WO2024111302A1 - Composite particles, negative electrode for secondary battery, and secondary battery - Google Patents

Abstract

Provided are: composite particles whereby it is possible to obtain a secondary battery with excellent low expansion characteristics and cycle characteristics, both of which are important properties of secondary batteries; a negative electrode for a secondary battery whereby it is possible to obtain a secondary battery having excellent cycle characteristics; and a secondary battery that includes the negative electrode for a secondary battery and has excellent cycle characteristics.　The composite particles include a matrix phase containing silicon particles, Si, O, and C, have at least one gap therein, and have an average porosity of 1%-80% as calculated by formula (1).　Formula (1): porosity (%) = (total pore volume/(specific volume + total pore volume)) × 100

Description

Composite particles, negative electrode for secondary battery, and secondary battery

The present invention relates to composite particles, a secondary battery negative electrode containing the composite particles, and a secondary battery containing the secondary battery 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 properties such as electric capacity, safety, and operational stability.
As such a secondary battery, lithium ion batteries are known that use a lithium intercalation compound that releases lithium ions from between layers as the negative electrode active material. For example, various lithium ion batteries that use a carbon material such as graphite that can absorb and release lithium ions between layers between crystal planes during charging and discharging as the negative electrode active material have been developed and are already in practical use.

Compared to carbon materials, silicon has a large theoretical electrical capacity, so the use of silicon as an anode active material has been considered in order to increase the capacity of lithium-ion secondary batteries. However, silicon has a large difference between its volume expansion and contraction when repeatedly charged and discharged, and silicon particles break down during repeated charging and discharging. As a result, there is a demand for improved cycle characteristics for secondary batteries that use silicon as the anode active material.

As a method for suppressing the volumetric change of silicon and improving the cycle characteristics of a secondary battery, a method of buffering the volumetric change of silicon by forming voids has been proposed.
For example, Patent Document 1 describes a battery negative electrode material that includes a silicon material region and a carbon material region made of a carbon material formed around the silicon material region at least partially with a gap therebetween, and the (002) average layer spacing d002 of the carbon material region is 0.365 nm or more and 0.390 nm or less as determined by powder X-ray diffraction using Cu-Kα radiation. It is described that the configuration of Patent Document 1 efficiently suppresses the expansion and contraction of silicon during charging and discharging, and provides a secondary battery with improved specific capacity and cycle durability.

Patent document 2 discloses a method for producing a composite material in which silicon is attached within the pore volume of a porous scaffold material. Examples of the porous scaffold material include porous carbon materials having micropores, mesopores, or macropores.

Patent document 3 describes composite particles for an energy storage device cell, each composite particle containing biomass-derived carbon and an active material, and describes that the biomass-derived carbon is porous.

Also, a method has been proposed in which the use of silicon having voids suppresses the volumetric change of the silicon, thereby improving the cycle characteristics of the secondary battery.
Patent Document 4 discloses silicon alloy particles that can produce porous silicon particles that undergo little change in volume even when Li ions are absorbed.
Patent Literature 5 discloses a particulate material consisting of a plurality of porous particles containing an electroactive material selected from silicon, germanium, or a mixture thereof, the particle material having a specific range of D50 particle size, intra-particle porosity, and pore size distribution measured by mercury porosimetry. It is described that the particulate material is an electroactive material that can be used to improve the charge/discharge capacity of a lithium ion battery.

Patent Document 6 discloses an anode active material including secondary particles that include a first particle that is a primary particle, the first particle including a first core and a first surface layer that is disposed on a surface of the first core and includes carbon, the first core including at least one of silicon and a silicon compound, and a metal compound, the metal compound including at least one of a metal oxide and a metal silicate, and it is described that the first core and the second core are porous cores including a large number of pores.
It is described that when the first core is a porous core, the diffusion of lithium ions proceeds quickly, whereas when the second core is a porous core, the diffusion of lithium ions proceeds quickly and the volume expansion of the second core during charging and discharging is suppressed.

JP 2019-125435 A JP 2020-514231 A JP 2022-500822 A JP 2021-167457 A JP 2017-533533 A JP 2019-508842 A

However, even if voids are provided in the negative electrode active material to buffer the expansion as in Patent Documents 1 to 6, the cycle characteristics tend to decrease.
On the other hand, silicon oxycarbide has been attracting attention as an active material that combines high capacity with good cycle characteristics. Silicon oxycarbide is known to have a lower volume expansion rate due to lithium absorption than silicon and silicon oxide, and to exhibit stable cycle characteristics.

The inventors focused on the combination of silicon particles and silicon oxycarbide, and conducted various studies with the aim of efficiently suppressing the expansion and contraction of the volume of silicon that occurs during charging and discharging, improving the cycle characteristics of secondary batteries, and reducing the expansion of battery cells, which led to the completion of the present invention.
That is, an object of the present invention is to provide composite particles which provide a secondary battery having low expansion and excellent cycle characteristics, which are important properties for a secondary battery.

The present invention has the following aspects.
[1]
Composite particles comprising silicon particles and a matrix phase containing Si, O and C, having one or more voids, and having an average porosity calculated by the following formula (1) of 1% or more and 80% or less.
Porosity (%) = (total pore volume / (specific volume + total pore volume)) × 100 (1)
[2]
The composite particle according to [1], having voids inside the silicon particle or at the interface between the silicon particle and the matrix phase.
[3]
The composite particle according to [1] or [2], wherein the porosity of the silicon particle having voids therein, calculated from the formula (1), is 10% or more and 80% or less.
[4]
The composite particle according to any one of [1] to [3], wherein a gap larger than 0 nm and not larger than 100 nm exists between the surface of at least one silicon particle and the matrix phase.
[5]
The composite particle according to any one of [1] to [4], wherein the porosity of the matrix phase calculated from the formula (1) is 20% or less.
[6]
The composite particle according to any one of [1] to [5], wherein the silicon element content is 20% by mass or more and 80% by mass or less.
[7]
The composite particle according to any one of [1] to [6], comprising nanosilicon particles having a particle diameter of 1000 nm or less.
[8]
The composite particle according to any one of [1] to [7], wherein the matrix phase contains at least silicon oxycarbide and a carbonaceous layer.

Further, the present invention has the following aspects.
[9]
A negative electrode for a secondary battery comprising the composite particle according to any one of [1] to [8].
[10]
A secondary battery comprising the negative electrode for secondary batteries according to [9] above.

The present invention provides composite particles that suppress expansion and provide a secondary battery with excellent cycle characteristics, which is one of its important properties, a secondary battery negative electrode that includes the composite particles, and a secondary battery that includes the secondary battery negative electrode.

In the following description, Si represents the same substance as "silicon", O represents the same substance as "oxygen", C represents the same substance as "carbon", and N represents the same substance as "nitrogen".
The composite particle of the present invention (hereinafter also referred to as "the present composite particle") contains silicon particles and a matrix phase containing Si, O, and C, has one or more voids, and has an average porosity calculated by the following formula (1) of 1% or more and 80% or less.
Porosity (%) = (total pore volume / (specific volume + total pore volume)) × 100 (1)

As mentioned above, silicon particles have a high capacity, but large volume changes occur when they absorb and release large amounts of lithium ions, which is thought to result in poor cycle characteristics. Therefore, a method has been proposed in which voids are provided in the silicon particles, the matrix phase, or between the two, so that the voids buffer the volume expansion and suppress damage to the carbon coating. However, if the voids are not appropriate, the buffering effect does not function sufficiently, and the active material cracks cause an increase in surface area, which is thought to increase the amount of SEI (Solid Electrolyte Interface) generated and reduce the initial Coulombic efficiency.

The voids proposed so far have not had a sufficient buffering effect, and in some cases have been insufficient to improve cycle performance. After investigating the reasons for this, the inventors concluded that the conventional voids alone were not able to adequately alleviate the expansion of silicon particles and the associated expansion of active material, or that even if voids were present, they were not fully utilized. As a result, they investigated the formation of appropriate voids that would adequately alleviate the expansion of active material. Furthermore, they conducted various studies to evaluate whether the formed voids could adequately alleviate the expansion of silicon particles and the associated expansion of active material.

As a result, the present inventors have found that the porosity defined by the above formula (1) may represent voids that can sufficiently alleviate the expansion of silicon particles and the associated expansion of the active material.
Furthermore, by using silicon oxycarbide, which has superior mechanical properties compared to conventionally used carbon, as the matrix, the composite particles have a strong expansion suppression effect and the porous structure is less likely to collapse with repeated charging and discharging. As a result, it was found that the expansion of the silicon particles is further suppressed, and the formed voids are fully utilized to buffer the expansion of the silicon particles, thereby suppressing the expansion of the entire composite particle containing silicon particles and silicon oxycarbide.

From the above study results, it was found that the present composite particles have a buffering effect that can sufficiently mitigate the expansion of silicon particles, and that the expansion of silicon particles is suppressed, and the expansion rate of the entire composite particle is also suppressed. Furthermore, it was found that a secondary battery using a secondary battery negative electrode containing the present composite particles combined with silicon oxycarbide, which has a porosity defined by the above formula (1) within a specific range, has improved cycle characteristics. This effect is thought to be due to the fact that the present composite particles have high pressure resistance strength, and that appropriate control of the voids effectively buffers volume expansion and contraction during charging and discharging, thereby improving the cycle characteristics of the secondary battery.

The silicon particles contained in the composite particles are zero-valent silicon particles, and the average particle size is preferably 1 nm to 1000 nm, more preferably 5 nm to 500 nm, and even more preferably 10 nm to 200 nm.

The average particle size here refers to the volume average particle size, and is the D50 value measured by dynamic light scattering using a laser diffraction particle size analyzer or similar. D50 is the particle size at which the cumulative volume distribution curve for silicon particles reaches 50% when drawn from the small diameter side.

The silicon particles having a large size become large lumps, and when the composite particles are used as the negative electrode active material to form a negative electrode for a secondary battery, the capacity retention rate of the negative electrode active material is likely to decrease because the pulverization phenomenon is likely to occur during charging and discharging. On the other hand, the silicon particles having a small size of less than 10 nm are too fine, so the silicon particles tend to aggregate with each other. Therefore, the dispersibility of the silicon particles in the composite particles may decrease. In addition, if the silicon particles are too fine, their surface activity energy increases, and there is a tendency for by-products and the like to increase on the surface of the silicon particles during high-temperature firing of the negative electrode active material. These may lead to a decrease in charge and discharge performance.
From this viewpoint, it is preferable that the silicon particles are within the above-mentioned range of average particle size, and that the number of large silicon particles exceeding 1000 nm and small silicon particles less than 10 nm, for example, is as small as possible.

From the viewpoint of suppressing expansion due to repeated charging and discharging, the silicon particles preferably contain nanosilicon particles having a particle diameter of 1000 nm or less, and more preferably contain nanosilicon particles having a particle diameter of 500 nm or less.
When the silicon particles contain nanosilicon particles having a particle diameter of 1000 nm or less, the content is preferably 20 mass% or more, and more preferably 25 mass% or more, with the mass of the entire silicon particles including the nanosilicon particles being 100 mass%.

The silicon particles can be produced by granulating silicon chunks by crushing, etc. The presence of these silicon particles can improve the charge/discharge capacity and initial coulombic efficiency when the composite particles are used in a secondary battery.
The silicon particles can be obtained by pulverizing a zero-valent silicon lump so that the average particle size falls within the above range.
Examples of the pulverizer used for pulverizing the silicon chunks to produce silicon particles include pulverizers such as a ball mill, a bead mill, a jet mill, etc. The pulverization may be wet pulverization using an organic solvent, and the organic solvent may be, for example, an alcohol or a ketone, but aromatic hydrocarbon solvents such as toluene, xylene, naphthalene, or methylnaphthalene may also be used.
The obtained silicon particles can be adjusted to a desired average particle size by controlling the bead mill conditions such as bead particle size, blending ratio, rotation speed, or grinding time, and by classification or the like.

The shape of the silicon particles is not particularly limited, but examples include spherical, sheet-like, and flat shapes.

The specific surface area of the silicon particles is preferably from 100 m ² /g to 1500 m ² /g from the viewpoints of capacitance and initial coulombic efficiency.
From the viewpoints of capacitance and initial coulombic efficiency, the specific surface area of the silicon particles is more preferably from 100 m ² /g to 1000 m ² /g, and further preferably from 100 m ² /g to 500 m ² /g.
The specific surface area is a value determined by the BET method, and can be determined by nitrogen gas adsorption measurement, for example, by using a specific surface area measuring device.
The specific surface area of silicon particles can be measured as follows: The amount of nitrogen adsorption at a relative pressure of 0.5 or less at liquid nitrogen temperature is determined at multiple points, and the specific surface area is calculated from a BET plot in a range where the heat of adsorption C value is positive and has a high linearity.

The shape of the silicon particles may be granular, needle-like, or flake-like, but crystalline is preferred. When the silicon particles are crystalline, the crystallite diameter obtained from the diffraction peak assigned to Si(111) in X-ray diffraction is preferably in the range of 5 nm to 30 nm from the viewpoint of initial coulombic efficiency and capacity retention. The crystallite diameter is more preferably 25 nm or less, and even more preferably 20 nm or less.

The composite particles have a matrix phase containing Si, O, and C. From the viewpoint of obtaining an advantageous balance between charge/discharge performance and capacity retention rate when the composite particles are used in a secondary battery, the O to Si content ratio of the matrix phase is preferably 0.1 to 2, more preferably 0.1 to 1.5, even more preferably 0.1 to 1.0, and particularly preferably 0.1 to 0.7.

From the viewpoint of the balance between charge/discharge performance and initial coulombic efficiency when the composite particles are used in a secondary battery, the C to Si content of the matrix phase is preferably 0.3 to 11, more preferably 0.3 to 8.
The content ratio is the molar ratio of Si, O, and C contained in the composite particle, and the content ratio of O to Si is the number of moles of O contained in the composite particle per mole of Si contained in the matrix phase. Similarly, the content ratio of C to Si is the number of moles of C contained in the matrix phase per mole of Si contained in the matrix phase.

From the viewpoint of the balance between charge/discharge capacity, initial coulombic efficiency, and capacity retention rate, the sum of the O content ratio to Si and the C content ratio to Si is preferably 1.2 or more, and more preferably 2.3 or more.

These content ratios can be obtained by measuring the content of each element and then converting it into a molar ratio (atomic number ratio). At this time, the contents of O and C can be quantified using an inorganic elemental analyzer, and the content of Si can be quantified using an inductively coupled plasma optical emission spectrometer (ICP-OES).
Although the content ratio is preferably measured by the above-mentioned method, it is also possible to perform local analysis of the composite particles, obtain the content ratio data obtained by the analysis at many measurement points, and infer the content ratio of the entire composite particles. Examples of local analysis include energy dispersive X-ray spectroscopy (SEM-EDX) and electron probe microanalyzer (EPMA).

The matrix phase containing Si, O, and C is preferably a three-dimensional network structure of a silicon oxycarbide (hereinafter also referred to as "SiOC") skeleton (hereinafter also referred to as "SiOC skeleton"), and is preferably a matrix having a carbonaceous phase composed only of C element along with the SiOC skeleton structure. The carbonaceous phase referred to here is C that is not contained in the three-dimensional skeleton of SiOC, and includes carbon that exists as free carbon, carbon that is bonded between C atoms in the carbonaceous phase, and C that bonds the SiOC skeleton and the carbonaceous phase.

As described below, the matrix phase preferably contains silicon oxycarbide and a calcined product of a carbon source resin, and more preferably contains silicon oxycarbide and a calcined product of a phenolic resin.

The SiOC skeleton in the matrix phase that constitutes this composite particle is characterized by high chemical stability, and by forming a composite structure with the carbonaceous phase, the electron transition resistance is reduced, which facilitates the diffusion of lithium ions. The silicon particles are tightly wrapped in a composite structure of the SiOC skeleton and the carbonaceous phase, preventing direct contact between the silicon particles and the electrolyte. Therefore, in the negative electrode active material that contains this composite particle, the silicon particles contained therein play a role as the main component in expressing charge/discharge performance, while chemical reactions between the silicon and the electrolyte during charge/discharge are avoided, thereby preventing performance degradation of the silicon particles to the greatest extent possible.

More specifically, when lithium ions approach SiOC, the distribution of electrons inside the SiOC fluctuates, forming electrostatic bonds and coordinate bonds between the SiOC and the lithium ions, which allows the lithium ions to be stored in the SiOC skeleton. And because the energy of these coordinate bonds is relatively low, lithium ion desorption reactions occur easily. In other words, it is believed that SiOC can reversibly cause lithium ion insertion and desorption reactions when it is charged and discharged.

The matrix phase contained in the present composite particle may contain nitrogen atoms (hereinafter also referred to as "N") in addition to the Si, O, and C. In the manufacturing method of the present composite particle described below, N can be introduced into the present composite particle by forming an atomic group containing N as a functional group in the molecule of the raw material used, such as a phenolic resin, a dispersant, a polysiloxane compound, other nitrogen compounds, and nitrogen gas used in the firing process. By containing N, the present composite particle tends to have excellent charge/discharge performance and capacity retention when used as a negative electrode active material.
When the present composite particle contains N, the N content is preferably 0.1 mass % or more, more preferably 0.5 mass % or more, and even more preferably 1 mass % or more, based on the total mass of Si, O, C and N being 100 mass %, from the viewpoints of charge/discharge performance and capacity retention rate.
From the viewpoint of charge/discharge performance and capacity retention, the content is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less.

The present composite particle has one or more voids, and the void ratio calculated by the following formula (1) is 1% or more and 80% or less.
Porosity (%) = (total pore volume / (specific volume + total pore volume)) × 100 (1)
As long as the porosity of the composite particle calculated by the formula (1) is within the above range, the voids may be present anywhere in the composite particle, and for example, it is preferable that the voids are present within the silicon particle, at the interface between the silicon particle and the matrix phase, or within the matrix phase. The voids may be present in one or more of these locations.

In the above formula (1), the total pore volume is the sum of the volumes of all the pores, and is calculated by converting the amount of gas adsorbed by the adsorbate at any relative pressure into the volume of the adsorbate in a liquid state. The unit is usually the volume per unit mass, and is expressed in ^cm3 /g.

The total pore volume of the composite particles can be calculated, for example, by image analysis using SEM observation, mercury intrusion porosimetry, or a specific surface area meter. Among these calculation methods, the method of calculation using a specific surface area meter is preferred from the viewpoint of accuracy. When measuring the total pore volume using a specific surface area meter, the pores present inside the particles may not be measured. Therefore, when measuring the total pore volume, the composite particles are crushed, the pores present inside the particles are exposed to the surface, and then the measurement is performed using a specific surface area meter, so that the total pore volume of the composite particles can be measured as accurately as possible. When crushing the composite particles for measuring the total pore volume, it is preferable to crush the composite particles until the average particle diameter after crushing is 2.0 μm to 1.0 μm in D50, and more preferably until the average particle diameter is 1.0 μm to 0.5 μm.

In the formula (1), the specific volume is the volume per unit mass of the composite particle excluding the pores and the surface recesses, and is calculated from the reciprocal of the true density. The unit of the specific volume is usually the volume per unit mass, and is expressed in ^cm3 /g.
The true density can usually be measured by a true density meter.

The definition of the porosity according to the above formula (1) is the value obtained by dividing the volume of the voids, including the internal pores and surface recesses, of the present composite particle by the apparent volume, including the pores and surface recesses of the present composite particle. In other words, since the voids include the internal pores and surface recesses, it is considered to be a more accurate porosity than the conventional definition of porosity.
As described below, the volume of voids is measured by exposing the pores present inside the particle to the surface, so that the volume can be measured accurately even for pores with complex shapes inside the particle.
Furthermore, since the voids throughout the entire particle, including the surface depressions, can be measured evenly and without unevenness, it is believed that the size of the voids in composite particles can be evaluated more accurately.
Since the porosity defined by the above formula (1) is the value obtained by dividing the more accurate volume of the composite particle measured as described above by the apparent volume of the composite particle, the obtained porosity is considered to be a value that more accurately reflects the actual state of the composite particle.

When voids exist within the silicon particles, the porosity of the silicon particles is calculated by the above formula (1), and from the viewpoint of improving cycle characteristics during charging and discharging, the value is preferably 80% or less, and more preferably 70% or less. From the viewpoint of suppressing expansion during charging and discharging, the porosity of the silicon particles calculated by the above formula (1) is more preferably 1% or more, even more preferably 5% or more, and particularly preferably 10% or more.

The porosity of the silicon particles can be calculated in the same manner as in the calculation of the porosity of the present composite particles, whereby the total pore volume can be measured, for example, using a specific surface area meter, and the true density can usually be measured with a true density meter.
In addition, when measuring the total pore volume, it is preferable to crush the silicon particles, expose the pores present inside the particles to the surface, and then measure using a specific surface area meter, as in the case of the present composite particles. When crushing the silicon particles in measuring the total pore volume, it is preferable to crush the silicon particles after crushing until the average particle size D50 is 1000 nm to 500 nm, and more preferably 500 nm to 10 nm.

When the voids are at the interface between the silicon particles and the matrix phase, from the viewpoint of expansion suppression, it is preferable that there be a void greater than 0 and not greater than 100 nm from the surface of at least one silicon particle in the composite particle. When the voids are at the interface between the silicon particles and the matrix phase, it is more preferable that there be a void of 5 nm or more from the surface of at least one silicon particle in the composite particle, and even more preferable that there be a void of 10 nm or more from the surface of at least one silicon particle in the composite particle, from the viewpoint of expansion suppression.
Furthermore, when the voids are at the interface between the silicon particles and the matrix phase, from the viewpoint of cycle characteristics, it is more preferable that there be voids of 100 nm or less from the surface of at least one silicon particle in the composite particle, and it is even more preferable that there be voids of 80 nm or less.

The voids at the interface between the silicon particles and the matrix phase can be measured by electron microscope observation and elemental distribution. From the elemental distribution, the distance at which the silicon particle/matrix interface exists can be calculated using image analysis software.

When the voids are present in the matrix phase, the porosity of the matrix phase is calculated by the formula (1), and from the viewpoint of cycle characteristics, the value is preferably 20% or less. From the viewpoint of cycle characteristics, the porosity of the matrix phase calculated by the formula (1) is more preferably 15% or less, and even more preferably 10% or less.
When the voids are present in the matrix phase, the void ratio in the matrix phase calculated by the formula (1) is more preferably 0.05% or more, and even more preferably 0.1% or more, from the viewpoint of suppressing expansion.

The porosity in the matrix phase can be calculated by removing the silicon particles from the composite particles to leave only the matrix phase, and then measuring the porosity using, for example, a specific surface area meter, and the true density can usually be measured using a true density meter. Alternatively, as shown in the examples, the matrix phase alone without adding silicon particles can be prepared, and the porosity of the matrix phase can be measured in the same manner as above.
Alternatively, the proportion of clear areas in any five areas excluding the Si particle area from an SEMM image of an arbitrary cross section of a composite particle having a silicon particle and a matrix phase may be binarized using image analysis software to calculate the porosity within the matrix phase.

When measuring the total pore volume, it is preferable to crush only the matrix phase excluding the silicon particles, as in the case of the present composite particles, and then measure using a specific surface area meter after exposing the pores present inside the matrix phase to the surface. When crushing the matrix phase when measuring the total pore volume, it is preferable to crush until the average particle size of the matrix phase after crushing is 2.0 μm to 1.0 μm in D50, and more preferably until it is 1.0 μm to 0.1 μm.

The porosity of the composite particles can be set within the above range by appropriately selecting the porosity within the silicon particles, the voids at the interface between the silicon particles and the matrix phase, and the porosity within the matrix phase. More specifically, by setting the porosity within the silicon particles, the voids at the interface between the silicon particles and the matrix phase, and the porosity within the matrix phase within the above ranges, the porosity of the composite particles finally obtained can be set within the above range.

The composite particles preferably have a structure in which silicon particles are uniformly dispersed in a matrix phase that has a three-dimensional network structure of a SiOC skeleton made up of the elements Si, O, and C. It is more preferable that the matrix phase has a composite structure with the carbonaceous phase.

The amount of silicon element contained in the present composite particle is preferably 20% by mass or more and 80% by mass or less, and more preferably 30% by mass or more and 70% by mass or less, from the viewpoint of improving the expansion coefficient during charge and discharge and the cycle characteristics, with the total mass of the silicon particles and the matrix phase being 100% by mass.
The amount of silicon element in the present composite particles can be determined, for example, by ICP measurement.

The specific surface area of the composite particles is preferably 0.01 m ² /g to 50 m ² /g. From the viewpoint of the amount of solvent absorbed during electrode preparation and the amount of binder used to maintain binding properties, the specific surface area of the composite particles is preferably 0.1 m ² /g or more, more preferably 1 m ² /g or more. The specific surface area of the composite particles is preferably 40 m ² /g or less, more preferably 30 m ² /g or less.
The specific surface area of the present composite particle is a value determined by the BET method. The nitrogen adsorption amount at a relative pressure of 0.5 or less at liquid nitrogen temperature is determined at multiple points, and the specific surface area can be calculated from the range in which the heat of adsorption C value is positive and highly linear from the BET plot.

The average particle diameter of the composite particles is preferably 0.5 μm to 20 μm, more preferably 2 μm to 15 μm. If the average particle diameter is too small, the specific surface area increases significantly, and when the secondary battery is used, the reversible charge/discharge capacity per unit volume may decrease due to an increase in the amount of SEI generated during charging/discharging. If the average particle diameter is too large, the electrode film may peel off from the current collector during preparation. The average particle diameter is the volume average particle diameter, the D50 value, as described above. The method for measuring D50 is the same as described above.
The particle size of the composite particles before classification is preferably in the range of 0.1 μm to 30 μm, and the particle size after fine particles are removed is preferably in the range of 0.5 μm to 30 μm.
The composite particles may be in the form of any of granules, needles, and flakes.

It is preferable that the present composite particle satisfies the following formula in terms of chemical shift value obtained from ²⁹ Si-NMR spectrum.
0.05<A/B<5
In the above formula, A represents the area intensity of the peak attributable to Si (0 valence) in the range of -70 ppm to -90 ppm, and B represents the area intensity of the peak attributable to SiO ₄ bonds in the range of -90 ppm to -130 ppm.

In the three-dimensional network structure of the SiOC skeleton, the bonds can be divided into three main types based on the type of O or C atom that bonds with Si and the number of bonds with each atom. The domains having the three types of bonds are SiO ₂ C ₂ , SiO ₃ C, and SiO ₄ , and silicon oxycarbide (SiOC) is formed when these domains are further randomly bonded. The chemical shift (solid-state NMR) of the SiO ₃ C domain is in the range of -60 ppm to -80 ppm with the center position at -70 ppm.

In the present composite particle, the chemical shift value obtained from the ^29Si -NMR spectrum satisfies the range of A/B, which means that the ratio of the zero-valent silicon particles in the present composite particle to the SiO ₄ present in the silicon oxycarbide is a ratio that makes it easy for the silicon particles to exhibit their performance, and when used as a secondary battery, the charge/discharge performance, particularly the cycle characteristics, are excellent. The range of A/B is more preferably 0.8≦A/B≦2.9, and even more preferably 0.9≦A/B≦2.8.

The ²⁹ Si-NMR spectrum can be easily obtained using a solid-state NMR device. In this specification, the solid-state NMR measurement is performed using, for example, a device (JNM-ECA600) manufactured by JEOL, Ltd. The A/B is obtained by performing a single pulse measurement with an 8 mm probe after 10 minutes of tuning with a solid-state NMR analyzer, Fourier transforming the obtained solid-state NMR spectrum data (accumulated 64 times), and performing waveform separation using the Gauss + Lorentz function. Next, based on the peak area obtained by waveform separation, the ratio of the area intensity, A, of the peak in the range of -70 ppm to -90 ppm to the area intensity, B, of the peak in the range of -90 ppm to -130 ppm is obtained.

In infrared analysis of the composite particles, it is preferable that there is no absorption spectrum derived from Si-H stretching vibration at 2000 cm ^-1 to 2200 cm ^-1 . If there is no absorption spectrum derived from Si-H stretching vibration at 2000 cm ^-1 to 2200 cm ^-1 , it means that lithium ions can be efficiently absorbed, which leads to an improvement in charge and discharge capacity.
The absence of an absorption spectrum derived from Si-H stretching vibration means that the absorption intensity of the absorption spectrum derived from Si-H stretching vibration in the range from 2000 cm ^-1 to 2200 cm ^-1 is 0.1% or less, and more preferably 0.05% or less, relative to the absorption intensity of the absorption spectrum in the range from 900 cm ^-1 to 1200 cm ^-1 .

When the matrix phase of the present composite particle has the carbonaceous phase, it is preferable that the Raman spectrum of the carbonaceous phase has a scattering peak near 1590 cm ⁻¹ which is assigned to the G band of a graphite long-period carbon lattice structure, and a scattering peak near 1330 cm ⁻¹ which is assigned to the D band of a graphite short-period carbon lattice structure having disorder or defects, and that the scattering peak intensity ratio I (G band/D band) is in the range of 0.7 to 2. The scattering peak intensity ratio I is more preferably 0.7 to 1.8. The fact that the scattering peak intensity ratio I is in the above range means that the following can be said about the carbonaceous phase in the matrix.

Some C atoms of the carbonaceous phase are bonded to some Si atoms in the SiOC skeleton. This carbonaceous phase is an important component that affects the charge and discharge characteristics. The carbonaceous phase is mainly formed in the SiOC skeleton composed of SiO ₂ C ₂ , SiO ₃ C, and SiO _4. Since the carbonaceous phase is bonded to some Si atoms of the SiOC skeleton, electron transfer between the inside of the SiOC skeleton and the Si atoms on the surface and the free carbon becomes easier. Therefore, when used as a secondary battery, the insertion and desorption reactions of lithium ions during charging and discharging proceed quickly, and it can be considered that the charge and discharge characteristics are improved. In addition, the composite particles may expand and contract due to the insertion and desorption reactions of lithium ions, but the presence of the carbonaceous phase 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.

The carbonaceous phase is preferably formed by thermal decomposition of the precursor silicon-containing compound and carbon source resin in an inert gas atmosphere when preparing the composite particles. Specifically, carbonizable sites in the molecular structures of the silicon-containing compound and 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 SiOC 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 as a carbonaceous phase is expected to reduce the resistance of the composite particles, and when used in the negative electrode of a secondary battery, the reaction inside the negative electrode active material will occur uniformly and smoothly, resulting in a secondary battery material with an excellent balance between charge/discharge performance and capacity retention rate. Although free carbon can be introduced only from Si-containing compounds, the use of a carbon source resin in combination is expected to increase the amount of free carbon present and its effect. The type of carbon source resin is preferably, for example, a carbon compound containing a six-membered carbon ring.

The state of the carbonaceous phase can be identified by a thermogravimetric differential thermal analyzer (TG-DTA) in addition to the Raman spectrum. Unlike the C atoms in the SiOC skeleton, the carbonaceous phase is easily 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 by using TG-DTA. In addition, 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 from the thermal weight loss behavior from the above measurement. The state of carbon can be determined using the temperature values of these behaviors.
On the other hand, the carbon atoms in the SiOC skeleton, i.e., the carbon atoms bonded to the Si atoms constituting the _SiO2C2 , _SiO3C , and _SiO4 , have very strong chemical bonds _and are therefore highly thermally stable, and are not likely to be thermally decomposed in the atmosphere within the measurement temperature range of the thermal analyzer. In addition, the carbon in the carbonaceous phase has properties similar to those of amorphous carbon, and is therefore thermally decomposed in the atmosphere at temperatures ranging from about 550°C to 900°C. As a result, a rapid weight loss occurs.
The maximum temperature for the TG-DTA measurement conditions is not particularly limited, but in order to completely complete the thermal decomposition reaction of carbon, it is preferable to carry out the TG-DTA measurement in the atmosphere under conditions of about 25° C. to about 1000° C. or higher.

The true density of the present composite particle is preferably more than 1.6 g/ ^cm3 and less than 2.4 g/ ^cm3 , and more preferably more than 1.7 g/ ^cm3 and less than 2.35 g/ ^cm3 . When the true density is within the above range, the composition ratio and porosity of each component constituting the present composite particle are in appropriate ranges, and when used as a negative electrode active material, charge/discharge performance is easily exhibited.

The composite particles may have a carbon coating.
The carbon coating preferably covers at least a portion of the surface of the composite particle, and the carbon coating is preferably a coating made of low crystalline carbon.
From the viewpoint of improving the chemical stability and thermal stability of the present composite particle, the amount of the carbon coating is preferably 0.1% by mass to 30% by mass, more preferably 1% by mass to 25% by mass, and even more preferably 5% by mass to 20% by mass, based on 100% by mass of the present composite particle including the carbon coating. Also, from the viewpoint of improving the chemical stability and thermal stability of the present active material, the average thickness of the carbon coating is preferably 10 nm to 300 nm.

From the viewpoint of improving the chemical stability and thermal stability of the composite particles, it is preferable that 1% or more of the surface of the composite particles has a carbon coating, and more preferably 10% or more of the surface of the composite particles has a carbon coating. The composite particles may have a carbon coating continuously or discontinuously on their surfaces.
The carbon coating is preferably formed on the surface of the composite particles by chemical vapor deposition.

From the viewpoint of improving electrical conductivity, the present composite particles preferably contain at least one element selected from the group consisting of Li, K, Na, Mg, Al, Fe, Ni, Ti and Bi.
Among these elements, it is preferable to contain Li, Na, Mg, Al, Fe, Ni and Ti, and it is more preferable to contain Li, Na, Mg, Al, Fe and Ni.

The composite particles may contain other components in addition to the silicon particles and matrix phase as necessary.

The present composite particles can be produced, for example, by a method including the following steps 1 to 3.
Step 1: A slurry of wet-milled silicon particles is mixed with a raw material that provides a matrix phase, stirred and dried to obtain a precursor.
Step 2: The precursor obtained in step 1 is calcined in an inert atmosphere at a maximum temperature in the range of 1000° C. to 1180° C. to obtain a calcined product.
Step 3: The fired product obtained in step 2 is pulverized to obtain the present composite particles.

Each step will be described below.
<Step 1>
The wet-milled silicon (0-valent) slurry used in step 1 can be prepared by pulverizing silicon particles in a wet powder mill using an organic solvent. A dispersant may be used to promote the pulverization of silicon particles in the organic solvent. The wet mill is not particularly limited, and examples thereof include a roller mill, a high-speed rotary mill, a container-driven mill, and a bead mill.
In the wet grinding, it is preferable to grind the silicon particles until they reach a desired average particle size.

The organic solvent used in the wet method is one that does not chemically react with silicon. Examples 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 dispersant may be aqueous or non-aqueous. In order to suppress excessive oxidation of the surface of the silicon particles, it is preferable to use a non-aqueous dispersant. Examples of the non-aqueous dispersant include polymeric types such as polyethers, 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 silicon in the silicon (zero valence) slurry is not particularly limited, but when the solvent and, if necessary, a dispersant are included, the amount of silicon particles is preferably in the range of 5% to 40% by mass, and more preferably 10% to 30% by mass, with the total amount of the dispersant and silicon particles being 100% by mass.

When the composite particles have voids in the silicon particles, a silicon slurry having voids may be used in step 1. Examples of methods for producing silicon particles having voids include the methods described in Patent Documents 4 and 5. The silicon particles can also be produced by anodization. The anodization method is a method for obtaining porous silicon by passing electricity through an aqueous hydrofluoric acid solution using a silicon anode. In this case, platinum is usually used as the cathode.
When silicon particles having voids are obtained by anodization, the porosity can be controlled by the strength and time of the current. They can also be produced by a solution method. The solution method is a method in which metal nanoparticles are formed on the surface of silicon particles, and the metal nanoparticles are etched with a hydrofluoric acid/hydrogen peroxide solution or the like to obtain porous Si. When porous silicon particles are obtained by the solution method, the porosity can be controlled by the treatment time and the concentration of hydrofluoric acid/hydrogen peroxide.

The raw material that provides the matrix phase used in step 1 is preferably a synthetic resin or natural chemical raw material that is carbonized by high-temperature firing in an inert atmosphere and has aromatic functional groups.

Synthetic resins include thermoplastic resins such as polyvinyl alcohol and polyacrylic acid, and thermosetting resins such as phenolic resin and furan resin. Natural chemical raw materials include coke and heavy oil, 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.

When the composite particles have voids in the matrix phase, an organic compound may be used in step 1. For example, the composite particles can be produced by adding an organic compound such as a dispersant as a sacrificial phase when mixing with the raw material that provides the matrix phase, separately from the production of the silicon slurry. The void ratio can be controlled by adjusting the amount added.
As organic compounds such as dispersants, compounds having a phenyl group, such as phenyl compounds, and further having solubility in polar solvents are preferred, and examples thereof include phenylalanine, salicylic acid, methyl salicylate, phthalic acid, maleic acid, quinone, benzoquinone, polyphenols, and polystyrene.

The raw material that gives the matrix phase and the silicon slurry are mixed uniformly, stirred, and then the mixture is desolvated and dried to obtain the precursor of the composite particles (hereinafter also referred to as "precursor"). The mixing is performed using a device that has a dispersing and mixing function. Examples include a stirrer, ultrasonic mixer, and premix disperser. In the desolvation and drying process aimed at distilling off the organic solvent, a dryer, reduced pressure dryer, spray dryer, etc. can be used.

The precursor preferably contains 3% to 97% by mass of silicon particles that are silicon (valence 0) and 3% to 97% by mass of the solid content of the raw material that provides the matrix phase, and more preferably contains 20% to 80% by mass of the solid content of the silicon particles and 20% to 80% by mass of the solid content of the carbon source resin. By subjecting the precursor of the negative electrode active material to the heat treatment described below, the mass may decrease and the ratio of nanosilicon in the negative electrode active material may change, so the content of silicon particles in the precursor may be set appropriately based on the content of silicon particles in the intended composite particle.

<Step 2>
Step 2 is a step in which the precursor obtained in step 1 is fired in an inert atmosphere at a maximum temperature range of 1000°C to 1180°C to completely decompose the thermally decomposable organic components, and the other main components are converted into a fired product suitable for the present composite particles by precisely controlling the firing conditions. Specifically, the raw materials that provide the matrix phase of the raw materials are partially converted into free carbon by the energy of the high-temperature treatment. That is, a matrix phase containing the fired product of the raw materials that provide the matrix phase is obtained by firing. The fired product referred to here is a product in which the composition or structure of the organic compounds such as the raw materials that provide the matrix phase are partially or completely changed by decomposition or conversion at high temperature.
The fired material of the raw material that gives the matrix phase may have all of the raw material that gives the matrix phase converted to carbon, or may have a portion converted to carbon with the remainder maintaining the structure of the raw material that gives the matrix phase.

In step 2, the precursor obtained in step 1 is fired in an inert atmosphere according to a firing program defined by the heating rate, holding time at a constant temperature, etc. The maximum temperature is the highest temperature that can be set, and it strongly influences the structure and performance of the fired product, the composite particle. In the present invention, the maximum temperature is set to 1000°C to 1180°C, which allows precise control of the microstructure of the composite particle and prevents oxidation of silicon particles due to firing at excessively high temperatures, resulting in better charge/discharge characteristics.

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

<Step 3>
Step 3 is a step of obtaining the present composite particles by pulverizing the fired product obtained in step 2 and classifying it as necessary. The pulverization may be performed in one step until the desired particle diameter is reached, or may be performed in several steps. For example, when the fired product is in the form of lumps or agglomerates of 10 mm or more and composite particles of 10 μm are to be produced, the fired product is coarsely pulverized with a jaw crusher, roll crusher, etc. to produce particles of about 1 mm, and then crushed to 100 μm with a glow mill, ball mill, etc., and to 10 μm with a bead mill, jet mill, etc. The particles produced by pulverization may contain coarse particles, and in order to remove them, or to remove fine powder and adjust the particle size distribution, classification is performed. The classifier used is a wind classifier, a wet classifier, etc., depending on the purpose, but when removing coarse particles, a classification method that passes through a sieve is preferable because it can reliably achieve the purpose. In addition, if the precursor mixture is controlled to a shape near the target particle diameter by spray drying or the like before firing, and the main firing is performed in that shape, it is of course possible to omit the pulverization step.

If the composite particles have voids at the interface between the silicon particles and the matrix phase, for example, voids can be created at the interface between the silicon particles and the matrix phase by adding a low molecular weight organic compound when mixing with the raw material that provides the matrix phase, separately from when making the silicon slurry. The void ratio can be controlled by adjusting the amount added.

In the manufacturing process, the porosity of the silicon particles, the porosity of the matrix phase, and the voids at the interface between the silicon particles and the matrix phase can be independently controlled, thereby making it possible to set the porosity of the composite particles within the above range.

The carbonaceous phase of the matrix phase of the composite particles mainly has an amorphous structure, but by controlling the sintering temperature in the manufacturing process, the interplanar spacing of the carbon 002 planes of the carbonaceous phase determined by XRD measurement can be set within the above range. For example, increasing the sintering temperature advances the carbonization reaction, narrowing the interplanar spacing of the carbon 002 planes.

When the composite particles contain at least one element selected from the group consisting of Li, K, Na, Mg, Al, Fe, Ni, Ti and Bi, they can be produced by adding a compound containing the element when mixing the silicon slurry with the raw material that provides the matrix phase in the manufacturing method described above.

When the composite particles have the carbon coating, the composite particles having the carbon coating can be obtained by coating at least a part of the surface of the fired product obtained by the above method with a carbon coating. The carbon coating is preferably an amorphous carbon coating obtained 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.
The pyrolytic carbon source gas may be acetylene, ethylene, acetone, alcohol, propane, methane, ethane, or the like.
Examples of the inert gas include nitrogen, helium, and argon, and nitrogen is usually used.

Since the present composite particle has excellent cycle characteristics, a secondary battery using the present composite particle as a secondary battery negative electrode exhibits good charge/discharge characteristics.
Specifically, the composite particles can be used as a negative electrode active material, and a slurry containing an organic binder and, if necessary, other components such as a conductive assistant can be applied to a copper foil current collector as a thin film to be used as a negative electrode. A carbon material can also be added to the slurry to prepare a negative electrode.
Examples of the carbon material include natural graphite, artificial graphite, and amorphous carbon such as hard carbon or soft carbon.

The present composite particles and a binder, which is an organic binding material, are kneaded together with a solvent using a dispersing device such as a stirrer, ball mill, super sand mill, or pressure kneader to prepare a negative electrode material slurry, which is then applied to a current collector to form a negative electrode layer. The negative electrode can also be obtained by forming the paste-like negative electrode material slurry into a shape such as a sheet or pellet, which is then integrated with a current collector. The negative electrode obtained by the above contains the present composite particles, and therefore becomes a secondary battery negative electrode with excellent initial coulombic efficiency. The negative electrode can be obtained, for example, by kneading the present composite particles and a binder, which is an organic binding material, 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. The negative electrode can also be obtained by forming the paste-like negative electrode material slurry into a shape such as a sheet or pellet, which is then integrated with a current collector.

Examples of the organic binder include styrene-butadiene rubber copolymers (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 (CMC).

Depending on their respective 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 negative electrode for a lithium ion secondary battery 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, 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 negative electrode active material of the present invention has high chemical stability and can employ an aqueous binder, making it easy to handle in practical use.

The negative electrode material slurry may also contain a conductive additive, if 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 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, and screen printing. After application, it is preferable to carry out a rolling process using a flat plate press, calendar roll, etc., as necessary.

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

The negative electrode layer formed on the current collector and the negative electrode layer integrated with the current collector are 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 pressed (pressurized). In the negative electrode using the composite particles of the present invention, 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, but if the 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 decreases, so an optimal range is selected.

A negative electrode containing the composite particles has excellent cycle characteristics and is therefore suitable for use in secondary batteries. Secondary batteries having such a negative electrode are preferably non-aqueous electrolyte secondary batteries and solid electrolyte secondary batteries, and exhibit excellent performance particularly when used as the negative electrode of a non-aqueous electrolyte secondary battery.

When used in a wet electrolyte secondary battery, for example, a secondary battery containing the composite particles can be constructed by placing a positive electrode and a negative electrode containing the negative electrode active material of the present invention opposite each other with a separator interposed therebetween and injecting an electrolyte solution.

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 (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, and porous carbon can be used alone or in combination.

As the separator, for example, a nonwoven fabric, cloth, microporous film, or a combination of these, whose main component is a polyolefin such as polyethylene or polypropylene, can be used. Note that if the positive and negative electrodes of the nonaqueous electrolyte secondary battery to be fabricated 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 containing the composite particles is not particularly limited, but usually, 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 sealed in an exterior body. The half cells used in the examples described below are mainly composed of the composite particles in the negative electrode, and a simple evaluation was performed using metallic lithium as the counter electrode, but this is to more clearly compare the cycle characteristics of the active material itself. By adding a small amount of the composite particles to a mixture mainly composed of graphite-based active material (capacity of about 340 mAh/g), the negative electrode capacity can be increased to about 400 to 700 mAh/g, which is significantly higher than the existing negative electrode capacity, and the cycle characteristics can be improved.

　Secondary batteries containing the present composite particles are used as, but are not limited to, paper-type batteries, button-type batteries, coin-type batteries, laminated-type batteries, cylindrical batteries, square batteries, etc. The present composite particles can also be applied to all electrochemical devices that use the insertion and removal of lithium ions as a charging and discharging mechanism, such as hybrid capacitors and solid-state lithium secondary batteries.

As described above, the composite particles of the present invention provide a secondary battery with excellent cycle characteristics, which is one of the important properties of a secondary battery. Therefore, the composite particles of the present invention can be suitably used in secondary batteries.

Although the present composite particle, the secondary battery negative electrode having the present composite particle, and the secondary battery including the negative electrode have been described above, the present invention is not limited to the configurations of the above-described embodiments.
The present composite particle, the secondary battery negative electrode having the present composite particle, and the secondary battery including the negative electrode may have any other configuration added to the configuration of the above embodiment, or may be replaced with any configuration that exhibits a similar function.

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 is subjected to a simple evaluation using the composite particles of the present invention as the negative electrode and metallic lithium as the counter electrode, in order to more clearly compare the cycle characteristics of the active material itself. By using such a configuration, it is possible to improve the cycle characteristics while suppressing the negative electrode capacity to about 400 to 700 mAh/g, which is much higher than the existing negative electrode capacity, by adding a small amount of the composite particles of the present invention to a mixture mainly composed of a graphite-based active material with a capacity of about 340 mAh/g.

"Synthesis Example 1: Method 1 for producing porous silicon particles"
The porous silicon particles were prepared by anodization. Silicon nanoparticles with a particle diameter of 100 nm were compressed into pellets made of In, and immersed in a 1.0 wt% hydrofluoric acid solution to remove the surface oxide film. The silicon particles were then introduced into an electrolysis cell, and electrolysis was performed for 30 minutes in an ethanol solution containing 50 wt% hydrofluoric acid at a bath temperature of 0°C and a constant current of 20 mA/ ^cm2 . A Pt plate was used as the counter electrode. After electrolytic etching, the silicon nanoparticles were washed with distilled water to obtain silicon microparticles having a porous structure.

"Synthesis Example 2: Method 2 for producing porous silicon particles"
Porous silicon particles were produced by a solution method. Silicon nanoparticles with a particle diameter of 100 nm were immersed in ethanol and subjected to ultrasonic treatment. 5 μmol of silver nitrate was added to a 0.02 M hydrofluoric acid-pure water mixed solution to form silver nanoparticles on the silicon surface. Next, a pure aqueous solution in which hydrofluoric acid and hydrogen peroxide were adjusted at a concentration ratio of 25:1 was added, and the particles were stirred for 5 minutes to form micropores. After washing with distilled water and drying, the particles were stirred in a 1 M nitric acid solution to obtain silicon microparticles with a porous structure.

Example 1
A silicon dispersant (DISPERBYK9077: manufactured by BYK Additives & Instruments) and a methyl ethyl ketone solvent were added at a mass ratio of 40% to the porous silicon particles having an average particle diameter of 100 nm prepared as in Synthesis Example 1 above, and mixed thoroughly in a stirrer to obtain a silicon slurry. A polysiloxane resin (SSA-500: manufactured by DIC Corporation) and a phenolic resin (Sumilite Resin: PR-53570, manufactured by Sumitomo Bakelite Co., Ltd.) were thoroughly mixed in a stirrer in a resin solid matter amount composition ratio of 5:5, and mixed with the silicon slurry so that the silicon element content after high-temperature firing was 50 mass%. After mixing, the mixture was desolvated in a nitrogen atmosphere in a 120°C oil bath, and dried under reduced pressure at 110°C for 10 hours to obtain a mixed dried product.

The mixed dried product was fired at a high temperature of 1100°C for 6 hours in a nitrogen atmosphere to obtain a black fired product. The black fired product was pulverized in a planetary ball mill to obtain composite particles with a D50 of 6.1 um. In order to calculate the silicon content of the produced composite particles, elemental analysis was performed, and the silicon content was 50 mass%. The silicon content was measured using an ICP-OES analyzer (Agilent 5110ICP-OES, manufactured by Agilent Technologies, Inc.).
The composite particles obtained as described above were further pulverized in a planetary ball mill (ball mill P-6 Classic Line: manufactured by FRITSCH) until D50 was 1 um or less, and the true density was measured using a true density meter (Ultrapyc 5000: manufactured by Anton Paar), and the specific volume was calculated from the reciprocal. In addition, the total pore volume was calculated using a specific surface area meter (Belsorp-mini X: manufactured by Microtrack Bell Co., Ltd.), and the porosity of the composite particles was calculated from the following formula (1).
The measurement was carried out three times, and the average porosity was 28%.
Porosity (%) = total pore volume (cm ³ /g) / [total pore volume (cm ³ /g) + specific volume (cm ³ /g)] (1)
The average porosity of the porous silicon particles produced in Synthesis Example 1 was also calculated by the above method and was found to be 50%.

The porosity in the matrix phase was calculated by the following method. Polysiloxane resin (SSA-500: manufactured by DIC Corporation) and phenolic resin (Sumilite Resin: PR-53570, manufactured by Sumitomo Bakelite Co., Ltd.) were thoroughly mixed in a stirrer with a resin solid matter composition ratio of 5:5. In a 120°C oil bath, the solvent was removed under a nitrogen atmosphere, and reduced pressure drying was performed at 110°C for 10 hours to obtain a mixed and dried product. The mixed and dried product was fired at a high temperature for 6 hours at 1100°C in a nitrogen atmosphere, and pulverized in a planetary ball mill until D50 was 1 um or less. The true density was measured using a true density meter, and the specific volume was calculated from the reciprocal, and the total pore volume was calculated from a specific surface area meter. The porosity of the matrix phase was calculated from the above formula. The measurement was performed three times, and the average porosity was 0.1%.
The average pore diameter was calculated using an image taken with a scanning electron microscope (JEM-7200, manufactured by JEOL Ltd.) Thirty pores that could be determined to be pores were arbitrarily selected from the 50,000x image, and the average pore diameter was found to be 10 nm.

To measure the expansion rate of the electrode, a half-cell was evaluated. Eight parts of the composite particles, one part of the conductive additive acetylene black, and one part of the organic binder were mixed and stirred for 10 minutes with a rotating and revolving type Awatori Rentaro (manufactured by Thinky Corporation) to prepare a slurry. The organic binder was a mixture of 0.75 parts of styrene-butadiene copolymer rubber (SBR resin), 0.25 parts of carboxymethylcellulose (CMC), and 10 parts of distilled water.
After applying the coating to a copper foil having a thickness of 20 μm using an applicator, the coating was dried at 110° C. under reduced pressure to obtain a thin electrode film. A circular electrode having a diameter of 14 mm was punched out, and pressed using a tablet molding machine so that the thickness became approximately 40 μm. The thickness was measured by calculating the average value of five points on the electrode using a tabletop micrometer (MF-501: manufactured by Nikon Corporation).

Battery characteristics were measured using a secondary battery charge/discharge tester (manufactured by Hokuto Denko Corporation), and charge/discharge characteristics were evaluated under set conditions of constant current, low voltage charge/discharge, and constant current charge/discharge, with room temperature at 25°C, cutoff voltage range of 0.005 to 1.5 V, and charge rate of 0.1 C. The expansion rate was calculated by disassembling the battery in the initial fully charged state, cleaning the electrodes with dimethyl carbonate, air drying, and measuring the electrode thickness at five points with a thickness meter, and calculating the expansion rate using the following formula. The number of measurements was N=3. The average expansion rate was 110%.
Expansion rate (%) = electrode thickness in initial fully charged state (um) / electrode thickness before charging and discharging (um)

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 with a discharge capacity design value of 450mAh/g. The composition of the active material powder was adjusted so that the charge capacity of the half cell was 1500mAh/g, and 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 in a volume ratio of 1/1 was used as the non-aqueous electrolyte, and a laminated lithium ion secondary battery was prepared using a 30μm thick polyethylene microporous film 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 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 the discharge capacity was determined. The capacity retention rate at room temperature for 100 cycles was 92%.

Examples 2 and 3
Batteries were produced in the same manner as in Example 1, except that the silicon slurry was mixed so that the silicon element content after high-temperature firing was 30 mass% in Example 2 and 70 mass% in Example 3, and the cycle characteristics and expansion coefficient after charge and discharge were evaluated. The results are shown in Table 1.

Example 4
Porous silicon was produced in the same manner as in Synthesis Example 1, except that electrolysis was performed for 5 minutes under a constant current condition of 2.0 mA/ ^cm2 . The same dispersant as in Example 1 was added at a mass ratio of 45% to the mass of the obtained porous silicon, and methyl ethyl ketone solvent was added to form a silicon slurry. The obtained silicon slurry was mixed with the same mixture of polysiloxane resin and phenol resin as in Example 1 so that the silicon element content after high-temperature firing was 30 mass%, and a battery was produced in the same manner as in Example 1, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

Example 5
A battery was fabricated in the same manner as in Example 1, except that silicon having a particle size of 500 nm was used as the raw material, and the cycle characteristics and expansion coefficient after charge and discharge were evaluated. The results are shown in Table 1.

Example 6
A battery was produced in the same manner as in Example 1, except that silicon with a particle diameter of 1000 nm was used as the raw material and the silicon slurry was mixed so that the silicon element content after high-temperature firing was 30 mass %, and the cycle characteristics and expansion coefficient after charge and discharge were evaluated. The results are shown in Table 1.

Example 7
A battery was produced in the same manner as in Example 1, except that phenylalanine was added as a low molecular weight organic compound in a mass ratio of 20% relative to the mass of silicon when the silicon slurry and resin were mixed, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

Example 8
Porous silicon was obtained in the same manner, except that electrolysis was performed for 5 minutes under a constant current condition of 2.0 mA/ ^cm2 . A battery was produced in the same manner as in Example 1, except that polystyrene was added at a mass ratio of 25% to the mass of silicon when mixing the silicon slurry and resin, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

Example 9
The silicon slurry was mixed so that the silicon element content after high-temperature firing was 55% by mass, and when the silicon slurry and the resin were mixed, 20% by mass of DISPERBYK9077 was added to the silicon mass. The battery was prepared in the same manner as in Example 1, and the cycle characteristics and expansion rate after charging and discharging were evaluated. The results are shown in Table 1.

Example 10
Porous silicon was produced in the same manner as in Synthesis Example 1, except that electrolysis was performed for 40 minutes under a constant current condition of 2.0 mA/ ^cm2 . A battery was produced in the same manner as in Example 1, except that 50% by mass of DISPERBYK9077 was added in terms of mass ratio to the mass of silicon when mixing the silicon slurry and resin, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

Example 11
Porous silicon was obtained in the same manner as in Synthesis Example 1, except that electrolysis was performed for 15 minutes under a constant current condition of 5.0 mA/ ^cm2 . 5% phenylalanine was added by mass ratio to the mass of the obtained porous silicon, and methyl ethyl ketone solvent was added to form a silicon slurry. The obtained silicon slurry was mixed with the same mixture of polysiloxane resin and phenolic resin as in Example 1 so that the silicon element content after high-temperature firing was 20% by mass. A battery was prepared in the same manner as in Example 1, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

Example 12
Porous silicon was obtained in the same manner as in Synthesis Example 1, except that electrolysis was performed for 15 minutes under a constant current condition of 5.0 mA/ ^cm2 . Using the obtained porous silicon, a battery was produced in the same manner as in Example 1, except that phenylalanine was added as a low molecular weight organic compound at a mass ratio of 10% relative to the mass of silicon when mixing the mixture of silicon slurry, polysiloxane resin, and phenolic resin, and the silicon element content after high-temperature firing was 80 mass%, and the cycle characteristics and expansion rate after charging and discharging were evaluated. The results are shown in Table 1.

Example 13
Porous silicon was obtained in the same manner as in Synthesis Example 1, except that electrolysis was performed for 15 minutes under a constant current condition of 20 mA/ ^cm2 . Using the obtained porous silicon, a battery was produced in the same manner as in Example 1, except that DISPERBYK9077 was added at a mass ratio of 100% relative to the mass of silicon when mixing the silicon slurry and resin, and the cycle characteristics and expansion rate after charging and discharging were evaluated. The results are shown in Table 1.

Example 14
A battery was produced in the same manner as in Example 1, except that the porous silicon produced in Synthesis Example 2 was used as the silicon, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

Example 15
A battery was prepared in the same manner as in Example 14, except that silicon with an average particle diameter of 500 nm was used as a raw material, a pure aqueous solution of hydrofluoric acid and hydrogen peroxide adjusted at a concentration ratio of 25:1 was added to the silicon particles, and the mixture was stirred for 20 minutes to prepare a slurry of silicon particles, and the silicon slurry was mixed so that the silicon element content after high-temperature firing was 70 mass%, and the cycle characteristics and expansion rate after charging and discharging were evaluated. The results are shown in Table 1.

Example 16
A battery was prepared in the same manner as in Example 14, except that silicon with an average particle diameter of 1000 nm was used as a raw material, a pure aqueous solution of hydrofluoric acid and hydrogen peroxide adjusted at a concentration ratio of 25:1 was added to the silicon particles, and the solution was stirred for 40 minutes to prepare a slurry of silicon particles, and the silicon slurry was mixed so that the silicon element content after high-temperature firing was 50 mass%. The cycle characteristics and expansion rate after charging and discharging were evaluated. The results are shown in Table 1.

Example 17
A pure aqueous solution of hydrofluoric acid and hydrogen peroxide adjusted to a concentration ratio of 25:1 is added to silicon particles, and stirred for 10 minutes to prepare a slurry of silicon particles. When mixing the silicon slurry and resin, 20% of DISPERBYK9077 is added by mass ratio to the mass of silicon. A battery is prepared in the same manner as in Example 14, and the cycle characteristics and expansion rate after charging and discharging are evaluated. The results are shown in Table 1.

Example 18
A pure aqueous solution of hydrofluoric acid and hydrogen peroxide adjusted to a concentration ratio of 3:1 was added to silicon particles, and the mixture was stirred for 1 minute to prepare a slurry of silicon particles. The silicon slurry was mixed so that the silicon element content after high-temperature firing was 50 mass %. Except for this, a battery was prepared in the same manner as in Example 14, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

Example 19
A battery was produced in the same manner as in Example 18, except that 10% by mass of polystyrene was added relative to the mass of silicon when mixing the silicon slurry and the resin, and the cycle characteristics and expansion coefficient after charge and discharge were evaluated. The results are shown in Table 1.

Comparative Example 1
A battery was produced in the same manner as in Example 1, except that silicon particles having an average particle size of 50 nm were used, and the cycle characteristics and expansion coefficient after charge and discharge were evaluated. The results are shown in Table 1.

Comparative Example 2
Porous silicon was produced in the same manner as in Synthesis Example 1, except that electrolysis was performed for 60 minutes under a constant current condition of 20 mA/ ^cm2 . The same dispersant as in Example 1 was added in a mass ratio of 50% to the obtained porous silicon, and a methyl ethyl ketone solvent was added to form a silicon slurry. The obtained silicon slurry was mixed with a mixture of the same polysiloxane resin and phenolic resin as in Example 1, so that the silicon element content after high-temperature firing was 10 mass%, and a battery was produced in the same manner as in Example 1, except that phenylalanine was added in a mass ratio of 30% relative to the silicon mass when the silicon slurry and resin were mixed, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

A battery was created in the same manner as in Example 1, except that no polysiloxane resin was used, and the cycle characteristics and expansion rate after charge and discharge were evaluated. The results are shown in Table 1.

The physical properties in the above examples and comparative examples were measured as follows.
Average particle size: Volume average particle size (D50), measured using a laser diffraction particle size distribution measuring device (Malvern Panalytical, Mastersizer 3000).

Porosity of composite particles and silicon particles: Grinding was performed using a planetary ball mill (ball mill P-6 classic line: manufactured by FRITSCH) until D50 was 1 um or less, and the true density was measured using a true density meter, and the specific volume was calculated from the reciprocal. In addition, the total pore volume was calculated using a specific surface area meter (Belsorp-mini X: manufactured by Microtrack Bell Co., Ltd.), and the porosity of the composite particles and silicon particles was calculated from the above formula (1). The measurement was performed three times, and the average value was taken as the porosity.
True density: Measured using a true density measuring device (Ultrapyc 5000 micro, manufactured by Anton Paar) with helium gas at a temperature of 25° C. and a measurement pressure of 115 kPa.
Total pore volume: A specific surface area meter (Belsorp-mini X: manufactured by Microtrackbel Co., Ltd.) was used, and nitrogen was used. The adsorption temperature was liquid nitrogen temperature (-196°C, 77K), the measurement range was 0.01-0.999 P/P0 (relative pressure), and the pretreatment conditions were 110°C under vacuum for 10 hours, after which the measurement was performed.

　Porosity of matrix phase: No silicon particles were added to the raw material used to provide the matrix phase in the examples or comparative examples, and the matrix phase was created under the same conditions and operations as in the examples or comparative examples. The resulting matrix phase was pulverized in a planetary ball mill until D50 was 1 um or less, and the true density was measured using the same true density meter under the same conditions as above, and the specific volume was calculated from its reciprocal. The total pore volume was also calculated using the same specific surface area meter under the same conditions as above. The porosity of the matrix phase alone was calculated from the above formula. The measurement was performed three times, and the average value was taken as the porosity.

　Voids at the interface between silicon particles and the matrix phase: Measurement conditions were accelerating voltage 5.0 kV, irradiation current 10.0 μA, WD: 10 nm, and calculations were performed using images taken with a scanning electron microscope (JEM-7200: manufactured by JEOL Ltd.). 50 silicon particles were randomly selected from the 50,000x image, and the average distance to the matrix layer was taken as the voids at the interface between the silicon particles and the matrix.

Average pore diameter: Measurement conditions were accelerating voltage 5.0 kV, irradiation current 10.0 μA, WD: 10 nm, and calculations were performed using images taken with a scanning electron microscope (JEM-7200: manufactured by JEOL Ltd.). Thirty random pores were selected, and the average value of their pore diameters was used as the average pore diameter.

Expansion coefficient of negative electrode: The battery in the initially fully charged state was disassembled, the electrodes were washed with dimethyl carbonate, and air-dried. The electrode thickness was measured at five points with a thickness meter, and the expansion coefficient was calculated using the following formula. The measurement was performed three times, and the average value was used as the expansion coefficient of the negative electrode.
Expansion rate (%) = electrode thickness in initial fully charged state (um) / electrode thickness before charging and discharging (um)

Battery characteristic evaluation: Battery characteristics were measured using a secondary battery charge/discharge test device (manufactured by Hokuto Denko Corporation). Charge/discharge characteristic evaluation tests were performed under the following set conditions: constant current, low voltage charge/discharge, and constant current charge/discharge, with a room temperature of 25°C, a cutoff voltage range of 0.005 to 1.5 V, and a charge rate of 0.1 C for the first to third charge/discharges, and 0.2 C from the fourth charge onwards. The charge rate after 10 charge/discharges was taken as the cycle characteristic.

As is clear from the above results, the secondary battery using the present composite particles has excellent cycle characteristics.
The reason for this is believed to be that the composite particles have voids that exhibit a buffering effect that can sufficiently alleviate the expansion of silicon particles, since the porosity defined by the above formula (1) is within a specific range. Furthermore, it is believed that the composite particles contain silicon oxycarbide, which has excellent mechanical properties, which further suppresses the expansion of silicon particles, and also effectively utilizes the voids of the composite particles, thereby suppressing the expansion of the entire composite particle. As a result, the cycle characteristics of a secondary battery using a secondary battery negative electrode containing the composite particles are improved. This improvement effect is believed to be the result of the expansion of the composite particles being appropriately buffered by the appropriate voids, while the expansion rate is also suppressed, and the increase in surface area and the generation of SEI due to cracking are suppressed.

Claims (10)

Composite particles comprising silicon particles and a matrix phase containing Si, O and C, having one or more voids, and having an average porosity calculated by the following formula (1) of 1% or more and 80% or less.
Porosity (%) = (total pore volume / (specific volume + total pore volume)) × 100 (1) The composite particle according to claim 1, which has voids inside the silicon particle or at the interface between the silicon particle and the matrix phase. The composite particle according to claim 2, wherein the porosity of the silicon particle having internal voids calculated from formula (1) is 10% or more and 80% or less. The composite particle according to claim 2, in which there is a gap between the surface of at least one silicon particle and the matrix phase that is greater than 0 nm and less than 100 nm. The composite particle according to claim 1 or 2, wherein the porosity of the matrix phase calculated from formula (1) is 20% or less. Composite particles according to claim 1 or 2, in which the silicon element is 20% by mass or more and 80% by mass or less. The composite particles according to claim 1 or 2, which contain silicon particles having a particle diameter of 1000 nm or less. The composite particle according to claim 1 or 2, wherein the matrix phase contains at least silicon oxycarbide and a carbonaceous layer. A negative electrode for a secondary battery comprising the composite particles according to claim 1 or 2. A secondary battery comprising the negative electrode for a secondary battery of claim 9.