound particles are coated with carbon, they aggregate to become secondary particles having a large particle size, and such secondary particles. It was difficult to detect the presence of primary particles having a particle size of 1 μm or less even when the particle size distribution was measured in a state containing the above. On the other hand, the present inventors measured the particle size distribution of the silicon compound particles in the state of the primary particles by using, for example, the above-mentioned measuring method, and determined the optimum abundance ratio of the primary particles of the silicon compound particles. By specifying, the negative electrode active material of the present invention was completed.

<Negative electrode for non-aqueous electrolyte secondary battery>
First, the negative electrode for a non-aqueous electrolyte secondary battery containing the negative electrode active material of the present invention will be described. FIG. 1 is a cross-sectional view showing an example of the configuration of a negative electrode for a non-aqueous electrolyte secondary battery (hereinafter, also referred to as “negative electrode”).

[Construction of negative electrode]
As shown in FIG. 1, the negative electrode 10 has a negative electrode active material layer 12 on the negative electrode current collector 11. The negative electrode active material layer 12 may be provided on both sides or only one side of the negative electrode current collector 11. Further, as long as the negative electrode active material of the present invention is used, the negative electrode current collector 11 may be omitted.

[Negative electrode current collector]
The negative electrode current collector 11 is made of an excellent conductive material and has 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).

The negative electrode current collector 11 preferably contains carbon (C) or sulfur (S) in addition to the main element. This is because the physical strength of the negative electrode current collector is improved. In particular, when the current collector has an active material layer that expands during charging, if the current collector contains the above elements, there is an effect of suppressing deformation of the electrode including the current collector. The content of the above-mentioned contained elements is not particularly limited, but is preferably 100 mass ppm or less. This is because a higher deformation suppressing effect can be obtained. The cycle characteristics can be further improved by such a deformation suppressing effect.

Further, the surface of the negative electrode current collector 11 may or may not be roughened. The roughened negative electrode current collector is, for example, a metal foil that has been electrolyzed, embossed, or chemically etched. The non-roughened negative electrode current collector is, for example, a rolled metal foil.

[Negative electrode active material layer]
The negative electrode active material layer 12 contains the negative electrode active material of the present invention capable of occluding and releasing lithium ions, and from the viewpoint of battery design, further, other materials such as a negative electrode binder (binder) and a conductive auxiliary agent. May include. The negative electrode active material contains negative electrode active material particles, and the negative electrode active material particles include silicon compound particles containing a silicon compound (SiO _x : 0.5 ≦ x ≦ 1.6).

Further, the negative electrode active material layer 12 may contain a mixed negative electrode active material material containing the negative electrode active material and the carbon-based active material of the present invention. As a result, the electrical resistance of the negative electrode active material layer is reduced, and the expansion stress associated with charging can be relaxed. As the carbon-based active material, for example, pyrolytic carbons, cokes, glassy carbon fibers, calcined organic polymer compounds, carbon blacks and the like can be used.

Further, in the mixed negative electrode active material, the ratio of the mass of the silicon-based negative electrode active material to the total mass of the negative electrode active material (silicon-based negative electrode active material) and the carbon-based active material of the present invention is 6% by mass or more. Is preferable. If the ratio of the mass of the silicon-based negative electrode active material to the total mass of the silicon-based negative electrode active material and the carbon-based negative electrode active material is 6% by mass or more, the battery capacity can be reliably improved.

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

Further, in the negative electrode active material of the present invention, the silicon compound particles contain at least one of Li ₂ SiO ₃ and Li ₄ SiO ₄ . In such a product, the SiO ₂ component portion, which is destabilized during insertion and desorption of lithium during charging and discharging of the battery, is modified in advance into another lithium silicate in the silicon compound, and therefore during charging. The generated irreversible capacity can be reduced.

Further, the battery characteristics are improved by the presence of at least one type of Li ₄ SiO ₄ and Li ₂ SiO ₃ inside the bulk of the silicon compound particles, but the battery characteristics are further improved when the above two types of Li compounds coexist. do. These lithium silicates can be quantified by NMR (Nuclear Magnetic Resonance) or XPS (X-ray photoelectron spectroscopy: X-ray photoelectron spectroscopy). XPS and NMR measurements can be performed, for example, under the following conditions.
XPS
・ Equipment: X-ray photoelectron spectrometer,
・ X-ray source: Monochromatic Al Kα ray,
・ X-ray spot diameter: 100 μm,
-Ar ion gun sputtering conditions: 0.5 kV / 2 mm x 2 mm.
²⁹ Si MAS NMR (Magic Angle Spinning Nuclear Magnetic Resonance)
-Equipment: Bruker 700 NMR spectrometer,
-Probe: 4 mm HR-MAS rotor 50 μL,
・ Sample rotation speed: 10 kHz,
-Measurement environment temperature: 25 ° C.

Further, the silicon compound particles have a half-value width (2θ) of the diffraction peak due to the Si (111) crystal plane obtained by X-ray diffraction using Cu—Kα rays of 1.2 ° or more, and the crystal plane thereof. The crystallite size corresponding to is preferably 7.5 nm or less. This peak appears near 2θ = 28.4 ± 0.5 ° when the crystallinity is high (when the half width is narrow). The lower the silicon crystallinity of the silicon compound in the silicon compound particles, the better. In particular, when the abundance of Si crystals is small, the battery characteristics can be improved and a stable Li compound can be produced.

Further, the negative electrode active material of the present invention has the maximum peak intensity value A of the Si and Li silicate regions given at a chemical shift value of -60 to -95 ppm obtained from the ²⁹ Si-MAS-NMR spectrum in the silicon compound particles. It is preferable that the peak intensity value B in the SiO ₂ region given at -96 to -150 ppm as the chemical shift value satisfies the relationship of A> B. If the amount of the silicon component or Li ₂ SiO ₃ is relatively large in the silicon compound particles based on the SiO ₂ component, the effect of improving the battery characteristics by inserting Li can be sufficiently obtained. The measurement conditions of ²⁹ Si-MAS-NMR may be the same as described above.

Further, in the negative electrode active material of the present invention, it is preferable that the negative electrode active material particles contain a carbon material in the surface layer portion. Since the negative electrode active material particles contain a carbon material in the surface layer portion, the conductivity can be improved. Therefore, when the negative electrode active material containing such negative electrode active material particles is used as the negative electrode active material of the secondary battery. , Battery characteristics can be improved.

Further, the average thickness of the carbon material on the surface layer of the negative electrode active material particles is preferably 10 nm or more and 5000 nm or less. When the average thickness of the carbon material is 10 nm or more, the conductivity is improved, and when the average thickness of the carbon material to be coated is 5000 nm or less, the negative electrode active material containing such negative electrode active material particles is lithium ion. When used as the negative electrode active material of the next battery, it is possible to suppress a decrease in battery capacity.

The average thickness of this carbon material can be calculated, for example, by the following procedure. First, the negative electrode active material particles are observed at an arbitrary magnification by a TEM (transmission electron microscope). The magnification is preferably such that the thickness of the carbon material can be visually confirmed so that the thickness can be measured. Subsequently, the thickness of the carbon material is measured at any 15 points. In this case, it is preferable to set the measurement position widely and randomly without concentrating on a specific place as much as possible. Finally, the average value of the thicknesses of the above 15 carbon materials is calculated.

The coverage of the carbon material is not particularly limited, but it is desirable that the coverage is as high as possible. When the coverage is 30% or more, the electrical conductivity is further improved, which is preferable. The method for coating the carbon material is not particularly limited, but the sugar carbonization method and the thermal decomposition method for hydrocarbon gas are preferable. This is because the coverage can be improved.

Further, it is preferable that the median diameter (D ₅₀ : particle diameter when the cumulative volume is 50%) of the primary particles of the silicon compound particles is 1.0 μm or more and 15 μm or less. This is because if the median diameter of the primary particles of the silicon compound particles is in the above range, lithium ions are easily occluded and discharged during charging and discharging, and the silicon compound particles are less likely to be broken. When the median diameter is 1.0 μm or more, the surface area per mass of the silicon compound particles can be reduced, and the increase in the irreversible capacity of the battery can be suppressed. On the other hand, when the median diameter is 15 μm or less, the particles are less likely to crack and a new surface is less likely to appear. The median diameter of the silicon compound particles can be measured by the same particle size measuring method as described above.

Further, in the negative electrode active material of the present invention, it is preferable that the BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more and 6 m ² / g or less. In particular, it is preferable that the BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more and 4 m ² / g or less. When the BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more, a sufficient contact area between the particles can be sufficiently secured, so that the conductivity of the negative electrode active material is improved. As a result, the cycle characteristics and initial efficiency of the battery are improved. When the BET specific surface area of the primary particles of the silicon compound particles is 6 m ² / g or less, particularly 4 m ² / g or less, the specific surface area of the silicon compound particles is in a more appropriate small range, and the elution of lithium ions can be further suppressed. As a result, the stability of the water-based negative electrode slurry is further improved.

Further, in the negative electrode active material of the present invention, the mode diameter in the volume-based particle size distribution of the primary particles of the silicon compound particles is 5 μm or more and 10 μm or less, and the distribution cumulative value in the volume-based particle size distribution is 99.9%. It is preferable that the particle size D _99.9 is 20 μm or less. When the mode diameter is 5 μm or more and 10 μm or less, the silicon compound particles have an appropriate size and a sufficient contact area between the particles can be secured, so that the conductivity of the negative electrode active material is improved. As a result, the cycle characteristics and initial efficiency of the battery are improved. Further, when D _99.9 is 20 μm or less, the local variation in the diameter of the negative electrode active material in the electrode can be reduced. As a result, the cycle characteristics are improved.

Further, for the negative electrode active material (silicon-based negative electrode active material) of the present invention, a test cell composed of a negative electrode containing a mixture of the silicon-based negative electrode active material and the carbon-based active material and counter-polar lithium is prepared, and the test cell is prepared. In each charge / discharge, charging / discharging consisting of charging in which a current is passed so as to insert lithium into the silicon-based negative electrode active material and discharge in which a current is passed so as to desorb lithium from the silicon-based negative electrode active material is carried out 30 times. When a graph showing the relationship between the differential value dQ / dV differentiated by the potential V of the negative electrode with the discharge capacity Q as the reference electrode and the potential V is drawn, the discharge after the Xth time (1 ≦ X ≦ 30) It is preferable that the potential V of the negative electrode at the time has a peak in the range of 0.40V to 0.55V. The above-mentioned peak in the V-dQ / dV curve is similar to the peak of the Kay material, and the discharge curve on the higher potential side rises sharply, so that the capacity is easily expressed when designing the battery. Further, it can be determined that a stable bulk is formed if the negative electrode active material has the above-mentioned peak after being charged and discharged within 30 times.

Further, as the negative electrode binder contained in the negative electrode active material layer, for example, any one or more of a polymer material, synthetic rubber and the like can be used. The polymer material is, for example, polyvinylidene fluoride, polyimide, polyamideimide, aramid, polyacrylic acid, lithium polyacrylate, carboxymethyl cellulose and the like. Examples of the synthetic rubber include styrene-butadiene rubber, fluororubber, ethylene propylene diene and the like.

As the negative electrode conductive auxiliary agent, for example, any one or more of carbon materials such as carbon black, acetylene black, graphite, kechen black, carbon nanotubes, and carbon nanofibers can be used.

The negative electrode active material layer is formed, for example, by a coating method. The coating method is a method in which the negative electrode active material particles and the above-mentioned binder, or if necessary, a conductive auxiliary agent and a carbon material are mixed, and then dispersed in an organic solvent, water, or the like for coating.

[Manufacturing method of negative electrode]
The negative electrode can be manufactured, for example, by the following procedure. First, a method for manufacturing a negative electrode active material used for a negative electrode will be described. First, silicon compound particles containing a silicon compound (SiO _x : 0.5 ≦ x ≦ 1.6) are prepared. Next, Li is inserted into the silicon compound particles to contain at least one of Li ₂ SiO ₃ and Li ₄ SiO ₄ . In this way, the negative electrode active material particles are produced. Next, from the prepared negative electrode active material particles, among the primary particles of the silicon compound particles, those having a ratio of the primary particles having a particle diameter of 1 μm or less of 15% or less on a volume basis are selected. Then, the negative electrode active material is produced by using the selected negative electrode active material particles.

More specifically, the negative electrode active material can be produced as follows. First, the raw material that generates silicon oxide gas is heated in the temperature range of 900 ° C. to 1600 ° C. under reduced pressure in the presence of the inert gas to generate silicon oxide gas. Considering the presence of surface oxygen of the metallic silicon powder and trace oxygen in the reaction furnace, it is desirable that the mixed molar ratio is in the range of 0.8 <metal silicon powder / silicon dioxide powder <1.3.

The generated silicon oxide gas is solidified and deposited on the adsorption plate. Next, the deposit of silicon oxide is taken out in a state where the temperature in the reaction furnace is lowered to 100 ° C. or lower, and pulverized by using a ball mill, a jet mill or the like to perform pulverization. The powder thus obtained may be classified. In the present invention, the particle size distribution of the silicon compound particles can be adjusted during the pulverization step and the classification step. As described above, silicon compound particles can be produced. The Si crystals in the silicon compound particles can be controlled by changing the vaporization temperature or by heat treatment after formation.

Here, a layer of carbon material may be formed on the surface layer of the silicon compound particles. A pyrolysis CVD method is desirable as a method for forming a layer of carbon material. A method of forming a layer of carbon material by a pyrolysis CVD method will be described.

First, the silicon compound particles are set in the furnace. Next, a hydrocarbon gas is introduced into the furnace to raise the temperature inside the furnace. The decomposition temperature is not particularly limited, but is preferably 1200 ° C. or lower, and more preferably 950 ° C. or lower. By setting the decomposition temperature to 1200 ° C. or lower, unintended disproportionation of the active material particles can be suppressed. After raising the temperature in the furnace to a predetermined temperature, a carbon layer is formed on the surface of the silicon compound particles. The hydrocarbon gas used as a raw material for the carbon material is not particularly limited, but it is desirable that _n ≦ 3 in the Cn _Hm composition. When n ≦ 3, the manufacturing cost can be lowered and the physical properties of the decomposition product can be improved.

Next, Li is inserted into the silicon active material particles prepared as described above to contain at least one of Li ₂ SiO ₃ and Li ₄ SiO ₄ . It is preferable to insert Li by the heat doping method.

In the modification by the heat doping method, for example, the silicon active material particles can be modified by mixing them with LiH powder or Li powder and heating them in a non-oxidizing atmosphere. As the non-oxidizing atmosphere, for example, an Ar atmosphere 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 the sealed container to make them uniform. Then, it is reformed by heating in the range of 700 ° C. to 750 ° C. In this case, in order to desorb Li from the silicon compound, the heated powder may be sufficiently cooled and then washed with alcohol, alkaline water, a weak acid or pure water. In the cleaning step after the modification in the bulk, fine powder may be removed from the negative electrode active material particles.

Further, Li may be inserted into the silicon active material particles by a redox method. In the modification by the redox method, for example, lithium can be inserted by first immersing the silicon active material particles in the solution A in which lithium is dissolved in an ether solvent. This solution A may further contain a polycyclic aromatic compound or a linear polyphenylene compound. After inserting lithium, the active lithium can be desorbed from the silicon active material particles by immersing the silicon active material particles in the solution B containing the polycyclic aromatic compound or its derivative. As the solvent of this solution B, for example, an ether solvent, a ketone solvent, an ester solvent, an alcohol solvent, an amine solvent, or a mixed solvent thereof can be used. Further, the silicon active material particles may be repeatedly immersed in the solution B. In this way, if the active lithium is desorbed after the lithium is inserted, the negative electrode active material having higher water resistance can be obtained. Further, in the step of immersing the silicon active material particles in the solution B, fine powder may be removed from the negative electrode active material particles.

As the ether solvent used for 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 thereof or the like is used. Can be used. Of these, tetrahydrofuran, dioxane and 1,2-dimethoxyethane are particularly preferable. These solvents are preferably dehydrated and preferably deoxidized.

Further, as the polycyclic aromatic compound contained in the solution A, one or more of naphthalene, anthracene, phenanthrene, naphthalene, pentacene, pyrene, picene, triphenylene, coronene, chrysen and derivatives thereof can be used. As the chain polyphenylene compound, one or more of biphenyl, terphenyl, and derivatives thereof can be used.

As the polycyclic aromatic compound contained in the solution B, one or more of naphthalene, anthracene, phenanthrene, naphthalene, pentacene, pyrene, picene, triphenylene, coronene, chrysene and derivatives thereof can be used.

Further, 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, tetraethylene glycol dimethyl ether and the like can be used. ..

As the ketone solvent, acetone, acetophenone and 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, isopropyl alcohol and the like can be used.

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

When the modification is performed by the heat doping method, the ²⁹ Si-MAS-NMR spectrum obtained from the silicon compound particles is different from that in the case of using the redox method. FIG. 2 shows an example of a ²⁹ Si-MAS-NMR spectrum measured from silicon compound particles when modified by a redox method. In FIG. 2, the peak given in the vicinity of −75 ppm is the peak derived from Li ₂ SiO ₃ , and the peak given in the vicinity of −80 to −100 ppm is the peak derived from Si. In addition, it may have a Li silicate peak other than Li ₂ SiO ₃ and Li ₄ SiO ₄ from -80 to -100 ppm.

Further, FIG. 3 shows an example of the ²⁹ Si-MAS-NMR spectrum measured from the silicon compound particles when the modification is performed by the thermal doping method. In FIG. 3, the peak given in the vicinity of −75 ppm is the peak derived from Li ₂ SiO ₃ , and the peak given in the vicinity of −80 to −100 ppm is the peak derived from Si. In addition, it may have a Li silicate peak other than Li ₂ SiO ₃ and Li ₄ SiO ₄ from -80 to -100 ppm. From the XPS spectrum, the peak of Li ₄ SiO ₄ can be confirmed.

Next, from the modified negative electrode active material particles, among the primary particles of the silicon compound, those having a ratio of the primary particles having a particle diameter of 1 μm or less of 15% or less on a volume basis are selected.

At this time, when the silicon compound particles are coated with a carbon material, the coated carbon material can be removed, the primary particles are dispersed, and then the particle size distribution of the primary particles can be measured and sorted. Further, when the silicon compound particles are not coated with the carbon material, the carbon material removal treatment may not be performed.

It should be noted that the selection of the negative electrode active material particles does not necessarily have to be performed each time the negative electrode active material is manufactured, and the ratio of the primary particles having a particle diameter of 1 μm or less among the primary particles of the silicon compound is 15% or less on a volume basis. After finding and selecting the production conditions that satisfy the conditions, the negative electrode active material can be produced under the same conditions as the selected conditions.

The negative electrode active material prepared as described above is mixed with other materials such as a negative electrode binder and a conductive auxiliary agent to form a negative electrode mixture, and then an organic solvent or water is added to form a slurry. Next, the above slurry is applied to the surface of the negative electrode current collector and dried to form a negative electrode active material layer. At this time, a heating press or the like may be performed if necessary. As described above, the negative electrode can be manufactured.

<Lithium-ion secondary battery>
Next, the lithium ion secondary battery containing the negative electrode active material of the present invention will be described. Here, as a specific example, a laminated film type lithium ion secondary battery will be taken as an example.

[Structure of laminated film type lithium ion secondary battery]
In the laminated film type lithium ion secondary battery 20 shown in FIG. 4, the wound electrode body 21 is mainly housed inside a sheet-shaped exterior member 25. This wound body has a separator between the positive electrode and the negative electrode, and is wound. There is also a case where a separator is provided between the positive electrode and the negative electrode to store the laminated body. In both electrode bodies, the positive electrode lead 22 is attached to the positive electrode, and the negative electrode lead 23 is attached to the negative electrode. The outermost peripheral portion of the electrode body is protected by a protective tape.

The positive and negative electrode leads are led out in one direction from the inside of the exterior member 25 toward the outside, for example. The positive electrode lead 22 is formed of a conductive material such as aluminum, and the negative electrode lead 23 is formed of a conductive material such as nickel or copper.

The exterior member 25 is, for example, a laminated film in which a fused layer, a metal layer, and a surface protective layer are laminated in this order. The laminated film consists of two films so that the fused layer faces the electrode body 21. The outer peripheral edges of the fused layer are fused or bonded together with an adhesive or the like. The fused portion is a film such as polyethylene or polypropylene, and the metal portion is an aluminum foil or the like. The protective layer is, for example, nylon.

A close contact film 24 is inserted between the exterior member 25 and the positive and negative electrode leads to prevent outside air from entering. This material is, for example, polyethylene, polypropylene, polyolefin resin.

[Positive electrode]
The positive electrode has, for example, a positive electrode active material layer on both sides or one side of the positive electrode current collector, similar to the negative electrode 10 in FIG.

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

The positive electrode active material layer contains any one or more of the positive electrode materials capable of occluding and releasing lithium ions, and contains other materials such as a binder, a conductive auxiliary agent, and a dispersant depending on the design. You can go out. In this case, the details regarding the binder and the conductive auxiliary agent are the same as those of the negative electrode binder and the negative electrode conductive auxiliary agent already described, for example.

As the positive electrode material, a lithium-containing compound is desirable. Examples of the lithium-containing compound include a composite oxide composed of lithium and a transition metal element, or a phosphoric acid compound having lithium and a transition metal element. Among these described positive electrode materials, a compound having at least one of nickel, iron, manganese, and cobalt is preferable. These chemical formulas are represented by, for example, Li _x M1O ₂ or Li _y M2PO ₄ . In the formula, M1 and M2 represent at least one transition metal element. The values of x and y show different values depending on the battery charge / discharge state, but are generally shown by 0.05 ≦ x ≦ 1.10 and 0.05 ≦ y ≦ 1.10.

Examples of the composite oxide having lithium and a transition metal element include lithium cobalt composite oxide (Li _x CoO ₂ ) and lithium nickel composite oxide (Li _x NiO ₂ ). Examples of the phosphoric acid compound having lithium and a transition metal element include a lithium iron phosphoric acid compound (LiFePO ₄ ) and a lithium iron manganese phosphoric acid compound (LiFe _1-u Mn _u PO ₄ (0 <u <1)). Can be mentioned. This is because when these positive electrode materials are used, a high battery capacity can be obtained and also excellent cycle characteristics can be obtained.

[Negative electrode]
The negative electrode has the same configuration as the negative electrode 10 for the lithium ion secondary battery of FIG. 1 described above, and has, for example, the negative electrode active material layers 12 on both sides of the current collector 11. It is preferable that the negative electrode has a larger negative electrode charge capacity than the electric capacity (charge capacity as a battery) obtained from the positive electrode active material. This is because the precipitation of lithium metal on the negative electrode can be suppressed.

The positive electrode active material layer is provided on a part of both sides of the positive electrode current collector, and the negative electrode active material layer is also provided on a part of both sides of the negative electrode current collector. In this case, for example, the negative electrode active material layer provided on the negative electrode current collector is provided with a region in which the opposite positive electrode active material layer does not exist. This is for stable battery design.

The non-opposed region, that is, 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 charging and discharging. Therefore, the state of the negative electrode active material layer is maintained as it is immediately after formation. As a result, the composition of the negative electrode active material can be accurately investigated with good reproducibility regardless of the presence or absence of charge / discharge.

[Separator]
The separator separates the positive electrode and the negative electrode, and allows lithium ions to pass through while preventing a current short circuit due to contact between the two electrodes. This separator is formed of, for example, a porous film made of synthetic resin or ceramic, and may have a laminated structure in which two or more kinds of porous films are laminated. Examples of the synthetic resin include polytetrafluoroethylene, polypropylene, polyethylene and the like.

[Electrolytic solution]
At least a part of the active material layer or the separator is impregnated with a liquid electrolyte (electrolyte solution). The electrolyte salt has an electrolyte salt dissolved in the solvent, and may contain other materials such as additives.

As the solvent, for example, a non-aqueous solvent can be used. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, 1,2-dimethoxyethane, and tetrahydrofuran. Among these, it is desirable to use at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate. This is because better characteristics can be obtained. Further, in this case, more advantageous characteristics can be obtained by combining a high-viscosity solvent such as ethylene carbonate and propylene carbonate with a low-viscosity solvent such as dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate. This is because the dissociative property and ion mobility of the electrolyte salt are improved.

When an alloy-based negative electrode is used, it is particularly desirable that the solvent contains at least one of a halogenated chain carbonate or a halogenated cyclic carbonate. As a result, a stable film is formed on the surface of the negative electrode active material during charging / discharging, particularly during charging. Here, the halogenated chain-chain carbonic acid ester is a chain-chain carbonic acid ester having halogen as a constituent element (at least one hydrogen is substituted with halogen). Further, the halogenated cyclic carbonate is a cyclic carbonate having halogen as a constituent element (that is, at least one hydrogen is substituted with halogen).

The type of halogen is not particularly limited, but fluorine is preferable. This is because it forms a better film than other halogens. Further, the larger the number of halogens, the more desirable. This is because the obtained film is more stable and the decomposition reaction of the electrolytic solution is reduced.

Examples of the halogenated chain carbonate include fluoromethylmethyl carbonate and difluoromethylmethyl carbonate. Examples of the halogenated cyclic carbonate include 4-fluoro-1,3-dioxolane-2-one, 4,5-difluoro-1,3-dioxolan-2-one and the like.

It is preferable to contain unsaturated carbon-bonded cyclic carbonate as a solvent additive. This is because a stable film is formed on the surface of the negative electrode during charging and discharging, and the decomposition reaction of the electrolytic solution can be suppressed. Examples of unsaturated carbon-bonded cyclic carbonates include vinylene carbonate and vinylethylene carbonate.

Further, it is preferable to contain sultone (cyclic sulfonic acid ester) as a solvent additive. This is because the chemical stability of the battery is improved. Examples of the sultone include propane sultone and propene sultone.

Further, the solvent preferably contains acid anhydride. This is because the chemical stability of the electrolytic solution is improved. Examples of the acid anhydride include propanedisulfonic acid anhydride.

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

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

[Manufacturing method of laminated film type secondary battery]
In the present invention, a negative electrode can be produced using the negative electrode active material produced by the above-mentioned method for producing a negative electrode active material of the present invention, and a lithium ion secondary battery can be produced using the produced negative electrode.

First, a positive electrode is manufactured using the above-mentioned positive electrode material. First, the positive electrode active material is mixed with a binder, a conductive auxiliary agent, etc., if necessary, to form a positive electrode mixture, and then dispersed in an organic solvent to form a positive electrode mixture slurry. Subsequently, the mixture slurry is applied to the positive electrode current collector with a coating device such as a knife roll or a die coater having a die head, and dried with hot air to obtain a positive electrode active material layer. Finally, the positive electrode active material layer is compression-molded with a roll press or the like. At this time, heating may be performed, or heating or compression may be repeated a plurality of times.

Next, the negative electrode active material layer is formed on the negative electrode current collector to prepare the negative electrode by using the same work procedure as the manufacturing of the negative electrode 10 for the lithium ion secondary battery described above.

When the positive electrode and the negative electrode are manufactured, the active material layers are formed on both sides of the positive electrode and the negative electrode current collector. At this time, the active material coating lengths on both sides of either electrode may be different (see FIG. 1).

Then, the electrolytic solution is adjusted. Subsequently, the positive electrode lead 22 is attached to the positive electrode current collector by ultrasonic welding or the like, and the negative electrode lead 23 is attached to the negative electrode current collector. Subsequently, the positive electrode and the negative electrode are laminated or wound via a separator to prepare a wound electrode body 21, and a protective tape is adhered to the outermost peripheral portion thereof. Next, the winding body is molded so as to have a flat shape. Subsequently, after sandwiching the wound electrode body between the folded film-shaped exterior members 25, the insulating portions of the exterior members are adhered to each other by a heat fusion method, and the wound electrode body is released in only one direction. Is enclosed. An adhesive film is inserted between the positive electrode lead and the negative electrode lead and the exterior member. A predetermined amount of the prepared electrolytic solution is charged from the release portion, and vacuum impregnation is performed. After impregnation, the release portion is adhered by a vacuum heat fusion method. As described above, the laminated film type lithium ion secondary battery 20 can be manufactured.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples of the present invention, but the present invention is not limited to these Examples.

(Example 1-1)
The laminated film type lithium ion secondary battery 20 shown in FIG. 4 was manufactured by the following procedure.

First, a positive electrode was prepared. The positive electrode active material is LiNi _0.7 Co _0.25 Al _0.05 O, which is a lithium nickel-cobalt composite oxide, in an amount of 95% by mass, a positive electrode conductive aid of 2.5% by mass, and a positive electrode binder (polyfluorovinylidene). : PVDF) 2.5% by mass was mixed to prepare a positive electrode mixture. Subsequently, the positive electrode mixture was dispersed in an organic solvent (N-methyl-2-pyrrolidone: NMP) to obtain a paste-like slurry. Subsequently, the slurry was applied to both sides of the positive electrode current collector with a coating device having a die head, and dried with a hot air type drying device. At this time, a positive electrode current collector having a thickness of 15 μm was used. Finally, compression molding was performed with a roll press.

Next, a negative electrode was prepared. First, the negative electrode active material was prepared as follows. A raw material in which metallic silicon and silicon dioxide were mixed was introduced into a reactor, vaporized in an atmosphere of 10 Pa of vacuum, deposited on an adsorption plate, cooled sufficiently, and then the sediment was taken out and pulverized with a ball mill. .. The value of x of SiO _x of the silicon compound particles thus obtained was 0.5. Subsequently, the particle size of the silicon compound particles was adjusted by classification. Then, by performing thermal decomposition CVD, the surface of the silicon compound particles was coated with a carbon material.

Subsequently, lithium was inserted into the silicon compound particles by the heat doping method to modify the particles. First, LiH powder and silicon compound particles were sufficiently mixed under an Ar atmosphere, sealed, and stirred together with the sealed container to make them uniform. Then, it was reformed by heating in the range of 700 ° C. to 750 ° C. Further, in order to desorb some of the active Li from the silicon compound, the heated silicon compound particles were sufficiently cooled and then washed with alcohol. By the above treatment, negative electrode active material particles were produced.

Here, a part of the negative electrode active material particles was taken out as a measurement sample, the carbon material coated on the negative electrode active material particles was removed to obtain silicon compound particles, and then the particle size distribution of the primary particles of the silicon compound particles was measured. First, the extracted negative electrode active material particles were heat-treated at 600 ° C. for 72 hours in the atmosphere to remove the coated carbon material, and only silicon compound particles were obtained, and then ultrasonic waves were applied to disperse the particles for 5 minutes. Then, the dispersed water in which the silicon compound particles were dispersed by the surface activator was dropped onto the laser diffraction type particle size distribution measuring device SALD-3100 (manufactured by Shimadzu Corporation) to measure the particle size distribution.

As a result, the ratio of the primary particles having a particle diameter of 1 μm or less among the primary particles of the silicon compound particles was 12% on a volume basis. Further, the mode diameter in the volume-based particle size distribution of the primary particles of the silicon compound particles was 6 μm, D _99.9 was 18 μm, and the median diameter D ₅₀ was 5.0 μm.

The BET specific surface area of the primary particles of the silicon compound particles was measured and found to be 5 m ² / g.

Next, the negative electrode active material particles for producing the negative electrode and the carbon-based active material were blended at a mass ratio of 1: 9 to prepare the negative electrode active material. Here, as the carbon-based active material, a mixture of natural graphite coated with a pitch layer and artificial graphite at a mass ratio of 5: 5 was used. The median diameter of the carbon-based active material was 20 μm.

Next, the prepared negative electrode active material, conductive auxiliary agent 1 (carbon nanotube, CNT), conductive auxiliary agent 2 (carbon fine particles having a median diameter of about 50 nm), styrene butadiene rubber (styrene butadiene copolymer, hereinafter referred to as SBR), Carboxymethyl cellulose (hereinafter referred to as CMC) was mixed at a dry mass ratio of 92.5: 1: 1: 2.5: 3 and then diluted with pure water to obtain a negative electrode mixture slurry. The above SBR and CMC are negative electrode binders (negative electrode binders).

Further, as the negative electrode current collector, an electrolytic copper foil having a thickness of 15 μm was used. The electrolytic copper foil contained carbon and sulfur at concentrations of 70 mass ppm each. Finally, the negative electrode mixture slurry was applied to the negative electrode current collector and dried at 100 ° C. for 1 hour in a vacuum atmosphere. After drying, the deposited amount (also referred to as area density) of the negative electrode active material layer per unit area on one side of the negative electrode was 5 mg / cm ² .

Next, the solvent (4-fluoro-1,3-dioxolane-2-one (FEC), ethylene carbonate (EC) and dimethyl carbonate (DMC)) was mixed, and then the electrolyte salt (lithium hexafluorophosphate: LiPF) was mixed. ₆ ) was dissolved to prepare an electrolytic solution. In this case, the composition of the solvent was FEC: EC: DMC = 10: 20: 70 in volume ratio, and the content of the electrolyte salt was 1.2 mol / kg with respect to the solvent.

Next, the secondary battery was assembled as follows. First, an aluminum lead was ultrasonically welded to one end of the positive electrode current collector, and a nickel lead was welded to one end of the negative electrode current collector. Subsequently, the positive electrode, the separator, the negative electrode, and the separator were laminated in this order and rotated in the longitudinal direction to obtain a rotating electrode body. The winding end portion was fixed with PET protective tape. As the separator, a laminated film (thickness 12 μm) sandwiched between a film containing porous polyethylene as a main component and a film containing porous polyethylene as a main component was used. Subsequently, after sandwiching the electrode body between the exterior members, the outer peripheral edges except one side were heat-sealed, and the electrode body was housed inside. As the exterior member, a nylon film, an aluminum foil, and an aluminum laminated film in which a polypropylene film was laminated were used. Subsequently, the adjusted electrolytic solution was injected through the opening, impregnated in a vacuum atmosphere, then heat-sealed and sealed.

The cycle characteristics and initial charge / discharge characteristics of the secondary battery produced as described above were evaluated.

The cycle characteristics were investigated as follows. First, in order to stabilize the battery, charging and discharging were performed for two cycles at 0.2 C in an atmosphere of 25 ° C., and the discharge capacity of the second cycle was measured. Subsequently, charging and discharging were performed until the total number of cycles reached 499 cycles, and the discharge capacity was measured each time. Finally, the discharge capacity at the 500th cycle obtained by charging / discharging at 0.2C was divided by the discharge capacity at the second cycle to calculate the capacity retention rate (hereinafter, also simply referred to as the maintenance rate). From the normal cycle, that is, from the 3rd cycle to the 499th cycle, charging and discharging were performed with a charge of 0.7 C and a discharge of 0.5 C.

When investigating the initial charge / discharge characteristics, the initial efficiency (hereinafter sometimes referred to as the initial efficiency) was calculated. The initial efficiency was calculated from the formula represented by the initial efficiency (%) = (initial discharge capacity / initial charge capacity) × 100. The atmospheric temperature was the same as when the cycle characteristics were examined.

(Examples 1-2 to 1-3, Comparative Examples 1-1 and 1-2)
A secondary battery was manufactured in the same manner as in Example 1-1, except that the amount of oxygen in the bulk of the silicon compound was adjusted. In this case, the amount of oxygen was adjusted by changing the ratio of metallic silicon and silicon dioxide in the raw material of the silicon compound and the heating temperature. The values of x of the silicon compound represented by SiO _x in Examples 1-1 to 1-3 and Comparative Examples 1-1 and 1-2 are shown in Table 1.

At this time, the silicon-based active material particles of Examples 1-1 to 1-3 and Comparative Examples 1-1 and 1-2 had the following properties. Li ₂ SiO ₃ and Li ₄ SiO ₄ were contained inside the silicon compound particles in the negative electrode active material particles. Further, in the silicon compound, the half width (2θ) of the diffraction peak due to the Si (111) crystal plane obtained by X-ray diffraction is 1.755 °, and the crystallite size due to the Si (111) crystal plane is It was 4.86 nm. The average thickness of the carbon material coated on the surface was 100 nm.

Further, in all the above Examples and Comparative Examples, peaks in the Si and Li silicate regions given at -60 to -95 ppm as chemical shift values obtained from the ²⁹ Si-MAS-NMR spectrum were expressed. Further, in all the above Examples and Comparative Examples, the maximum peak intensity values A of the Si and Li silicate regions given at -60 to -95 ppm as the chemical shift values obtained from the ²⁹ Si-MAS-NMR spectrum and -96 to -96 to The relationship with the peak intensity value B in the SiO ₂ region given at −150 ppm was A> B.

Further, a 2032 size coin battery type test cell was prepared from the negative electrode and the counter electrode lithium prepared as described above, and the discharge behavior thereof was evaluated. More specifically, first, constant current and constant voltage charging was performed up to 0 V with counter electrode Li, and charging was terminated when the current density reached 0.05 mA / cm ² . Then, constant current discharge was performed up to 1.2V. The current density at this time was 0.2 mA / cm ² . This charging / discharging is repeated 30 times, and from the data obtained in each charging / discharging, a graph is drawn with the vertical axis as the capacity change rate (dQ / dV) and the horizontal axis as the voltage (V), and V is 0.4 to 0. It was confirmed whether a peak could be obtained in the range of .55 (V). As a result, the above peak was not obtained in Comparative Example 1 in which x of SiOx was less than 0.5. In the other examples and comparative examples, the peak was obtained within 30 times of charging and discharging, and the above peak was obtained in all charging and discharging from the first charging and discharging to the 30th charging and discharging.

Table 1 shows the evaluation results of Examples 1-1 to 1-3 and Comparative Examples 1-1 and 1-2.

As shown in Table 1, in the silicon compound represented by SiOx, when the value of x was out of the range of 0.5 ≦ x ≦ 1.6, the battery characteristics deteriorated. For example, as shown in Comparative Example 1-1, when there is not enough oxygen (x = 0.3), the initial efficiency is improved, but the capacity retention rate is significantly deteriorated. On the other hand, as shown in Comparative Example 1-2, when the amount of oxygen was large (x = 1.8), the conductivity was lowered and the capacity of the silicon oxide was not substantially developed, so the evaluation was stopped.

(Example 2-1 and Example 2-2)
A secondary battery was produced under the same conditions as in Example 1-2 except that the type of lithium silicate contained inside the silicon compound particles was changed as shown in Table 2, and the cycle characteristics and initial efficiency were evaluated.

(Comparative Example 2-1)
A secondary battery was prepared under the same conditions as in Example 1-2 except that lithium was not inserted into the silicon compound particles, and the cycle characteristics and initial efficiency were evaluated.

The results of Example 2-1 and Example 2-2 and Comparative Example 2-1 are shown in Table 2.

Since the silicon compound contains a stable lithium silicate such as Li ₂ SiO ₃ and Li ₄ SiO ₄ , the capacity retention rate and the initial efficiency are improved. In particular, when both Li ₂ SiO ₃ and Li ₄ SiO ₄ are contained in lithium silicate, the capacity retention rate and the initial efficiency are further improved. On the other hand, in Comparative Example 2-1 which was not modified and did not contain lithium in the silicon compound, the capacity retention rate and the initial efficiency were lowered.

(Examples 3-1 to 3-9, Comparative Examples 3-1 to 3-2)
Examples except that the ratio of primary particles of 1 μm or less, BET specific surface area, mode diameter in particle size distribution, _D99.9 , and median diameter _D50 among the primary particles of silicon compound particles were changed as shown in Table 3. A secondary battery was prepared under the same conditions as 1-2, and the cycle characteristics and initial efficiency were evaluated. The particle size distribution of the primary particles of the silicon compound particles was adjusted in the pulverization step and the classification step after the silicon compound particles were produced. The BET specific surface area of the primary particles of the silicon compound particles can be adjusted (controlled) by adjusting the precipitation chamber temperature of the reactor and the vapor concentration of the SiO gas. The higher the precipitation chamber temperature and the higher the vapor concentration of SiO gas, the smaller the BET specific surface area. Here, the vapor concentration of SiO gas can be changed depending on the production conditions. For example, the vapor concentration of SiO gas can be increased by increasing the reaction temperature or increasing the amount of raw materials charged.

Further, here, in order to evaluate the stability of the aqueous slurry containing the negative electrode active material particles, 30 g of a part of the prepared negative electrode mixture slurry is taken out separately from the one for making a secondary battery and stored at 20 ° C. Then, the time from the preparation of the negative electrode mixture slurry to the generation of gas was measured.

Table 3 shows the results of Examples 3-1 to 3-9 and Comparative Examples 3-1 to 3-2.

As can be seen from Table 3, as shown in Examples 3-1 to 3-9 and 1-2, when the ratio of the primary particles of 1 μm or less of the silicon compound particles is 15% or less, the negative electrode mixture slurry is used. Among them, lithium ions are difficult to elute from the negative electrode active material, so that the negative electrode mixture slurry is stabilized and the time until gas generation is 36 hours or more. On the other hand, in Comparative Examples 3-1 and 3-2, since the ratio of the primary particles of the silicon compound particles to 1 μm or less is larger than 15%, lithium ions are easily eluted from the negative electrode active material, and the negative electrode mixture slurry. Became unstable, and it took 6 hours to generate gas. Further, as compared with Comparative Examples 3-1 and 3-2, in Examples 3-1 to 3-9, the capacity retention rate and the initial efficiency were improved. Further, in particular, in Examples 3-2, 3-3, and 3-5 in which the ratio of the primary particles of 1 μm or less of the silicon compound particles is 8% or less, the time until gas generation is 72 hours or more. It was found that the length became longer and the negative electrode mixture slurry became more stable. Moreover, in the case of these examples, a particularly good capacity retention rate and initial efficiency were obtained.

Further, in Examples 3-1 to 3-3 and 1-2 in which the BET specific surface area of the primary particles of the silicon compound particles satisfies 1 m ² / g or more and 6 m ² / g or less, the BET specific surface area is 6 m ² / g. The capacity retention rate and the initial efficiency were further improved as compared with the large Example 3-7 and the Example 3-8 having a BET specific surface area of less than 1 m ² / g.

Further, in Examples 3-1 to 3-3 and 1-2 in which the mode diameter in the particle size distribution of the primary particles of the silicon compound particles is 5 to 10 μm and D _99.9 is 20 μm or less, the mode diameter is The capacity retention rate and the initial efficiency were further improved as compared with Example 3-4 having a mode diameter of less than 5, Example 3-5 having a mode diameter larger than 10, and Example 3-6 having a _D99.9 of 20 μm or more.

In Examples 3-1 to 3-3 and 1-2 in which the median diameter of the primary particles of the silicon compound particles is 1.0 μm or more and 15 μm or less, the capacity is higher than that in Example 3-9 in which the median diameter exceeds 15 μm. The maintenance rate and initial efficiency have been improved. When the median diameter D ₅₀ of the primary particles of the silicon compound particles is 1.0 μm or less, the proportion of the primary particles of 1 μm or less greatly exceeds 15%. It is considered that elution is likely to occur and the battery characteristics are deteriorated.

(Examples 4-1 to 4-6)
A secondary battery was prepared under the same conditions as in Example 1-2 except that the crystallinity of the silicon compound particles was changed as shown in Table 4, and the cycle characteristics and the initial efficiency were evaluated. The crystallinity in the silicon compound particles can be controlled by changing the vaporization temperature of the raw material or by heat treatment after the silicon compound particles are formed.

In particular, a high capacity retention rate was obtained with a low crystallinity material having a half width of 1.2 ° or more and a crystallinity size of 7.5 nm or less due to the Si (111) plane.

(Example 5-1)
Under the same conditions as in Example 1-2, except that the relationship between the maximum peak intensity value A of the Si and Li silicate regions and the peak intensity value B derived from the SiO ₂ region of the silicon compound is A <B. The next battery was prepared and the cycle characteristics and initial efficiency were evaluated. In this case, by reducing the amount of lithium inserted at the time of reforming, the amount of Li ₂ SiO ₃ was reduced, and the intensity A of the peak derived from Li ₂ SiO ₃ was reduced.

As can be seen from Table 5, the battery characteristics were improved when the relationship of peak intensities was A> B.

(Example 6-1)
In the V-dQ / dV curve obtained by charging and discharging 30 times in the above test cell, a negative electrode active material in which a peak was not obtained in the range of V of 0.40V to 0.55V was used in any charging and discharging. A secondary battery was produced under the same conditions as in Example 1-2 except that the cycle characteristics and the initial efficiency were evaluated.

In order for the discharge curve shape to rise sharper, it is necessary for the silicon compound (SiOx) to exhibit the same discharge behavior as silicon (Si). Since the silicon compound has a relatively gentle discharge curve in which the peak does not appear in the above range after 30 times of charging and discharging, the initial efficiency is slightly lowered when the secondary battery is used. If the peak is expressed by charging / discharging within 30 times, a stable bulk is formed, and the capacity retention rate and the initial efficiency are improved.

(Examples 7-1 to 7-4)
A secondary battery was prepared under the same conditions as in Example 1-2 except that the average thickness of the carbon material coated on the surface of the silicon-based active material particles was changed, and the cycle characteristics and the initial efficiency were evaluated. The average thickness of the carbon material can be adjusted by changing the CVD conditions.

As can be seen from Table 7, when the film thickness of the carbon layer is 10 nm or more, the conductivity is particularly improved, so that the capacity retention rate and the initial efficiency can be improved. On the other hand, when the film thickness of the carbon layer is 5000 nm or less, the amount of silicon compound particles can be sufficiently secured in the battery design, so that the battery capacity does not decrease.

(Example 8-1)
A secondary battery was produced under the same conditions as in Example 1-2 except that the modification method was changed to the redox method, and the cycle characteristics and initial efficiency were evaluated.

Modification by the redox method was performed as follows. First, the negative electrode active material particles coated with the carbon material were immersed in a solution (solution A) in which lithium pieces and naphthalene, which is an aromatic compound, were dissolved in tetrahydrofuran (hereinafter, also referred to as THF). This solution A was prepared by dissolving naphthalene in a THF solvent at a concentration of 0.2 mol / L, and then adding 10% by mass of lithium pieces to the mixed solution of THF and naphthalene. The temperature of the solution when immersing the negative electrode active material particles was 20 ° C., and the immersing time was 20 hours. Then, the negative electrode active material particles were collected by filtration. Through the above treatment, lithium was inserted into the negative electrode active material particles.

Subsequently, the obtained silicon compound particles were heat-treated at 600 ° C. for 24 hours under an argon atmosphere to stabilize the Li compound.

Next, the negative electrode active material particles were washed, and the negative electrode active material particles after the washing treatment were dried under reduced pressure. In this way, the negative electrode active material particles were modified.

Good capacity retention and initial efficiency were obtained even when the redox method was used.

(Example 9-1)
A secondary battery was produced under the same conditions as in Example 1-2 except that the ratio of the mass of the silicon-based active material particles in the negative electrode active material was changed, and the rate of increase in battery capacity was evaluated.

FIG. 5 shows a graph showing the relationship between the ratio of silicon-based active material particles to the total amount of negative electrode active material and the rate of increase in battery capacity of the secondary battery. The graph shown by A in FIG. 5 shows the rate of increase in battery capacity when the proportion of silicon compound particles is increased in the negative electrode active material of the negative electrode of the present invention. On the other hand, the graph shown by B in FIG. 5 shows the rate of increase in battery capacity when the proportion of silicon compound particles not doped with Li is increased. As can be seen from FIG. 5, when the proportion of the silicon compound is 6% by mass or more, the rate of increase in battery capacity becomes larger than in the conventional case, and the volume energy density increases particularly remarkably.

The present invention is not limited to the above embodiment. The above-described embodiment is an example, and the present invention can be anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits the same function and effect. Is included in the technical scope of.

10 ... Negative electrode, 11 ... Negative electrode current collector, 12 ... Negative electrode active material layer,
20 ... Lithium secondary battery (laminated film type), 21 ... Winding electrode body,
22 ... Positive electrode lead, 23 ... Negative electrode lead, 24 ... Adhesive film,
25 ... Exterior member.

Claims (10)

Negative electrode active material A negative electrode active material for an aqueous negative electrode slurry containing particles.
The negative electrode active material particles contain silicon compound particles containing a silicon compound (SiO _x : 0.5 ≦ x ≦ 1.6).
The silicon compound particles contain at least one of Li ₂ SiO ₃ and Li ₄ SiO ₄ .
Among the primary particles of the silicon compound particles, the ratio of the primary particles having a particle diameter of 1 μm or less is 15% or less on a volume basis.
The BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more and 6 m ² / g or less.
D _99. The mode diameter of the primary particles of the silicon compound particles in the volume-based particle size distribution is 5 μm or more and 10 μm or less, and the cumulative distribution value in the volume-based particle size distribution is 99.9% . A negative electrode active material characterized in that ₉ is 20 μm or less . The negative electrode active material according to claim 1, wherein the ratio of the primary particles having a particle diameter of 1 μm or less to the primary particles of the silicon compound particles is 8% or less on a volume basis. The negative electrode active material according to claim 1 or 2 , wherein the BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more and 4 m ² / g or less. The silicon compound particles have a half-value width (2θ) of 1.2 ° or more of the diffraction peak caused by the Si (111) crystal plane obtained by X-ray diffraction using Cu—Kα rays, and the crystal plane has a half-value width (2θ) or more. The negative electrode active material according to any one of claims 1 to 3 , wherein the corresponding crystal face size is 7.5 nm or less. In the silicon compound particles, the maximum peak intensity value A of the Si and Li silicate regions given at -60 to -95 ppm as the chemical shift value obtained from the ²⁹ Si-MAS-NMR spectrum, and -96 to-as the chemical shift value. The negative electrode active material according to any one of claims 1 to 4 , wherein the peak intensity value B in the SiO ₂ region given at 150 ppm satisfies the relationship A> B. Claim 1 to claim 1, wherein the median diameter, which is the particle diameter at which the cumulative distribution value of the primary particles of the silicon compound particles in the volume-based particle size distribution is 50%, is 1.0 μm or more and 15 μm or less. 5. The negative electrode active material according to any one of 5. The negative electrode active material according to any one of claims 1 to 6 , wherein the negative electrode active material particles contain a carbon material in the surface layer portion. The negative electrode active material according to claim 7 , wherein the carbon material has an average thickness of 10 nm or more and 5000 nm or less. A mixed negative electrode active material material comprising the negative electrode active material according to any one of claims 1 to 8 and a carbon-based active material. A method for producing a negative electrode active material for an aqueous negative electrode slurry containing negative electrode active material particles containing silicon compound particles.
A step of producing silicon compound particles containing a silicon compound (SiO _x : 0.5 ≦ x ≦ 1.6) and
A step of inserting lithium into the silicon compound particles to contain at least one of Li ₂ SiO ₃ and Li ₄ SiO ₄ .
To produce negative electrode active material particles by
From the negative electrode active material particles, the proportion of the primary particles having a particle diameter of 1 μm or less among the primary particles of the silicon compound is 15% or less on a volume basis.
The BET specific surface area of the primary particles of the silicon compound particles is 1 m ² / g or more and 6 m ² / g or less.
D _99. The mode diameter of the primary particles of the silicon compound particles in the volume-based particle size distribution is 5 μm or more and 10 μm or less, and the cumulative distribution value in the volume-based particle size distribution is 99.9% . Including the step of selecting those having ₉ of 20 μm or less .
A method for producing a negative electrode active material, which comprises producing a negative electrode active material using the selected negative electrode active material particles.

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