JP7485230B2 - Nanosilicon, nanosilicon slurry, method for producing nanosilicon, active material for secondary battery, and secondary battery - Google Patents

Description

The present invention relates to nanosilicon, nanosilicon slurry, and a method for producing the nanosilicon. The present invention also relates to an active material for secondary batteries containing the nanosilicon, a negative electrode containing the active material for secondary batteries, and a secondary battery containing the negative electrode.

Non-aqueous electrolyte secondary batteries are used in portable devices, hybrid and electric vehicles, home storage batteries, and the like, and are required to have a good balance of multiple characteristics such as electric capacity, safety, and operational stability.
Furthermore, in recent years, with the miniaturization of various electronic devices and communication devices and the rapid spread of hybrid automobiles and the like, there has been a strong demand for the development of lithium ion secondary batteries that have higher capacity and further improved various battery characteristics, such as cycle characteristics and discharge rate characteristics, as a driving power source for these devices and the like.

Since silicon has a large theoretical electric capacity, its use as an anode active material has been considered in order to increase the capacity of lithium-ion secondary batteries. However, silicon has a large difference between its volume expansion and contraction when repeatedly charged and discharged, and silicon particles break down during repeated charging and discharging. As a result, secondary batteries using silicon as the anode active material have poor cycle characteristics.

Non-Patent Document 1 describes that such destruction of silicon particles is suppressed in silicon particles having a particle size of 150 nm or less.
Furthermore, Patent Document 1 describes a negative electrode active material for secondary batteries that uses silicon particles with nano-order particle sizes.

JP 2021-180124 A

Xiao et al. , ACS Nano, 6, 1522-1531 (2012)

However, even in the case of a secondary battery active material using silicon particles having a particle size described in Non-Patent Document 1 or Patent Document 1, repeated charging and discharging caused particle destruction and the cycle characteristics were insufficient. Moreover, in the case of a secondary battery active material using silicon particles having a small particle size, the electric capacity and initial coulombic efficiency were insufficient.
Therefore, there is a demand for the development of an active material for lithium secondary batteries using silicon particles, which has improved cycle characteristics, electric capacity and initial coulombic efficiency.

The present inventors have focused on the particle size of silicon particles and the degree of surface oxidation of silicon particles, and have investigated active materials for secondary batteries excellent in cycle characteristics, electric capacity, and initial Coulombic efficiency. As a result, they have found an active material for secondary batteries excellent in cycle characteristics, electric capacity, and initial Coulombic efficiency of lithium secondary batteries.
That is, the present invention relates to a secondary battery active material used in lithium ion secondary batteries and a secondary battery containing the secondary battery active material as a negative electrode active material, and aims to provide silicon particles used in the secondary battery active material that provides a secondary battery excellent in cycle performance, initial coulombic efficiency, and capacity retention rate.
A further object of the present invention is to provide a method for producing silicon particles used in the active material for secondary batteries.

The present invention has the following aspects.
[1] Nanosilicon having a specific surface area of 100 to 400 m ² /g and an oxygen atom content of 5 to 45 atom % relative to silicon atoms.
[2] The nanosilicon according to [1] above, having a crystallite diameter of 5 to 14 nm.
[3] The nanosilicon according to [1] or [2], having a volume average particle diameter of 10 to 200 nm.

The present invention also has the following aspects.
[4] A nanosilicon slurry comprising the nanosilicon according to any one of [1] to [3] above, a dispersant and a solvent.

Further, the present invention has the following aspects.
[5] A method for producing nanosilicon, comprising wet-milling silicon powder in a gas atmosphere having a dew point temperature of −60° C. or lower, a temperature of 60° C. or lower, and a non-aqueous solvent having a moisture concentration of 10,000 ppm or lower.
[6] The method for producing nanosilicon according to [5], wherein the wet grinding is carried out in a non-aqueous solvent containing at least one surfactant selected from the group consisting of cationic surfactants, anionic surfactants, and amphoteric surfactants.
[7] The method for producing nanosilicon according to [5] or [6], wherein the wet pulverization involves wet pulverizing silicon powder having a volume average particle size of 1 to 20 μm and an oxygen atom content of 10 atom% or less relative to silicon atoms in a non-aqueous solvent.
[8] The method for producing nanosilicon according to any one of [5] to [7], wherein the purity of the silicon powder in the wet grinding is 99 mass% or more.

The present invention also has the following aspects.
[9] A method for producing an active material for a secondary battery, comprising mixing the nanosilicon obtained by the method for producing nanosilicon according to any one of [5] to [8] above with a resin, drying the mixture, and then firing the mixture under an inert gas atmosphere.
[10] An active material for secondary batteries containing nanosilicon according to any one of [1] to [3] above.
[11] A negative electrode for a secondary battery, comprising the active material for a secondary battery according to [10] above.
[12] A secondary battery comprising the negative electrode for secondary batteries according to [11] above.

According to the present invention, there is provided an active material for a secondary battery used in a lithium ion secondary battery, and a secondary battery containing the active material for a secondary battery as a negative electrode active material, which provides a secondary battery excellent in cycle performance, initial coulombic efficiency, and capacity retention rate.
The present invention further provides a method for producing silicon particles used in the active material for secondary batteries.

The nanosilicon of the present invention (hereinafter also referred to as "the present nanosilicon") has a specific surface area of 100 to 400 ^m2 /g and 5 to 45 atom% of oxygen atoms relative to silicon atoms. Here, "5 to 45 atom% of oxygen atoms relative to silicon atoms" means that the value obtained by dividing the number of oxygen atoms in the present nanosilicon by the number of silicon atoms, expressed as atom%, is 5 to 45, and the same applies below.
As mentioned above, it is believed that the destruction of silicon particles having a small particle size is suppressed even if the volume changes due to repeated charging and discharging.On the other hand, it is believed that the surface area of silicon particles having a small particle size is large, and the surface oxidation rate of silicon particles is large.As a result, it is believed that the secondary battery active material containing silicon particles having a small particle size is inferior in electric capacity and initial coulombic efficiency, although the deterioration of cycle characteristics due to repeated charging and discharging is suppressed.

This nanosilicon has a volume average particle size of nano-order, and its small particle size improves cycle characteristics. In addition, its surface area is equal to or greater than that of conventional silicon particles, but the oxygen atom content is kept low, which is believed to have improved the electrical capacity and initial coulombic efficiency.

The volume average particle size of the nano order means that the volume average particle size is in nanometer units, and the volume average particle size is usually 1 to 999 nm. If the silicon particles exceed 1000 nm, the dispersibility of the silicon slurry may deteriorate, the pressure during stirring may increase, and the productivity may decrease. From these viewpoints, the volume average particle size of the present nanosilicon is preferably 10 to 200 nm, more preferably 10 to 100 nm, and even more preferably 20 to 70 nm.
The volume average particle size is the value of D50, which can be measured using a laser diffraction particle size analyzer or the like. D50 can be measured by dynamic light scattering using a laser particle size analyzer or the like. The volume average particle size of the present nanosilicon is the particle size at which the cumulative volume distribution curve is drawn from the small diameter side in the particle size distribution and the cumulative 50% is the average particle size.

The specific surface area of the nanosilicon is from 100 to 400 m ² /g.
The specific surface area is a value determined by the BET method, and can be determined by nitrogen gas adsorption measurement, for example, by using a specific surface area measuring device.
The specific surface area of the present nanosilicon is more preferably from 100 to 300 m ² /g, and further preferably from 100 to 230 m ² /g, from the viewpoints of electric capacity and initial coulombic efficiency.

The amount of oxygen atoms in the nanosilicon is 5 to 45 atm% relative to silicon atoms as described above. Even if the average particle size of the nanosilicon is small as in the above range, it is believed that by using the nanosilicon having the amount of oxygen atoms in the above range as the negative electrode active material of a secondary battery, a secondary battery excellent in cycleability, initial coulombic efficiency, and capacity retention rate can be obtained.
From the viewpoint of initial coulombic efficiency and capacity retention, the amount of oxygen atoms in the nanosilicon is preferably 5 to 30 atm%, more preferably 5 to 25 atm%, and further preferably 5 to 15%. The amount of oxygen atoms was obtained by calcining the nanosilicon at a temperature of 300 to 800° C. to volatilize the additives, and then performing composition analysis using SEM-EDS (JSM-7900F, manufactured by JEOL).
The nanosilicon may contain carbon atoms and nitrogen atoms in addition to silicon atoms and oxygen atoms. When nitrogen atoms are contained, the amount is preferably 5 mass % or less from the viewpoint of the decrease in capacity due to the formation of silicon nitride.

The shape of the nanosilicon may be granular, needle-like, or flake-like as long as the nanosilicon satisfies the above-mentioned specific surface area and oxygen atomic weight, but crystalline is preferred. When the nanosilicon is crystalline, it is preferable from the viewpoint of initial coulombic efficiency and capacity maintenance rate if the crystallite diameter obtained from the diffraction peak assigned to Si (111) in X-ray diffraction (hereinafter also referred to as "crystallite diameter") is in the range of 5 to 14 nm. The crystallite diameter is more preferably 12 nm or less, and even more preferably 10 nm or less.

From the viewpoint of charge/discharge performance when used as a negative electrode active material, the nanosilicon preferably has a length in the long axis direction of 30 to 300 nm and a thickness of 1 to 60 nm. From the viewpoint of charge/discharge performance when used as a negative electrode active material, a needle-like or flake-like shape with a so-called aspect ratio, which is the ratio of length to thickness, of 0.5 or less is preferred.
The morphology of the nanosilicon can be measured by dynamic light scattering to determine the average particle size, but the aspect ratio of the sample can be more easily and precisely identified by using an analytical means such as a transmission electron microscope (TEM) or a field emission scanning electron microscope (FE-SEM). In the case of a negative electrode active material containing the secondary battery material of the present invention, the sample can be cut with a focused ion beam (FIB) and the cross section can be observed with an FE-SEM, or the sample can be sliced and the state of the nanosilicon can be identified by TEM observation.
The aspect ratio of the nanosilicon is a calculation result based on 50 particles of the main part of the sample within the field of view of the TEM image.

When the present nanosilicon is used as an active material for a secondary battery, the present nanosilicon may be used as an active material as it is, or may be used as an active material for a secondary battery in which the matrix phase contains the present nanosilicon.
From the viewpoint of the stability of the active material for secondary batteries, it is preferable to use an active material in which the matrix phase contains the present nanosilicon as the active material for secondary batteries.

When the active material containing the nanosilicon is used as an active material for a secondary battery, the matrix phase is a material capable of absorbing and releasing lithium ions. The material capable of absorbing and releasing is a material that can absorb lithium ions into the matrix phase when the battery is charged and release the lithium ions from the matrix phase when the battery is discharged. The cycle of absorbing and releasing is repeated in the lithium secondary battery.
Examples of materials capable of absorbing and releasing lithium ions include graphite, silicon dioxide, titanium oxide, and compounds containing silicon, oxygen, and carbon. The matrix phase is preferably composed of these materials, and more preferably composed of compounds containing silicon, oxygen, and carbon from the viewpoints of improving the initial efficiency and capacity retention rate.
Compounds containing silicon, oxygen, and carbon include silicon oxycarbide.

Silicon oxycarbide is composed of compounds containing silicon, oxygen, and carbon, and among these, a three-dimensional network structure of a silicon-oxygen-carbon skeleton and a structure containing free carbon are preferred. Here, free carbon is carbon that is not contained in the three-dimensional silicon-oxygen-carbon skeleton. Free carbon includes carbon that exists as a carbon phase, carbon that is bonded to carbon in a carbon phase, and carbon that is bonded to a silicon-oxygen-carbon skeleton and a carbon phase.

The silicon oxycarbide is preferably represented by the following formula (1):
SiOxCy (1)
In formula (1), x represents the molar ratio of oxygen to silicon, and y represents the molar ratio of carbon to silicon.
When the active material containing the present nanosilicon and the matrix phase is used in a secondary battery, from the viewpoint of achieving an advantageous balance between charge/discharge performance and capacity retention rate, 1≦x<2 is preferable, 1≦x≦1.9 is more preferable, and 1≦x≦1.8 is even more preferable.
Furthermore, when the active material containing the present nanosilicon and a matrix phase is used in a secondary battery, from the viewpoint of the balance between charge/discharge performance and initial coulombic efficiency, 1≦y≦20 is preferable, and 1.2≦y≦15 is more preferable.

The x and y can be determined by measuring the mass content of each element and then converting it into a molar ratio (atomic number ratio). In this case, the oxygen and carbon contents can be quantified using an inorganic elemental analyzer, and the silicon content can be quantified using an inductively coupled plasma optical emission spectrometer (ICP-OES).
It is preferable to measure x and y by the above-described method, but it is also possible to perform a local analysis of the active material, obtain a large number of measurement points for the content ratio data obtained thereby, and infer the content ratio of the entire active material. Examples of local analysis include energy dispersive X-ray spectroscopy (SEM-EDX) and electron probe microanalyzer (EPMA).

When the matrix phase is a silicon oxycarbide phase made of silicon oxycarbide and has a three-dimensional network structure of a silicon-oxygen-carbon skeleton and a structure containing free carbon, the silicon-oxygen-carbon skeleton in the silicon oxycarbide phase has high chemical stability, forms a composite structure with the free carbon, and exhibits small volumetric changes in response to the absorption and release of lithium. The nanosilicon is tightly wrapped in a composite structure of the silicon-oxygen-carbon skeleton and free carbon, which further suppresses the volumetric changes of the nanosilicon in response to the absorption and release of lithium. As a result, when an active material containing the nanosilicon and a matrix phase is used as a negative electrode active material, the nanosilicon in the negative electrode plays a role as a main component in expressing charge and discharge performance, while the silicon oxycarbide phase further suppresses particle destruction associated with volumetric changes of the nanosilicon during charge and discharge, thereby further improving the cycleability of the lithium secondary battery.

Furthermore, when the compound that constitutes the silicon oxycarbide phase has a three-dimensional network structure of a silicon-oxygen-carbon skeleton and a structure that contains free carbon, the approach of lithium ions causes a change in the internal electron distribution of the silicon-oxygen-carbon skeleton, and electrostatic bonds and coordinate bonds are formed between the silicon-oxygen-carbon skeleton and the lithium ions. These electrostatic bonds and coordinate bonds allow lithium ions to be stored in the silicon-oxygen-carbon skeleton. Meanwhile, because the coordinate bond energy is relatively low, lithium ion desorption reactions occur easily. In other words, it is believed that the silicon-oxygen-carbon skeleton can reversibly cause lithium ion insertion and desorption reactions during charging and discharging.

The silicon oxycarbide may contain nitrogen in addition to silicon, oxygen, and carbon. In the manufacturing method of the active material containing the nanosilicon and the matrix phase described later, nitrogen atoms contained in the raw materials used, such as nitrogen compounds such as phenolic resins or polysiloxane compounds, other dispersants, and nitrogen gas used in the firing process, can be introduced into the silicon oxycarbide phase as an atomic group containing nitrogen atoms as functional groups in the molecule. When the silicon oxycarbide phase contains nitrogen, the charge/discharge performance and capacity retention rate tend to be excellent when the active material containing the nanosilicon and the matrix phase is used as the negative electrode active material.
When the compound constituting the silicon oxycarbide phase is a compound containing silicon, oxygen, carbon and nitrogen, the silicon oxycarbide phase preferably contains a compound represented by the following formula (2).
SiOaCbNc (2)
In formula (2), a and b have the same meanings as above, and c represents the molar ratio of nitrogen to silicon.
When the silicon oxycarbide phase contains a compound represented by formula (2), from the viewpoint of charge/discharge performance and capacity retention rate when the active material containing the present nanosilicon is used in a secondary battery, it is preferable that 1≦a≦2, 1≦b≦20, and 0<c≦0.5, and it is more preferable that 1≦a≦1.9, 1.2≦b≦15, and 0<c≦0.4.

Similarly to x and y, a, b and c can be determined by measuring the mass contents of the elements and then converting them into molar ratios (atomic number ratios).
As with x and y, it is preferable to measure a, b, and c by the above-described method, but it is also possible to perform a local analysis of the active material, obtain a large number of measurement points for the content ratio data obtained thereby, and infer the content ratio of the entire active material. Examples of local analysis include energy dispersive X-ray spectroscopy (SEM-EDX) and electron probe microanalyzer (EPMA).

If the average particle size of the active material containing the present nanosilicon and the matrix phase is too small, the specific surface area will increase significantly, and when the active material is used as the negative electrode active material in a secondary battery, the reversible charge/discharge capacity per unit volume may decrease due to an increase in the amount of solid-phase interface electrolyte decomposition products (hereinafter, also referred to as "SEI") generated during charging and discharging. If the average particle size is too large, there is a risk of peeling off from the current collector during electrode film production.
Therefore, in the case of an active material in which the matrix phase contains the present nanosilicon, the volume average particle size is preferably 2 μm or more and 15 μm or less. The volume average particle size of an active material in which the matrix phase contains the present nanosilicon is more preferably 2.5 μm or more, and particularly preferably 3.0 μm or more. The volume average particle size of the active material is more preferably 12 μm or less, and particularly preferably 10 μm or less. The volume average particle size is the value of D50.

The specific surface area of the active material in which the matrix phase contains the present nanosilicon is preferably 0.3 m ² /g or more and 10 m ² /g or less. The specific surface area of the active material in which the matrix phase contains the present nanosilicon is more preferably 0.5 m ² /g or more, and particularly preferably 1.0 m ² /g or more. The specific surface area of the active material is more preferably 9.0 m ² /g or less, and particularly preferably 8.0 m ² /g or less. When the specific surface area is in the above range, the amount of solvent absorbed during electrode preparation can be appropriately maintained, and the amount of binder used to maintain binding properties can also be appropriately maintained. The specific surface area is a value determined by the BET method as described above, and can be determined by nitrogen gas adsorption measurement, for example, can be measured using a specific surface area measuring device.

In the case of an active material in which the matrix phase contains silicon oxycarbide, it is preferable that the silicon oxycarbide has free carbon composed only of carbon elements along with a silicon-oxygen-carbon skeletal structure. When the silicon oxycarbide has free carbon, a scattering peak is observed in the Raman spectrum of the active material at 1590 cm ⁻¹ which is attributed to the G band of the graphite long-period carbon lattice structure, and at about 1330 cm ⁻¹ which is attributed to the D band of the graphite short-period carbon lattice structure having disorder or defects. The intensity ratio I(G band)/I(D band) of the scattering peak intensity of the D band, I(G band), to the scattering peak intensity of the D band, I(D band), is preferably 0.7 or more and 2 or less. The scattering peak intensity ratio I(G band)/I(D band) is more preferably 0.7 or more and 1.8 or less. The fact that the scattering peak intensity ratio I(G band)/I(D band) is in the above range means that the following can be said about the free carbon in the matrix.

Some carbon atoms of the free carbon are bonded to some silicon atoms in the silicon-oxygen-carbon skeleton. This free carbon is an important component that affects the charge-discharge characteristics. The free carbon is mainly formed in the silicon-oxygen-carbon skeleton composed of SiO ₂ C ₂ , SiO ₃ C, and SiO ₄ , and is bonded to some silicon atoms of the silicon-oxygen-carbon skeleton, so that electron transfer between the silicon atoms inside the silicon-oxygen-carbon skeleton and on the surface and the free carbon becomes easier. Therefore, when the active material containing the nanosilicon in the matrix phase is used as the negative electrode active material in a secondary battery, the insertion and desorption reaction of lithium ions during charging and discharging proceeds quickly, and the charge-discharge characteristics are improved. In addition, the insertion and desorption reaction of lithium ions may cause the active material to expand and contract, but the presence of free carbon in its vicinity is thought to have the effect of mitigating the expansion and contraction of the entire active material, greatly improving the capacity retention rate.

Free carbon is formed by thermal decomposition of the silicon-containing compound and the carbon source resin in an inert gas atmosphere when producing the silicon oxycarbide phase. Specifically, carbonizable sites in the molecular structures of the silicon-containing compound and the carbon source resin are converted into carbon components by high-temperature thermal decomposition in an inactive atmosphere, and some of these carbons bond to part of the silicon-oxygen-carbon skeleton. The carbonizable components are preferably hydrocarbons, more preferably alkyls, alkylenes, alkenes, alkynes, and aromatics, and even more preferably aromatics.

In addition, the presence of free carbon is expected to reduce the resistance of the active material, and when the active material is used as the negative electrode of a secondary battery, it is believed that the reaction inside the active material will occur uniformly and smoothly, resulting in an active material for secondary batteries with an excellent balance between charge/discharge performance and capacity retention rate. Although free carbon can be introduced only from silicon-containing compounds, the use of a carbon source resin in combination is expected to increase the amount of free carbon present and its effect. There are no particular limitations on the type of carbon source resin, but a carbon compound containing a six-membered carbon ring is preferred.

The state of the free carbon can be identified by a thermogravimetric differential thermal analyzer (TG-DTA) in addition to Raman spectroscopy. Unlike carbon atoms in a silicon-oxygen-carbon skeleton, free carbon is easily thermally decomposed in the atmosphere, and the amount of carbon present can be determined from the amount of thermal weight loss measured in the presence of air. In other words, the amount of carbon can be quantified using TG-DTA.
In addition, from the thermal weight loss behavior, the changes in the thermal decomposition temperature behavior, such as the decomposition reaction start temperature, the decomposition reaction end temperature, the number of thermal decomposition reaction species, and the temperature of the maximum weight loss amount in each thermal decomposition reaction species, can be easily understood. The state of carbon can be determined using the temperature values of these behaviors. On the other hand, the carbon atoms in the silicon-oxygen-carbon skeleton, that is, the carbon atoms bonded to the silicon atoms constituting the SiO ₂ C ₂ , SiO ₃ C, and SiO ₄ , have a very strong chemical bond and are therefore highly thermally stable, and are not considered to be thermally decomposed in the air within the temperature range of the thermal analysis device measurement. In addition, since the carbon in the silicon oxycarbide phase of the active material has properties similar to those of amorphous carbon, it is thermally decomposed in the air in the temperature range of about 550°C to 900°C. As a result, a rapid weight loss occurs. The maximum temperature of the measurement conditions for TG-DTA is not particularly limited, but in order to completely terminate the thermal decomposition reaction of carbon, it is preferable to perform the TG-DTA measurement in the air under conditions of about 25°C to about 1000°C or higher.

The surface of the active material may be coated with a coating material, which is preferably a material that is expected to have electronic conductivity, lithium ion conductivity, and the effect of inhibiting decomposition of the electrolyte.
The coating material includes electron conductive materials such as carbon, titanium, nickel, etc. Among these, from the viewpoint of improving the chemical stability and thermal stability of the negative electrode active material, carbon is preferred, and low crystalline carbon is more preferred.

When the coating material is low-crystalline carbon, the average thickness of the coating layer is preferably 10 nm to 300 nm, and the content of low-crystalline carbon is preferably 1 to 30 mass % relative to 100 mass % of the total amount of the active material.
When the coating material is carbon, the carbon coating is preferably formed on the surface of the active material by a vapor deposition method. The amount of the carbon coating is preferably 1% by mass to 10% by mass, based on the total mass of the active material and the carbon coating being 100% by mass, from the viewpoint of improving the chemical stability and thermal stability of the active material.
The mass of the active material is the mass of the nanosilicon when the active material is composed only of the nanosilicon, and is the total amount of the nanosilicon and the matrix phase when the active material is composed of the nanosilicon. For example, when the matrix phase is composed of silicon oxycarbide, it is the total amount of the nanosilicon and the silicon oxycarbide. When the silicon oxycarbide contains nitrogen, it is the total amount including the nitrogen. When the active material contains another third component described later, it is the total amount including the third component.

The active material may contain other necessary third components in addition to those mentioned above.
The third component may be a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al (hereinafter, also referred to as a "metal silicate compound").
A silicate compound is generally a compound containing an anion having a structure in which one or several silicon atoms are at the center and electronegative ligands surround the silicon atom, and a metal silicate compound is a salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al and a compound containing the above anion.
Known compounds containing the anion include silicate ions such as orthosilicate ion (SiO ₄ ^4- ), metasilicate ion (SiO ₃ ^2- ), pyrosilicate ion (Si ₂ O ₇ ^6- ), and cyclic silicate ion (Si ₃ O ₉ ^6- or Si ₆ O ₁₈ ^12- ). The silicate compound is preferably a silicate compound which is a salt of metasilicate ion and at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al. Of the metals, Li or Mg is preferred.

The metal silicate compound has at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al, and may have two or more of these metals. When having two or more metals, one silicate ion may have multiple metals, or may be a mixture of silicate compounds having different metals. In addition, the metal silicate compound may have other metals as long as it has at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al.
The metal silicate compound is preferably a lithium silicate compound or a magnesium silicate compound, more preferably lithium metasilicate (Li ₂ SiO ₃ ) or magnesium metasilicate (MgSiO ₃ ), and particularly preferably magnesium metasilicate (MgSiO ₃ ).

The method for producing the present nanosilicon includes wet-grinding silicon powder in a gas atmosphere having a dew point temperature of -60°C or lower, a temperature of 60°C or lower, and a non-aqueous solvent having a moisture concentration of 10,000 ppm or lower (hereinafter also referred to as "the present production method").
The dew point temperature is the temperature at which condensation occurs when gas is cooled, and is an index of the humidity in the gas. "In a gas atmosphere with a dew point temperature of -60°C or lower" refers to a gas atmosphere in which condensation occurs only when the gas is cooled to -60°C, and represents a state in which the moisture content in the gas is low. If the dew point temperature exceeds -60°C, a large amount of moisture will be mixed into the solvent during wet grinding, causing poor dispersion, and the silicon particles will gel, resulting in reduced productivity.

The dew point temperature can be approximately calculated using, for example, the saturated water vapor pressure table of JIS Z 8806 "Humidity - Measurement method".
The gas atmosphere is usually an inert gas atmosphere, and from the viewpoint of handling, a nitrogen atmosphere is preferable. In the case of nitrogen, a dew point temperature of −60° C. or lower means that the moisture content in the nitrogen is 10.67 ppm.
In this manufacturing method, silicon powder is wet-pulverized in a non-aqueous solvent having a water concentration of 10,000 ppm or less at a temperature of 60° C. or less under the above-mentioned gas atmosphere. From the viewpoint of suppressing evaporation of the solvent and gelation of the silicon particles, the temperature is preferably 40° C. or less.

From the viewpoint of suppressing volatilization of the solvent and silicon oxidation, the temperature of the nonaqueous solvent is preferably 40° C. or less.
Examples of non-aqueous solvents include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and diisobutyl ketone; alcohols such as ethanol, methanol, normal propyl alcohol, and isopropyl alcohol; and aromatics such as benzene, toluene, and xylene.
From the viewpoint of reducing the initial efficiency due to oxidation of silicon, the water concentration of the nonaqueous solvent is preferably 10,000 ppm or less, more preferably 5,000 ppm or less, further preferably 1,000 ppm or less, and most preferably 600 ppm or less.
The water concentration of the nonaqueous solvent can be controlled within the above range, for example, by adding a dehydrating agent to the nonaqueous solvent before use and filtering off the dehydrating agent after a certain period of time has elapsed, or by distilling the nonaqueous solvent before use, or by controlling the dew point of the atmosphere, or the like.
The concentration of the silicon particles in the non-aqueous solvent is preferably 5 to 50% by mass.

Silicon particles are wet-milled in a non-aqueous solvent having the above-mentioned temperature and water concentration.
The silicon particles used are composed of zero-valent silicon. If the volume average particle diameter of silicon particles is too small, dispersibility may be poor and productivity may be reduced, so the volume average particle diameter of silicon particles is preferably 1 μm or more, more preferably 3 μm or more. If the volume average particle diameter of silicon particles is too large, productivity may be reduced, so the volume average particle diameter is preferably 20 μm or less, preferably 10 μm or less, more preferably 5 μm or less. The volume average particle diameter is the same as D50, and the measurement method is also the same as above.
From the viewpoint of reducing the amount of oxygen atoms in the obtained nanosilicon, the oxygen atoms in the silicon particles used are preferably 10 atm % or less, more preferably 5 atm % or less, and even more preferably 2 atm % or less, relative to the silicon atoms.

If the purity of the silicon particles used is less than 99% by mass, when the nanosilicon is used as the negative electrode active material, the metal may easily dissolve, making it difficult to handle as a battery. Therefore, the purity of the silicon particles used is preferably 99% by mass or more, more preferably 99.9% by mass, and even more preferably 99.99% by mass.

Examples of the mill used for wet milling include a ball mill, a bead mill, and a jet mill.
The nanosilicon having an average particle size of nano-order can be obtained by controlling the grinding conditions such as bead particle size of 0.5 mm or less, bead filling rate of 50 to 95 vol%, rotor peripheral speed of 2 to 14 m/s, or grinding time of 0.5 to 24 h, and classifying the same. The bead particle size is preferably 0.2 mm or less. The rotor peripheral speed of the bead mill is preferably 4 to 12 m/s, more preferably 6 to 12 m/s.

In the wet grinding, a dispersant may be added to promote grinding of silicon particles. The type of dispersant may be aqueous or non-aqueous, and non-aqueous dispersants are preferred. The type of non-aqueous dispersant may be a polymer type such as polyether, alcohol, polyalkylene polyamine, or polycarboxylic acid partial alkyl ester, a low molecular type such as polyhydric alcohol ester or alkyl polyamine, or an inorganic type such as polyphosphate.
When the dispersant is added, its amount is preferably in the range of 5% by mass to 60% by mass, and more preferably 5% by mass to 30% by mass, based on the mass of the silicon particles.

Furthermore, in the wet grinding, in order to improve the dispersibility of the silicon particles used and promote the grinding of the silicon particles, at least one surfactant selected from the group consisting of cationic surfactants, anionic surfactants, and amphoteric surfactants may be added.
Examples of the cationic surfactant include aliphatic amine salts, aliphatic quaternary ammonium salts, aromatic quaternary ammonium salts, and heterocyclic quaternary ammonium salts. The amine value of the cationic surfactant is, for example, 1 to 100 mgKOH/g, preferably 5 to 80 mgKOH/g, more preferably 10 to 48 mgKOH/g, and particularly preferably 35 to 48 mgKOH/g. When the amine value is within the above range, re-aggregation of silicon particles nano-sized by pulverization can be suppressed, and the viscosity of the slurry can be suppressed, resulting in excellent cycle characteristics, charge/discharge capacity, and initial coulombic efficiency in the battery. Specifically, DISPERBYK9077 (manufactured by BYK Additives & Instruments, DISPERBYK is a registered trademark) can be used as the cationic surfactant.

Examples of anionic surfactants include carboxylates, sulfonates, sulfates, and phosphates. The acid value of the anionic surfactant is, for example, 1 to 200 mgKOH/g, preferably 10 to 180 mgKOH/g, and more preferably 50 to 150 mgKOH/g. When the acid value is within the above range, the wettability of the silicon particles to the dispersion medium is improved, so that the viscosity of the slurry can be suppressed, resulting in excellent cycle characteristics, charge/discharge capacity, and initial coulombic efficiency in the battery. Specifically, DISPERBYK111 (manufactured by BYK Additives & Instruments) can be used as the anionic surfactant.

The surfactant may be an amphoteric surfactant having the above amine value and acid value, or a cationic surfactant and an anionic surfactant having the above amine value and acid value may be used in combination. The amphoteric surfactant is a surfactant that exhibits the properties of an anionic surfactant in the alkaline range and the properties of a cationic surfactant in the acidic range, and examples of such surfactants include compounds containing carboxylates, amino acids, and betaines. Specific examples of amphoteric surfactants that can be used include ANTI-TERRA (registered trademark)-U100 (manufactured by BYK Additives & Instruments).
When the surfactant is added, the amount added is preferably from 5 to 60% by mass, more preferably from 5 to 40% by mass, and further preferably from 5 to 20% by mass, based on the mass of the silicon particles.

When the matrix phase is an active material for secondary batteries containing the present nanosilicon (hereinafter also referred to as the present active material), the present active material can be produced, for example, by mixing the present nanosilicon with a resin, drying the mixture, and then firing it under an inert gas atmosphere.
As described above, the matrix phase is composed of a material capable of absorbing and releasing lithium ions, so the resin to be mixed with the present nanosilicon may be any resin that can become a material capable of absorbing and releasing lithium ions by baking. As described above, materials capable of absorbing and releasing lithium ions include graphite, silicon dioxide, titanium oxide, and compounds containing silicon, oxygen, and carbon, so the resin to be mixed with the present nanosilicon may be a polysiloxane resin or a carbon source resin.

When the present nanosilicon and a resin are mixed, from the viewpoint of uniformly mixing the two, it is preferable to use the present nanosilicon as a nanosilicon slurry in which the present nanosilicon is dispersed in a solvent.
From the viewpoint of dispersibility, the nanosilicon slurry is preferably a nanosilicon slurry containing the present nanosilicon, a dispersant and a solvent (hereinafter, also referred to as "the present nanosilicon slurry").
The dispersant contained in the nanosilicon slurry is the same as that described above, and the solvent is water or the same non-aqueous solvent as the non-aqueous solvent described above. The preferred dispersant and non-aqueous solvent are also the same as those described above. The nanosilicon slurry may also contain a surfactant. The surfactant may be at least one selected from the group consisting of the cationic surfactant, the anionic surfactant, and the amphoteric surfactant, as described above.
The nanosilicon slurry preferably comprises the nanosilicon, a dispersant and a non-aqueous solvent.
The present nanosilicon slurry is more preferably a slurry in which the present nanosilicon is dispersed in the non-aqueous solvent without separating the present nanosilicon obtained by using the dispersant in the present manufacturing method. In this case, the amount of the present nanosilicon is preferably in the range of 5% by mass to 40% by mass, more preferably 10% by mass to 30% by mass, based on the total amount of the non-aqueous solvent and, if necessary, the dispersant and the present nanosilicon being 100% by mass.

When the matrix phase of the active material contains silicon oxycarbide, it is preferable to use a mixture of a polysiloxane compound and a carbon source resin as the resin.When the mixture of a polysiloxane compound and a carbon source resin is mixed with the nanosilicon, it is preferable to use the nanosilicon slurry.
The present nanosilicon slurry is mixed with a mixture of a polysiloxane compound and a carbon source resin, the solvent is removed to obtain a precursor, the obtained precursor is calcined to obtain a calcined product, and if necessary, is pulverized to obtain the present active material having the desired average particle size or specific surface area.

The polysiloxane compound may be a resin containing at least one of a polycarbosilane structure, a polysilazane structure, a polysilane structure, and a polysiloxane structure. The resin may contain only these structures, or may be a composite resin having at least one of these structures as a segment and chemically bonded to other polymer segments. The form of the composite may be graft copolymerization, block copolymerization, random copolymerization, alternating copolymerization, etc. For example, a composite resin having a graft structure in which a polysiloxane segment is chemically bonded to the side chain of a polymer segment, a composite resin having a block structure in which a polysiloxane segment is chemically bonded to the end of a polymer segment, etc. may be mentioned.

A polysiloxane compound in which the polysiloxane segment has a structural unit represented by the following general formula (S-1) and/or the following general formula (S-2) is preferred. In particular, it is more preferred that the polysiloxane compound has a carboxy group, an epoxy group, an amino group, or a polyether group on the side chain or terminal of the siloxane bond (Si-O-Si) main skeleton.

なお、前記一般式（Ｓ－１）および（Ｓ－２）中、Ｒ^１は置換基を有してもよい芳香族炭化水素基またはアルキル基、エポキシ基、カルボキシ基などを表す。Ｒ^２およびＲ^３は、それぞれアルキル基、シクロアルキル基、アリール基またはアラルキル基、エポキシ基、カルボキシ基などを示す。

In the general formulae (S-1) and (S-2), ^R1 represents an aromatic hydrocarbon group or an alkyl group which may have a substituent, an epoxy group, a carboxy group, etc. ^R2 and ^R3 each represent an alkyl group, a cycloalkyl group, an aryl group or an aralkyl group, an epoxy group, a carboxy group, etc.

Examples of the alkyl group include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, neopentyl, tert-pentyl, 1-methylbutyl, 2-methylbutyl, 1,2-dimethylpropyl, 1-ethylpropyl, hexyl, isohexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 2,2-dimethylbutyl, 1-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-2-methylpropyl, and 1-ethyl-1-methylpropyl. Examples of the cycloalkyl group include cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl.

Examples of aryl groups include phenyl groups, naphthyl groups, 2-methylphenyl groups, 3-methylphenyl groups, 4-methylphenyl groups, 4-vinylphenyl groups, and 3-isopropylphenyl groups.

Examples of aralkyl groups include benzyl groups, diphenylmethyl groups, and naphthylmethyl groups.

Examples of polymer segments other than the polysiloxane segment contained in the polysiloxane compound include vinyl polymer segments such as acrylic polymers, fluoroolefin polymers, vinyl ester polymers, aromatic vinyl polymers, and polyolefin polymers, as well as polymer segments such as polyurethane polymer segments, polyester polymer segments, and polyether polymer segments. Among these, vinyl polymer segments are preferred.

The polysiloxane compound may be a composite resin in which polysiloxane segments and polymer segments are bonded in a structure shown in the following structural formula (S-3), or may have a three-dimensional mesh-like polysiloxane structure.

なお式中、炭素原子は重合体セグメントを構成する炭素原子であり、２個のケイ素原子はポリシロキサンセグメントを構成するケイ素原子である。

In the formula, the carbon atom is a carbon atom that constitutes a polymer segment, and the two silicon atoms are silicon atoms that constitute a polysiloxane segment.

The polysiloxane segment of the polysiloxane compound may have a functional group capable of reacting by heating, such as a polymerizable double bond, in the polysiloxane segment. By subjecting the polysiloxane compound to heat treatment before thermal decomposition, the crosslinking reaction proceeds, and the compound becomes solid, making it easier to carry out the thermal decomposition treatment.

Examples of polymerizable double bonds include vinyl groups and (meth)acryloyl groups. Preferably, there are two or more polymerizable double bonds in the polysiloxane segment, more preferably 3 to 200, and even more preferably 3 to 50. In addition, by using a composite resin having two or more polymerizable double bonds as the polysiloxane compound, the crosslinking reaction can be easily carried out.

The polysiloxane segment may have a silanol group and/or a hydrolyzable silyl group. Examples of the hydrolyzable group in the hydrolyzable silyl group include a halogen atom, an alkoxy group, a substituted alkoxy group, an acyloxy group, a phenoxy group, a mercapto group, an amino group, an amide group, an aminooxy group, an iminoxy group, and an alkenyloxy group. When these groups are hydrolyzed, the hydrolyzable silyl group becomes a silanol group. In parallel with the thermosetting reaction, a hydrolysis condensation reaction proceeds between the hydroxyl group in the silanol group and the hydrolyzable group in the hydrolyzable silyl group, thereby obtaining a solid polysiloxane compound.

In the present invention, the silanol group refers to a silicon-containing group having a hydroxyl group directly bonded to a silicon atom. In the present invention, the hydrolyzable silyl group refers to a silicon-containing group having a hydrolyzable group directly bonded to a silicon atom, and specific examples thereof include groups represented by the following general formula (S-4).

なお式中、Ｒ^４はアルキル基、アリール基またはアラルキル基等の１価の有機基を、Ｒ^５はハロゲン原子、アルコキシ基、アシロキシ基、アリルオキシ基、メルカプト基、アミノ基、アミド基、アミノオキシ基、イミノオキシ基またはアルケニルオキシ基である。またｂは０から２の整数である。

In the formula, ^R4 is a monovalent organic group such as an alkyl group, an aryl group, or an aralkyl group, ^R5 is a halogen atom, an alkoxy group, an acyloxy group, an allyloxy group, a mercapto group, an amino group, an amido group, an aminooxy group, an iminoxy group, or an alkenyloxy group, and b is an integer of 0 to 2.

Examples of alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, neopentyl, tert-pentyl, 1-methylbutyl, 2-methylbutyl, 1,2-dimethylpropyl, 1-ethylpropyl, hexyl, isohexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 2,2-dimethylbutyl, 1-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-2-methylpropyl, and 1-ethyl-1-methylpropyl.

Examples of aryl groups include phenyl groups, naphthyl groups, 2-methylphenyl groups, 3-methylphenyl groups, 4-methylphenyl groups, 4-vinylphenyl groups, and 3-isopropylphenyl groups.

Examples of aralkyl groups include benzyl groups, diphenylmethyl groups, and naphthylmethyl groups.

Examples of halogen atoms include fluorine atoms, chlorine atoms, bromine atoms, iodine atoms, etc.

Examples of alkoxy groups include methoxy groups, ethoxy groups, propoxy groups, isopropoxy groups, butoxy groups, sec-butoxy groups, and tert-butoxy groups.

Examples of acyloxy groups include formyloxy groups, acetoxy groups, propanoyloxy groups, butanoyloxy groups, pivaloyloxy groups, pentanoyloxy groups, phenylacetoxy groups, acetoacetoxy groups, benzoyloxy groups, and naphthoyloxy groups.

Examples of allyloxy groups include phenyloxy groups and naphthyloxy groups.

Examples of alkenyloxy groups include vinyloxy groups, allyloxy groups, 1-propenyloxy groups, isopropenyloxy groups, 2-butenyloxy groups, 3-butenyloxy groups, 2-pentenyloxy groups, 3-methyl-3-butenyloxy groups, and 2-hexenyloxy groups.

Examples of polysiloxane segments having structural units represented by the general formula (S-1) and/or the general formula (S-2) include those having the following structures:

The polymer segment may have various functional groups as necessary, as long as they do not impair the effects of the present invention. Examples of such functional groups include a carboxyl group, a blocked carboxyl group, a carboxylic anhydride group, a tertiary amino group, a hydroxyl group, a blocked hydroxyl group, a cyclocarbonate group, an epoxy group, a carbonyl group, a primary amide group, a secondary amide, a carbamate group, and a functional group represented by the following structural formula (S-5).

The polymer segment may also have a polymerizable double bond such as a vinyl group or a (meth)acryloyl group.

The polysiloxane compound is preferably produced, for example, by the methods shown in (1) to (3) below.

(1) A method in which a polymer segment containing a silanol group and/or a hydrolyzable silyl group is prepared in advance as a raw material for the polymer segment, and this polymer segment is mixed with a silane compound having both a silanol group and/or a hydrolyzable silyl group and a polymerizable double bond to carry out a hydrolysis condensation reaction.

(2) A polymer segment containing a silanol group and/or a hydrolyzable silyl group is prepared in advance as a raw material for the polymer segment. A polysiloxane is also prepared in advance by subjecting a silane compound having both a silanol group and/or a hydrolyzable silyl group and a polymerizable double bond to a hydrolysis and condensation reaction. The polymer segment and the polysiloxane are then mixed together to carry out a hydrolysis and condensation reaction.

(3) A method in which the polymer segment, a silane compound having a silanol group and/or a hydrolyzable silyl group, and a polymerizable double bond, and a polysiloxane are mixed together, and a hydrolysis condensation reaction is carried out.
A polysiloxane compound can be obtained by the above method.
Examples of polysiloxane compounds include the Ceranate (registered trademark) series (organic-inorganic hybrid coating resin; manufactured by DIC Corporation) and the CompoCerane SQ series (silsesquioxane hybrid; manufactured by Arakawa Chemical Industries, Ltd.).

The carbon source resin is preferably a synthetic resin or natural chemical raw material having aromatic functional groups, which has good miscibility with polysiloxane compounds and can be carbonized by high-temperature baking in an inert atmosphere.

Examples of synthetic resins include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid, and thermosetting resins such as phenolic resins and furan resins. Natural chemical raw materials include heavy oils, particularly tar pitches, such as coal tar, light tar oil, medium tar oil, heavy tar oil, naphthalene oil, anthracene oil, coal tar pitch, pitch oil, mesophase pitch, oxygen-crosslinked petroleum pitch, and heavy oil, but the use of phenolic resins is more preferable from the standpoint of cheap availability and the elimination of impurities.

In particular, the carbon source resin is preferably a resin containing an aromatic hydrocarbon moiety, and the resin containing an aromatic hydrocarbon moiety is preferably a phenol resin, an epoxy resin, or a thermosetting resin, and the phenol resin is preferably a resol type.
The phenolic resin may be, for example, SUMILITE RESIN series (resole type phenolic resin, manufactured by Sumitomo Bakelite Co., Ltd.).

The nanosilicon slurry, preferably the nanosilicon slurry, is mixed with the mixture of the polysiloxane compound and the carbon source resin, and the solvent is removed to obtain a precursor.
The mixture containing the polysiloxane compound and the carbon source resin is preferably in a state in which the polysiloxane compound and the carbon source resin are uniformly mixed. The mixing is performed using a device having the functions of dispersion and mixing. Examples of the device having the functions of dispersion and mixing include a stirrer, an ultrasonic mixer, and a premix disperser. In the work of desolvation and drying for the purpose of distilling off the organic solvent, a dryer, a reduced pressure dryer, a spray dryer, etc. can be used.

The precursor preferably contains 3% to 50% by mass of the nanosilicon, 15% to 85% by mass of a solid content of the polysiloxane compound, and 3% to 70% by mass of a solid content of the carbon source resin, and more preferably contains 8% to 40% by mass of the solid content of the silicon oxide particles, 20 to 70% by mass of a solid content of the polysiloxane compound, and 3% to 60% by mass of a solid content of the carbon source resin.

The precursor obtained above is calcined in an inert gas atmosphere to completely decompose the thermally decomposable organic components, resulting in a calcined product. The calcination temperature can be set, for example, at a maximum temperature in the range of 900°C to 1200°C, which allows the thermally decomposable organic components to be completely decomposed. The polysiloxane compound and carbon source resin are also converted by the energy of the high-temperature treatment into a silicon oxycarbide phase having a silicon-oxygen-carbon skeleton and free carbon.

Firing is carried out according to a firing program that specifies factors such as the heating rate and holding time at a constant temperature. The maximum temperature reached is the highest temperature that can be set, and it has a strong influence on the structure and performance of the fired product. The maximum temperature reached allows precise control of the microstructure of this active material, which retains the chemically bonded state of silicon and carbon in the silicon oxycarbide phase, resulting in better charge and discharge characteristics.

The calcination method is not particularly limited, but a reaction device having a heating function in an inert gas atmosphere may be used, and processing can be carried out by a continuous method or a batch method. The calcination device can be appropriately selected according to the purpose from among a fluidized bed reactor, a rotary furnace, a vertical moving bed reactor, a tunnel furnace, a batch furnace, a rotary kiln, etc.

The obtained fired product is pulverized and classified as necessary to obtain the active material. The pulverization may be performed in one step to the desired particle size, or may be performed in several steps. For example, when preparing an active material of about 10 μm, the fired product of lumps or aggregates of 10 mm or more is coarsely pulverized with a jaw crusher, roll crusher, etc. to particles of about 1 mm, then crushed to about 100 μm with a glow mill, ball mill, etc., and to about 10 μm with a bead mill, jet mill, etc. The particles produced by pulverization may contain coarse particles, and classification is performed to remove them, or to remove fine powder and adjust the particle size distribution. The classifier used is a wind classifier, a wet classifier, etc., depending on the purpose, but when removing coarse particles, a classification method that passes through a sieve is preferable because it can reliably achieve the purpose. In addition, if the precursor mixture is controlled to a shape close to the target particle size by spray drying or the like before firing, and fired in that shape, it is possible to omit the grinding process.

When the present active material contains a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al, a salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al is added to a suspension obtained by mixing a slurry of the present silicon oxide particles with a mixture of a polysiloxane compound and a carbon source resin, and then the same operation as above is carried out to obtain the present active material containing the silicate compound.
Examples of the salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg, and Al include halides such as fluorides, chlorides, and bromides, hydroxides, and carbonates of these metals.

The salt of the metal may be a salt of two or more metals, one salt may have a plurality of metals, or a mixture of salts having different metals.
The amount of the metal salt added to the suspension is preferably 0.01 to 0.4 in terms of molar ratio relative to the number of moles of silicon oxide particles.
When the metal salt is soluble in an organic solvent, the metal salt may be dissolved in an organic solvent and added to the suspension and mixed. When the metal salt is insoluble in an organic solvent, the metal salt particles may be dispersed in an organic solvent and then added to the suspension and mixed. From the viewpoint of improving the dispersion effect, the metal salt is preferably nanoparticles having an average particle size of 100 nm or less. As the organic solvent, alcohols, ketones, etc. can be suitably used, but aromatic hydrocarbon solvents such as toluene, xylene, naphthalene, and methylnaphthalene can also be used.

By uniformly dispersing the metal salt in the suspension, the metal salt molecules can be brought into sufficient contact with the nanosilicon. When silicon oxide is present on or around the surface of the nanosilicon, the silicate compound can be present near the surface of the nanosilicon by bringing the metal salt molecules into sufficient contact with the nanosilicon under conditions in which the metal salt molecules and the nanosilicon undergo a solid-phase reaction. In order to make the concentration of the silicate compound near the surface of the nanosilicon higher than the concentration of silicon oxycarbide, it is important to improve the contact state between the metal salt and the silicon oxide particles.
In addition, the metal salt molecules can be attached to the vicinity of the nanosilicon surface by surface modification using an organic additive. The molecular structure of the organic additive is not particularly limited, but it is preferable that the organic additive has a molecular structure that can form a physical or chemical bond with the dispersant present on the surface of the nanosilicon. The physical or chemical bond can be an electrostatic action, a hydrogen bond, an intermolecular van der Waals force, an ionic bond, a covalent bond, etc. During high-temperature baking, the metal salt molecules undergo a solid-phase reaction with the silicon oxide on the surface of the nanosilicon, so that the surface of the nanosilicon can be covered with the silicate compound.

The present active material is excellent in cycle property, initial coulombic efficiency and capacity retention rate, and a secondary battery using the present active material as a negative electrode exhibits excellent characteristics.
Specifically, a slurry containing the active material, an organic binder, and other components such as a conductive assistant, if necessary, can be applied to a copper foil collector in the form of a thin film to form a negative electrode. Alternatively, a carbon material such as graphite can be added to the slurry to form a negative electrode.
Examples of the carbon material include natural graphite, artificial graphite, and amorphous carbon such as hard carbon or soft carbon.

For example, the active material and the organic binder are mixed together with a solvent using a dispersing device such as a mixer, ball mill, super sand mill, or pressure kneader to prepare a negative electrode material slurry, which is then applied to a current collector to form a negative electrode layer. Alternatively, the negative electrode material can be obtained by forming the paste-like negative electrode material slurry into a sheet, pellet, or other shape and integrating it with a current collector.

Examples of the organic binder include styrene-butadiene rubber copolymers (hereinafter also referred to as "SBR"); ethylenically unsaturated carboxylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, (meth)acrylonitrile, and hydroxyethyl (meth)acrylate, and unsaturated carboxylic acid copolymers such as (meth)acrylic copolymers consisting of ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid; and polymeric compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide, and carboxymethylcellulose (hereinafter also referred to as "CMC").

Depending on their physical properties, these organic binders may be dispersed or dissolved in water, or dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP). The content of the organic binder in the negative electrode layer of the lithium ion secondary battery negative electrode is preferably 1% to 30% by mass, more preferably 2% to 20% by mass, and even more preferably 3% to 15% by mass.

When the content of the organic binder is 1% by mass or more, the adhesion is better and the destruction of the negative electrode structure due to the expansion and contraction during charging and discharging is more suppressed, whereas when the content is 30% by mass or less, the increase in the electrode resistance is more suppressed.
Within this range, the present active material has high chemical stability and can employ an aqueous binder, making it easy to handle in practical applications.

In addition, the negative electrode material slurry may be mixed with a conductive additive as necessary. Examples of conductive additives include carbon black, graphite, acetylene black, and oxides and nitrides that exhibit electrical conductivity. The amount of conductive additive used may be about 1% by mass to 15% by mass relative to the negative electrode active material of the present invention.

The material and shape of the current collector may be, for example, a strip of copper, nickel, titanium, stainless steel, or the like in the form of foil, perforated foil, mesh, or the like. Porous materials such as porous metal (foamed metal) and carbon paper may also be used.

Methods for applying the negative electrode material slurry to the current collector include, for example, metal mask printing, electrostatic painting, dip coating, spray coating, roll coating, doctor blade, gravure coating, screen printing, etc. After application, it is preferable to perform a rolling process using a flat plate press, calendar roll, etc. as necessary.

In addition, the negative electrode material slurry can be made into a sheet or pellet form, and integrated with the current collector by, for example, rolling, pressing, or a combination of these.

The negative electrode layer formed on the current collector or the negative electrode layer integrated with the current collector is preferably heat-treated according to the organic binder used. For example, when using a water-based styrene-butadiene rubber copolymer (SBR), heat treatment at 100 to 130°C is sufficient, and when using an organic binder with a polyimide or polyamideimide as the main skeleton, heat treatment at 150 to 450°C is preferable.

This heat treatment removes the solvent and hardens the binder, increasing strength and improving adhesion between particles and between the particles and the current collector. It is preferable to carry out these heat treatments in an inert atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere, to prevent oxidation of the current collector during treatment.

In addition, after the heat treatment, the negative electrode is preferably subjected to pressure treatment. In the negative electrode using the present active material, the electrode density is preferably 1 g/cm ³ to 1.8 g/cm ³ , more preferably 1.1 g/cm ³ to 1.7 g/cm ³ , and even more preferably 1.2 g/cm ³ to 1.6 g/cm ^3. The higher the electrode density, the more the adhesion and the volume capacity density of the electrode tend to improve. On the other hand, if the electrode density is too high, the voids in the electrode are reduced, which weakens the effect of suppressing the volume expansion of silicon, etc., and the capacity retention rate may decrease. Therefore, the optimal range of the electrode density is selected.

The secondary battery of the present invention contains the active material in the negative electrode. As secondary batteries having a negative electrode containing the active material, non-aqueous electrolyte secondary batteries and solid electrolyte secondary batteries are preferred, and the active material exhibits excellent performance particularly when used as the negative electrode of a non-aqueous electrolyte secondary battery.

When the secondary battery of the present invention is used, for example, as a wet electrolyte secondary battery, it can be constructed by arranging a positive electrode and a negative electrode containing the negative electrode active material of the present invention opposite each other via a separator and injecting an electrolyte.

The positive electrode can be obtained by forming a positive electrode layer on the surface of a current collector in the same manner as the negative electrode. In this case, the current collector can be a strip of metal or alloy such as aluminum, titanium, or stainless steel in the form of foil, perforated foil, mesh, or the like.

The positive electrode material used in the positive electrode layer is not particularly limited. When a lithium ion secondary battery is produced among non-aqueous electrolyte secondary batteries, for example, a metal compound, a metal oxide, a metal sulfide, or a conductive polymer material capable of doping or intercalating lithium ions may be used. For example, lithium cobalt oxide ( _LiCoO2 ), lithium nickel oxide ( _LiNiO2 ), lithium manganese oxide ( _LiMnO2 ), and composite oxides thereof ( _{LiCoxNiyMnzO2} , x+y+z= ₁ ), lithium manganese spinel ( _LiMn2O4 ), lithium vanadium compounds, _V2O5 , _{V6O13 , VO2 , MnO2 , TiO2 , MoV2O8 , TiS2} , _V2S5 , _VS2 , _MoS2 , _{MoS3 , Cr3O8 , Cr2O5} , olivine type _LiMPO4 (wherein M is Co, Ni, Mn or Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene and polyacene, porous carbon, etc. can be used alone or in combination.

The separator may be, for example, a nonwoven fabric, cloth, microporous film, or a combination of these, whose main component is a polyolefin such as polyethylene or polypropylene. If the positive and negative electrodes of the nonaqueous electrolyte secondary battery are not in direct contact with each other, there is no need to use a separator.

As the electrolyte, for example, a so-called organic electrolyte can be used in which a lithium salt such as LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , or LiSO ₃ CF ₃ is dissolved in a non-aqueous solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, or ethyl acetate, or a mixture of two or more components.

The structure of the secondary battery of the present invention is not particularly limited, but typically, the positive and negative electrodes and a separator, which is provided as necessary, are wound into a flat spiral shape to form a wound electrode assembly, or are stacked as flat plates to form a stacked electrode assembly, and these electrode assembly are enclosed in an exterior body. Note that the half cells used in the examples of the present invention are mainly composed of the active material in the negative electrode, and a simple evaluation was performed using metallic lithium in the counter electrode, in order to more clearly compare the cycle characteristics of the active material itself.

Secondary batteries using this active material are used as, but are not limited to, paper-type batteries, button-type batteries, coin-type batteries, laminated batteries, cylindrical batteries, square batteries, etc. The above-mentioned negative electrode active material of the present invention can also be applied to general electrochemical devices that use the insertion and removal of lithium ions as a charging and discharging mechanism, such as hybrid capacitors and solid lithium secondary batteries.

As described above, when the present active material is used as the negative electrode active material of a secondary battery, it gives a secondary battery excellent in cycle performance, initial coulombic efficiency and capacity retention rate.
The present active material can be used as a negative electrode by the above-mentioned method to produce a secondary battery having the negative electrode.

The above describes the present nanosilicon, the present nanosilicon slurry containing the present nanosilicon, the method for producing the present nanosilicon, the present active material containing the present nanosilicon, and the secondary battery containing the present active material in the negative electrode, but the present invention is not limited to the configurations of the above embodiments.
The present nanosilicon, the present nanosilicon slurry, the present active material, and the secondary battery containing the present active material in the negative electrode may have any other configuration added to the configuration of the above embodiment, or may be replaced with any configuration that exhibits a similar function.
Furthermore, in the configurations of the above-described embodiments of the method for producing nanosilicon and the method for producing active material, any other steps may be added, or any step that exhibits a similar function may be substituted.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these.
In addition, the half-cell used in the examples of the present invention has a negative electrode mainly made of the present active material, and a simple evaluation is performed using metallic lithium as the counter electrode, in order to more clearly compare the cycle characteristics of the active material itself.

Synthesis Example 1: Preparation of polysiloxane compound (synthesis of methyltrimethoxysilane condensate (a1))
A reaction vessel equipped with a stirrer, a thermometer, a dropping funnel, a cooling tube, and a nitrogen gas inlet was charged with 1,421 parts by mass of methyltrimethoxysilane (hereinafter, referred to as "MTMS") and heated to 60° C. Next, a mixture of 0.17 parts by mass of iso-propyl acid phosphate ("Phoslex A-3" manufactured by SC Organic Chemical Co., Ltd.) and 207 parts by mass of deionized water was dropped into the reaction vessel over a period of 5 minutes, and the mixture was stirred at a temperature of 80° C. for 4 hours to carry out a hydrolysis and condensation reaction of MTMS.
The condensate obtained by the hydrolysis-condensation reaction was distilled at a temperature of 40 to 60° C. and under a reduced pressure of 40 to 1.3 kPa. The phrase "under a reduced pressure of 40 to 1.3 kPa" means that the reduced pressure condition at the start of methanol distillation is 40 kPa, and the pressure is finally reduced to 1.3 kPa. The same applies to the following description. By removing the methanol and water produced during the reaction, 1,000 parts by mass of a liquid containing a condensate of MTMS having a number average molecular weight of 1,000 to 5,000 (hereinafter also referred to as "a1"). The content of the active ingredient in the obtained liquid was 70% by mass.
The effective component is calculated by dividing the theoretical yield (parts by mass) when all methoxy groups of a silane monomer such as MTMS undergo a condensation reaction by the actual yield (parts by mass) after the condensation reaction, i.e., [theoretical yield (parts by mass) when all methoxy groups of a silane monomer undergo a condensation reaction/actual yield (parts by mass) after the condensation reaction].

(Production of Curable Resin Composition)
A reaction vessel equipped with a stirrer, a thermometer, a dropping funnel, a cooling tube, and a nitrogen gas inlet was charged with 150 parts by mass of butanol (hereinafter also referred to as "BuOH"), 105 parts by mass of phenyltrimethoxysilane (hereinafter also referred to as "PTMS"), and 277 parts by mass of dimethyldimethoxysilane (hereinafter also referred to as "DMDMS"), and the temperature was raised to 80°C.
Next, at the same temperature, a mixture containing 21 parts by mass of methyl methacrylate (hereinafter also referred to as "MMA"), 4 parts by mass of butyl methacrylate (hereinafter also referred to as "BMA"), 3 parts by mass of butyric acid (hereinafter also referred to as "BA"), 2 parts by mass of methacryloyloxypropyltrimethoxysilane (hereinafter also referred to as "MPTS"), 3 parts by mass of BuOH, and 0.6 parts by mass of butylperoxy-2-ethylhexanoate (hereinafter also referred to as "TBPEH") was dropped into the reaction vessel over 6 hours. After the dropwise addition was completed, the mixture was further reacted at the same temperature for 20 hours to obtain an organic solvent solution of a vinyl polymer (a2) having a number average molecular weight of 10,000 and having a hydrolyzable silyl group.

Next, a mixture of 0.04 parts by mass of iso-propyl acid phosphate ("Phoslex A-3" manufactured by SC Organic Chemical Co., Ltd.) and 112 parts by mass of deionized water was added dropwise over a period of 5 minutes, and the mixture was further stirred at the same temperature for 10 hours to cause a hydrolysis condensation reaction, thereby obtaining a liquid containing a composite resin in which the hydrolyzable silyl group of the vinyl polymer (a2) and the hydrolyzable silyl group and silanol group of the polysiloxane derived from the PTMS and DMDMS were bonded to each other.
Next, 472 parts by mass of the MTMS condensate (a1) obtained in Synthesis Example 1 and 80 parts by mass of deionized water were added to this liquid, and the mixture was stirred at the same temperature for 10 hours to cause a hydrolysis condensation reaction, and the produced methanol and water were removed by distillation under the same conditions as in Synthesis Example 1. Next, 250 parts by mass of BuOH was added, and 1,000 parts by mass of a curable resin composition having a nonvolatile content of 60.1% by mass was obtained.

Example 1
70 g of commercially available silicon powder (manufactured by Kojundo Kagaku) with a silicon purity of 99.9% by mass, a volume average particle size of 3.2 μm, and 3.5 atom% oxygen relative to silicon, 28 g of DISPERBYK9077 (manufactured by BYK Additives & Instruments, DISPERBYK is a registered trademark), and 280 g of methyl ethyl ketone (hereinafter also referred to as "MEK") were mixed in a stirring tank and stirred well. The stirring tank was covered with a lid and nitrogen gas with a dew point of -70 ° C. was supplied to the tank, and the tank was placed under an inert gas atmosphere. The initial moisture concentration was 0.058% by mass. This mixture was wet-milled for 1.5 hours using a bead mill (Ultra Apec Mill UAM-015 manufactured by Hiroshima Metal & Machinery Co., Ltd.) to obtain nanosilicon uniformly dispersed in the dispersion medium. The diameter of the beads in the bead mill was 0.2 mm, and the temperature of the mixed liquid during wet pulverization was not more than 40° C. Nanosilicon was obtained with a specific surface area of 105 m ² /g, a crystallite size of 13.5 nm, an oxygen content of 12.5 atom % relative to silicon, and a volume average particle size of 180 nm.

The nanosilicon-containing slurry, the curable resin composition of Synthesis Example 1, and a phenolic resin (Sumitomo Bakelite, Sumilite Resin PR-53570) were mixed in a three-necked separable flask so that the composition after firing was calculated to be SiOC/C/Si = 10/40/50. One neck was covered, and a nitrogen inlet tube and a solvent trap were connected to the remaining two necks. Nitrogen was introduced into the flask, and while stirring the mixture with a magnetic stirrer, the flask was heated to 120°C in an oil bath, and the solvent was distilled off until the stirrer stopped moving. It was then cooled to room temperature, and a dried resin product, which is a firing precursor, was obtained. The dried resin product, which is a firing precursor, was then fired in a nitrogen atmosphere at a temperature of 1050°C for 6 hours, and a black solid product was obtained. The black solid product obtained was pulverized with a planetary ball mill to obtain a negative electrode active material powder.

For the obtained negative electrode active material powder, the powder X-ray diffraction (XRD) measurement using Cu-Kα rays did not detect a diffraction peak at 2θ = 28.4 ° attributed to the Si (111) crystal plane. The content of nitrogen element was 0.2 mass% from the energy dispersive X-ray analysis (EDS) results. In addition, the Raman scattering analysis measurement results showed a peak at about 1590 cm ^-1 attributed to the G band of carbon and a peak at about 1330 cm ^-1 attributed to the D band, and the intensity ratio I (G band) / I (D band) was 1.4.

A mixed slurry containing 80% by mass of the above-mentioned negative electrode active material powder, 10% by mass of acetylene black as a conductive additive, and 10% by mass of a mixture of CMC and SBR as a binder was prepared and formed into a film on copper foil. It was then dried under reduced pressure at 110°C and a Li metal foil was used as the counter electrode to prepare a half cell. The charge and discharge characteristics of this half cell were evaluated using a two-phase battery charge and discharge measuring device (manufactured by Hokuto Co., Ltd.) with a cutoff voltage range of 0.005 to 1.5 V. The measurement results of the charge and discharge characteristics were an initial discharge capacity of 1630 mAh/g, an initial efficiency of 86%, and a retention rate after 5 cycles of 91%.

For the evaluation of the full cell, a positive electrode film was prepared using a single-layer sheet using _LiCoO2 as the positive electrode active material and aluminum foil as the current collector, and a negative electrode film was prepared by mixing graphite powder and active material powder with a discharge capacity design value of 450mAh/g. A non-aqueous electrolyte solution was used in which lithium hexafluorophosphate was dissolved at a concentration of 1 mol/L in a 1/1 volumetric mixture of ethylene carbonate (hereinafter also referred to as "EC") and diethyl carbonate (hereinafter also referred to as "DEC"), and a laminated lithium ion secondary battery was prepared using a 30 μm thick polyethylene microporous film as a separator. The laminated lithium ion secondary battery was charged at a constant current of 1.2 mA (0.25c based on the positive electrode) at 25°C until the voltage of the test cell reached 4.2V, and after reaching 4.2V, the current was reduced to keep the cell voltage at 4.2V, and the discharge capacity was determined. The capacity retention rate after 300 cycles was 90%, with one cycle being a charge and discharge within a voltage range of 2.5V to 4.2V. After charging and discharging, the laminated cell was disassembled in a glove box in an argon atmosphere to remove the negative electrode, which was washed with an EC/DEC mixed solution and then left to dry, after which the thickness of the electrode film was measured. The rate of change in the thickness of the negative electrode film before and after charging and discharging was taken as the negative electrode expansion rate. The negative electrode expansion rate was 19%. The results are shown in Table 1.

Examples 2 to 12
Wet milling was performed in the same manner as in Example 1, except that the weight percentage of the additive in the mixed solution during wet milling with a bead mill and the milling time for wet milling were set to the conditions shown in Table 1. A negative electrode active material was produced using the nanosilicon produced. A half cell was produced using this in the same manner as in Example 1, and the charge/discharge characteristics were evaluated. The results are shown in Table 1.

Example 13
Wet milling was performed in the same manner as in Example 1, except that the initial water concentration of the mixed solution during wet milling with a bead mill was 0.49 mass%, and the nanosilicon thus produced was used to produce a negative electrode active material. A half cell was produced using this in the same manner as in Example 1, and the charge/discharge characteristics were evaluated. The results are shown in Table 1.

Examples 14 and 18
Wet milling was carried out in the same manner as in Example 1, except that the volume average particle size of the raw silicon powder when wet milled in a bead mill and the amount of oxygen relative to silicon in the raw silicon powder were different, and the nanosilicon produced was used to produce a negative electrode active material. A half cell was produced using this in the same manner as in Example 1, and the charge/discharge characteristics were evaluated. The results are shown in Table 1.

Example 19
Wet milling was carried out in the same manner as in Example 1, except that the raw silicon powder used in wet milling had a silicon purity of 99% by weight, a volume average particle size of 4.6 μm, and an amount of oxygen relative to silicon of 1.5 atom%, and the nanosilicon produced was used to produce a negative electrode active material. A half cell was produced using this in the same manner as in Example 1, and the charge/discharge characteristics were evaluated. The results are shown in Table 1.

Comparative Example 1
Wet milling was carried out in the same manner as in Example 1, except that no additive was added. Viscosity increased significantly, gelation occurred during milling, and operation of the bead mill was stopped. Due to gelation of the nanosilicon-containing slurry, it was not suitable for the next step, and it was not possible to continue.

Comparative Example 2
Wet pulverization was carried out using a bead mill in the same manner as in Example 1, except that the initial water concentration was 2.51% by mass and the liquid temperature was 60° C. or higher. Viscosity increased significantly, gelation occurred during pulverization, and the operation of the bead mill was stopped. Due to gelation of the nanosilicon-containing slurry, it was not suitable for the next step and could not be continued.

Comparative Example 3
Wet milling was carried out using a bead mill in the same manner as in Example 1, except that the dew point of the nitrogen gas supplied to the stirring tank was -40°C or higher. Viscosity increased significantly, gelation occurred during the milling process, and the operation of the bead mill was stopped. Due to gelation of the nanosilicon-containing slurry, it was not suitable for the next step and could not be continued.

Comparative Example 4
A negative electrode active material was prepared in the same manner as in Example 1, except that the precursor of the negative electrode active material was calcined in air, and then a half cell was prepared. The measurement results of the charge and discharge characteristics showed that the initial discharge capacity was 1090 mAh/g and the initial efficiency was 61.2%. The results are shown in Table 1.

Comparative Example 5
Wet milling, preparation of anode active material, and preparation of half cells were carried out in the same manner as in Example 1, except that silicon powder with a volume average particle size of 0.5 μm and an oxygen content of 16 atom% relative to silicon was used as the raw material. The lumps could not be broken up, the separator (screen) was clogged, and the pressure inside the bead mill rose, causing the bead mill to stop operating. It was not possible to continue milling any further.

Comparative Example 6
Wet milling was carried out in the same manner as in Example 1, except that silicon powder with a volume average particle size of 30 μm and an oxygen content of 0.3 atom% relative to silicon was used as the raw material. The separator of the bead mill was clogged because the coarse particles could not be milled, resulting in screen blockage. The pressure inside the bead mill rose, and the operation of the bead mill was stopped. It was not possible to continue further.

[Evaluation method]
The evaluation methods are as follows:
Volume average particle size: Measured using a laser diffraction particle size distribution measuring device (Malvern Panalytical, Mastersizer 3000).
Specific surface area: Measured by the BET method using a specific surface area measuring device (BELSORP-mini, manufactured by BELJAPAN) by nitrogen adsorption measurement. ²⁹ Si-NMR: JNM-ECA600, manufactured by JEOL RESONANCE, was used.

Battery characteristic evaluation: Battery characteristics were measured using a secondary battery charge/discharge tester (manufactured by Hokuto Denko Corporation). The charge/discharge characteristics were evaluated under the set conditions of constant current/constant voltage charge/constant current discharge, with room temperature at 25°C, cutoff voltage range of 0.005 to 1.5V, and charge/discharge rate of 0.1C (1st to 3rd cycles) and 0.2C (after 4th cycle). When switching between charge and discharge, the battery was left in an open circuit for 30 minutes. Discharge capacity, charge capacity, initial coulombic efficiency and cyclability (in this application, this refers to the capacity retention rate after 5 cycles of charge/discharge of a full cell at 25°C), and negative electrode expansion rate were determined as follows. Charge capacity and discharge capacity of active material: Calculated by charge/discharge measurement of half cell. Initial Coulombic efficiency of active material (%)=initial discharge capacity (mAh/g)/initial charge capacity (mAh/g) Capacity retention rate (%@5th)=5th negative electrode discharge capacity (mAh/g)/negative electrode initial discharge capacity (mAh/g), obtained by measuring a full cell (lamicelle).

As is clear from the above results, when this nanosilicon is used as the negative electrode active material, the cycleability, initial coulombic efficiency, and capacity retention rate are all high, and the balance of these secondary battery properties is excellent. Furthermore, secondary batteries containing this active material as the negative electrode active material have excellent battery properties.

Claims (8)

Nanosilicon having a specific surface area of 100 to 400 m ² /g, 5 to 45 atom % of oxygen atoms relative to silicon atoms, a crystallite diameter of 5 to 14 nm, and a volume average particle diameter of 10 to 200 nm. A nanosilicon slurry comprising the nanosilicon of claim 1, a dispersant and a solvent. The method for producing nanosilicon according to claim 1, wherein silicon powder having a volume average particle size of 1 to 20 μm and an oxygen atom ratio of 10 atom% or less to silicon atoms is wet-pulverized in a non-aqueous solvent having a water concentration of 10,000 ppm or less and containing a cationic surfactant having an amine value of 1 to 100 mgKOH/g at a temperature of 60° C. or less in a gas atmosphere having a dew point temperature of −60° C. or less. The method for producing nanosilicon according to claim 3 , wherein the purity of the silicon powder obtained in the wet grinding is 99% by mass or more. A method for producing an active material for a secondary battery, comprising mixing the nanosilicon obtained by the method for producing nanosilicon according to claim 3 or 4 with a resin, drying the mixture, and then firing the mixture in an inert gas atmosphere. An active material for secondary batteries containing the nanosilicon according to claim 1. A negative electrode for a secondary battery comprising the active material for a secondary battery according to claim 6 . A secondary battery comprising the negative electrode for secondary batteries according to claim 7 .