JP7632757B2 - Composite particles, method for producing composite particles, negative electrode active material for secondary battery, negative electrode and secondary battery - Google Patents

Description

The present invention relates to composite particles, a method for producing the composite particles, a negative electrode active material for a secondary battery comprising the composite particles, a negative electrode comprising the negative electrode active material, and a secondary battery comprising the negative electrode.

In response to environmental concerns about energy demand, the use of lithium-ion batteries (LIBs) is expanding as they are rapidly deployed in electric vehicles (EVs). Graphite, the mainstream LIB anode material, has a low theoretical capacity density (372 mAh/g), so there has been active development of high-capacity active materials using silicon, tin, and oxides to improve battery capacity. However, these materials have a problem in that the volume expands and contracts significantly as they absorb and release lithium ions, and the active material becomes pulverized with repeated charging and discharging, deteriorating the charge and discharge characteristics.

Patent Document 1 describes a negative electrode active material comprising a carbonaceous matrix containing a plurality of nanoparticles, the nanoparticles including a silicon core, an oxide film layer containing SiOx (0<x≦2) disposed on the silicon core, and a coating layer containing LiF covering at least a portion of the surface of the oxide film layer.

Patent Document 2 describes a negative electrode active material for a non-aqueous electrolyte secondary battery, which has negative electrode active material particles, the negative electrode active material particles containing silicon compound (SiOx: 0.5≦x≦1.6) particles, at least a portion of the negative electrode active material particles being coated with a carbon material and containing one or more types selected from Li ₂ SiO ₃ and Li ₂ Si ₂ O ₅ , and when the negative electrode active material particles are measured by X-ray diffraction using Cu-Kα rays, the intensity Ia of a peak attributable to a Si crystal structure obtained by the X-ray diffraction at about 2θ=28.4°, the peak intensity Ib of a peak attributable to Li ₂ SiO ₃ obtained by the X-ray diffraction, and the peak intensity Ic of a peak attributable to Li ₂ Si ₂ O ₅ obtained by the X-ray diffraction satisfy Ib/Ia≦4.8 and Ic/Ia≦6.0.

Patent Document 3 describes a Si-O-C-Li composite comprising nanosilicon, a silicon-oxygen-lithium compound, and a carbon coating, the silicon-oxygen-lithium compound being partially crystallized, the composite further comprising silicon oxide, the composite comprising nanosilicon dispersed in the silicon-oxygen-lithium compound to form fused particles, the fused particles dispersed in a sea-island format in a matrix of silicon oxide to form composite particles, and a structure in which the carbon coating is coated on the surfaces of the composite particles.

Japanese Patent Application Publication No. 2022-125285 Japanese Patent Application Publication No. 2020-113495 Japanese Patent No. 7094957

However, the negative electrode active material described in the above Patent Document 1 is made by coating Si particles with LiF, which may not be able to withstand the silicon expansion that accompanies the absorption and release of lithium ions. In order to suppress such Si expansion, it is thought that it is necessary to provide a structure such as a matrix phase.

In the negative electrode active material of Patent Document 2, Li needs to be sufficiently diffused to the inside to improve performance, but this requires an increased amount of Li to be added. Increasing the amount of Li added leads to increased costs and may reduce the initial efficiency of a secondary battery when stored in air for a long period of time. For this reason, it is preferable to reduce the amount of Li added as much as possible. In addition, in order to improve performance, it is necessary to control the structure of the active material with a small amount of Li. Specifically, it is preferable for Li to be selectively present on the surface of the active material, which suppresses surface side reactions with the electrolyte and improves battery performance.

The composite described in Patent Document 3 only covers silicon with a silicon-oxy-lithium compound, silicon oxide, and a carbon coating, which cannot withstand the expansion inhibition of silicon. Patent Document 3 also describes that "the obtained composite has a stable structure, and can effectively prevent components such as air from penetrating the inside of the particles and causing the active ingredient to become ineffective, and the structure and properties do not deteriorate even when stored for a long period of time," but this has not been substantiated by quantitative evaluation in the examples, and it cannot necessarily be said that the quality is stable even when stored for a long period of time as described above.

In the conventional techniques described in Patent Documents 1 to 3, only _SiO2 or LixSiyOz exists around the Si particles, which may not sufficiently suppress the expansion of Si. In order to improve the charge/discharge characteristics, it is necessary to control the metal element concentration inside the active material, and further, in order to suppress the surface side reaction with the electrolyte, it is necessary to control the structure of the active material surface.

The present invention aims to provide composite particles that can suppress surface side reactions with an electrolyte and Si expansion during repeated charging and discharging, and further improve the initial charge-discharge efficiency and cycle characteristics of the active material, a method for producing the composite particles, a negative electrode active material for a secondary battery containing the composite particles, a negative electrode containing the negative electrode active material, and a secondary battery containing the negative electrode. In addition, the present invention aims to provide composite particles and a negative electrode active material for a secondary battery containing the same, which are stable in quality and do not decrease in initial efficiency when used as a secondary battery when stored in air for a long period of time by controlling the amount of Li added.

As a result of intensive research to solve the above problems, the inventors have discovered that it is possible to prepare a composite particle structure containing a specific metal silicate and a matrix phase containing Si and O elements that suppresses the expansion of Si during charging and discharging. Furthermore, they have discovered that by controlling the concentration of the metal portion of this metal silicate and selectively distributing the metal elements, it is possible to suppress side reactions on the surface, leading to improved charging and discharging performance.

The present invention has the following aspects.
[1] Composite particles comprising at least one silicon particle and a matrix phase containing Si and O elements outside the silicon particle, and further containing a silicate compound, the silicate compound containing at least one metal element selected from the group consisting of Li, K, Na, Ca, Mg, and Al.
[2] The composite particle according to [1], wherein the metal ion element concentration of the metal element is 0.1 to 40.0 atom % based on the total elements of the composite particle.
[3] The composite particle according to [1] or [2], wherein when a ratio A of a metal ion element to all elements present in a region of 30% inward from the surface of the composite particle is taken as a ratio B of a metal ion element to all elements present in the other region, the metal ion element abundance ratio satisfies 0.5<A/(A+B)<1.0.
[4] The composite particle according to any one of [1] to [3], wherein the silicate compound contains Li element.
[5] The composite particle according to any one of [1] to [4], wherein the silicate compound has a crystal structure represented by LixSiyOz (wherein x, y, and z are integers greater than 0).
[6] The composite particle according to any one of [1] to [5], wherein the silicate compound has a crystal structure represented by LixSiyOz (where x+4y-2z=0).
[7] The composite particle according to any one of [1] to [6], wherein the silicate compound has a crystal structure of at least one selected from _the group consisting of Li ₂ SiO ₃ , Li _{4 SiO 4} , and Li ₂ Si 2 O 5.
[8] The composite particle according to any one of [1] to [7], wherein the Li element is 20 mass % or less based on the total mass of the composite particle.
[9] The composite particle according to any one of [1] to [8], wherein a peak intensity C corresponding to the (111) plane of the silicon particle in the X-ray diffraction spectrum of the silicon particle and a peak intensity D corresponding to the (111) plane in the X-ray diffraction spectrum of Li ₂ SiO ₃ satisfy 0<D/C≦0.6.
[10] The composite particle according to any one of [1] to [9], wherein in the X-ray diffraction spectrum of the silicon particle, at a peak where 2θ corresponds to the (111) plane of the _silicon particle, the crystallite size x satisfies 10 nm < x < 30 nm, and in the X-ray diffraction peak where 2θ corresponds to the (111) plane of Li 2 SiO ₃ , the crystallite size y satisfies 1 nm < y < 40 nm.
[11] The composite particles according to any one of [1] to [10], wherein the average particle diameter x is 1 μm<x<15 μm and the specific surface area n is 0.1 ^{m 2} /g<n<20 m ² /g.
[12] The composite particle according to any one of [1] to [11], wherein the matrix phase contains Si and O elements as well as C element.
[13] A method for producing a composite particle according to any one of [1] to [12], comprising the following steps 1 and 2:
Step 1: mixing at least one metal element compound selected from the group consisting of Li, K, Na, Ca, Mg, and Al with an intermediate derived from a polysiloxane resin containing silicon particles; and Step 2: firing at 900° C. or less in a non-oxygen atmosphere. [14] A negative electrode active material for secondary batteries comprising the composite particles according to any one of [1] to [12].
[15] A negative electrode comprising the negative electrode active material for a secondary battery according to [14].
[16] A secondary battery comprising the negative electrode according to [15].

The composite particles of the present invention can suppress surface side reactions and can maintain high charge/discharge performance such as capacity retention rate and initial coulombic efficiency in secondary batteries. In addition, the composite particles of the present invention have quality stability and do not decrease in initial efficiency when used as a secondary battery when stored in air for a long period of time.

Hereinafter, embodiments of the present invention will be described in detail.
In this specification, "mass" is synonymous with "weight." Furthermore, in this specification, the term "to" indicating a range of values is used to mean that the values before and after the range are included as the lower and upper limits, unless otherwise specified.

[Composite particles]
The composite particle of the present embodiment has at least one silicon particle and a matrix phase containing Si and O elements outside the silicon particle, and further contains a silicate compound, and the silicate compound contains at least one metal element selected from the group consisting of Li, K, Na, Ca, Mg, and Al.
The composite particle of the present embodiment is characterized in that a plurality of silicon particles are dispersed and embedded in a matrix phase, and further in that the silicate compound is distributed as a component. The silicate compound may be present outside the matrix phase as a silicate layer, or may be present inside the silicon particles or the matrix phase, or may be both.

(Silicon particles)
The silicon particles in the composite particles of this embodiment are composed of zero-valent silicon and have an average particle size of 300 nm or less. The average particle size of the silicon particles is preferably 5 nm to 300 nm, more preferably 20 nm to 250 nm, and even more preferably 50 nm to 200 nm. When the average particle size of the silicon particles is within this range, the charge/discharge performance, such as the capacity retention rate and the initial coulombic efficiency, of the secondary battery can be maintained at a high level.
The average particle size of the silicon particles can be obtained by observing the composite particles of this embodiment with a TEM and calculating the average particle size of 50 silicon particles randomly selected from the TEM image at a magnification of 50,000.

The silicon particles constituting the composite particles of this embodiment preferably satisfy the following relationship: peak intensity C corresponding to the (111) plane of the silicon particles in the X-ray diffraction spectrum and peak intensity D corresponding to the (111) plane in the X-ray diffraction spectrum of Li ₂ SiO ₃ are 0<D/C≦0.6. The peak intensity range is more preferably 0.05<D/C≦0.5. When this peak intensity range is satisfied, the charge/discharge performance such as the capacity retention rate and the initial coulombic efficiency in the secondary battery can be maintained at a high level.

In the peak where 2θ in the X-ray diffraction spectrum corresponds to the (111) plane of the silicon crystal structure, the crystallite size x obtained by Scherrer's calculation using the peak half-width is preferably 10 nm<x<30 nm, and in the X-ray diffraction peak corresponding to the (111) plane of Li ₂ SiO ₃ , the crystallite size y is preferably 1 nm<y<40 nm. When the crystallite size satisfies this range, the charge/discharge performance such as the capacity retention rate and the initial coulombic efficiency in the secondary battery can be maintained at a high level.

From the viewpoint of charge/discharge capacity, initial coulombic efficiency, and cycle characteristics, the specific surface area of the silicon particles is preferably 50 m ² /g to 400 ^{m 2} /g, more preferably 100 m ² /g to 300 ^{m 2} /g, and even more preferably 150 m ² /g to 230 m ² /g. The specific surface area is a value determined by the BET method, and can be determined by nitrogen gas adsorption measurement, for example, by using a specific surface area measuring device. In addition, in order to remove organic deposits on the silicon particles, it is preferable to perform a heat treatment at 500°C for 2 hours in a nitrogen atmosphere before measuring the specific surface area.

The silicon particles are not particularly limited in shape and may be, for example, granular, needle-like, or flaky, but from the viewpoint of charge/discharge performance when used as a secondary battery, the length in the major axis direction is preferably 30 nm to 300 nm, and the thickness is preferably 1 nm to 60 nm. From the viewpoint of charge/discharge performance when used as a secondary battery, a needle-like or flaky shape having an aspect ratio, which is the ratio of thickness to length, of 0.5 or less is preferred.
The morphology of silicon particles can be measured by the average particle size using dynamic light scattering, but the aspect ratio of the sample can be more easily and precisely identified by using analytical means such as a transmission electron microscope (TEM) or a field emission scanning electron microscope (FE-SEM). In the case of silicon particles, the sample can be cut with a focused ion beam (FIB) and the cross section can be observed with a FE-SEM, or the sample can be sliced and the state can be identified by TEM observation. The aspect ratio of silicon particles is a calculation result based on the main part of 50 particles of the sample within the field of view of the TEM image.

(Matrix phase)
The matrix phase present outside the silicon particles contains at least the elements Si and O. The matrix phase preferably has a composition represented by SiOx (1<x<2), and further preferably contains carbonaceous matter (C element) as a component.
The matrix phase is a three-dimensional network structure of a silicon-oxygen skeleton containing at least silicon and oxygen elements. It is also preferable that the matrix phase contains silicon, carbon, and oxygen elements and is a three-dimensional network structure of a silicon-oxygen-carbon skeleton. This three-dimensional network structure of a silicon-oxygen-carbon skeleton has relatively high chemical stability, has a composite structure with carbon (carbonaceous phase), and exhibits small volumetric change with respect to the absorption and release of lithium. In particular, it is preferable that the three-dimensional network structure of the matrix phase contains a silicon oxycarbide (SiOC) structure. This three-dimensional network structure may contain a nitrogen element in addition to silicon, carbon, and oxygen elements.

(Silicate Compound)
The silicate compounds are generally compounds containing anions having a structure in which one or more silicon atoms are at the center and electronegative ligands surround the silicon atoms, and the silicate compounds containing at least one metal (element) selected from the group consisting of Li, K, Na, Ca, Mg, and Al are salts of these metals and compounds containing anions.

Examples of compounds containing the above anions include silicate ions such as orthosilicate ion (SiO ₄ ⁴⁻ ), metasilicate ion (SiO ₃ ²⁻ ), pyrosilicate ion (Si ₂ O ₇ ⁶⁻ ), and cyclic silicate ion (Si ₃ O ₉ ⁶⁻ or Si ₆ O ₁₈ ¹²⁻ ).

The silicate compound is preferably a lithium silicate compound, a magnesium silicate compound, or a sodium silicate compound, more preferably a lithium silicate compound or a magnesium silicate compound, and particularly preferably a lithium silicate compound.

The lithium silicate compound preferably contains a crystal structure represented by Li _x Si _y O _z (where x, y, and z are integers greater than 0), and more preferably contains a crystal structure represented by Li _x Si _y O _z (x+4y-2z=0). Examples of such lithium silicate compounds include lithium metasilicate (Li ₂ SiO ₃ , Li ₂ Si ₂ O ₅ , Li ₄ SiO ₄ ), and it is preferable that the lithium silicate compound contains at least one crystal structure selected from the group consisting of Li ₂ SiO ₃ , Li ₄ SiO ₄ , and Li ₂ Si ₂ O ₅ .

The magnesium silicate compound may, for example, be magnesium metasilicate (MgSiO _{3 ,} Mg ₂ SiO ₄ ).

Examples of sodium silicate compounds include sodium metasilicate (Na ₂ SiO ₃ , Na ₄ SiO ₄ , Na ₂ Si ₂ O ₅ , Na ₂ Si ₄ O ₉ ).

The silicate compound may have two or more of the above metals. When two or more metals are contained, one silicate ion may have a plurality of metals, or a mixture of silicate compounds having different metals may be used. The silicate compound may also have a metal other than the above metals. When the silicate compound is in a crystalline state, it can be detected by powder X-ray diffraction measurement (XRD), and when it is amorphous, it can be confirmed by solid-state ^29Si -NMR measurement.

The content of the above metal elements is preferably 25% by mass or less, more preferably 0.1 to 20% by mass, even more preferably 0.2 to 18% by mass, and particularly preferably 0.3 to 15% by mass, based on the total mass of the composite particles. When the content of the metal elements is within this range, surface side reactions of the composite particles can be suppressed, and the charge/discharge performance when used as a secondary battery can be further improved. It is preferable that the metal elements are uniformly dispersed (distributed) within the composite particles, rather than being localized as metal ions in only a few areas. The content of the metal elements can be measured, for example, by Auger electron spectroscopy (AES) or an ICP-OES analyzer.

The silicate compound preferably contains Li element, and the content of Li element is preferably 20 mass% or less, more preferably 0.1 to 19 mass%, even more preferably 0.2 to 17 mass%, and particularly preferably 0.3 to 15 mass%, relative to the total mass of the composite particle. When the content of Li element is within this range, surface side reactions of the composite particle can be suppressed, and the charge/discharge performance when formed into a secondary battery can be further improved.

In the composite particle of this embodiment, the metal ion element concentration of the metal element in the total elements of the composite particle is preferably 0.1 to 40.0 atom%, more preferably 0.3 to 35.0 atom%, and particularly preferably 0.5 to 30.0 atom%. When the metal ion element concentration is within this range, side reactions on the surface of the composite particle can be suppressed, and the charge/discharge performance when used as a secondary battery can be further improved. The metal ion element concentration of the metal element can also be measured, for example, by Auger electron spectroscopy (AES).

In the composite particle of this embodiment, a concentration gradient of the metal ion element may exist, and it is preferable that the concentration of the surface part is higher than that of the inside of the composite particle. Specifically, when the ratio A of the metal ion element to all elements present in the region 30% in the inward direction from the surface part of the composite particle is taken as the ratio B of the metal ion element to all elements present in the other region, it is preferable that the metal ion element abundance ratio satisfies 0.5<A/(A+B)<1.0. The metal ion element abundance ratio is more preferably 0.6<A/(A+B)<0.99, and even more preferably 0.7<A/(A+B)<0.98. When the metal ion element abundance ratio is within this range, side reactions on the surface can be suppressed, and the charge/discharge performance when used as a secondary battery can be further improved.

The composite particles of this embodiment have an average particle size x of preferably 1 μm<x<15 μm, more preferably 2 μm<x<13 μm, and even more preferably 3 μm<x<10 μm, and a specific surface area n of preferably 0.1 m ² /g<n<20 m ² /g, more preferably 0.5 m ² /g<n<18 m ² /g, and even more preferably 0.7 m ² /g<n<15 m ² /g.
When the average particle diameter and specific surface area of the composite particles are within these ranges, side reactions on the surface can be suppressed and the charge/discharge performance of the secondary battery can be further improved. In particular, it is preferable that the average particle diameter x is 1 μm<x<15 μm and the specific surface area n is 0.1 m ² /g<n<20 m ² /g.

[Negative electrode active material for secondary batteries]
The negative electrode active material for a secondary battery of this embodiment includes the composite particles of this embodiment. The composite particles of this embodiment themselves have the performance as a negative electrode active material, but the negative electrode active material for a secondary battery may contain components other than the composite particles.

The negative electrode active material for secondary batteries may have a carbon layer on the surface of the composite particles. That is, at least a part of the surface of the composite particles may have a carbon coating. The carbon layer (carbon coating) is preferably a coating made of low crystalline carbon. From the viewpoint of improving chemical stability and thermal stability, the amount of the carbon layer (carbon coating) is preferably 0.1% by mass or more and 30% by mass or less, more preferably 1% by mass or more and 25% by mass or less, and even more preferably 5% by mass or more and 20% by mass or less, based on the mass of the composite active material for secondary batteries being 100% by mass. The composite active material for secondary batteries may have a carbon layer (carbon coating) on its surface continuously or intermittently. It is preferable that the carbon layer (carbon coating) is present on the surface of the active material by chemical vapor deposition.

The true density of the negative electrode active material for secondary batteries is preferably 1.6 g/cm ³ or more and 2.6 g/cm ³ or less. From the viewpoint of improving the energy density of the resulting secondary battery, the true density is more preferably 1.75 g/cm ³ or more, and further preferably 1.80 g/cm ³ or more. The true density is a value measured using a true density measuring device.

The average particle size of the negative electrode active material for secondary batteries is preferably 2 μm or more and 15 μm or less. If the average particle size is 2 μm or more, a significant increase in the specific surface area can be suppressed, and when used as a secondary battery, the reversible charge/discharge capacity per unit volume can be improved by suppressing the amount of SEI generated during charging and discharging. In addition, if the average particle size is 15 μm or less, the adhesion strength with the current collector is improved, thereby suppressing peeling from the current collector.

The specific surface area of the negative electrode active material for secondary batteries is preferably 0.1 m ² /g or more and 10 m ² /g or less. The specific surface area is more preferably 0.3 m ² /g or more, and particularly preferably 0.5 m ² /g or more. When the specific surface area is in the above range, the amount of solvent absorbed during electrode preparation can be appropriately maintained, and the amount of binder used to maintain binding properties can also be appropriately maintained. The specific surface area is a value determined by the BET method, and can be determined by nitrogen gas adsorption measurement, for example, using a specific surface area measurement device.

[Method of manufacturing composite particles]
The method for producing composite particles of the present embodiment includes the following steps 1 and 2.
Step 1: Mixing at least one metal element compound selected from the group consisting of Li, K, Na, Ca, Mg and Al with an intermediate derived from a polysiloxane resin containing silicon particles. Step 2: Sintering at 900°C or less in a non-oxygen atmosphere. The composite particles of this embodiment are not limited to those obtained by the above manufacturing method. The description of the manufacturing method below is only one aspect of the manufacturing method of the composite particles of this embodiment, and may include steps other than the above steps 1 and 2, and the order of steps 1 and 2 is not particularly limited.

First, the silicon particles used in step 1 are obtained by wet grinding the raw silicon. Specifically, the raw silicon is dispersed in a solvent such as an organic solvent in the wet grinding to obtain a silicon slurry, which is a dispersion of silicon particles. The silicon slurry can be prepared while grinding the raw silicon in a wet powder grinding device. It is preferable to add a dispersant to the organic solvent to promote the grinding of the raw silicon. Examples of wet grinding devices include roller mills, high-speed rotary grinders, container-driven mills, and bead mills. The preferred range of the average particle size of the silicon particles in the obtained silicon slurry is as described above.

Examples of the above 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.

The above-mentioned dispersants include aqueous and non-aqueous dispersants, with non-aqueous dispersants being preferred. Examples of non-aqueous dispersants include polymeric types such as polyethers, alcohols, polyalkylene polyamines, and polycarboxylic acid partial alkyl esters, low molecular weight types such as polyhydric alcohol esters and alkyl polyamines, and inorganic types such as polyphosphates. The solid concentration of the silicon particles and dispersant in the silicon slurry is not particularly limited, but is preferably in the range of 5% to 40% by mass, and more preferably 10% to 30% by mass, with the total amount of the solvent, dispersant, and silicon particles being 100% by mass. The amount of dispersant added is preferably in the range of 2% to 60% by mass, and more preferably 5% to 50% by mass, with respect to the total mass of the silicon particles.

Next, the polysiloxane compound is mixed uniformly with the silicon slurry obtained by the wet grinding process, and then the process is followed by desolvation, drying, and high-temperature treatment to obtain an intermediate derived from polysiloxane resin containing silicon particles. Specifically, the matrix phase, which is a three-dimensional network structure of a silicon-oxygen skeleton, is mixed with the silicon particles and homogenized, and then the process is followed by desolvation, drying, and high-temperature treatment to obtain an intermediate.

Examples of the polysiloxane compound include resins containing at least one of a polycarbosilane structure, a polysilazane structure, a polysilane structure, and a polysiloxane structure. The resin may contain only these structures, or may be a composite resin having at least one of these structures as a segment and chemically bonded to other polymer segments. The form of the composite may be graft copolymerization, block copolymerization, random copolymerization, alternating copolymerization, etc. Examples include 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.

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

In the above general formulas (S-1) and (S-2), ^R1 represents an aromatic hydrocarbon group or an alkyl group which may have a substituent, an epoxy group, a carboxy group, 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, naphthyl, 2-methylphenyl, 3-methylphenyl, 4-methylphenyl, 4-vinylphenyl, 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 olefin polymers, as well as polymer segments such as urethane polymer segments, ester polymer segments, and ether 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.

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

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

Examples of polymerizable double bonds include vinyl groups and (meth)acryloyl groups. It is preferable that 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.

The silanol group in the present invention is a silicon-containing group having a hydroxyl group directly bonded to a silicon atom. The hydrolyzable silyl group in the present invention is a silicon-containing group having a hydrolyzable group directly bonded to a silicon atom, and specifically includes, for example, a group represented by the following general formula (S-4).

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

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

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

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 above general formula (S-1) and/or the above general formula (S-2) include those having the following structures:

^R6 in the structural formulae (1) to (3) above has the same meaning as ^R1 above. ^R7 and ^R8 in the structural formulae (4) to (8) above have the same meaning as ^R2 and ^R3 above, respectively.

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

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

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

(3) A method in which a polymer segment, a silane compound having both 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. This method produces a polysiloxane compound.

In addition, commercially available polysiloxane compounds may be used, such as the Ceranate (registered trademark) series (organic/inorganic hybrid coating resin; manufactured by DIC Corporation) and the Compoceran SQ series (silsesquioxane hybrid; manufactured by Arakawa Chemical Industries, Ltd.).

In order to obtain a polysiloxane resin having a silicon-oxygen-carbon skeleton, a carbon source resin that is compatible with the polysiloxane compound may be used. Examples of such carbon source resins include thermosetting resins and thermoplastic resins that contain a molecular structure with a benzene ring or aromatic functional group.

Examples of thermosetting resins include phenolic resins such as novolac-type phenolic resins and resole-type phenolic resins; epoxy resins such as bisphenol-type epoxy resins and novolac-type epoxy resins; melamine resins; urea resins; aniline resins; cyanate resins; furan resins; ketone resins; unsaturated polyester resins; and urethane resins.

Thermoplastic resins are not particularly limited, and examples include polyethylene, polystyrene, polyacrylonitrile, acrylonitrile-styrene (AS) resin, acrylonitrile-butadiene-styrene (ABS) resin, polypropylene, vinyl chloride, methacrylic resin, polyethylene terephthalate, polyamide, polycarbonate, polyacetal, polyphenylene ether, polybutylene terephthalate, polyphenylene sulfide, polysulfone, polyethersulfone, polyetheretherketone, polyetherimide, polyamideimide, polyimide, and polyphthalamide.

Then, by removing the solvent, drying and performing high-temperature treatment, an intermediate derived from polysiloxane resin containing silicon particles is obtained. Drying is performed, for example, using a dryer, a reduced pressure dryer, a spray dryer, etc. The drying temperature is preferably 80°C or higher, and preferably 150°C or lower. Drying may be performed while reducing the pressure. The high-temperature treatment is performed according to a firing program that is specified by the heating rate, the holding time at a constant temperature, etc. The treatment temperature is preferably, for example, a maximum reaching temperature of 1000°C or higher. The treatment equipment can be appropriately selected according to the purpose from among fluidized bed reactors, rotary furnaces, vertical moving bed reactors, tunnel furnaces, batch furnaces, rotary kilns, etc.

(Step 1)
Next, at least one metal element compound selected from the group consisting of Li, K, Na, Ca, Mg, and Al is mixed with the intermediate derived from the polysiloxane resin containing silicon particles obtained above (mixing step). Note that in the description of step 1 below, some steps may be omitted, and there is no particular restriction on the mixing method.

The metal element compound is not particularly limited as long as it contains the above-mentioned elements, and metal element compounds such as oxides, hydroxides, oxyhydroxides, nitrates, oxalates, acetates, and carbonates can be used. Specifically, when the metal element is Li, lithium oxide and anhydrous lithium acetate are typical examples, and when the metal element is Mg, anhydrous magnesium acetate is typical. In this embodiment, when the metal element is Li, anhydrous lithium acetate is preferable, and when the metal element is Mg, anhydrous magnesium acetate is preferable.
These metal element compounds may be added by dispersing them in a suitable organic solvent (polar or non-polar) such as ethanol, MEK, etc. The amount of the metal element compound added is preferably 0.1 to 30% by mass based on the total mass of the composite particles.

After adding the metal element compound, mixing is performed to uniformly disperse the metal element in the polysiloxane resin-derived intermediate. The mixing method can be performed, for example, with a stirrer equipped with a stirring blade, and the stirring time is, for example, 1 minute to 2 hours, preferably 2 minutes to 1 hour. Stirring may be performed at room temperature, or while heating to about 30 to 50°C. In addition, instead of dispersing in an organic solvent and then mixing, the powdered metal element compound may be added directly to the polysiloxane resin-derived intermediate and mixed. In the case of mixing in powder, it can be performed with a ball mill or a bead mill, and the mixing time is, for example, 1 minute to 2 hours, preferably 2 minutes to 1 hour. Stirring may be performed at room temperature, or while heating to about 30 to 50°C.

After the mixing in step 1, the solvent is removed and the mixture is dried. The solvent can be removed by a conventional method such as filtration. Drying can be performed, for example, using a dryer, a vacuum dryer, or a spray dryer. The drying temperature is preferably 80°C or higher and 150°C or lower. Drying may be performed under reduced pressure.

(Step 2)
After the drying, the mixture is fired at 900°C or less in an oxygen-free atmosphere (firing step). The firing apparatus may be selected from fluidized bed reactors, rotary furnaces, vertical moving bed reactors, tunnel furnaces, batch furnaces, rotary kilns, etc., depending on the purpose. The firing is performed according to a firing program that is determined by the heating rate, holding time at a constant temperature, etc. The firing temperature is preferably, for example, a maximum reaching temperature of 900°C or less. This allows composite particles to be obtained that contain the metal element of the metal element compound added in the mixing step as a compound.

The non-oxidizing atmosphere during the firing process is not particularly limited in terms of the type of gas used, and examples include nitrogen, argon, hydrogen, and nitrogen/hydrogen mixed gas.

The fired product obtained above may be further pulverized and classified as necessary. The pulverization may be performed in one stage until the desired particle size is reached, or may be performed in several stages. In addition, in the case of lumps or agglomerated particles with a particle size of 10 mm or more, the particles may be coarsely pulverized using a jaw crusher, roll crusher, etc., and then pulverized using a glow mill, ball mill, etc., and further pulverized using a bead mill, jet mill, etc. Classification may be performed using a wind classifier, wet classifier, etc.

The composite particles thus obtained may be used as they are as the negative electrode active material for secondary batteries. When the negative electrode active material for secondary batteries further has carbon (carbon coating) on the surface, the obtained active material is coated with a carbon coating in a chemical vapor deposition apparatus at a temperature range of 700°C to 1000°C in a flow of pyrolytic carbon source gas and carrier inert gas. Examples of pyrolytic carbon source gas include acetylene, ethylene, acetone, alcohol, propane, methane, and ethane. Examples of inert gas include nitrogen, helium, and argon, and nitrogen is usually used.

In the method for producing composite particles of this embodiment, a metal element compound may be added to the intermediate after the high-temperature treatment, and the mixture may be mixed and fired, or a metal element compound may be added to an intermediate that has not been subjected to high-temperature treatment, and the mixture may be mixed and fired.
Furthermore, the composite particles obtained by firing may be washed with a liquid in order to remove impurities and unreacted substances, and the liquid is preferably one having an OH group, such as water or alcohol.

[Negative electrode]
The negative electrode of this embodiment is not particularly limited as long as it contains the above-mentioned negative electrode active material for secondary batteries, but may also contain other components such as an organic binder and a conductive assistant.
The negative electrode of this embodiment can be produced by applying a slurry containing the composite particles of this embodiment, an organic binder, and other components such as conductive assistants as necessary, onto a copper foil current collector in the form of a thin film. The negative electrode can also be produced by adding a carbon material such as graphite to the slurry. Examples of the carbon material include natural graphite, artificial graphite, and amorphous carbon such as hard carbon or soft carbon.

For example, the negative electrode active material for secondary batteries and a binder, which is an organic binding material, are mixed together with a solvent using a dispersing device such as a stirrer, ball mill, super sand mill, or pressure kneader to prepare a negative electrode material slurry, which is then applied to a current collector to form a negative electrode layer. Alternatively, the negative electrode material slurry in paste form can be formed into a sheet, pellet, or other shape and integrated with the current collector to obtain the negative electrode layer.

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% by mass to 30% by mass, more preferably 2% by mass to 20% by mass, and even more preferably 3% by mass to 15% by mass.

When the content of the organic binder is 1% by mass or more, adhesion is better and destruction of the negative electrode structure due to expansion and contraction during charging and discharging is more suppressed. On the other hand, when the content is 30% by mass or less, an increase in electrode resistance is more suppressed.

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, for example, 1% by mass to 15% by mass with respect to the negative electrode active material of this embodiment.

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

[Secondary battery]
The secondary battery of the present embodiment includes the negative electrode of the present embodiment. As the secondary battery, non-aqueous electrolyte secondary batteries and solid electrolyte secondary batteries are preferable, and the negative electrode of the present embodiment exhibits excellent performance particularly when used as the negative electrode of a non-aqueous electrolyte secondary battery.

When the secondary battery of this embodiment 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 for a secondary battery of this embodiment 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 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, either alone or as a mixture of two or more components.

The structure of the secondary battery of this embodiment 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 plate group, or are stacked as flat plates to form a stacked electrode plate group, and these electrode plate groups are enclosed in an exterior body. Note that the half cells used in the examples of the present invention are mainly composed of the present negative electrode active material in the negative electrode, 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.

The secondary battery of this embodiment is used as, but is not limited to, a paper battery, a button battery, a coin battery, a laminated battery, a cylindrical battery, a square battery, etc. The negative electrode active material of this embodiment described above can also be applied to electrochemical devices in general that use the insertion and removal of lithium ions as a charging and discharging mechanism, such as hybrid capacitors and solid lithium secondary batteries.

The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto. In the examples, [%] and [parts] represent "mass %" and "mass parts" unless otherwise specified. Silicon particles were obtained by the following method, composite particles were produced, and secondary battery evaluation was performed.

(Preparation of silicon particles)
Synthesis Example 1
In a 150 ml container of a small bead mill device, zirconia beads with a particle size of 0.1 mm and 100 m methyl ethyl ketone (hereinafter also referred to as "MEK") solvent were placed at a filling rate of 60%. Then, silicon powder with an average particle size of 5 μm and a cationic dispersant liquid (BYK Japan Co., Ltd.: BYK145) were placed, and wet grinding was performed using a bead mill at a rotation speed of 1500 revolutions/min and a grinding time of 4 hours to obtain a dark brown liquid-like silicon slurry with a solid concentration of 30% by mass. When the shape and size of the obtained silicon particles were confirmed by TEM observation, the average particle size of the silicon particles was 50 nm.

(Preparation and evaluation of composite particles)
Example 1
The silicon particles (average particle size = 50 nm) prepared in the above Synthesis Example 1 were mixed with polysiloxane resin (CERANATE SSA-500: manufactured by DIC Corporation) so that the silicon element content after high-temperature firing with the silicon slurry was 50%, and the solvent was removed in a nitrogen atmosphere in an oil bath at 120 ° C., and reduced pressure drying was performed at 110 ° C. for 10 hours to obtain a mixed dried product. Next, the mixed dried product was fired at a high temperature at 1100 ° C. / 6 hours in a nitrogen atmosphere to obtain a black fired product. Then, the black fired product was pulverized with a planetary ball mill (ball mill P-6 classic line: manufactured by FRITSCH) to obtain a composite particle intermediate. Lithium oxide (LiO ₂ ) (manufactured by Wako Pure Chemical Industries, Ltd.) was added to the prepared composite particle intermediate in an amount of 0.1 equivalent to the silicon particles, and mixed with a planetary ball mill. The mixture was sintered at a high temperature of 800° C. for 2 hours in a nitrogen atmosphere, washed with distilled water, and dried under reduced pressure to obtain composite particles with an average particle size of 6.4 μm and a specific surface area (SSA) of 5.1 m ² /g.
The amount of metal ion elements relative to the total mass of the obtained composite particles was measured using an ICP-OES analyzer (ICAP-7400, manufactured by Thermo Fisher Scientific Co., Ltd.), and the amount of metal elements present was 4.2% by mass.
The metal ion element concentration distribution ratio between the surface and interior of the composite particle was measured by Auger electron spectroscopy (AES), and the result was that A/(A+B) was 0.9, where A is the ratio of the metal ion element to all elements in a region 30% inward from the particle surface and B is the ratio of the metal ion element to all elements in the other regions.
Identification of metal silicate species and measurement of crystallite size and intensity ratio were performed using an XRD device (Ultima5: manufactured by Rigaku Co., Ltd.) and calculated using the Scherrer equation based on the half-width of the diffraction peaks (2θ = 28.4°, 26.9°) belonging to the silicon (111) crystal plane and Li ₂ SiO ₃ (111) crystal plane from the results using Cu-Ka radiation. The Si crystallite size was 22 nm, the Li ₂ SiO ₃ crystallite size was 21 nm, and the peak intensity ratio (D/C) was 0.28.
In order to measure the initial efficiency and cycle characteristics of the electrode, a half cell was evaluated. The composite particles (8 parts) obtained above, acetylene black (1 part) as a conductive assistant, and an organic binder (1 part, consisting of SBR (0.75 parts) + CMC (0.25 parts)) were mixed and stirred for 10 minutes with a rotating and revolving type foaming blender to prepare a slurry. After coating a copper foil with a thickness of 20 μm using an applicator, the film was dried under reduced pressure at 110 ° C. to obtain a thin electrode film. A circular electrode with a diameter of 14 mm was punched out and pressed using a tablet molder so that the thickness was about 40 μm. The thickness was measured by calculating the average value of five points on the electrode using a thickness gauge. In a dry room with extremely low moisture content (dew point -40°C or less), Li foil was placed opposite the electrode of the present invention via a 25 μm polypropylene separator, and an electrolyte (Kishida Chemical Co., Ltd., 1 mol/L LiPF ₆ , diethyl carbonate:ethylene carbonate = 1:1 (capacity ratio)) was adsorbed to prepare an evaluation half battery (CR2032 type). Battery characteristics were measured using a secondary battery charge/discharge tester (Hokuto Denko Co., Ltd.), and an evaluation test of charge/discharge characteristics was performed under the set conditions of constant current/low voltage charge/discharge/constant current charge/discharge at room temperature of 25°C, cutoff voltage range of 0.005-1.5 V, and charge rate of 0.1 C (1-3 times) and 0.2 C (after 4 cycles). The initial efficiency was 86%.
In order to evaluate the atmospheric stability of the samples, composite particles stored in a vial at room temperature in the atmosphere were evaluated using a half cell. The initial efficiency of the sample stored in the atmosphere for 7 days was 85%, and the initial efficiency of the sample stored in the atmosphere for 14 days was also 85%.
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, active material powder, and binder at 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 and diethyl carbonate 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 having a thickness of 30 μm as a separator. The laminated lithium ion secondary battery was charged at a constant current of 1.2 mA (0.25c based on the positive electrode) at room temperature 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 at room temperature was 89%.

Example 2
Polysiloxane resin (CERANATE SSA-934: manufactured by DIC Corporation) and phenolic resin (SUMILITE RESIN: PR-53570, manufactured by Sumitomo Bakelite Co., Ltd.) were thoroughly mixed in a stirrer in a resin solid matter composition ratio (5:5), and mixed with the silicon slurry so that the silicon element content after high-temperature baking was 50% to produce a mixed and dried product. The procedure was the same as in Example 1, except that when the Li source was added, the amount of Li ₂ O added was 0.05 equivalents relative to the silicon particles.

Example 3
The same procedure as in Example 2 was repeated except that the amount of Li ₂ O added was 0.2 equivalents relative to the silicon particles.

Example 4
The same procedure as in Example 1 was carried out except that the amount of Li ₂ O added was 0.1 equivalent relative to the silicon particles and the firing temperature was 800° C./hour.

Example 5
The same procedure as in Example 2 was repeated except that Li ₂ O was changed to Li ₂ CO ₃ , the amount added was 1.0 equivalent relative to the silicon particles, and the firing temperature was 800° C./4 h.

Example 6
The same procedure as in Example 2 was repeated except that the amount of Li ₂ O added was 0.07 equivalent relative to the silicon particles and the firing temperature was 800° C./hour.

Example 7
The same procedure as in Example 2 was repeated except that the amount of Li ₂ O added was 0.25 equivalents relative to the silicon particles and the firing temperature was 800° C. for 6 hours.

Example 8
The same procedure as in Example 2 was repeated except that the amount of Li ₂ O added was 0.3 equivalents relative to the silicon particles and the firing temperature was 800° C./4 hours.

Example 9
The black fired product was ground in a planetary ball mill at 220 rpm for 10 minutes, and the procedure was the same as in Example 2 except that the amount of Li ₂ O added was 0.5 equivalents relative to the silicon particles and the firing temperature was 800° C./6 hours.

Example 10
The same procedures as in Example 2 were carried out except that the amount of Li ₂ O added was 0.4 equivalents relative to the silicon particles and the firing temperature was other than 700° C./6 hours.

Example 11
The same procedure as in Example 2 was repeated except that the amount of Li ₂ O added was 0.5 equivalents relative to the silicon particles and the firing temperature was 900° C./2 hours.

Example 12
The same procedure as in Example 2 was repeated except that the amount of Li ₂ O added was 0.1 equivalent relative to the silicon particles and the firing temperature was 800° C./24 hours.

Example 13
The same procedure as in Example 2 was repeated, except that Li ₂ O was changed to Na ₂ CO ₃ in an amount of 0.5 equivalents relative to the silicon particles.

Example 14
The same procedure as in Example 2 was repeated, except that Li ₂ O was changed to K ₂ CO ₃ in an amount of 0.5 equivalents relative to the silicon particles.

Comparative Example 1
The same procedure as in Example 2 was carried out except that Li ₂ O was not added.

Comparative Example 2
The same procedure as in Example 2 was repeated except that no polysiloxane resin or phenolic resin was used, the amount of Li ₂ O added was 0.1 equivalent relative to the silicon particles, and the firing temperature was 800° C./2 hours.

Comparative Example 3
The same procedure as in Example 2 was repeated except that no silicon particles were used and the amount of Li ₂ O added was 0.3 equivalents relative to the material after sintering at 1100° C. for 6 hours.

Comparative Example 4
The same procedure as in Example 2 was repeated except that Li ₂ O was changed to Fe ₂ O ₃ , the amount of Fe 2 O 3 added was 0.1 equivalent relative to the silicon particles, and the firing temperature was 800° C./2 h.

As can be seen from Table 1 above, the composite particles of the present invention (Examples 1 to 14) have higher capacity retention and initial (coulombic) efficiency when used in secondary batteries than Comparative Examples 1 to 4, and there is almost no decrease in initial efficiency when stored in air for a long period of time. This shows that the composite particles of the present invention can maintain high charge/discharge performance, such as capacity retention and initial coulombic efficiency, in secondary batteries.

Claims (15)

A composite particle having at least one silicon particle and a matrix phase outside the silicon particle, further containing a silicate compound containing a metal element , the matrix phase having a three-dimensional network structure of a silicon-oxygen skeleton optionally containing a nitrogen element or a three-dimensional network structure of a silicon-oxygen-carbon skeleton optionally containing a nitrogen element, the metal element being at least one selected from the group consisting of Li, K, Na, Ca, Mg and Al,
Composite particles, wherein the ratio A (atom %) of a metal ion element to all elements present in a region 30% inward from the surface of the composite particle is defined as A, and the ratio B (atom %) of a metal ion element to all elements present in the other region is defined as B, such that the metal ion element abundance ratio satisfies 0.5<A/(A+B)<1.0. The composite particle according to claim 1, wherein the metal ion element concentration of the metal element is 0.1 to 40.0 atom% of the total elements of the composite particle. The composite particle according to claim 1, wherein the silicate compound contains Li element. The composite particle according to claim 3, wherein the silicate compound has a crystal structure represented by LixSiyOz (where x, y, and z are integers greater than 0). The composite particle according to claim 4, wherein the silicate compound has a crystal structure represented by LixSiyOz (where x+4y-2z=0). The composite particle according to claim 5, wherein the silicate compound has a crystal structure that is at least one selected from the group consisting of Li2SiO3, Li4SiO4, and Li2Si2O5. The composite particle according to claim 1, wherein the Li element is 20 mass% or less with respect to the total mass of the composite particle. The composite particle according to claim 1, wherein a peak intensity C corresponding to the (111) plane of the silicon particle in the X-ray diffraction spectrum of the silicon particle and a peak intensity D corresponding to the (111) plane in the X-ray diffraction spectrum of Li2SiO3 satisfy 0<D/C≦0.6. The composite particle according to claim 1, wherein the crystallite size x is 10 nm < x < 30 nm at the peak in the X-ray diffraction spectrum of the silicon particle corresponding to the (111) plane of the silicon particle, and the crystallite size y is 1 nm < y < 40 nm at the X-ray diffraction peak corresponding to the (111) plane of Li2SiO3. The composite particles according to claim 1, wherein the average particle diameter x is 1 μm<x<15 μm and the specific surface area n is 0.1 m2/g<n<20 m2/g. The composite particle according to claim 1, wherein the matrix phase contains Si and O elements as well as C elements. A method for producing the composite particle according to claim 1, comprising the following steps 1 and 2:
Step 1: A step of mixing at least one metal element compound selected from the group consisting of Li, K, Na, Ca, Mg, and Al with an intermediate derived from a polysiloxane resin containing silicon particles. Step 2: A step of firing at 900° C. or less in a non-oxygen atmosphere. A negative electrode active material for a secondary battery comprising the composite particles according to any one of claims 1 to 11. A negative electrode comprising the negative electrode active material for a secondary battery according to claim 13. A secondary battery comprising the negative electrode according to claim 14.