WO2021005689A1 - Negative-electrode active material for lithium-ion secondary battery, negative electrode for lithium-ion secondary battery, lithium-ion secondary battery, and method for manufacturing negative-electrode active material for lithium-ion secondary battery - Google Patents

Abstract

Provided is a negative-electrode active material for a lithium-ion secondary battery, said material including: silicon oxide particles; and carbon present on the entirety or part of the surfaces of the silicon oxide particles. In a particle size distribution curve obtained by means of the laser diffraction/scattering method, the ratio (vD50/nD50) of the particle diameter (vD50) when the accumulation from the small-diameter side is 50% on a volume basis and the particle diameter (nD50) when the accumulation from the small-diameter side is 50% on the basis of the particle number is at least 1.3.

Description

Method for manufacturing negative electrode active material for lithium ion secondary battery, negative electrode for lithium ion secondary battery, negative electrode active material for lithium ion secondary battery, and negative electrode active material for lithium ion secondary battery

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

Currently, graphite is mainly used as the negative electrode active material of lithium ion secondary batteries, but it is known that graphite has a theoretical capacity limit of 372 mAh / g in discharge capacity. In recent years, with the increasing performance of mobile devices such as mobile phones, laptop computers, and tablet terminals, and the expansion of their use as power sources for electric vehicles, negative electrode active materials that can achieve even higher capacities for lithium-ion secondary batteries. Development is desired.

Against the background of the above circumstances, studies have been made to use a substance having a theoretical capacity higher than that of graphite as a negative electrode active material. Among them, silicon oxide has a large capacity, is inexpensive, and has good workability, so research on its use as a negative electrode active material is particularly active.

For example, in Patent Document 1, a peak derived from Si (111) is observed in X-ray diffraction, and the size of a silicon crystal obtained by the Scheller method based on the half-value width of the diffraction line is 1 to 500 nm. Disclosed is a negative electrode active material in which the surface of particles having a structure in which fine crystals of silicon are dispersed in a silicon-based compound is coated with carbon.
According to the technique of Patent Document 1, silicon microcrystals or fine particles are dispersed in an inert and strong substance such as silicon dioxide, and carbon for imparting conductivity to at least a part of the surface is fused. It is said that by allowing the surface to be conductive, the structure becomes stable against changes in the volume of silicon due to occlusion and release of lithium, and as a result, long-term stability is obtained and the initial efficiency is improved. ing.

Further, in Patent Document 2, the surface of the silicon oxide particles is coated with a graphite film, the graphite coating amount is 3 to 40% by weight, the BET specific surface area is 2 to 30 m ² / g, and the graphite film is Raman. Negative electrode active materials having a spectrum peculiar to the graphite structure are disclosed in which Raman shifts in the spectral spectrum are around 1330 cm ^-1 and 1580 cm ^-1 .
According to the technology of Patent Document 2, the lithium ion secondary that can reach the characteristic level required by the market by controlling the physical properties of the graphite film that coats the surface of the material that can occlude and release lithium ions within a specific range. It is said that the negative electrode of the battery can be obtained.

Further, Patent Document 3 discloses a negative electrode active material in which the surface of silicon oxide particles represented by the general formula SiO _x is coated with a carbon film treated with thermal plasma.
According to the technique of Patent Document 3, it is said that a negative electrode active material having excellent cycle characteristics can be obtained by solving the expansion of an electrode and the expansion of a battery due to gas generation, which are drawbacks of silicon oxide.

Japanese Patent No. 3952180 Japanese Patent No. 4171897 Japanese Unexamined Patent Publication No. 2011-90869

In the future, as a negative electrode active material for application to lithium ion secondary batteries suitable for improving the performance of mobile devices, etc., in addition to being able to simply store a large amount of lithium ions (that is, having a high charge capacity), it was stored. It is necessary to be able to release more lithium ions (that is, high charge / discharge efficiency). Furthermore, suppressing deterioration due to expansion and contraction of the electrodes of the lithium ion secondary battery is also an important issue.
The present invention has been made in view of the above requirements, and is a negative electrode active material for a lithium ion secondary battery capable of improving the initial charge / discharge efficiency of the lithium ion secondary battery and suppressing deterioration, and lithium using the negative electrode active material. An object of the present invention is to provide a negative electrode for an ion secondary battery, a lithium ion secondary battery, and a method for producing a negative electrode active material for a lithium ion secondary battery.

Specific means for solving the above problems are as follows.
<1> Silicon oxide particles and carbon present on a part or all of the surface of the silicon oxide particles are provided, and the accumulation from the small diameter side is the volume basis in the particle size distribution curve obtained by the laser diffraction / scattering method. The ratio (vD50 / nD50) of the particle size (vD50) when it becomes 50% and the particle size (nD50) when the accumulation from the small diameter side becomes 50% on a number basis is 1.3 or more. Negative electrode active material for lithium ion secondary batteries.
<2> The negative electrode active material for a lithium ion secondary battery according to <1>, wherein the proportion of particles having a particle diameter of 3.0 μm or more is 50% or more on a volume basis.
<3> The lithium ion secondary battery according to <1> or <2>, wherein the carbon content is 0.5% by mass to 10.0% by mass of the total of the silicon oxide particles and the carbon. Negative electrode active material for.
<4> The negative electrode active material for a lithium ion secondary battery according to any one of <1> to <3>, which has a BET specific surface area of 0.1 m ² / g to 15 m ² / g.
<5> When CuKα beam with a wavelength of 0.15406 nm is used as the radiation source, the X-ray diffraction peak intensity (P _SiO2 ) of 2θ = 20 ° to 25 ° derived from SiO ₂ and 2θ = 27 derived from Si. In any one of <1> to <4>, the ratio ( _PSi / _PSiO2 ) to the X-ray diffraction peak intensity ( _PSi ) of ° to 29 ° is in the range of 1.0 to 2.6. The negative electrode active material for a lithium ion secondary battery described.
<6> A current collector and a negative electrode material layer containing the negative electrode active material for a lithium ion secondary battery according to any one of <1> to <5> provided on the current collector. Negative electrode for lithium-ion secondary batteries.
A lithium ion secondary battery comprising a positive electrode <7>, a negative electrode for a lithium ion secondary battery according to <6>, and an electrolyte.
<8> A method for producing a negative electrode active material for a lithium ion secondary battery, which comprises a step of adhering carbon to a part or all of the surface of silicon oxide particles. The negative electrode active material for a lithium ion secondary battery is laser diffraction. -In the particle size distribution curve obtained by the scattering method, the particle size (vD50) when the accumulation from the small diameter side is 50% on a volume basis and the particle size when the accumulation from the small diameter side is 50% on a number basis (vD50). A method for producing a negative electrode active material for a lithium ion secondary battery, wherein the ratio (vD50 / nD50) to nD50) is 1.3 or more.
<9> Production of the negative electrode active material for a lithium ion secondary battery according to <8> for producing the negative electrode active material for a lithium ion secondary battery according to any one of <1> to <5>. Method.

According to the present invention, a negative electrode active material for a lithium ion secondary battery capable of improving the initial charge / discharge efficiency and suppressing deterioration of the lithium ion secondary battery, a negative electrode for a lithium ion secondary battery using the active material, and lithium. A method for producing an ion secondary battery and a negative electrode active material for a lithium ion secondary battery is provided.

Hereinafter, a mode for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, the components (including element steps and the like) are not essential unless otherwise specified. The same applies to the numerical values and their ranges, and does not limit the present invention.

In the present disclosure, the term "process" includes not only a process independent of other processes but also the process if the purpose of the process is achieved even if the process cannot be clearly distinguished from the other process. ..
In the present disclosure, the numerical range indicated by using "-" includes the numerical values before and after "-" as the minimum value and the maximum value, respectively.
In the numerical range described stepwise in the present disclosure, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical range described stepwise. .. Further, in the numerical range described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.
In the present disclosure, each component may contain a plurality of applicable substances. When a plurality of substances corresponding to each component are present in the composition, the content rate or content of each component is the total content rate or content of the plurality of substances present in the composition unless otherwise specified. Means quantity.
In the present disclosure, a plurality of types of particles corresponding to each component may be contained. When a plurality of particles corresponding to each component are present in the composition, the particle size of each component means a value for a mixture of the plurality of particles present in the composition unless otherwise specified.
In the present disclosure, the term "layer" or "membrane" is used only in a part of the region in addition to the case where the layer or the membrane is formed in the entire region when the region in which the layer or the membrane exists is observed. The case where it is formed is also included.
In the present disclosure, the term "laminated" refers to stacking layers, and two or more layers may be bonded or the two or more layers may be removable.

<Negative electrode active material for lithium ion secondary batteries>
The negative electrode active material for the lithium ion secondary battery of the present embodiment (hereinafter, may be simply referred to as “negative electrode active material”) is present on the silicon oxide particles and a part or all of the surface of the silicon oxide particles. Particle size (vD50) when the accumulation from the small diameter side is 50% on a volume basis in the particle size distribution curve obtained by the laser diffraction / scattering method, and the accumulation from the small diameter side is 50 on a number basis. The ratio (vD50 / nD50) to the particle size (nD50) when it becomes% is 1.3 or more.

As a result of the studies by the present inventors, it is possible to improve the initial charge / discharge efficiency of the lithium ion secondary battery, suppress the expansion of the battery, and improve the cycle characteristics by using the negative electrode active material satisfying the above-mentioned particle size distribution conditions. I found out.
The reason is not always clear, but in addition to improving the initial charge / discharge efficiency when the negative electrode active material contains silicon oxide particles, small-diameter particles are present in the silicon oxide particles in an appropriate ratio. As a result, it is considered that the increase in the specific surface area of the entire particle and the improvement of the dispersibility in the carbon-based negative electrode active material used in combination contribute to the suppression of electrode expansion and the improvement of cycle characteristics.

It is considered that the value of vD50 / nD50 in the particle size distribution of the negative electrode active material tends to increase as the proportion of small-diameter particles increases. The value of vD50 / nD50 is preferably 1.4 or more, more preferably 1.5 or more, and particularly preferably 2.0 or more. The upper limit of the value of vD50 / nD50 is not particularly limited, but is preferably 5.0 or less, more preferably 4.0 or less, and further preferably 3.0 or less. When vD50 / nD50 is 5.0 or less, gas generation due to the reaction between the negative electrode active material and the electrolytic solution tends to be suppressed.

The negative electrode active material has a particle size (vD10) when the cumulative amount from the small diameter side is 10% on a volume basis in the particle size distribution curve obtained by the laser diffraction / scattering method, and 10% on a number basis. The ratio (vD10 / nD10) to the particle size (nD10) at that time is preferably 1.2 or more, more preferably 1.3 or more, and further preferably 1.4 or more. The upper limit of the value of vD10 / nD10 is not particularly limited, but is preferably 5.0 or less, more preferably 4.0 or less, and further preferably 3.0 or less.

The negative electrode active material has a particle size (vD90) when the accumulation from the small diameter side is 90% on a volume basis in the particle size distribution curve obtained by the laser diffraction / scattering method, and the accumulation from the small diameter side is 90% on a number basis. The ratio (vD90 / nD90) to the particle size (nD90) at that time is preferably 1.2 or more, more preferably 1.3 or more, and further preferably 1.4 or more. The upper limit of the value of vD90 / nD90 is not particularly limited, but is preferably 5.0 or less, more preferably 4.0 or less, and further preferably 3.0 or less.

In the negative electrode active material, the proportion of particles having a particle diameter of 3.0 μm or more is preferably 50% or more on a volume basis. That is, in the particle size distribution curve obtained by the laser diffraction / scattering method, the particle size (vD50, hereinafter also referred to as volume average particle size) when the accumulation from the small diameter side is 50% on a volume basis is 3.0 μm or more. Is preferable.

The upper limit of the volume average particle size of the negative electrode active material is not particularly limited, but is preferably 20.0 μm or less, more preferably 15.0 μm or less, and further preferably 10.0 μm or less.

At least a part of the negative electrode active material may form secondary particles. It is considered that the secondary particles of the negative electrode active material are formed, for example, by combining some particles when carbon is attached to the surface of the silicon oxide particles. The degree of formation of secondary particles can be controlled by, for example, the particle size distribution of silicon oxide particles before carbon is attached (ratio of small-diameter particles, etc.), the method of attaching carbon (type of device, etc.), and the like. ..
In the present disclosure, the "secondary particles" mean particles formed by fixing a plurality of particles to the extent that they do not separate under normal use conditions of the negative electrode active material.

Whether or not at least a part of the negative electrode active material forms secondary particles can be determined by, for example, observation with an electron microscope.

When the negative electrode active material contains secondary particles, the state of each secondary particle is not particularly limited. For example, secondary particles in a state in which particles having a particle diameter of 1.0 μm or less are attached to particles having a particle diameter of 5.0 μm or more may be included.

When the negative electrode active material contains secondary particles, the ratio is not particularly limited. For example, it may be 20% by mass or less of the whole silicon oxide particles, 10% by mass or less, or 5% by mass or less.

In the present disclosure, the particle size distribution and particle size of the negative electrode active material including the secondary particles mean the values measured in that state (not the state of the primary particles) of the particles forming the secondary particles.

The method of adjusting the particle size so that the negative electrode active material satisfies the above-mentioned particle size distribution condition is not particularly limited. For example, after pulverizing the massive silicon oxide as described in Examples described later, it can be adjusted by sieving or the like as necessary.

Since the negative electrode active material of the present embodiment occupies a relatively large proportion of small-diameter particles, it can be expected to realize, for example, a reduction in the amount of fine powder generated during production and omission of a step of removing fine powder. Therefore, it is also advantageous from the viewpoint of manufacturing efficiency.

(Silicon oxide particles)
The silicon oxide constituting the silicon oxide particles may be an oxide containing a silicon element, and examples thereof include silicon oxide, silicon dioxide, and silicon sulfite. The silicon oxide contained in the silicon oxide particles may be only one type or a combination of two or more types.

Among the silicon oxides, silicon oxide and silicon dioxide are generally represented as silicon monoxide (SiO) and silicon dioxide (SiO ₂ ), respectively, but in a surface state (eg, the presence of an oxide film) or Depending on the production status of the compound, the actual measurement value (or conversion value) of the contained element may be represented by the composition formula SiO _x (x is 0 <x ≦ 2), and in this case as well, the silicon oxide according to the present disclosure is used. To do. The value of x in the composition formula can be calculated, for example, by quantifying the oxygen contained in the silicon oxide by the inert gas melting-non-dispersion infrared absorption method. In addition, when a silicon oxide disproportionation reaction (2SiO → Si + SiO ₂ ) is involved in the manufacturing process of the negative electrode active material, it is shown in a state where silicon and silicon dioxide (in some cases, silicon oxide) are contained in the chemical reaction. In this case as well, the silicon oxide according to the present disclosure is used.

(carbon)
Carbon is present on part or all of the surface of the silicon oxide particles. The presence of carbon on part or all of the surface of the silicon oxide particles imparts conductivity to the silicon oxide particles, which are insulators, and improves the efficiency of the charge / discharge reaction. Therefore, it is considered that the initial discharge capacity and the initial charge / discharge efficiency are improved. Hereinafter, silicon oxide particles in which carbon is present on a part or all of the surface may be referred to as "SiO-C particles".

In the present disclosure, examples of carbon present on a part or all of the surface of the silicon oxide particles include graphite, amorphous carbon and the like. The organic substances and conductive particles described later do not correspond to "carbon" in the present disclosure.
The mode in which carbon is present on a part or all of the surface of the silicon oxide particles is not particularly limited. For example, continuous or discontinuous coating and the like can be mentioned.
The presence or absence of carbon in the negative electrode active material can be confirmed by, for example, laser Raman spectroscopy with an excitation wavelength of 532 nm.

The carbon content is preferably 0.5% by mass to 10.0% by mass of the total of the silicon oxide particles and carbon. With such a configuration, the initial discharge capacity and the initial charge / discharge efficiency tend to be further improved. The carbon content is more preferably 1.0% by mass to 9.0% by mass, further preferably 2.0% by mass to 8.0% by mass, and particularly preferably 3.0% by mass to 7.0% by mass. ..

The carbon content (mass basis) can be determined, for example, by high-frequency firing-infrared analysis. In the high-frequency firing-infrared analysis method, for example, a carbon-sulfur simultaneous analyzer (LECO Japan GK, CSLS600) can be applied. When the negative electrode active material contains an organic substance described later, the carbon content is reduced by heating the negative electrode active material to a temperature higher than the decomposition temperature (for example, 300 ° C.) to remove the mass loss derived from the organic substance in advance. Can be measured.

Carbon preferably has low crystallinity. In the present disclosure, "low crystallinity" of carbon means that the R value of the negative electrode active material obtained by the method shown below is 0.5 or more.
R value of the negative electrode active material, in a profile obtained by laser Raman spectroscopy of the excitation wavelength 532 nm, when the intensity of a peak appearing near 1360 cm ^-1 Id, the intensity of the peak appearing in the vicinity of 1580 cm ^-1 and Ig , Means the intensity ratio Id / Ig (also referred to as D / G) of both peaks.

Here, the peak appearing in the vicinity of 1360 cm ^-1 is usually a peak identified to correspond to an amorphous structure, and means, for example, a peak observed in 1300 cm ^-1 to 1400 cm ^-1 . Also, the peak appearing near 1580 cm ^-1, generally a peak identified as corresponding to the graphite crystal structure, for ^example, refers to peaks observed at 1530cm ^-1 ~ 1630cm -1.
Incidentally, R value Raman spectrum measuring apparatus (e.g., NSR-1000 type, manufactured by JASCO Corporation) was used, based in the range of ^{1050 cm} -1 ~ 1750 cm ^-1 with respect to the measurement range ^{(830cm -1} ~ ^1940cm -1) It can be obtained by performing line correction.

The R value of the negative electrode active material is preferably 0.5 to 2.5, more preferably 0.7 to 2.3, and even more preferably 0.8 to 2.0. When the R value is 0.5 to 2.5, the surface of the silicon oxide particles is sufficiently covered with low crystalline carbon in which carbon crystallites are diffusely oriented, so that the reactivity with the electrolytic solution can be reduced. Cycle characteristics tend to improve.

The method of imparting carbon to the surface of the silicon oxide particles is not particularly limited. Specific examples thereof include a wet mixing method, a dry mixing method, and a chemical vapor deposition method.

When carbon is added by a wet mixing method, for example, a silicon oxide particle and a carbon raw material (carbon source) dissolved or dispersed in a solvent are mixed, and the carbon source is the surface of the silicon oxide particle. There is a method of carbonizing the carbon source by adhering to the silicon, removing the solvent if necessary, and then heat-treating in an inert atmosphere.

When carbon is added by a dry mixing method, for example, silicon oxide particles and a carbon source are mixed in a solid state to form a mixture, and the mixture is heat-treated in an inert atmosphere to obtain carbon as the carbon source. There is a method of making it. When the silicon oxide particles and the carbon source are mixed, a treatment for applying mechanical energy (for example, a mechanochemical treatment) may be performed.

When carbon is added by the chemical vapor deposition method, a known method can be applied. For example, carbon can be imparted to the surface of the silicon oxide particles by heat-treating the silicon oxide particles in an atmosphere containing a gas in which the carbon source is vaporized.

When carbon is added to the surface of silicon oxide particles by a wet mixing method or a dry mixing method, the carbon source used is not particularly limited as long as it is a substance that can be converted to carbon by heat treatment. Specifically, polymer compounds such as phenol resin, styrene resin, polyvinyl alcohol, polyvinyl chloride, polyvinyl acetate, and polybutyral; ethylene heavy end pitch, coal pitch, petroleum pitch, coal tar pitch, asphalt decomposition pitch, etc. Pitches such as PVC pitch produced by thermal decomposition of polyvinyl chloride (PVC) and the like, and pitches such as naphthalene pitch produced by polymerizing naphthalene and the like in the presence of a super strong acid; polysaccharides such as starch and cellulose can be mentioned. These carbon sources may be used alone or in combination of two or more.

When carbon is added to the surface of silicon oxide particles by a chemical vapor deposition method, the carbon source used is an aliphatic hydrocarbon, an aromatic hydrocarbon, an alicyclic hydrocarbon, or the like, which is gaseous or easily gasified. It is preferable to use a possible substance. Specific examples thereof include methane, ethane, propane, toluene, benzene, xylene, styrene, naphthalene, cresol, anthracene, and derivatives thereof. These carbon sources may be used alone or in combination of two or more.

The heat treatment temperature at the time of carbonizing the carbon source is not particularly limited as long as the temperature at which the carbon source is carbonized, and is preferably 700 ° C. or higher, more preferably 800 ° C. or higher, and more preferably 900 ° C. or higher. It is more preferable to have. The heat treatment temperature is preferably 1300 ° C. or lower, preferably 1200 ° C. or lower, from the viewpoint of obtaining low crystallinity carbon and producing silicon crystals in a desired size by a disproportionation reaction described later. It is more preferable that the temperature is 1100 ° C. or lower.

The heat treatment time for carbonizing the carbon source can be selected depending on the type and amount of the carbon source used. For example, 1 hour to 10 hours is preferable, and 2 hours to 7 hours is more preferable.

The heat treatment for carbonizing the carbon source is preferably carried out in an inert atmosphere such as nitrogen or argon. The heat treatment apparatus is not particularly limited, and examples thereof include a heating apparatus capable of processing by a continuous method, a batch method, or the like. Specifically, it can be selected from a fluidized bed reactor, a rotary furnace, a vertical mobile bed reactor, a tunnel furnace, a batch furnace, and the like.

If the heat-treated product obtained by the heat treatment is in a state where a plurality of particles are agglomerated, further crushing treatment may be performed. Further, if adjustment to a desired average particle size is required, further pulverization treatment may be performed.

(X-ray diffraction peak intensity ratio)
The negative electrode active material, 2 [Theta] that when using CuKα radiation of a wavelength 0.15406nm as a radiation source, an X-ray diffraction peak intensity of 2θ = 20 ° ~ 25 ° derived from SiO _{2 (P SiO2),} derived from the Si The ratio ( _PSi / _PSiO2 ) to the X-ray diffraction peak intensity ( _PSi ) of = 27 ° to 29 ° is preferably in the range of 1.0 to 2.6.

The ratio of the X-ray diffraction peak intensity ( _PSi / _PSiO2 ) of the negative electrode active material is a value measured with carbon, organic matter, conductive particles, etc. attached to the silicon oxide particles, but these adhere to the silicon oxide particles. It may be a value measured in a non-existing state.

As a negative electrode active material having an X-ray diffraction peak intensity ratio ( _PSi / _PSiO2 ) in the range of 1.0 to 2.6, a silicon oxide having a structure in which silicon crystallites are present in the silicon oxide. Negative electrode active material containing particles can be mentioned.

Silicon oxide particles having a structure in which silicon crystallites are dispersed in silicon oxide cause, for example, a disproportionation reaction (2SiO → Si + SiO ₂ ) of silicon oxide to cause silicon in the silicon oxide particles. It can be produced by generating crystallites. By controlling the degree to which silicon crystallites are formed in the silicon oxide particles, the ratio of the X-ray diffraction peak intensities can be controlled to a desired value.

The advantage of having silicon crystallites present in the silicon oxide particles by the disproportionation reaction of the silicon oxide can be considered as follows. The above-mentioned SiO _x (x is 0 <x ≦ 2) tends to be inferior in the initial charge / discharge characteristics because lithium ions are trapped during the initial charge. This is because the lithium ions are trapped by the dangling bonds (unpaired electron pairs) of oxygen existing in the amorphous SiO ₂ phase. Therefore, it is considered preferable from the viewpoint of improving charge / discharge characteristics to suppress the generation of dangling bonds of active oxygen atoms by reconstructing the amorphous SiO ₂ phase by heat treatment.

When the ratio of the X-ray diffraction peak intensity ( _PSi / P _SiO2 ) of the negative electrode active material is 1.0 or more, the silicon crystallites in the silicon oxide particles are sufficiently grown and the ratio of SiO ₂ is large. It does not become too much, a sufficient initial discharge capacity can be obtained, and a decrease in charge / discharge efficiency due to an irreversible reaction tends to be suppressed. On the other hand, when the ratio of the X-ray diffraction peak intensity ( _PSi / _PSiO2 ) is 2.6 or less, the crystallites of the produced silicon do not become too large and the expansion and contraction are easily relaxed, and the initial discharge capacity is lowered. Tends to be suppressed. From the viewpoint of obtaining a negative electrode active material having better charge / discharge characteristics, the ratio of X-ray diffraction peak intensities ( _PSi / _PSiO2 ) is preferably in the range of 1.5 to 2.0.

The ratio of the X-ray diffraction peak intensities of the negative electrode active material ( _PSi / _PSiO2 ) can be controlled, for example, by the conditions of the heat treatment that causes the disproportionation reaction of the silicon oxide. For example, by raising the heat treatment temperature or lengthening the heat treatment time, the formation and enlargement of silicon crystallites can be promoted, and the ratio of the X-ray diffraction peak intensity can be increased. On the other hand, by lowering the heat treatment temperature or shortening the heat treatment time, the formation of silicon crystallites can be suppressed, and the ratio of the X-ray diffraction peak intensities can be reduced.

When silicon oxide particles are produced by the disproportionation reaction of silicon oxide, the silicon oxide used as a raw material is, for example, a mixture of silicon dioxide and metallic silicon is heated to cool the gas of silicon monoxide produced. It can be obtained by a known sublimation method for precipitating. It can also be obtained from the market as silicon oxide, silicon monoxide and the like.

Whether or not silicon crystallites are present in the silicon oxide particles can be confirmed by, for example, powder X-ray diffraction (XRD) measurement. When silicon crystallites are present in the silicon oxide particles, powder X-ray diffraction (XRD) measurement using CuKα rays with a wavelength of 0.15406 nm is performed, and the vicinity of 2θ = 28.4 °. A diffraction peak derived from Si (111) is observed.

When silicon crystallites are present in the silicon oxide particles, the size of the silicon crystallites is preferably 8.0 nm or less, more preferably 6.0 nm or less. When the size of the silicon crystallites is 8.0 nm or less, the silicon crystallites are difficult to localize in the silicon oxide particles and tend to be dispersed throughout the particles. Therefore, in the silicon oxide particles. Lithium ions are easily diffused, and a good charging capacity is easily obtained. The size of silicon crystallites is preferably 2.0 nm or more, more preferably 3.0 nm or more. When the size of the silicon crystallite is 2.0 nm or more, the reaction between the lithium ion and the silicon oxide is well controlled, and good charge / discharge efficiency can be easily obtained.

The size of silicon crystallites is the size of silicon single crystals contained in silicon oxide particles, and Si (111) obtained by powder X-ray diffraction analysis using CuKα rays with a wavelength of 0.15406 nm as the radiation source. It can be obtained from the half-value width of the diffraction peak near 2θ = 28.4 ° from which it is derived, using Scherrer's equation.

The method for forming silicon crystallites in the silicon oxide particles is not particularly limited. For example, it can be produced by heat-treating silicon oxide particles in a temperature range of 700 ° C. to 1300 ° C. in an inert atmosphere to cause a disproportionation reaction (2SiO → Si + SiO ₂ ). The heat treatment for causing the disproportionation reaction may be performed in the same step as the heat treatment for imparting carbon to the surface of the silicon oxide particles.

The heat treatment conditions for causing the disproportionation reaction of the silicon oxide are, for example, to carry out the silicon oxide in a temperature range of 700 ° C. to 1300 ° C., preferably 800 ° C. to 1200 ° C. in an inert atmosphere. Can be done. From the viewpoint of producing silicon crystals having a desired size, the heat treatment temperature is preferably more than 900 ° C, more preferably 950 ° C or higher. The heat treatment temperature is preferably less than 1150 ° C, more preferably 1100 ° C or lower.

When the negative electrode active material is obtained through a pulverization step, the particle size distribution may be adjusted by performing a classification treatment after pulverization. The classification method is not particularly limited, and can be selected from dry classification, wet classification, sieving and the like. From the viewpoint of productivity, it is preferable to perform crushing and classification at once. For example, a jet mill and cyclone coupling system can classify particles before they reaggregate, making it easy to obtain the desired particle size distribution shape.

When necessary (for example, when the aspect ratio of the negative electrode active material cannot be adjusted to a desired range only by pulverization treatment), the negative electrode active material after pulverization is further subjected to surface modification treatment to adjust the aspect ratio. You may. The apparatus for performing the surface modification treatment is not particularly limited. For example, a mechanofusion system, a nobilta, a hybridization system and the like can be mentioned.

(BET specific surface area)
The BET specific surface area of the negative electrode active material is preferably 0.1 m ² / g to 15 m ² / g, more preferably 0.5 m ² / g to 10 m ² / g, and 1.0 m ² / g. more preferably from ~ 7.0m ² / ^g, particularly preferably 1.0m ² /g~4.0m 2 / g. When the BET specific surface area of the negative electrode active material is 15 m ² / g or less, the increase in the initial irreversible capacity of the obtained lithium ion secondary battery tends to be suppressed. In addition, the amount of binder used when producing the negative electrode can be reduced. When the specific surface area of the negative electrode active material is 0.1 m ² / g or more, a sufficient contact area between the negative electrode active material and the electrolytic solution is sufficiently secured, and good charge / discharge efficiency tends to be obtained. The BET specific surface area of the negative electrode active material is measured by the BET method (nitrogen gas adsorption method).

(Powder electrical resistance)
The powder electrical resistance of the negative electrode active material is preferably 100 Ω · cm or less, more preferably 80 Ω · cm or less, and further preferably 50 Ω · cm or less at a pressure of 10 MPa. When the electric resistance of the powder is 100 Ω · cm or less, the movement of electrons during charging / discharging is less likely to be hindered, lithium is easily stored and released, and the cycle characteristics are excellent. The powder electrical resistance of the negative electrode active material can be measured using, for example, a powder electrical resistance device (MSP-PD51 type 4 probe probe, Mitsubishi Chemical Analytech Co., Ltd.). The powder electrical resistance of the negative electrode active material may be 0.1 Ω · cm or more, preferably 1 Ω · cm or more, and more preferably 10 Ω · cm or more at a pressure of 10 MPa.

From the viewpoint of reducing the powder electrical resistance value of the negative electrode active material, the negative electrode active material preferably contains conductive particles described later. By adhering the conductive particles to the surface of the SiO—C particles to form a protrusion structure, the resistance value of the entire negative electrode active material can be reduced.

(organic matter)
The negative electrode active material may contain an organic substance. When the negative electrode active material contains an organic substance, the initial discharge capacity, the initial charge / discharge efficiency, and the recovery rate after charge / discharge tend to be further improved. It is considered that this is because the specific surface area of the negative electrode active material is lowered by containing an organic substance, and the reaction with the electrolytic solution is suppressed. The organic substance contained in the negative electrode active material may be only one type or two or more types.

The content of organic matter is preferably 0.1% by mass to 5.0% by mass of the entire negative electrode active material. When the content of the organic substance is within the above range, the effect of improving the recovery rate after charging / discharging tends to be sufficiently obtained while suppressing the decrease in conductivity. The content of the organic substance in the entire negative electrode active material is more preferably 0.2% by mass to 3.0% by mass, and further preferably 0.3% by mass to 1.0% by mass.

Whether or not the negative electrode active material contains an organic substance is determined by, for example, heating a sufficiently dried negative electrode active material to a temperature higher than the temperature at which the organic substance decomposes and lower than the temperature at which the carbon decomposes (for example, 300 ° C.). It can be confirmed by measuring the mass of the negative electrode active material after the organic matter is decomposed. Specifically, when the mass of the negative electrode active material before heating is A (g) and the mass of the negative electrode active material after heating is B (g), it is represented by {(AB) / A} × 100. When the rate of change of the mass is 0.1% or more, it can be determined that the negative electrode active material contains an organic substance.

The rate of change in the mass is preferably 0.1% to 5.0%, more preferably 0.3% to 1.0%. When the rate of change is 0.1% or more, a sufficient amount of organic matter is present on the surface of the SiO—C particles, so that the effect of containing the organic matter tends to be sufficiently obtained.

The type of organic matter is not particularly limited. For _example, water-soluble cellulose derivative to a derivative of starch of C _{6 H} 10 _{O 5} as a basic _structure, C _{6 H} 10 _{O 5} a basic structure to viscous _{polysaccharide,} a C _{6 H} 10 _{O 5} as a basic structure, polyuronide and At least one selected from the group consisting of water-soluble synthetic resins can be mentioned.

Specific examples of the starch derivative having C ₆ H ₁₀ O ₅ as a basic structure include hydroxyalkyl starches such as acetate starch, phosphoric acid starch, carboxymethyl starch and hydroxyethyl starch. Specific examples of the viscous polysaccharide having C ₆ H ₁₀ O ₅ as a basic structure include pullulan and dextrin. Examples of the water-soluble cellulose derivative having C ₆ H ₁₀ O ₅ as a basic structure include carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and the like. Examples of polyuronide include pectic acid, alginic acid and the like. Examples of the water-soluble synthetic resin include water-soluble acrylic resin, water-soluble epoxy resin, water-soluble polyester resin, water-soluble polyamide resin, and more specifically, polyvinyl alcohol, polyacrylic acid, polyacrylic acid salt, and polyvinyl. Examples thereof include sulfonic acid, polyvinyl sulfonate, poly4-vinylphenol, poly4-vinylphenol salt, polystyrene sulfonic acid, polystyrene sulfonate, polyaniline sulfonic acid and the like. The organic substance may be used in the state of a metal salt, an alkylene glycol ester or the like.

From the viewpoint of reducing the specific surface area of the negative electrode active material, the organic substance is a part or all of the SiO-C particles (when the conductive particles described later are present on the surface of the SiO-C particles, the surface thereof). It is preferable that the state is covered with.

The method for allowing the organic substance to be present on a part or all of the surface of the SiO—C particles is not particularly limited. For example, the organic substance can be attached to the SiO-C particles by putting the SiO-C particles in a liquid in which the organic substance is dissolved or dispersed and stirring the mixture as necessary. After that, the SiO-C particles to which the organic substance is attached are taken out from the liquid and dried if necessary, so that the SiO-C particles to which the organic substance is attached to the surface can be obtained.
In the above method, the temperature of the liquid during stirring is not particularly limited and can be selected from, for example, 5 ° C to 95 ° C. The temperature at the time of drying is not particularly limited and can be selected from, for example, 50 ° C to 200 ° C. The content of the organic substance in the solution is not particularly limited and can be selected from, for example, 0.1% by mass to 20% by mass.

(Conductive particles)
The negative electrode active material may contain conductive particles. Since the negative electrode active material contains conductive particles, even if the silicon oxide particles expand and contract, the conductive particles come into contact with each other, so that conduction can be easily ensured. In addition, the resistance value of the entire negative electrode active material tends to decrease. As a result, the decrease in capacity due to repeated charging and discharging is suppressed, and the cycle characteristics tend to be maintained well.

From the viewpoint of ensuring continuity by contact between the negative electrode active materials, it is preferable that the conductive particles are present on the surface of the SiO—C particles. Hereinafter, the particles in which the conductive particles are present on the surface of the SiO-C particles may be referred to as "CP / SiO-C particles".

The type of conductive particles is not particularly limited. For example, at least one selected from the group consisting of granular graphite and carbon black is preferable, and granular graphite is preferable from the viewpoint of improving cycle characteristics. Examples of granular graphite include particles such as artificial graphite, natural graphite, and MC (mesophase carbon). Examples of carbon black include acetylene black, ketjene black, thermal black, furnace black and the like, and acetylene black is preferable from the viewpoint of conductivity.

Granular graphite is preferably more crystalline than carbon present on the surface of silicon oxide particles from the viewpoint of improving both battery capacity and charge / discharge efficiency. Specifically, for granular graphite, the average interplanar spacing (d ₀₀₂ ) obtained by measuring based on the Gakushin method is preferably 0.335 nm to 0.347 nm, and 0.335 nm to 0.345 nm. It is more preferably 0.335 nm to 0.340 nm, and particularly preferably 0.335 nm to 0.337 nm. When the average interplanar spacing of the granular graphite is 0.347 nm or less, the crystallinity of the granular graphite is high, and both the battery capacity and the charge / discharge efficiency tend to be improved. On the other hand, since the theoretical value of graphite crystals is 0.335 nm, when the average interplanar spacing of granular graphite is close to this value, both the battery capacity and the charge / discharge efficiency tend to improve.

The shape of the granular graphite is not particularly limited, and may be flat graphite or spheroidal graphite. From the viewpoint of improving cycle characteristics, flat graphite is preferable. Examples of flat graphite include graphite having a scale-like, scaly-like, and lump-like shape.

The aspect ratio of the conductive particles is not particularly limited, but from the viewpoint of ensuring continuity between the conductive particles and improving the cycle characteristics, the average value of the aspect ratio is preferably 0.3 or less. It is more preferably 2 or less. The average value of the aspect ratios of the conductive particles is preferably 0.001 or more, and more preferably 0.01 or more.

The aspect ratio of the conductive particles is a value measured by observation by SEM. Specifically, for each of the 100 conductive particles arbitrarily selected in the SEM image, the length in the major axis direction is A, and the length in the minor axis direction (in the case of flat graphite, the length in the thickness direction) is set. It is a value calculated as B / A when B is set. The average value of the aspect ratio is the arithmetic mean value of the aspect ratio of 100 conductive particles.

The conductive particles may be either primary particles (singular particles) or secondary particles (granulated particles) formed from a plurality of primary particles. Further, the flat graphite may be porous graphite particles.

The content of the conductive particles is preferably 1.0% by mass to 10.0% by mass, preferably 2.0% by mass to 9.0% by mass, based on the entire negative electrode active material from the viewpoint of improving the cycle characteristics. More preferably, it is more preferably 3.0% by mass to 8.0% by mass.

The content of conductive particles can be determined, for example, by high-frequency firing-infrared analysis. In the high-frequency firing-infrared analysis method, for example, a carbon-sulfur simultaneous analyzer (CSLS600, LECO Japan GK) can be used. Since this measurement also includes the carbon content of the SiO—C particles, it may be subtracted from the separately measured carbon content obtained.

The method for producing the negative electrode active material containing the conductive particles is not particularly limited, and examples thereof include a wet method and a dry method.

As a method for producing a negative electrode active material containing conductive particles by a wet method, for example, SiO-C particles are added to a particle dispersion liquid in which conductive particles are dispersed in a dispersion medium, and after stirring, a dryer or the like is used. Then, a method of producing by removing the dispersion medium can be mentioned. The dispersion medium used is not particularly limited, and water, an organic solvent, or the like can be used. The organic solvent may be a water-soluble organic solvent such as alcohol or a water-insoluble organic solvent. The dispersion medium may contain a dispersant from the viewpoint of increasing the dispersibility of the conductive particles and making the adhesion of the SiO—C particles to the surface more uniform. The dispersant can be selected according to the type of dispersion medium used. For example, when the dispersion medium is an aqueous system, carboxymethyl cellulose is preferable from the viewpoint of dispersion stability.

Examples of the method for producing the negative electrode active material containing the conductive particles by the dry method include a method in which the conductive particles are added together with the carbon source when the carbon source of carbon is applied to the surface of the silicon oxide particles. .. Specifically, for example, a method of mixing a carbon source and conductive particles with silicon oxide particles and applying mechanical energy (for example, mechanochemical treatment) can be mentioned.

If necessary, the obtained negative electrode active material may be further classified. The classification process can be performed using a sieving machine or the like.

The negative electrode active material (SiO-based negative electrode active material) of the present embodiment may be used in combination with another negative electrode active material, if necessary. For example, it may be used in combination with a carbon-based negative electrode active material conventionally known as an active material for the negative electrode of a lithium ion secondary battery. Depending on the type of carbon-based negative electrode active material used in combination, improvement of charge / discharge efficiency, improvement of cycle characteristics, effect of suppressing expansion of electrodes, etc. can be obtained. The carbon-based negative electrode active material used in combination with the negative electrode active material of the present embodiment may be only one type or two or more types.

Examples of the carbon-based negative electrode active material include natural graphite such as scaly natural graphite and spherical natural graphite obtained by spheroidizing scaly natural graphite, and a negative electrode active material made of a carbon material such as artificial graphite and amorphous carbon. Further, these carbon-based negative electrode active materials may have carbon (carbon or the like described above) on a part or all of the surface thereof.

When the negative electrode active material of the present embodiment is used in combination with the carbon-based negative electrode active material, the ratio (A: B) of the negative electrode active material (A) and the carbon-based negative electrode active material (B) of the present embodiment is determined. It can be adjusted as appropriate according to the purpose. For example, from the viewpoint of the effect of suppressing the expansion of the electrode, it is preferably 0.1: 99.9 to 20:80, and 0.5: 99.5 to 15:85 on a mass basis. More preferably, it is more preferably 1:99 to 10:90.

<Manufacturing method of negative electrode active material for lithium ion secondary battery>
The method for producing a negative electrode active material for a lithium ion secondary battery of the present embodiment is a method for producing a negative electrode active material for a lithium ion secondary battery, which comprises a step of adhering carbon to a part or all of the surface of silicon oxide particles. Yes, the negative electrode active material for lithium ion secondary batteries has the particle size (vD50) when the accumulation from the small diameter side is 50% on a volume basis in the particle size distribution curve obtained by the laser diffraction / scattering method, and the particle size from the small diameter side. The ratio (vD50 / nD50) to the particle size (nD50) when the cumulative number is 50% on a number basis is 1.3 or more.

According to the above method, it is possible to produce a negative electrode active material capable of improving the initial charge / discharge efficiency of the lithium ion secondary battery, suppressing electrode expansion, and improving the cycle characteristics.

The above method may be for producing the negative electrode active material of the above-described embodiment. That is, the negative electrode active material produced by the above method may be the negative electrode active material of the above-described embodiment.

In the above method, the method of carrying out the step of adhering carbon to a part or all of the surface of the silicon oxide particles is not particularly limited. For example, carbon can be added to the surface of the silicon oxide particles by the method described above.

The above method may include a step other than the step of adhering carbon to a part or all of the surface of the silicon oxide particles, if necessary. For example, pulverization of silicon oxide particles, particle size adjustment, heat treatment for disproportionation reaction, adhesion of organic substances or conductive particles, and the like may be carried out.

<Negative electrode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery of the present embodiment (hereinafter, may be abbreviated as "negative electrode") includes a current collector and a negative electrode material layer containing the above-mentioned negative electrode active material provided on the current collector. , Have.

The negative electrode can be produced, for example, by forming a negative electrode material layer on the current collector using the above-mentioned composition containing the negative electrode active material.
Examples of the composition containing the negative electrode active material include a mixture of the negative electrode active material with an organic binder, a solvent, a thickener, a conductive auxiliary agent, a carbon-based negative electrode active material, and the like.

Specific examples of the organic binder include styrene-butadiene copolymers; ethylenic properties such as methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, and hydroxyethyl (meth) acrylate. A (meth) acrylic copolymer obtained by copolymerizing an unsaturated carboxylic acid ester with an ethylenically unsaturated carboxylic acid such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid; polyvinylidene fluoride, Polymer compounds such as polyethylene oxide, polyepicchlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, and polyamideimide; and the like. In addition, "(meth) acrylate" means "acrylate" and the corresponding "methacrylate". The same applies to other similar expressions such as "(meth) acrylic copolymer". The organic binder may be one dispersed or dissolved in water, or one dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP). One type of organic binder may be used alone, or two or more types may be used in combination.

From the viewpoint of adhesion, the organic binder having a main skeleton of polyacrylonitrile, polyimide or polyamide-imide is preferable among the organic binders, the heat treatment temperature at the time of producing the negative electrode is low, and the flexibility of the electrode is excellent. An organic binder having a main skeleton of polyacrylonitrile is more preferable. Examples of the organic binder having polyacrylonitrile as a main skeleton include those obtained by adding acrylic acid that imparts adhesiveness and a linear ether group that imparts flexibility to a polyacrylonitrile skeleton.

The content of the organic binder in the negative electrode material layer is preferably 0.1% by mass to 20% by mass, more preferably 0.2% by mass to 20% by mass, and 0.3% by mass. It is more preferably to 15% by mass. When the content of the organic binder in the negative electrode material layer is 0.1% by mass or more, good adhesion is obtained, and the negative electrode is suppressed from being destroyed by expansion and contraction during charging and discharging. .. On the other hand, when the content of the organic binder in the negative electrode material layer is 20% by mass or less, an increase in electrode resistance can be suppressed.

Specific examples of the thickener include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein and the like. One type of thickener may be used alone, or two or more types may be used in combination.

Specific examples of the solvent include N-methylpyrrolidone, dimethylacetamide, dimethylformamide, γ-butyrolactone and the like. One type of solvent may be used alone, or two or more types may be used in combination.

Specific examples of the conductive auxiliary agent include carbon black, acetylene black, oxides showing conductivity, and nitrides showing conductivity. One type of conductive auxiliary agent may be used alone, or two or more types may be used in combination. The content of the conductive auxiliary agent in the negative electrode material layer is preferably 0.1% by mass to 20% by mass.

Examples of the material of the current collector include aluminum, copper, nickel, titanium, stainless steel, porous metal (foam metal), carbon paper, and the like. Examples of the shape of the current collector include a foil shape, a perforated foil shape, and a mesh shape.

As a method of forming a negative electrode material layer on a current collector using a composition containing a negative electrode active material, a coating liquid containing a negative electrode active material is applied onto the current collector to remove volatile substances such as a solvent. , A method of pressure molding, a method of integrating the negative electrode material layer formed into a sheet shape, a pellet shape, or the like and a current collector, and the like.
Examples of the method of applying the coating liquid to the current collector include a metal mask printing method, an electrostatic coating method, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a gravure coating method, and a screen printing method. .. The pressurization treatment after coating can be performed by a flat plate press, a calendar roll, or the like.
The integration of the negative electrode material layer and the current collector can be, for example, integration by a roll, integration by a press, or a combination thereof.

The negative electrode material layer formed on the current collector or the negative electrode material layer integrated with the current collector may be heat-treated according to the type of the organic binder used. For example, when using an organic binder having polyacrylonitrile as the main skeleton, it is preferable to heat-treat at 100 ° C. to 180 ° C., and when using an organic binder having polyimide or polyamide-imide as the main skeleton, 150 ° C. It is preferable to heat-treat at ° C to 450 ° C.
By this heat treatment, the solvent is removed and the strength is increased by curing the organic binder, and the adhesion between the negative electrode active materials and the adhesion between the negative electrode active material and the current collector can be improved. In addition, these heat treatments are preferably carried out in an inert atmosphere such as helium, argon or nitrogen or in a vacuum atmosphere in order to prevent oxidation of the current collector during the treatment.

Further, it is preferable that the negative electrode material layer is pressed (pressurized) before the heat treatment. The electrode density can be adjusted by pressurizing. Electrode density may, for ^example, more preferably is preferably 1.4g / cm 3 ~ 1.9g / cm 3, a 1.5g / cm 3 ~ 1.85g / cm 3, 1.6g / cm It is more preferably ³ to 1.8 g / cm ³ . Regarding the electrode density, the higher the value, the more the volume capacity of the negative electrode tends to improve, and the adhesion between the negative electrode active materials and the adhesion between the negative electrode active material and the current collector tend to improve. ..

<Lithium-ion secondary battery>
The lithium ion secondary battery of the present embodiment includes a positive electrode, the above-mentioned negative electrode, and an electrolyte.
A lithium ion secondary battery is manufactured, for example, by arranging a negative electrode and a positive electrode in a battery container so as to face each other via a separator, and injecting an electrolytic solution obtained by dissolving an electrolyte in an organic solvent into the battery container. can do.

The positive electrode can be obtained by forming a positive electrode material layer on the surface of the current collector in the same manner as the negative electrode. As the current collector at the positive electrode, the same current collector as at the negative electrode can be used.

The material used for the positive electrode (also referred to as the positive electrode material) may be a compound capable of doping or intercalating lithium ions, and may be lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or lithium manganate (LiMnO). ₂ ) and the like.

For the positive electrode, for example, a positive electrode material, an organic binder such as polyvinylidene fluoride, and a solvent such as N-methyl-2-pyrrolidone and γ-butyrolactone are mixed to prepare a positive electrode coating solution, and this positive electrode coating solution is prepared. Can be applied to at least one surface of a current collector such as an aluminum foil, then the solvent is dried and removed, and if necessary, pressure treatment is performed.
A conductive auxiliary agent may be added to the positive electrode coating liquid. Examples of the conductive auxiliary agent include carbon black, acetylene black, oxides exhibiting conductivity, nitrides exhibiting conductivity, and the like. These conductive auxiliaries may be used alone or in combination of two or more.

As _{the electrolyte, LiPF 6, LiClO 4, LiBF} 4, LiClF 4, LiAsF 6, LiSbF 6, LiAlO 4, LiAlCl 4, LiN (CF 3 SO 2) 2, LiN (C 2 F 5 SO 2) 2, LiC ( CF ₃ SO ₂ ) ₃ , LiCl, LiI and the like can be mentioned.

Examples of the organic solvent that dissolves the electrolyte include propylene carbonate, ethylene carbonate, diethyl carbonate, ethylmethyl carbonate, vinylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane and 2-methyltetrahydrofuran.

Examples of the separator include a paper separator, a polypropylene separator, a polyethylene separator, a glass fiber separator, and the like.

The manufacturing method of the lithium ion secondary battery is not particularly limited. For example, a cylindrical lithium ion secondary battery can be manufactured by the following steps. First, the two electrodes, the positive electrode and the negative electrode, are wound around the separator. The obtained spiral winding group is inserted into the battery can, and the tab terminal previously welded to the current collector of the negative electrode is welded to the bottom of the battery can. The electrolytic solution is injected into the obtained battery can. Further, the tab terminal welded to the current collector of the positive electrode in advance is welded to the lid of the battery, and the lid is arranged on the upper part of the battery can via the insulating gasket. A lithium ion secondary battery can be obtained by crimping and sealing the portion where the lid and the battery can are in contact with each other.

The form of the lithium ion secondary battery is not particularly limited, and examples thereof include lithium ion secondary batteries such as paper type batteries, button type batteries, coin type batteries, laminated batteries, cylindrical batteries, and square batteries.

The negative electrode active material of the present embodiment is not limited to the lithium ion secondary battery, and can be applied to all electrochemical devices having a charging / discharging mechanism of inserting and removing lithium ions.

Hereinafter, the above-described embodiment will be described in more detail based on the examples, but the above-described embodiment is not limited to the following examples. Unless otherwise specified, "%" is based on mass.

[Comparative Example 1]
(Preparation of negative electrode active material)
Massive silicon oxide (High Purity Chemical Laboratory Co., Ltd., standard 10 mm to 30 mm square) was coarsely pulverized in a mortar. Then, after further pulverizing with a jet mill (lab type, Nippon Pneumatic Industries Co., Ltd.), fine particles are removed by sizing with a 300M (300 mesh) test sieve so that the volume average particle diameter (vD50) is around 5 μm. Then, silicon oxide particles were obtained.

1000 g of the obtained silicon oxide particles were charged into a batch heating furnace (rotary kiln furnace). Next, the temperature was raised to 950 ° C. under the heating condition of 300 ° C./hour and maintained, and then a mixed gas of acetylene gas (carbon source) / nitrogen gas flowed in at 10 L / min (partial pressure of acetylene gas: 10%). ), A chemical vapor deposition treatment (CVD) was carried out under the condition of 2 hours. After the treatment, the temperature was lowered to obtain a chemically vapor deposition-treated product. The above heat treatment was performed under the condition that a disproportionation reaction of silicon oxide occurs.

The obtained chemically vapor-deposited product was crushed with a mortar and sieved with a 300M (300 mesh) test sieve to obtain a negative electrode active material in which carbon was attached to the surface of silicon oxide particles.

The particle size distribution of the obtained negative electrode active material was measured. Specifically, the measurement sample (5 mg) was placed in a 0.01 mass% aqueous solution of a surfactant (Esomin T / 15, Lion Corporation) and dispersed with a vibration stirrer. The obtained dispersion was placed in a sample water tank of a laser diffraction type particle size distribution measuring device (SALD3000J, Shimadzu Corporation), circulated by a pump while applying ultrasonic waves, and measured by a laser diffraction type. The measurement conditions were as follows. Hereinafter, in the examples, the particle distribution was measured in the same manner.
-Light source: Red semiconductor laser (690 nm)
・ Absorbance: 0.10 to 0.15
-Refractive index: 2.00 to 0.20

From the obtained particle size distribution, the particle size (vD10, vD50, vD90) when the volume-based accumulation from the small diameter side is 10%, 50%, 90%, and the number-based accumulation from the small diameter side is 10%. , 50% and 90% of the particle size (nD10, nD50, nD90) were obtained, respectively. The results are shown in Table 1.

[Example 1]
Carbon was attached to the silicon oxide particles (before the execution of CVD) used in Comparative Example 1 in the same manner as in Comparative Example 1 except that the fine powder removed after pulverization was blended in an amount of 5% by mass of the whole. Silicon oxide particles (negative electrode active material) were prepared and their particle size distribution was measured. The results are shown in Table 1.

[Example 2]
Carbon adhered to the silicon oxide particles used in Comparative Example 1 (before the execution of CVD) in the same manner as in Comparative Example 1 except that the fine powder removed after pulverization was blended in an amount of 10% by mass of the whole. Silicon oxide particles (negative electrode active material) were prepared and their particle size distribution was measured. The results are shown in Table 1.

<Measurement of BET specific surface area>
Using a high-speed specific surface area / pore distribution measuring device (ASAP2020, Micromeritics Japan LLC), nitrogen adsorption at liquid nitrogen temperature (77K) was measured by the 5-point method, and the BET method (relative pressure range: 0. The specific surface area (m ² / g) of the negative electrode active material was calculated from 05 to 0.2).

<Measurement of X-ray diffraction peak intensity ratio>
The negative electrode active material was analyzed using a powder X-ray diffraction measuring device (MultiFlex (2 kW), Rigaku Co., Ltd.) in the same manner as described above. In the negative electrode active material, the ratio of the X-ray diffraction peak intensity of 2θ = 27 ° to 29 ° ( _PSi / _PSiO2 ) derived from Si to the X-ray diffraction peak intensity of 2θ = 20 ° to 25 ° derived from SiO ₂ is determined. Calculated.

<Measurement of carbon content>
The carbon content (mass%) of the negative electrode active material was measured by a high-frequency firing-infrared analysis method. The high-frequency firing-infrared analysis method is an analysis method in which a sample is heated and burned in an oxygen stream in a high-frequency furnace, carbon and sulfur in the sample are converted into CO ₂ and SO ₂ , respectively, and quantified by an infrared absorption method. The measuring device, measuring conditions, etc. are as follows.
・ Equipment: Carbon sulfur simultaneous analyzer (CSLS600, LECO Japan GK)
・ Frequency: 18MHz
・ High frequency output: 1600W
-Sample mass: Approximately 0.05 g
・ Analysis time: Automatic mode is used in the setting mode of the device ・ Auxiliary material: Fe + W / Sn
-Standard sample: Leco 501-024 (C: 3.03% ± 0.04 S: 0.055% ± 0.002) 97
-Number of measurements: 2 times (The content rate value in the table is the average value of the 2 measurements)

<Evaluation of battery performance>
Using the negative electrode active material prepared in Examples and Comparative Examples, a lithium ion secondary battery for evaluation was prepared by the following method, and the battery performance was evaluated. The results are shown in the table.

5 parts by mass of the negative electrode active material produced in Examples and Comparative Examples and 95 parts by mass of artificial graphite manufactured by Hitachi Kasei Co., Ltd. as a carbon-based negative electrode active material were mixed, and carboxymethyl cellulose (CMC) 1 was mixed with 98 parts by mass of this mixture. A mass part and 1 part by mass of styrene-butadiene rubber (SBR) were added and kneaded to prepare a composition for a negative electrode. This negative electrode composition was applied to the glossy surface of the electrolytic copper foil so that the coating amount was 10 mg / cm ² , pre-dried at 90 ° C. for 2 hours, and the density was 1.65 g / cm ^{3 by} a roll press. Adjusted to be. Then, a negative electrode was prepared by drying at 120 ° C. for 4 hours in a vacuum atmosphere.

Ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) (volume ratio 1: 1: 1) and vinylene carbonate (VC) containing the prepared negative electrode, metallic lithium as the counter electrode, and 1 M LiPF ₆ as the electrolytic solution. ) (1.0% by mass), a polyethylene micropore membrane having a thickness of 25 μm as a separator, and a copper plate having a thickness of 250 μm as a spacer were used to prepare a 2016 type coin cell.

(1) Battery capacity (initial discharge capacity and initial charge / discharge efficiency)
The prepared battery is placed in a constant temperature bath held at 25 ° C., charged at a constant current of 0.45 mA / cm ² until it reaches 0 V, and then a value corresponding to a current of 0.09 mA / cm ² at a constant voltage of 0 V. The battery was further charged until it decayed to, and the initial charge capacity was measured. After charging, the battery was discharged after a 30-minute rest period. The discharge was performed at 0.45 mA / cm ² until it reached 1.5 V, and the initial discharge capacity was measured. At this time, the capacity was converted to the mass of the negative electrode active material used. The value obtained by multiplying the value obtained by dividing the initial discharge capacity by the initial charge capacity by 100 was calculated as the initial charge / discharge efficiency (%). In addition, one charge and one discharge are combined into one cycle.

(2) Electrode expansion coefficient (during charging and discharging)
Before evaluating the battery performance, the thickness of the prepared negative electrode was measured using a micrometer. Then, the charge / discharge test of the first cycle was carried out under the same conditions as described above, and a battery stopped in the charged state and a battery stopped in the discharged state were prepared. Next, the above two types of batteries were disassembled, the negative electrode was taken out, and the thickness was measured using a micrometer.
From the measurement results, the electrode expansion coefficient during charging (%) and the electrode expansion coefficient during discharge (%) were calculated according to the following equations.
Electrode expansion coefficient during charging (%) = (negative electrode thickness in charged state) / (negative electrode thickness before battery performance evaluation) x 100
Electrode expansion coefficient during discharge (%) = (negative electrode thickness in discharged state) / (negative electrode thickness before battery performance evaluation) x 100

(3) 10-cycle capacity retention rate A cycle test was conducted in which a charge / discharge test was performed 10 times under the same conditions as above. The cycle characteristics calculated by the following formula were evaluated.
10-cycle capacity retention rate (%) = (10th cycle discharge capacity) / (1st cycle discharge capacity) x 100

 

 

As shown in Table 1, in the particle size distribution curve obtained by the laser diffraction / scattering method, the particle size (vD50) when the accumulation from the small diameter side is 50% on a volume basis and the accumulation from the small diameter side are on the number basis. The battery of the example using silicon oxide particles having a ratio (vD50 / nD50) of 1.3 or more to the particle size (nD50) at 50% as the negative electrode active material does not satisfy this condition. Compared with the battery of the comparative example in which the physical particles were used as the negative electrode active material, the expansion of the electrode was suppressed and the 10-cycle capacity retention rate was improved.

All documents, patent applications, and technical standards described herein are to the same extent as if the individual documents, patent applications, and technical standards were specifically and individually stated to be incorporated by reference. Incorporated herein by reference.

Claims (9)

It is provided with silicon oxide particles and carbon present on a part or all of the surface of the silicon oxide particles, and the accumulation from the small diameter side is 50% on a volume basis in the particle size distribution curve obtained by the laser diffraction / scattering method. For silicon oxide particles in which the ratio (vD50 / nD50) of the particle size (vD50) at the time of becoming and the particle size (nD50) when the accumulation from the small diameter side is 50% on the basis of the number is 1.3 or more. There is a negative electrode active material for lithium ion secondary batteries. The negative electrode active material for a lithium ion secondary battery according to claim 1, wherein the proportion of particles having a particle diameter of 3.0 μm or more is 50% or more on a volume basis. The negative electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the carbon content is 0.5% by mass to 10.0% by mass of the total of the silicon oxide particles and the carbon. Active material. The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the BET specific surface area is 0.1 m ² / g to 15 m ² / g. When using CuKα radiation of a wavelength 0.15406nm as a radiation source, X-rays diffraction peak intensity of 2θ = 20 ° ~ 25 ° derived from SiO ₂ and _{(P SiO2), 2θ = 27} ° ~ 29 derived from Si The lithium according to any one of claims 1 to 4, wherein the ratio (PS _i / P _{SiO 2} ) to the X-ray diffraction peak intensity (PS _i ) of ° is in the range of 1.0 to 2.6. Negative electrode active material for ion secondary batteries. Lithium ion having a current collector and a negative electrode material layer containing the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5 provided on the current collector. Negative electrode for secondary batteries. A lithium ion secondary battery including a positive electrode, a negative electrode for a lithium ion secondary battery according to claim 6, and an electrolyte. A method for producing a negative electrode active material for a lithium ion secondary battery, which comprises a step of adhering carbon to a part or all of the surface of silicon oxide particles. The negative electrode active material for a lithium ion secondary battery is a laser diffraction / scattering method. The particle size (vD50) when the accumulation from the small diameter side is 50% on the volume basis and the particle size (nD50) when the accumulation from the small diameter side is 50% on the number basis in the particle size distribution curve obtained by A method for producing a negative electrode active material for a lithium ion secondary battery, wherein the ratio (vD50 / nD50) is 1.3 or more. The method for producing a negative electrode active material for a lithium ion secondary battery according to claim 8, for producing the negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 5.