JP7491482B2 - Active material for secondary battery, method for producing 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 method for producing the same. The present invention also relates to 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 porous silicon-containing carbon-based composite material obtained by carbonizing metallic silicon or a silicon-containing compound and a silicon-free organic compound having a softening point or melting point together in an inert gas or in vacuum at a specific temperature.

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 particle destruction due to this volume change, Patent Document 3 discloses a structure that has a coating layer containing silicon, oxygen, and carbon, even without silicon nanoparticles, and has a specific BET specific surface area and particle size that satisfy specific conditions.

WO2008/081883 JP 2015-18626 A WO2020/179409

However, even when the material disclosed in the above document is used as the negative electrode for a secondary battery, the cycleability is still insufficient. Therefore, there is a need for further improvement in the cycleability and initial efficiency of lithium secondary batteries using silicon oxide, which has a large electric 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 having a core-shell composite structure with a silicon oxide-containing particle as a core and a silicon oxycarbide-containing phase as a shell layer.
[2] The active material for a secondary battery according to [1], wherein the shell layer has an average thickness of 5 nm to 500 nm.
[3] The active material for a secondary battery according to either [1] or [2], wherein the silicon oxide-containing particles have an average particle size of 1 μm or more and 20 μm or less.
[4] The active material for a secondary battery according to any one of [1] to [3] above, which has a peak at 2θ of about 28.4° in X-ray diffraction, which is attributed to Si(111).
[5] The active material for a secondary battery according to any one of [1] to [4] above, which has a specific surface area of 0.3 m ² /g or more and 10 m ² /g or less.
[6] The active material for a secondary battery according to any one of [1] to [5], wherein the silicon oxycarbide-containing phase further contains nitrogen atoms.
[7] The active material for a secondary battery according to any one of [1] to [6], which has scattering peaks at about 1590 cm ⁻¹ and 1330 cm ⁻¹ which belong to the G band and D band of a carbon structure in a Raman spectrum, and the intensity ratio I (G band)/I (D band) of these scattering peaks is 0.7 to 2.
[8] The active material for a secondary battery according to any one of [1] to [7], wherein a carbon coating is provided on a surface of the core-shell composite structure, and the amount of the carbon coating is 1 mass % or more and 10 mass % or less, with the total mass of the core-shell composite structure being 100 mass %.
[9] The active material for a secondary battery according to any one of [1] to [8] 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.
[10] A method for producing an active material for a secondary battery according to any one of [1] to [9] above, comprising the following steps (1) to (3):
(1) a step of obtaining a precursor for preparing the shell layer, (2) a step of applying the precursor for preparing the shell layer to the surface of the silicon oxide-containing particle and drying the precursor, and (3) a step of calcining at a high temperature in an inert gas atmosphere at a calcination temperature of 900° C. to 1300° C. to obtain a negative electrode active material. [11] The method for producing an active material for a secondary battery according to the above [10], further comprising the following step (4):
(4) A step of coating the negative electrode active material with a carbon film in a chemical vapor deposition apparatus at a temperature range of 700° C. to 1000° C. in a flow of a pyrolytic carbon source gas and a carrier inert gas to obtain a negative electrode active material.
[12] A secondary battery comprising the active material for secondary batteries according to any one of [1] to [9] 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") has a core-shell composite structure in which a silicon oxide-containing particle serves as a core (hereinafter also referred to as "the core") and a silicon oxycarbide-containing phase serves as a shell layer (hereinafter also referred to as "the shell layer").
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. By using the former as the core and the latter as the shell layer, it is thought that a secondary battery active material has been obtained that provides a secondary battery that maintains high capacity and has excellent volume expansion and cycle characteristics.

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.

The silicon oxide-containing particles may contain other components in addition to the silicon oxide, such as metal tin, tin oxide, germanium, germanium oxide, and the like.
In addition, silicon oxide may generate zero-valent silicon by disproportionation reaction, and the silicon oxide-containing particles may contain zero-valent silicon generated by disproportionation from the viewpoint of increasing the capacity and initial efficiency of the present active material obtained. When the silicon oxide-containing particles contain zero-valent silicon, 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.
Although zero-valent silicon may be added separately to the silicon oxide-containing particles, it is preferable that the zero-valent silicon contained in the silicon oxide-containing particles is zero-valent silicon produced by the disproportionation reaction, since the crystallites of zero-valent silicon are relatively small.

If the average particle size of the silicon oxide-containing particles is too large, the silicon oxide-containing particles will become large lumps, and when this active material is used as a negative electrode active material, the silicon oxide-containing particles will cause large expansion and contraction of the negative electrode active material during charging and discharging. As a result, stress will be concentrated in a part of the matrix, which will easily cause the structure of the active material to collapse, and the capacity retention rate of the negative electrode active material will tend to decrease. On the other hand, if the average particle size of the silicon oxide-containing particles is too small, the silicon oxide-containing particles will easily aggregate with each other. This may reduce the dispersibility of the silicon oxide-containing particles in the negative electrode active material. In addition, if the silicon oxide-containing particles are too fine, their specific surface area will be high, and there will be a tendency for by-products and the like to increase on the surface of the silicon oxide-containing particles during high-temperature sintering 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-containing particles serving as the core is preferably 20 μm or less, more preferably 15 μm or less, and from the viewpoints of particle dispersibility and specific surface area, the average particle size of the silicon oxide-containing particles is preferably 1 μm or more, more preferably 2 μm 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 silicon oxide-containing particles can be prepared, for example, by pulverizing silicon oxide so as to have an average particle size within the above 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-containing 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-containing particles fall within the above range.

The shape of the silicon oxide-containing particles may be any of granular, acicular, and flake-like.
The morphology of the silicon oxide-containing particles can be determined by measuring the average particle size using a dynamic light scattering method, but the aspect ratio of a 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, 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 can be calculated from 50 particles in the main part of the sample within the field of view of the TEM image.

The shell layer is made of a silicon oxycarbide-containing phase. Silicon oxycarbide is composed of a compound 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 the carbon phase, and carbon that is bonded to the silicon-oxygen-carbon skeleton and the carbon phase.
The silicon oxycarbide-containing phase may contain titanium oxide, lithium titanate, aluminum oxide, transition metal oxides, and the like in addition to silicon oxycarbide.

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 silicon oxycarbide-containing phase has a structure containing a three-dimensional network of silicon-oxygen-carbon skeleton and free carbon, the silicon-oxygen-carbon skeleton in the silicon oxycarbide phase has high chemical stability, and the structure containing free carbon results in small volume changes with respect to the absorption and release of lithium. When the silicon oxide-containing particles are at least partially coated with a structure containing a silicon-oxygen-carbon skeleton and free carbon, volume changes of the silicon oxide-containing particles with respect to the absorption and release of lithium are suppressed. As a result, when this active material is used as a negative electrode, the silicon oxide-containing particles in the negative electrode play a role as a main component for the expression of charge and discharge performance, while the silicon oxycarbide-containing phase suppresses particle destruction associated with volume changes of the silicon oxide-containing particles during charge and discharge, improving the cycleability of the lithium secondary battery.

Furthermore, when the compound that constitutes the silicon oxycarbide-containing 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 silicon oxycarbide 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-containing phase contains nitrogen, the present active material tends to have excellent charge/discharge performance and capacity retention when used as a negative electrode active material.
When the compound constituting the silicon oxycarbide-containing phase is a compound containing silicon, oxygen, carbon and nitrogen, the silicon oxycarbide-containing 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-containing 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 has a core-shell composite structure having the core and the shell layer. The core-shell composite structure is a structure in which the surface of the core is covered with the shell. The shell layer does not need to cover the entire surface of the core, but only needs to cover at least a part of the core. The coverage of the shell layer is preferably 70% or more, more preferably 80% or more, and particularly preferably 100%. The coverage is determined by SEM observation and EDS element mapping.
The thickness of the shell layer is preferably from 5 nm to 500 nm, and more preferably from 10 nm to 400 nm, from the viewpoint of the balance between the charge/discharge capacity, the initial coulombic efficiency, and the cycle characteristics.

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 1 μm or more and 30 μm or less. The average particle size of the present active material is more preferably 2 μm or more, and particularly preferably 3 μm or more. The average particle size of the present active material is more preferably 25 μm or less, and particularly preferably 20 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 1.0 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 within 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 the silicon oxycarbide-containing phase. When the active material has free carbon in the silicon oxycarbide-containing phase, a scattering peak at 1590 cm ⁻¹ attributed to the G band of the graphite long-period carbon lattice structure and a scattering peak at about 1330 cm ⁻¹ attributed to the D band of the graphite short-period carbon lattice structure having disorder or defects 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 to 2. The scattering peak intensity ratio, I (G band) / I (D band), is more preferably 0.7 to 1.8. 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-containing phase. Specifically, carbonizable sites in the molecular structures of the silicon-containing compound and the carbon source resin become carbon components through 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 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, the carbon in the silicon oxycarbide-containing 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 core-shell composite structure, the coating being preferably made of a material that is expected to have electron 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 an amorphous 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 amount of the carbon coating is 1 to 10 mass % with the total mass of the core-shell composite structure being 100 mass %.
The carbon coating is preferably formed on the surface of the core-shell composite structure by vapor deposition.
The total mass of the core-shell composite structure is the total mass of the core and shell layer constituting the active material. In the case where the silicon oxycarbide-containing layer phase is the shell layer containing nitrogen, the total mass includes nitrogen. In the case where the core-shell composite structure contains a third component described below, the total mass includes the third component.

When a silicon-containing active material is used as a negative electrode active material, lithium reacts with silicon oxide present in the silicon-containing active material during initial charging to produce lithium oxide. The lithium oxide produced cannot be reversibly returned to the positive electrode during discharge. It is believed that this irreversible reaction results in the loss of lithium, resulting in a decrease in the initial charge/discharge efficiency.
In order to inhibit the production of such lithium oxides, the present active material may contain a third component other than the silicon oxide-containing particles and the silicon oxycarbide-containing phase.

Such a third component includes 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 an ABO type oxide 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 an ABO type oxide 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 ( _{Li2SiO3 , Li2Si2O5 , Li4SiO4} ) or magnesium metasilicate (MgSiO3 _{, Mg2SiO4} ), _and particularly preferably magnesium metasilicate ( _{MgSiO3 , Mg2SiO4} ).

When the present active material contains the metal silicate compound, the metal silicate compound is preferably distributed throughout the core-shell structure, and more preferably distributed uniformly.
The presence of the metal silicate compound in the core-shell structure can be confirmed by a high-resolution transmission electron microscope (hereinafter, also referred to as "HR-TEM"). In detail, the active material can be sliced by a focused ion beam (FIB) and observed with an HR-TEM.
If the metal silicate compound is in a crystalline state, it can be detected by powder X-ray diffraction measurement (XRD), and if it is amorphous, it can be confirmed by solid-state ²⁹ Si-NMR measurement.

When the present active material has the metal silicate compound and a surface coating, the present active material preferably has a multilayer composite structure in which, from the inside, the present core made of silicon oxide and the metal silicate compound, the present shell layer, and the surface coating are arranged in this order.
The surface coating is preferably an amorphous carbon coating.

The active material can be obtained, for example, by a manufacturing method (hereinafter also referred to as the "manufacturing method") including the following steps (1) to (3): (1) a step of obtaining a precursor for producing the shell layer (hereinafter also referred to as the "precursor") (hereinafter also referred to as the "precursor preparation step"); (2) a step of applying the precursor to the surface of the silicon oxide-containing particles and drying it (hereinafter also referred to as the "application and drying step"); and (3) a step of obtaining the active material by high-temperature firing at a firing temperature of 900°C to 1300°C in an inert atmosphere (hereinafter also referred to as the "firing step").

The present precursor is a compound that is applied to the surface of a silicon oxide-containing particle, dried, and then fired to become the present shell layer, and is a compound that contains the above-mentioned silicon oxycarbide.
The precursor may be, for example, a mixture of a polysiloxane compound and a carbon source resin. The precursor may contain components contained in the silicon oxycarbide-containing phase other than the polysiloxane compound and the carbon source resin, as necessary.

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. It may be a resin containing only these structures, or 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, there is a composite resin having a graft structure in which a polysiloxane segment is chemically bonded to the side chain of a polymer segment, and a composite resin having a block structure in which a polysiloxane segment is chemically bonded to the end of a polymer segment.

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 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.

なお構造式（Ｓ－３）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 structural formula (S-3), 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).

一般式（Ｓ－４）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 general formula (S-4), ^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 (a) to (c) below.

(a) 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.

(b) 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 condensation reaction. The polymer segment and the polysiloxane are then mixed together to carry out a hydrolysis condensation reaction.

(c) 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.).

In this precursor preparation process, the polysiloxane compound and the carbon source resin are mixed to produce the precursor. A stirrer, ultrasonic mixer, premix disperser, etc. are used for mixing. The precursor may also be a slurry in which the mixture of the polysiloxane compound and the carbon source resin is dispersed in a solvent, or a solution in which the mixture is dissolved. The solvent used may be water or an organic solvent.

Examples of organic 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.

When the mixture of the polysiloxane compound and the carbon source resin is made into a slurry, a dispersant may be added to the slurry. The type of dispersant includes aqueous and non-aqueous dispersants, and non-aqueous dispersants are preferred.
Examples of the types of non-aqueous dispersants include polymeric types such as polyethers, alcohols, polyalkylene polyamines, and polycarboxylic acid partial alkyl esters; low molecular types such as polyhydric alcohol esters and alkyl polyamines; and inorganic types such as polyphosphates.

When the precursor is a slurry or a solution, it is preferable that the polysiloxane compound and the carbon source resin are in a uniformly mixed state. 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.
The concentration of the mixture of the polysiloxane compound and the carbon source resin in the slurry or solution of the mixture of the polysiloxane compound and the carbon source resin is not particularly limited, but the amount of the mixture of the polysiloxane compound and the carbon source resin is preferably in the range of 70% by mass to 99% by mass, and more preferably 80% by mass to 95% by mass, where the total amount of the solvent and, if necessary, a dispersant, and the mixture of the polysiloxane compound and the carbon source resin is 100% by mass.

In the present coating and drying step, the present precursor obtained in the present precursor preparation step is first coated on the surface of silicon oxide-containing particles.
As described above, the silicon oxide-containing 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.
Silicon oxide may be crushed, classified, etc. to obtain silicon oxide-containing particles having a desired average particle size. The crushing and classification methods are as described above.

Examples of the application method include, when the precursor is a slurry or solution, spraying the precursor and silicon oxide-containing particles, applying the precursor with a roll or brush, or immersing the precursor in the solution.
When the present precursor does not contain the solvent, examples of the method include a method of immersing the present precursor in a slurry or solution of silicon oxide-containing particles, or a method of applying the present precursor onto silicon oxide-containing particles by vapor deposition.

Among the above methods, it is preferable to use a slurry as the present precursor and to immerse and mix silicon oxide-containing particles in the slurry of the present precursor, or to mix a slurry of silicon oxide-containing particles with the present precursor in slurry form.
The mixing is preferably carried out using a device having the functions of dispersion and mixing, such as a stirrer, ultrasonic mixer, premix disperser, etc.

The silicon oxide-containing particle slurry can be prepared by grinding the 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-containing particles. Examples of wet grinding devices include roller mills, high-speed rotary grinders, container-driven mills, and bead mills.

The organic solvent may be, for example, the same as the solvent described above, 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.

In addition, when the silicon oxide-containing particles are made into a slurry, a dispersant may be added. As with the above, the type of dispersant may be an aqueous or non-aqueous dispersant, with non-aqueous dispersants being preferred. Examples of the type of non-aqueous dispersant include polymeric types such as polyethers, alcohols, polyalkylene polyamines, and polycarboxylic acid partial alkyl esters, low molecular types such as polyhydric alcohol esters and alkyl polyamines, and inorganic types such as polyphosphates.
The concentration of the solid content of the silicon oxide-containing particles in the slurry of silicon oxide-containing particles is not particularly limited, but when the above-mentioned solvent and, if necessary, a dispersant are contained, the amount of silicon oxide is preferably in the range of 5 mass% to 40 mass%, and more preferably 10 mass% to 30 mass%, where the total amount of the dispersant and silicon oxide is 100 mass%.

The precursor is applied to the surface of silicon oxide-containing particles by the above-mentioned method, and the silicon oxide-containing particles to which the precursor is applied are dried.
When a slurry of silicon oxide-containing particles or a slurry of the present precursor is used for coating, it is preferable to dry the material until the solvent used is removed.
For the drying to remove the solvent, a dryer, a reduced pressure dryer, a spray dryer, or the like can be used.
When the solvent is removed by drying, the solvent may be completely removed, or the product may be subjected to the firing step described below with a small amount of the solvent remaining.

The dried product obtained in the coating and drying process preferably contains 20% to 90% by mass of the silicon oxide-containing particles, 5% to 30% by mass of the solid content of the polysiloxane compound, and 0% to 20% by mass of the solid content of the carbon source resin, and more preferably contains 30% to 85% by mass of the solid content of the silicon oxide-containing particles, 10 to 25% by mass of the solid content of the polysiloxane compound, and 0% to 10% by mass of the solid content of the carbon source resin.

In the firing process, the dried material obtained in the coating and drying process is fired at a high temperature of 900°C to 1300°C in an inert atmosphere. In the firing process, the thermally decomposable organic components are completely decomposed, and the active material is obtained. By setting the firing temperature at 900°C to 1300°C, the thermally decomposable organic components can be completely decomposed. In addition, the polysiloxane compound and the carbon source resin are converted by the energy of the high-temperature treatment into a silicon oxycarbide-containing 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 firing temperature is the maximum temperature set in the firing process, and it has a strong influence on the structure and performance of the fired product. The firing temperature allows precise control of the microstructure of this active material, which retains the chemically bonded state of silicon and carbon in the silicon oxycarbide-containing phase, resulting in better charge/discharge characteristics.

The calcination method is not particularly limited, but it is sufficient to use a reaction device having a heating function in an inert atmosphere, and treatment can be performed by a continuous method or a batch method. The calcination device can be appropriately selected from a fluidized bed reaction furnace, a rotary furnace, a vertical moving bed reaction furnace, a tunnel furnace, a batch furnace, a rotary kiln, etc. depending on the purpose.
The inert gas may be nitrogen, helium, argon, or the like.

The active material obtained in the firing process may be pulverized and classified as necessary. The pulverization may be performed in one step to the desired particle size, or may be performed in several steps. For example, when lumps or aggregates of 10 mm or more are to be made into particles of about 10 μm, the particles are coarsely pulverized with a jaw crusher, roll crusher, etc. to about 1 mm, then crushed to about 100 μm with a glow mill, ball mill, etc., and crushed 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 dried product obtained in the coating and drying process is controlled to a shape close to the target particle size by spray drying, etc. before the firing process, and is subjected to the firing process in that shape, it is possible to omit the pulverization process.

When the present active material has the metal silicate compound, a salt of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al is added to a slurry obtained by mixing a slurry of silicon oxide particles with the present precursor, and after drying, the present active material having the silicate compound is obtained in a main firing process.
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 when the metal salt is added to the slurry of the silicon oxide-containing particles and the present precursor is preferably 0.01 to 0.5 in terms of molar ratio to the number of moles of silicon oxide in the silicon oxide-containing 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 slurry of the silicon oxide-containing particles and the precursor, and then 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 slurry of the silicon oxide-containing particles and the precursor, and then 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.

When the core-shell composite has the silicate compound, the metal salt can be uniformly dispersed in the slurry of the silicon oxide-containing particles and the precursor to obtain the core-shell composite having the silicate compound. Under the condition that the slurry of the precursor and the slurry of the silicon oxide-containing particles react with the metal salt in a solid phase, the metal salt molecules are sufficiently contacted with the silicon oxide particles and the silicon oxycarbide to make the silicate compound exist in the composite structure.
In addition, the metal salt molecules can be surface-modified using an organic additive, and can be attached near the surface of silicon oxide-containing particles. The molecular structure of the organic additive is not particularly limited, but it is preferable that the molecular structure can be physically or chemically bonded with the dispersant present on the surface of the silicon oxide-containing particles. The physical or chemical bond can be electrostatic action, hydrogen bond, intermolecular van der Waals force, ionic bond, covalent bond, etc. During high-temperature firing, the metal salt molecules can undergo solid-phase reaction with the _SiO2 phase in silicon oxide to form a metal silicate compound in the silicon oxide-containing particles.

When the active material has a coating of amorphous carbon on the surface of the core-shell composite structure, the production method may include, after the main firing step, (4) a step of coating with a carbon coating in a chemical vapor deposition apparatus at a temperature range of 700° C. to 1000° C. in a flow of a pyrolytic carbon source gas and a carrier inert gas to obtain a negative electrode active material.
Examples of the pyrolytic carbon source gas include propane, acetylene, ethylene, acetone, etc. Examples of the inert gas include nitrogen, helium, argon, etc., 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.
When the present active material is used as a negative electrode active material, the present active material may be used alone, or a mixed active material of the present active material with another active material may be used, or the present active material may be used as a composite active material in which the present active material is dispersed in a matrix phase.

Examples of other active materials include silicon oxide particles, graphite, silicon particles, metal tin particles, and tin oxide.
The matrix phase is a material capable of absorbing and releasing lithium ions. The material capable of absorbing and releasing lithium ions is a material that can absorb lithium ions into the matrix phase when the battery is charged and release lithium ions from the matrix phase when the battery is discharged. The above-mentioned cycle of absorption and release is repeated in the lithium secondary battery.
Examples of materials capable of absorbing and releasing lithium ions include graphite, silicon oxide, titanium oxide, tin 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.
The silicon, oxygen and carbon-containing compound may be the silicon oxycarbide contained in the silicon oxycarbide-containing phase, and the preferred silicon oxycarbide is the same as that described above. The silicon oxycarbide may contain a nitrogen atom, and the preferred compound when it contains nitrogen is the same as that described above.

A slurry containing the present active material, a mixed active material or a composite active material containing the present active material, an organic binder, and other components such as a conductive assistant as required can be applied to a copper foil collector in the form of a thin film to form a negative electrode. A carbon material such as graphite can also be added to the slurry containing the present active material 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 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 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.

Although the present active material, the secondary battery containing the present active material in the negative electrode, and the method for producing the present active material 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.
In the method for producing the active material of the present invention, any other step may be added or may be replaced with any other step that exhibits a similar function.

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: Preparation of Polysiloxane Compound Synthesis Example 1 (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)].

Synthesis Example 2 (Production of Curable Resin Composition (1))
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 (a2-1) 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.

Synthesis Example 3 (Production of Curable Resin Composition (2))
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 BuOH, 249 parts by mass of PTMS and 263 parts by mass of DMDMS, and the temperature was raised 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.

Example 1
The polysiloxane resin (curable resin composition (1)) having an average molecular weight of 3500 prepared in the above Synthesis Example 2 and the phenolic resin having an average molecular weight of 3000 were mixed in an isopropyl alcohol solvent so that the weight ratio of the resin solids was 90/10, and stirred to prepare 100 parts of a mixed solution with a solid concentration of 3 mass%. After preparation, 100 parts of silicon oxide particles (commercially available product) having an average particle size of 3 μm were added and thoroughly mixed in a stirrer. The obtained resin mixture slurry containing silicon oxide particles was spray-dried with a small spray dryer (nitrogen circulation). A black solid was obtained by high-temperature firing at 950 ° C. for 4 hours in a nitrogen atmosphere, and a black powder was prepared by crushing with a planetary ball mill.
20 parts of the obtained black powder were placed in a CVD apparatus (desktop rotary kiln: manufactured by Takasago Industrial Co., Ltd.), and while introducing a mixed gas of ethylene gas at 0.2 L/min and nitrogen gas at 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 produce active material particles.

When the cross section of the black powder before the carbon coating treatment was observed with an FE-SEM, it was confirmed that a thin film of silicon oxy-covered phase with a thickness of about 25 nm was formed as a shell layer on the surface of the silicon oxide core particle. The active material particles after the carbon coating treatment had an average particle size of 3.0 μm, a specific surface area of 3.4 m ² /g, and a carbon coating amount of 2.1% according to carbon thermal analysis.
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) measurement showed that the nitrogen content was 0.2% by mass. In addition, the Raman scattering analysis measurement 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 an initial discharge capacity of 1587 mAh/g and an initial coulombic efficiency of 74.9%.

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 45 ° 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 91%, with one cycle being charge and discharge within a voltage range of 2.5 V to 4.2 V. The results are shown in Table 1.

Examples 2 to 5
Similarly to Example 1, the polysiloxane resin (curable resin composition (1)) having an average molecular weight of 3500 prepared in Synthesis Example 2 and the phenolic resin having an average molecular weight of 3000 were mixed and stirred in an isopropyl alcohol solvent so that the weight ratio of the resin solids was 90/10, and the solid concentration was changed to 6% by mass in Example 2, 12% by mass in Example 3, 30% by mass in Example 4, and 50% by mass in Example 5 to obtain a mixed solution. After preparing 100 parts of the mixed solution, 100 parts of silicon oxide (commercially available product) having an average particle size of 3 μm was added, and a slurry was prepared and spray-dried. An active material powder having a carbon coating was obtained under the same conditions as in Example 1. The particle size and specific surface area were measured, and the charge/discharge performance in a half cell and a full cell was evaluated using the obtained active material powder. Various evaluation results are shown in Table 1.

Examples 6 to 9
The polysiloxane resin (curable resin composition (1)) and the phenolic resin shown in Example 1 were mixed and stirred in an isopropyl alcohol solvent so that the weight ratio of the resin solid was 90/10, so that the thickness of the silicon oxy-covered phase on the silicon oxide particle surface after high-temperature baking was 103 nm. A mixed solution was prepared. 100 parts of the prepared mixed solution were added to 100 parts of silicon oxide particles having an average particle size of 1.5 μm in Example 6, 5 μm in Example 7, 11 μm in Example 8, and 16 μm in Example 9 to prepare a slurry. After spray drying, an active material powder having a carbon coating was obtained under the same conditions as in Example 1. The particle size and specific surface area were measured, and the charge/discharge performance in half cells and full cells was evaluated using the obtained active material powder. Various evaluation results are shown in Table 1.

Examples 9 to 12
As in Example 3, the polysiloxane resin (curable resin composition (1)) having an average molecular weight of 3500 prepared in Synthesis Example 2 and the phenolic resin having an average molecular weight of 3000 were mixed and stirred in an isopropyl alcohol solvent so that the weight ratio of the resin solids was 100/0 in Example 10, 50/50 in Example 11, and 30/70 in Example 12. A mixed solution with a solid concentration of 12% by mass was prepared. 100 parts of silicon oxide (commercial product) having an average particle size of 3 μm was added to 100 parts of the prepared mixed solution, and a slurry was prepared and spray-dried. The remaining operations were the same as in Example 1 to obtain an active material powder having a carbon coating. The particle size and specific surface area were measured, and the charge/discharge performance in a half cell and a full cell was evaluated using the obtained active material powder. Various evaluation results are shown in Table 1.

Examples 13 and 14
The polysiloxane resin (curable resin composition (2)) having an average molecular weight of 3500 prepared in the synthesis example 3 and the phenolic resin having an average molecular weight of 3000 were mixed with an isopropyl alcohol solvent and stirred so that the weight ratio of the resin solid was 90/10, to prepare a mixed solution having a solid concentration of 12% by mass. 100 parts of silicon oxide particles (commercially available product) having an average particle size of 3 μm were added to 100 parts of the prepared mixed solution, and thoroughly mixed in a stirrer. The resin mixed slurry containing the obtained silicon oxide particles was spray-dried with a small spray dryer (nitrogen circulation). In a nitrogen atmosphere, a black solid was obtained by high-temperature firing under conditions of 1050 ° C. for 4 hours in Example 13 and 1100 ° C. for 4 hours in Example 14. The obtained black solid was crushed with a planetary ball mill to prepare a black powder. The remaining operations were the same as those in Example 1 to obtain an active material powder having a carbon coating. The particle size and specific surface area were measured, and the charge/discharge performance in a half cell and a full cell was evaluated using the obtained active material powder. The results of various evaluations are shown in Table 1.

Examples 15 and 16
A slurry containing silicon oxide particles was prepared in the same manner as in Example 3, and then LiCl was added as a raw material to the slurry 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 spray drying was performed. The subsequent conditions were the same as in Example 1, and an active material powder having a carbon coating was produced. The content of Li element in the obtained active material particles was 2.5 mass% in Example 15 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 1.

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. When the amount of carbon coating after the treatment was measured by a thermal analyzer, it was found to have increased by 2.0% from the weight before the treatment. As a result of measuring the powder X-ray diffraction (XRD) of the obtained active material particles using Cu-Kα rays, 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 1.

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 carbon-coated black powder were measured, and the charge-discharge performance in a half cell and a full cell was evaluated using the carbon-coated black powder. The various evaluation results are shown in Table 1.

Comparative Example 3
Instead of silicon oxide particles, silicon particles (commercially available) were classified to an average particle size of 3 μm, and a slurry containing silicon particles was prepared under the same conditions as in Example 3, and spray-dried. After drying, the slurry was fired 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°, which is attributed 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 1.

[Evaluation method]
In Table 1, the evaluation methods are as follows.
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 performed in air at room temperature using an X-ray diffractometer (Rigaku Corporation, SmartLab).

Measurement of nitrogen content: An oxygen/nitrogen analyzer (EMGA-920) was used.
Measurement of thickness of silicon oxycarbide-containing phase: The cross-sectional structure of the grain was processed using a cross-section polisher (CP method) and was measured by FE-SEM observation.
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, in addition to maintaining a high capacity, the expansion rate of the negative electrode is low, and both the cycleability (or capacity retention rate) and the initial coulombic efficiency are high, and the balance of these secondary battery properties is excellent. Furthermore, secondary batteries containing this active material as a negative electrode active material have excellent battery properties.

Claims (10)

An active material for a secondary battery, comprising a core-shell composite structure having a silicon oxide-containing particle as a core and a silicon oxycarbide-containing phase as a shell layer, the shell layer having an average thickness of 5 nm to 500 nm, and having scattering peaks at about 1590 cm ^-1 and 1330 cm ^-1 which belong to a G band and a D band of a carbon structure derived from free carbon composed only of carbon element in a Raman spectrum, and an intensity ratio I(G band)/I(D band) of these scattering peaks being 0.7 to 2. The active material for secondary batteries according to claim 1, wherein the average particle size of the silicon oxide-containing particles is 1 μm or more and 20 μm or less. 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). 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-containing phase further contains nitrogen atoms. The active material for secondary batteries according to claim 1, wherein the surface of the core-shell composite structure has a carbon coating, and the amount of the carbon coating is 1% by mass or more and 10% by mass or less, with the total mass of the core-shell composite structure being 100% by mass. The active material for secondary batteries according to claim 1, wherein the core-shell composite structure has a silicate compound of at least one metal selected from the group consisting of Li, K, Na, Ca, Mg and Al. 1. A method for producing an active material for a secondary battery, the active material having a core-shell composite structure having a silicon oxide-containing particle as a core and a silicon oxycarbide-containing phase as a shell layer, the shell layer having an average thickness of 5 nm to 500 nm, the method comprising the steps of:
(1) a step of obtaining a precursor for preparing the shell layer; (2) a step of applying the precursor for preparing the shell layer to the surface of the silicon oxide-containing particles and drying the same; and (3) a step of calcining the precursor at a high temperature in an inert gas atmosphere at a calcination temperature of 900° C. to 1300° C. to obtain an active material for a secondary battery. The method for producing an active material for a secondary battery according to claim 8, further comprising the following step (4):
(4) In a chemical vapor deposition apparatus, in a flow of a pyrolytic carbon source gas and a carrier inert gas,
A process of obtaining a negative electrode active material by coating with a carbon film at a temperature range of 700°C to 1000°C. A secondary battery comprising the active material for secondary batteries according to claim 1 in the negative electrode.