JP7491478B2 - Active material for secondary battery and secondary battery - Google Patents

Description

The present invention relates to an active material for a secondary battery and a secondary battery. More specifically, the present invention relates to an active material for a secondary battery and a secondary battery containing the active material for a secondary battery in 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.

Silicon and amorphous silicon oxide have attracted much attention as negative electrode active materials for lithium ion secondary batteries because of their large capacity, but they have poor cycle performance due to their large volume expansion during repeated charging and discharging. In addition, lithium secondary batteries using silicon oxide have low initial efficiency, so various improvements have been attempted to use silicon oxide as a negative electrode active material.
For example, Patent Document 1 proposes a silicon composite in which silicon crystallites are dispersed in silicon oxide as a negative electrode active material for secondary batteries.

However, silicon oxide contains many oxygen atoms that generate irreversible lithium silicate during charging, which poses the problem of low initial efficiency. As a result, when a battery was actually fabricated, an excessive battery capacity was required for the positive electrode, and an increase in battery capacity commensurate with the increase in active material capacity was not observed.

In order to solve the above problems, Patent Document 2 proposes a silicon oxide for use as a negative electrode material for non-aqueous electrolyte secondary batteries, in which silicon crystallites are dispersed via siloxane bonds and have a structure having minute spaces between the silicon crystallites.
However, when silicon oxide is used as a negative electrode active material for lithium ion secondary batteries, a rapid decrease in charge/discharge capacity after repeated charge/discharge cycles is thought to be due to a large volume change caused by the absorption and release of a large amount of lithium, which leads to particle destruction.

In order to suppress the destruction of particles accompanying this volume change, Patent Document 3 attempts to suppress the large volume change by using silicon flakes having a hyperporous structure as an active material.
However, it is believed that providing such pores increases the specific surface area of the active material, resulting in a decrease in cycle characteristics.

JP 2004-323284 A JP 2015-18626 A JP 2019-519067 A

Therefore, there has yet to be sufficient improvement in the cycleability and initial efficiency of lithium secondary batteries using silicon oxide, which has a large electrical capacity.

The present inventors have investigated a secondary battery active material using silicon oxide that suppresses particle destruction due to volume change of silicon oxide, improves the cycle performance of lithium secondary batteries, and has a high electric capacity, and as a result, have found a secondary battery composite active material that improves the cycle performance, initial coulombic efficiency, and capacity retention rate of lithium secondary batteries.
That is, the present invention relates to 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, and aims to provide an active material for a secondary battery that provides a secondary battery excellent in cycle performance, initial coulombic efficiency, and capacity retention rate.

The present invention has the following aspects.
[1] An active material for a secondary battery, which is a composite particle having a silicon oxycarbide phase and at least two silicon oxide particles in the silicon oxycarbide phase.
[2] The active material for a secondary battery according to [1], wherein the content of the silicon oxide particles is 1% by mass or more and 60% by mass or less.
[3] The active material for a secondary battery according to [1] or [2] above, which has a peak at 2θ of about 28.4° in X-ray diffraction, which is attributed to Si(111).
[4] The active material for a secondary battery according to any one of [1] to [3], wherein the silicon oxide particles have an average particle size of 5 μm or less.
[5] The active material for a secondary battery according to any one of [1] to [4], wherein the composite particles have an average particle size of 2 μm or more and 15 μm or less.
[6] The active material for a secondary battery according to any one of [1] to [5] above, which has a specific surface area of 0.3 m ² /g or more and 10 m ² /g or less.
[7] The active material for a secondary battery according to any one of [1] to [6], wherein the silicon oxycarbide phase further contains nitrogen atoms.
[8] The active material for a secondary battery according to any one of [1] to [7], which has a scattering peak at 1590 cm ⁻¹ assigned to a G band of a carbon structure and a scattering peak at about 1330 cm ⁻¹ assigned to a D band in a Raman spectrum, and an intensity ratio of the scattering peaks, I (G band) / I (D band), is 0.7 or more and 2 or less.
[9] The active material for a secondary battery according to any one of [1] to [8], wherein the composite particles have a surface having a carbon coating, and the amount of the carbon coating is 1% by mass or more and 10% by mass or less.
[10] The active material for a secondary battery according to any one of [1] to [9] above, which contains a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al.

The present invention also has the following aspects.
[11] A secondary battery comprising the active material for secondary batteries according to any one of [1] to [10] in a negative electrode.

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 cycleability, volume expansion, and initial coulombic efficiency.

The active material for secondary batteries of the present invention (hereinafter also referred to as "the active material") is a composite particle having a silicon oxycarbide phase and at least two or more silicon oxide particles (hereinafter also referred to as "the silicon oxide particles") in the silicon oxycarbide phase.
As mentioned above, silicon oxide has a high capacity, but a large volume change occurs when a large amount of lithium is absorbed and released, which is thought to result in poor cycle performance. On the other hand, silicon oxycarbide phase has a relatively low capacity, but the volume change is small when lithium is absorbed and released, and it has excellent cycle characteristics. It is thought that by combining the two, the present active material has obtained an active material for secondary batteries that provides a secondary battery with excellent volume expansion and cycle characteristics while maintaining a high capacity.

Silicon oxide is a general term for amorphous silicon oxide obtained by heating a mixture of silicon dioxide and metallic silicon to produce silicon monoxide gas, which is then cooled and precipitated, and is represented by the following general formula (1).
SiON (1)
In the formula (1), n is from 0.4 to 1.8, and preferably from 0.5 to 1.6.

Since the active material is a composite particle in which silicon oxide particles are contained in a matrix of a silicon oxycarbide phase, it is considered that the particle size of the silicon oxide has a large effect on the performance of the active material. If the average particle size of the silicon oxide particles exceeds 5 μm, the silicon oxide becomes a large mass, and when the active material is used as a negative electrode active material, the silicon oxide particles cause a large expansion and contraction of the negative electrode active material during charging and discharging. As a result, stress is concentrated in a part of the matrix, so that the structure of the active material is easily collapsed, and the capacity retention rate of the negative electrode active material tends to decrease. On the other hand, small silicon oxide particles of less than 300 nm are too fine, so the silicon oxide particles tend to aggregate with each other. Therefore, the dispersibility of the silicon oxide particles in the negative electrode active material may decrease. In addition, if the silicon oxide particles are too fine, their specific surface area increases, and there is a tendency for by-products and the like to increase on the surface of the silicon oxide particles during high-temperature baking of the negative electrode active material. These may lead to a decrease in charge and discharge performance.

From the above viewpoints, the average particle size of the silicon oxide particles is preferably 3 μm or less, and more preferably 2 μm or less. From the viewpoints of particle dispersibility and specific surface area, the average particle size of the silicon oxide particles is preferably 300 nm or more, and more preferably 200 nm or more.
Here, the average particle size is the value of D50, which can be measured using a laser diffraction particle size analyzer, etc. D50 can be measured by a dynamic light scattering method using a laser particle size analyzer, etc. The average particle size of the present silicon oxide particles is the particle size at which the cumulative volume distribution curve reaches 50% in the particle size distribution, when the cumulative volume distribution curve is drawn from the small diameter side.

The present silicon oxide particles can be obtained by pulverizing silicon oxide or the like so that the average particle size falls within the above-mentioned range.
Examples of the mill used for the pulverization include a ball mill, a bead mill, a jet mill, etc. The pulverization may be wet pulverization using an organic solvent, and as the organic solvent, for example, alcohols, ketones, etc. can be suitably used, but aromatic hydrocarbon solvents such as toluene, xylene, naphthalene, and methylnaphthalene can also be used.
The obtained silicon oxide particles can be classified or the like by controlling the bead mill conditions such as bead particle size, blending ratio, rotation speed or grinding time, thereby making it possible to make the average particle size of the present silicon oxide particles fall within the above range.

The silicon oxide particles may be in the form of any of granules, needles, and flakes.
The morphology of the silicon oxide particles can be measured by dynamic light scattering to determine the average particle size, but samples with the above aspect ratios can be identified more easily and precisely by using 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, a 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 silicon oxide particles can be identified by TEM observation.
The aspect ratio of the silicon oxide particles is a calculation result based on 50 particles in the main part of the sample within the field of view of the TEM image.

The silicon oxycarbide phase is composed of compounds containing silicon, oxygen, and carbon, and preferably has a three-dimensional network structure of a silicon-oxygen-carbon skeleton and a structure containing free carbon. 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 (2):
SiOxCy (2)
In formula (2), x represents the molar ratio of oxygen to silicon, and y represents the molar ratio of carbon to silicon.
When the present active material is used in a secondary battery, from the viewpoint of obtaining 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.
When the present active material is used in a secondary battery, from the viewpoint of the balance between the charge/discharge performance and the 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 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).
Although it is preferable to measure x and y by the above-described method, 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 compound constituting the silicon oxycarbide phase is a compound containing silicon, oxygen, and carbon, 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 silicon oxide particles are tightly wrapped in a composite structure of the silicon-oxygen-carbon skeleton and free carbon, suppressing volumetric changes in the silicon oxide particles in response to the absorption and release of lithium. As a result, when the active material is used as a negative electrode, the silicon oxide particles in the negative electrode play a role as a main component in expressing charge and discharge performance, while the silicon oxycarbide phase suppresses particle destruction associated with volumetric changes in the silicon oxide particles during charge and discharge, 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. Nitrogen can be introduced into the silicon oxycarbide phase by forming the raw materials used in the manufacturing method of the present active material described below, such as phenolic resins, dispersants, polysiloxane compounds, other nitrogen compounds, and nitrogen gas used in the firing process into atomic groups containing nitrogen as functional groups in the molecules. When the silicon oxycarbide phase contains nitrogen, the charge/discharge performance and capacity retention rate tend to be excellent when the present active material is used as a 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 (3).
SiOxCyNz (3)
In formula (3), x and y are as defined above, and z represents the molar ratio of nitrogen to silicon.
When the silicon oxycarbide phase contains the compound represented by formula (3), from the viewpoint of charge/discharge performance and capacity retention rate when the active material is used in a secondary battery, it is preferable that 1≦x≦2, 1≦y≦20, and 0<z≦0.5, and it is more preferable that 1≦x≦1.9, 1.2≦y≦15, and 0<z≦0.4.

Similarly to x and y, z can be determined by measuring the mass content of the elements and then converting it into a molar ratio (atomic number ratio).
As with x and y, z is preferably measured by the method described above, but it is also possible to perform local analysis of the active material, obtain a large number of measurement points for 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).

The active material is a composite particle having the silicon oxide particles in the silicon oxycarbide phase, and the composite particle in which the silicon oxide is dispersed in the silicon oxycarbide phase is preferred. The number of the silicon oxide particles dispersed in the silicon oxycarbide phase is 2 or more, and there is no particular upper limit.

Silicon oxide may generate silicon as a simple substance by disproportionation reaction, but the present active material may have a structure containing silicon as a simple substance generated by disproportionation from the viewpoint of high capacity and high initial efficiency. When the present active material contains silicon as a simple substance, when the present active material is subjected to X-ray diffraction analysis, a peak at 2θ of about 28.4°, which is assigned to the Si(111) plane, is detected in the X-ray diffraction pattern.
Silicon may be added separately to the present active material, but it is preferable that the silicon contained in the present active material is one produced by the disproportionation reaction and has relatively small crystallites.

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

The specific surface area of the active material is preferably 0.3 m ² /g or more and 10 m ² /g or less. The specific surface area of the active material is more preferably 0.5 m ² /g or more, and particularly preferably 1 m ² /g or more. The specific surface area of the active material is more preferably 9 m ² /g or less, and particularly preferably 8 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, and can be determined by nitrogen gas adsorption measurement, for example, using a specific surface area measurement device.

The active material preferably has free carbon composed of only carbon element together with silicon-oxygen-carbon skeletal structure in silicon oxycarbide phase. When the active material has free carbon, a scattering peak at 1590 cm ⁻¹ attributed to G band of graphite long period carbon lattice structure and a scattering peak at about 1330 cm ⁻¹ attributed to D band of graphite short period carbon lattice structure having disorder or defect are observed in the Raman spectrum of the active material. The intensity ratio of the scattering peak intensity of the D band, I (G band), to the scattering peak intensity of the D band, I (D band), I (G 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 this active material is used in a secondary battery, the insertion and desorption reaction of lithium ions during charging and discharging proceeds quickly, and it is considered that the charge-discharge characteristics are improved. In addition, the insertion and desorption reaction of lithium ions may cause the negative electrode active material to expand and contract slightly, 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 this active material, and when this active material is used as the negative electrode of a secondary battery, it is believed that the reaction inside this 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, the carbon in the silicon oxycarbide phase of the active material has properties similar to those of amorphous carbon, so it is thermally decomposed in the air at a 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 active material preferably has a coating on the surface of the composite particle, the coating being preferably made of a material that is expected to have electronic conductivity, lithium ion conductivity, and the effect of inhibiting decomposition of the electrolyte.
Examples of the coating include coatings of 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, a carbon coating is preferred, and a low-crystalline carbon coating is more preferred.

In the case of a carbon coating, from the viewpoint of improving the chemical stability and thermal stability of the present active material, it is preferable that the average thickness of the coating is 10 nm or more and 300 nm or less, or that the content of the carbon coating is 1 to 10 mass % with the mass of the present active material being 100 mass %.
In the case of a carbon coating, the carbon coating is preferably formed on the surface of the active material by vapor deposition.
The mass of the active material is the total amount of the silicon oxide particles and silicon oxycarbide phase that constitute the active material. If the silicon oxycarbide phase contains nitrogen, the total amount also includes nitrogen. If the active material contains a third component such as silicon oxide, which will be described later, the total amount also includes 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 "the 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 the present 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 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 the silicate compound has two or more metals, one silicate ion may have multiple metals, or it may be a mixture of silicate compounds having different metals. In addition, the 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 silicate compound is preferably _a lithium silicate compound or a _magnesium silicate compound, more preferably lithium metasilicate ( _{Li2SiO3 , Li2Si2O5 , Li4SiO4} ) or magnesium metasilicate (MgSiO3 _{, Mg2SiO4} ), _and particularly preferably magnesium metasilicate ( _{MgSiO3 , Mg2SiO4} ).

The silicate compound may be present in either the silicon oxycarbide phase or the silicon oxide particles, or may be present in both. When the silicate compound is present in both the silicon oxycarbide phase and the silicon oxide, it is preferred that the concentration of the silicate compound in the silicon oxide particles is higher than the concentration in the silicon oxycarbide.
When the silicate compound is in a crystalline state, it can be detected by powder X-ray diffraction measurement (XRD), and when it is in an amorphous state, it can be confirmed by solid-state ²⁹ Si-NMR measurement.

The present active material can be obtained, for example, by the following method.
As described above, the silicon oxide particles can be produced by heating a mixture of silicon dioxide and metallic silicon to produce silicon monoxide gas, which is then cooled and precipitated. Commercially available silicon oxide may also be used.
When producing the present active material, silicon oxide may be crushed, classified, etc. to have a desired average particle size to obtain the present silicon oxide. The crushing and classification methods are as described above.

The slurry of the silicon oxide particles obtained above is mixed with a mixture of a polysiloxane compound and a carbon source resin to form a suspension, and the solvent is removed to obtain a precursor. The precursor obtained is fired to obtain a fired product, which is then pulverized as necessary to obtain the present active material having the desired average particle size or specific surface area.
The present silicon oxide particle slurry can be prepared by grinding silicon oxide particles in a wet powder grinding device using an organic solvent. A dispersant may be added to the organic solvent to promote grinding of the silicon oxide particles. Examples of wet grinding devices include roller mills, high-speed rotary grinders, container-driven mills, and bead mills.

Examples of the organic solvent 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.

The type of dispersant may be aqueous or non-aqueous, with non-aqueous dispersants being preferred. Examples of non-aqueous dispersants include polymeric types such as polyethers, alcohols, polyalkylene polyamines, and polycarboxylic acid partial alkyl esters, low molecular weight types such as polyhydric alcohol esters and alkyl polyamines, and inorganic types such as polyphosphates. The concentration of silicon oxide solids in the silicon oxide slurry is not particularly limited, but when the solvent and, if necessary, a dispersant are included, the amount of silicon oxide is preferably in the range of 5% to 40% by mass, and more preferably 10% to 30% by mass, with the total amount of the dispersant and silicon oxide being 100% by mass.

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.

Polysiloxane compounds 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) are 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 end 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は置換基を有してい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 formulas (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, or the like. ^R2 and ^R3 each represent an alkyl group, a cycloalkyl group, an aryl group or an aralkyl group, an epoxy group, a carboxy group, or the like.

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 constituting the polymer segment, and the two silicon atoms are silicon atoms constituting the 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 slurry of the silicon oxide particles is mixed with a mixture of a polysiloxane compound and a 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 silicon oxide particles, 15% to 85% by mass of the solid content of the polysiloxane compound, and 3% to 70% by mass of the solid content of the carbon source resin, and more preferably contains 8% to 40% by mass of the silicon oxide particles, 20 to 70% by mass of the solid content of the polysiloxane compound, and 3% to 60% by mass of the 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 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, which is a composite particle having a silicon oxycarbide phase and at least two or more silicon oxide particles in the silicon oxycarbide phase. The pulverization may be performed in one step until the desired particle size is reached, 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 into particles of about 1 mm using a jaw crusher, roll crusher, etc., and then pulverized to about 100 μm using a glow mill, ball mill, etc., and to about 10 μm using a bead mill, jet mill, etc. The particles produced by pulverization may contain coarse particles, and classification is performed to remove these particles and to remove fine powder and adjust the particle size distribution. The classifier used may be 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 have a shape close to the target particle size by spray drying or the like before firing, and then fired in that shape, it is possible to omit the pulverization step.

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 silicon oxide particles. When silicon oxide is present on the surface or periphery of the silicon oxide particles, the metal salt molecules can be brought into sufficient contact with the silicon oxide particles under conditions in which the metal salt molecules and the silicon oxide particles undergo a solid-phase reaction, so that the silicate compound can be present in the silicon oxide particles. In order to increase the concentration of the silicate compound in the silicon oxide particles to a level higher than that in 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 silicon oxide particle 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 silicon oxide particles. 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 firing, the metal salt molecules react with the silicon oxide particles in a solid phase, thereby forming the silicate compound in the silicon oxide particles.

If the active material has a carbon coating on the surface of the composite particles, the method is followed by coating with a carbon coating in a chemical vapor deposition apparatus in a flow of a pyrolyzable carbon source gas and a carrier inert gas at a temperature range of 700° C. to 1000° C.
The pyrolytic carbon source gas may be acetylene, ethylene, acetone, alcohol, propane, methane, ethane, or the like.
Examples of the inert gas include nitrogen, helium, and argon, and nitrogen is usually used.

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 binder, which is an organic binding material, are kneaded together with a solvent using a dispersing device such as a stirrer, 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 depending on 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 nonaqueous 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 type batteries, cylindrical type batteries, square type 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.

Although the present active material and the secondary battery containing the present active material in the negative electrode have been described above, the present invention is not limited to the configurations of the above-mentioned embodiments.
In the present active material and the secondary battery containing the present active material in the negative electrode, any other configuration may be added to the configuration of the above embodiment, or any other configuration that exerts the same 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 silicon-containing active material of the present invention, 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 abbreviated 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 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 distillation of methanol 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)].

Preparation of Curable Resin Composition (1) Into a reaction vessel equipped with a stirrer, a thermometer, a dropping funnel, a cooling tube, and a nitrogen gas inlet, 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") were charged and heated 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 (a2-1) 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 (a2-1) and the hydrolyzable silyl group and silanol group having the polysiloxane derived from the PTMS and DMDMS were bonded together.
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 to obtain 1,000 parts by mass of a curable resin composition (1) having a nonvolatile content of 60.1% by mass.

Preparation of Curable Resin Composition (2) Into a reaction vessel equipped with a stirrer, a thermometer, a dropping funnel, a cooling tube and a nitrogen gas inlet, 150 parts by mass of BuOH, 249 parts by mass of PTMS and 263 parts by mass of DMDMS were charged and heated to 80°C.
Next, at the same temperature, a mixture containing 18 parts by mass of MMA, 14 parts by mass of BMA, 7 parts by mass of BA, 1 part by mass of acrylic acid (hereinafter also referred to as "AA"), 2 parts by mass of MPTS, 6 parts by mass of BuOH, and 0.9 parts by mass of TBPEH was added dropwise into the reaction vessel over 5 hours. After completion of the addition, the mixture was further reacted at the same temperature for 10 hours to obtain an organic solvent solution of a vinyl polymer (a2-2) having a hydrolyzable silyl group and a number average molecular weight of 20 and 100.

Next, a mixture of 0.05 parts by mass of iso-propyl acid phosphate ("Phoslex A-3" manufactured by SC Organic Chemical Industry Co., Ltd.) and 147 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 (a2-2) and the hydrolyzable silyl group and silanol group of the polysiloxane derived from the PTMS and DMDMS were bonded to each other.
Next, 76 parts by mass of 3-glycidoxypropyltrimethoxysilane, 231 parts by mass of the MTMS condensate (a1) obtained in Synthesis Example 1, and 56 parts by mass of deionized water were added to this liquid, and the mixture was stirred at the same temperature for 15 hours to cause a hydrolysis and 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 to obtain 1,000 parts by mass of a curable resin composition (2) having a nonvolatile content of 60.0% by mass.

Synthesis Example 2: Pulverization Conditions for Silicon Oxide Particles In a 150 ml container of a small bead mill device, zirconia beads having a particle size of 0.2 mm were placed as a pulverization medium, and 100 ml of methyl ethyl ketone solvent was placed in a container of a 150 ml small bead mill device so that the filling rate was 60%. Then, 100 parts by mass of silicon oxide powder (commercially available product) having an average particle size of 5 μm and 20 parts by mass of a cationic dispersant liquid (BYK Japan Co., Ltd.: BYK145) were placed, and wet pulverization was performed using a bead mill under the pulverization conditions described in Table 1, to obtain a slurry of dark brown liquid silicon oxide, SiO-1 to SiO-3, with a solid concentration of 30% by mass. The average particle size of each silicon oxide was measured by laser light scattering measurement, and the results were as shown in Table 1.

In addition, a small ball mill device with a container diameter of 150 mm and zirconia balls as the grinding media was used, and methanol was used as the grinding solvent. 100 parts by mass of silicon oxide powder (commercially available) with an average particle size of 5 μm and 20 parts of a cationic dispersant liquid (BYK145, manufactured by BYK Japan KK) were placed in the grinding container, and the silicon oxide was ground under the grinding conditions shown in Table 1, to obtain a slurry of silicon oxide powders, SiO-4 to SiO-7, with a solid concentration of 30% by mass. The average particle sizes of SiO-4 to SiO-7 are shown in Table 1.

Example 1
The polysiloxane resin having an average molecular weight of 3500 (curable resin composition (1)) prepared in Synthesis Example 1 and the phenolic resin having an average molecular weight of 3000 were mixed in a weight ratio of 90/10 of resin solids, and the slurry of SiO-1 obtained in Synthesis Example 2 was added so that the content of silicon oxide particles in the product after high-temperature baking was 50 mass%, and thoroughly mixed in a stirrer. The obtained resin mixed suspension containing silicon oxide particles was subjected to desolvation under nitrogen follow conditions in an oil bath at 120°C. Thereafter, the mixture was dried under reduced pressure at 110°C for 10 hours using a vacuum dryer, and finally baked at a high temperature at 900°C for 4 hours in a nitrogen atmosphere to obtain a black solid. The obtained black solid was pulverized with a planetary ball mill to produce a black powder. 20 g of this black powder was placed in a CVD apparatus (desktop rotary kiln: manufactured by Takasago Industrial Co., Ltd.), and a carbon coating treatment was performed on the surface of the black powder by chemical vapor deposition at 850° C. for 1 hour while introducing a mixed gas of ethylene gas at 0.2 L/min and nitrogen gas at 0.8 L/min, thereby producing active material particles.

The amount of carbon coating on the active material powder after the carbon coating treatment was measured by a thermal analyzer and found to have increased by 2.1% compared to the weight before the treatment. The average particle size of the active material powder obtained was about 2.9 μm, and the specific surface area was 6.5 m ² /g.
The powder X-ray diffraction (XRD) measurement using Cu-Kα radiation did not detect a diffraction peak at 2θ=28.4° attributed to the Si(111) crystal plane. The energy dispersive X-ray spectroscopy (EDS) result showed that the nitrogen element content was 0.2 mass%. In addition, the Raman scattering analysis measurement result 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.

80 parts by mass of the active material particles obtained above were mixed with 10 parts by mass of acetylene black as a conductive assistant and 10 parts by mass of a mixture of CMC and SBR as a binder to prepare a slurry. The obtained slurry was formed into a film on a copper foil. After drying under reduced pressure at 110°C, a coin-type lithium ion battery was produced as a half cell with Li metal foil as the counter electrode. The charge and discharge characteristics of the produced half cell were evaluated at 25°C using a secondary battery charge and discharge tester (manufactured by Hokuto Co., Ltd.). The cutoff voltage range was 0.005 to 1.5 V. The charge and discharge measurement results showed that the initial discharge capacity was 1180 mAh/g and the initial coulombic efficiency was 67.5%.

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 450 mAh / g. A non-aqueous electrolyte solution in which lithium hexafluorophosphate was dissolved at a concentration of 1 mol / L in a mixture of ethylene carbonate (hereinafter also referred to as "EC") and diethyl carbonate (hereinafter also referred to as "DEC") at a volume ratio of 1 / 1 was used as the non-aqueous electrolyte, and a laminated lithium ion secondary battery was prepared using a polyethylene microporous film with a thickness of 30 μm as the 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 4.2 V, and after reaching 4.2 V, the current was reduced to keep the cell voltage at 4.2 V and charging was performed, and the discharge capacity was obtained. 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 2.

Examples 2 and 3
A silicon oxide slurry in which silicon oxide was SiO-2 in Example 2 and SiO-3 in Example 3 was used was used, and active materials were obtained under the same conditions as in Example 1. The particle size and specific surface area of the obtained active materials were measured, and the charge/discharge performance in half cells and full cells was evaluated using the obtained active materials. The various evaluation results are shown in Table 2.

Examples 4 to 8
SiO-4 was used as silicon oxide, and the silicon oxide content was 5 mass% in Example 4, 10 mass% in Example 5, 30 mass% in Example 6, 50 mass% in Example 7, and 58 mass% in Example 8, and active material particles were obtained under the same conditions as in Example 1. The obtained active material particles were used to evaluate the charge/discharge performance in a half cell and a full cell. The obtained various evaluation results are shown in Table 2.

Examples 9 and 10
Using SiO-4 as silicon oxide, under the same conditions as in Example 1, a resin mixed suspension containing silicon oxide particles was desolvated under nitrogen follow conditions in an oil bath at 120 ° C., and then fired at high temperature. The pulverization conditions of the fired product were changed to obtain black powder, which was then put into a CVD device (desktop rotary kiln: manufactured by Takasago Kogyo Co., Ltd.) and a mixed gas of ethylene gas 0.3 L / min and nitrogen gas 0.7 L / min was introduced. In Example 9, carbon coating treatment was performed on the black powder surface by chemical vapor deposition at 850 ° C. for 1 hour, and in Example 10, at 850 ° C. for 2 hours, to produce active material particles of Examples 9 and 10. The particle size and specific surface area of the obtained active material were measured, and the charge and discharge performance in half cells and full cells was evaluated using the obtained active material particles. Various evaluation results are shown in Table 2.

Examples 11 to 13
The polysiloxane resin (curable resin composition (2)) having an average molecular weight of 3500 prepared in the synthesis example 1 and the phenolic resin having an average molecular weight of 3000 were mixed in a mass ratio of resin solids of 100/0 in Example 11, 50/50 in Example 12, and 30/70 in Example 13, and the slurry of SiO-4 obtained in synthesis example 2 was added so that the content of silicon oxide particles in the product after high-temperature baking was 50 mass%, and the resin mixed suspension containing silicon oxide particles was desolvated under nitrogen follow conditions in an oil bath at 120 ° C. The subsequent conditions were the same as in Example 1, and active material particles were prepared. The particle size and specific surface area of the obtained active material were measured, and the charge and discharge performance in half cells and full cells was evaluated using the obtained active material particles. Various evaluation results are shown in Table 2.

Example 14
Using SiO-5 as silicon oxide, active material particles were obtained under the same conditions as in Example 1. The obtained active material particles were used to evaluate the charge/discharge performance in a half cell and a full cell. The obtained evaluation results are shown in Table 2.

Examples 15 and 16
Silicon oxide slurry was used in which silicon oxide was SiO-6 in Example 15 and SiO-7 in Example 16, and a resin mixed suspension containing silicon oxide particles was mixed. LiCl was added as a raw material to the resin mixed suspension containing silicon oxide particles so that the molar ratio of Li ⁺ /silicon oxide relative to the amount of silicon oxide particles was 30/100 in Example 15 and 50/100 in Example 16, and the solvent was removed under nitrogen follow conditions in an oil bath at 120°C. The subsequent conditions were the same as in Example 1, and an active material powder was produced. The content of Li element in the obtained active material particles was 2.5 mass% in the example and 5.1 mass% in Example 16. The particle size and specific surface area of the obtained active material particles were measured, and the charge/discharge performance in half cells and full cells was evaluated using the obtained active material particles. Various evaluation results are shown in Table 2.

Example 17
Using SiO-7 as silicon oxide, a resin mixed suspension containing silicon oxide particles was desolvated under nitrogen flow conditions in an oil bath at 120°C under the same conditions as in Example 1, and the active material particles were obtained by firing at a high temperature of 1000°C for 4 hours in a nitrogen atmosphere. The active material particles obtained were subjected to powder X-ray diffraction (XRD) measurement using Cu-Kα radiation, and a diffraction peak at 2θ=28.4° assigned to the Si(111) crystal plane was detected. The particle size and specific surface area of the obtained active material particles were measured, and the charge/discharge performance in half cells and full cells was evaluated using the obtained active material particles. Various evaluation results are shown in Table 2.

Comparative Example 1
20 g of silicon oxide powder with an average particle size of 5 μm was put into a CVD apparatus (desktop rotary kiln: manufactured by Takasago Kogyo Co., Ltd.), and while introducing a mixed gas of ethylene gas 0.2 L/min and nitrogen gas 0.8 L/min, a carbon coating treatment was performed on the surface of the black powder by chemical vapor deposition at 850 ° C for 1 hour to prepare active material particles. The amount of carbon coating after the treatment was measured by a thermal analyzer, and it was found to have increased by 2.0% from the weight before the treatment. The obtained active material particles were subjected to powder X-ray diffraction (XRD) measurement using Cu-Kα rays, and a diffraction peak at 2θ = 28.4 ° assigned to the Si (111) crystal plane was not detected. The particle size and specific surface area of the obtained active material particles were measured, and the charge and discharge performance in half cells and full cells was evaluated using the obtained active material particles. Various evaluation results are shown in Table 2.

Comparative Example 2
The curable resin composition (1) prepared in Synthesis Example 1 was dried under reduced pressure at 110° C., and then baked at a high temperature of 1,100° C. for 4 hours in a nitrogen atmosphere to obtain a black solid. The obtained black solid was pulverized in a planetary ball mill to produce a black powder, which was then subjected to a carbon coating treatment under the same CVD conditions as in Comparative Example 1.
The particle size and specific surface area of the obtained black powder after the carbon coating treatment were measured, and the charge/discharge performance in a half cell and a full cell was evaluated using the black powder after the carbon coating treatment. The various evaluation results are shown in Table 2.

Comparative Example 3
Instead of silicon oxide particles, commercially available silicon particles (manufactured by Alfa Aesar) with an average particle size of 100 nm were used, and a resin mixed suspension containing silicon particles was subjected to desolvation under nitrogen flow conditions in an oil bath at 120°C under the same conditions as in Example 1, and then the mixture was baked at a high temperature of 1000°C for 4 hours in a nitrogen atmosphere to obtain a black solid. The obtained active material particles were subjected to powder X-ray diffraction (XRD) measurement using Cu-Kα radiation, and a diffraction peak at 2θ = 28.4° assigned to the Si (111) crystal plane was strongly detected. The particle size and specific surface area of the obtained active material particles were measured, and the charge/discharge performance in half cells and full cells was evaluated using the obtained active material particles. Various evaluation results are shown in Table 2.

Comparative Example 4
A phenolic resin having an average molecular weight of 3000 was mixed with the slurry of SiO-4 obtained in Synthesis Example 2 so that the content of silicon oxide particles in the product after high-temperature firing was 50% by mass, and the solvent was removed in an oil bath at 120°C under nitrogen flow conditions. Active material particles were obtained under the same conditions as in Example 1, except that CVD carbon coating was not performed. The charge/discharge performance in a half cell and a full cell was evaluated using the obtained active material particles. The various evaluation results obtained are shown in Table 2.

[Evaluation method]
In Table 2, the evaluation methods are as follows.
Average particle size D50: 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.

Raman scattering spectrum measurement: The measurement device used was an NRS-5500 (manufactured by JASCO Corporation). The measurement conditions were as follows: excitation laser wavelength 532 nm, objective lens magnification 100 times, and measurement wave number range 3500 to 100 cm ⁻¹ .
Powder X-ray diffraction (XRD): Measurements were carried out in air using an X-ray diffractometer (Rigaku Corporation, SmartLab).

Measurement of nitrogen content: An oxygen/nitrogen analyzer (EMGA-920) was used.
Measurement of carbon coating amount: The weight loss was measured in air using a thermal analyzer (Thermo Plus EVO2, manufactured by Rigaku Corporation) and calculated.

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 times) 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 300 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 (% @ 300th) = 300th negative electrode discharge capacity (mAh/g) / negative electrode initial discharge capacity (mAh/g), measured by full cell (laminated cell). Negative electrode expansion rate: After 300 cycles of charging and discharging the full cell, the negative electrode was removed, washed with EC/DEC mixed solution, and then left to dry, and 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 the negative electrode expansion rate.

As is clear from the above results, when this active material is used as a negative electrode active material, a high capacity is maintained, the expansion rate of the negative electrode is low, the cycleability (or capacity retention rate) and the initial coulombic efficiency are both high, and the balance of these secondary battery properties is excellent. Furthermore, a secondary battery containing this active material as a negative electrode active material has excellent battery properties.

Claims (11)

Composite particles having a silicon oxycarbide phase and at least two silicon oxide particles in the silicon oxycarbide phase, the content of the silicon oxide particles being 1% by mass or more and 60% by mass or less,
the silicon oxycarbide phase has a silicon-oxygen-carbon skeletal structure and free carbon composed only of carbon element;
The active material for a secondary battery has, in a Raman spectrum, scattering peaks at about 1590 cm ⁻¹ assigned to a G band and at about 1330 cm ⁻¹ assigned to a D band of a carbon structure derived from free carbon composed only of carbon elements , and an intensity ratio of the scattering peaks, I (G band)/I (D band), of 0.7 or more and 2 or less. The active material for a secondary battery according to claim 1 , wherein the free carbon contains a carbon component derived from a carbon source resin. The active material for secondary batteries according to claim 1, which has a peak at 2θ of around 28.4° in X-ray diffraction, which is attributed to Si(111). The active material for a secondary battery according to claim 1, wherein the average particle size of the silicon oxide particles is 5 μm or less. The active material for a secondary battery according to claim 1, wherein the average particle size of the composite particles is 2 μm or more and 15 μm or less. 2. The active material for a secondary battery according to claim 1, which has a specific surface area of 0.3 m ² /g or more and 10 m ² /g or less. The active material for a secondary battery according to claim 1, wherein the silicon oxycarbide phase further contains nitrogen atoms. The active material for a secondary battery according to claim 1, wherein the surface of the composite particles has a carbon coating, and the amount of the carbon coating is 1% by mass or more and 10% by mass or less. The active material for a secondary battery according to claim 1, which contains a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al. A secondary battery comprising the active material for secondary batteries according to claim 1 in the negative electrode. The method for producing an active material for a secondary battery according to claim 1, comprising the steps of: mixing a slurry of silicon oxide particles with a mixture of a polysiloxane compound and a carbon source resin, removing the solvent to obtain a precursor; and calcining the precursor to obtain a calcined product.