WO2025070505A1 - Negative electrode active material for secondary batteries, and secondary battery - Google Patents

Abstract

A negative electrode active material for secondary batteries includes silicon-containing particles, which include silicate phases and silicon phases that are dispersed in the silicate phase. If a cross-section of the silicon-containing particles is measured by X-ray photon spectroscopy (XPS), a N atom ratio R_N of the silicon-containing particles quantified on the basis of a N1s spectrum is 1% to 4% inclusive.

Description

Negative electrode active material for secondary battery, and secondary battery

This disclosure relates to a negative electrode active material for a secondary battery, and a secondary battery including the negative electrode active material for a secondary battery.

A negative electrode active material capable of absorbing and releasing lithium ions is used in the negative electrode of a secondary battery, such as a lithium-ion secondary battery, and graphite is generally used as such a negative electrode active material. In recent years, composite materials containing silicon, which have a higher capacity density than graphite, have been considered for the negative electrode active material (for example, Patent Document 1).

International Publication No. 2020/110917

Materials containing silicon are promising as high-capacity negative electrode materials for secondary batteries. However, materials containing silicon expand and contract significantly during charging and discharging, which can easily induce side reactions.

Silicon-containing materials that have a silicate phase and a silicon phase dispersed in the silicate phase are prone to cracking in the silicate phase as the silicon phase expands and contracts during charging and discharging. Meanwhile, a side reaction can occur in which the electrolyte reacts with the silicate phase to produce hydrogen fluoride (HF). The HF produced erodes the silicon phase through cracks in the silicate phase, deactivating it. As a result, the capacity retention rate during charge and discharge cycles is prone to decrease.

In view of the above, one aspect of the present disclosure relates to a negative electrode active material for a secondary battery, comprising: a silicon-containing particle, the silicon-containing particle comprising a silicate phase and a silicon phase dispersed within the silicate phase; and when a cross section of the silicon-containing particle is measured by X-ray photon spectroscopy (XPS), an N atomic ratio R _N of the silicon-containing particle, quantified based on an N1s spectrum, is 1% or more and 4% or less.

Another aspect of the present disclosure relates to a secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, the negative electrode including the above-mentioned negative electrode active material for secondary batteries.

This disclosure makes it possible to suppress the decrease in capacity retention rate that accompanies charging and discharging of a secondary battery.

The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

FIG. 1 is a cross-sectional view illustrating a schematic example of a negative electrode active material (silicon-containing material) according to an embodiment of the present disclosure. 1 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away;

Below, examples of embodiments of the present disclosure are described, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values and materials may be exemplified, but other numerical values and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when a lower limit and an upper limit are exemplified for numerical values of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more of the materials may be used in combination.

　In addition, the present disclosure encompasses a combination of the features of two or more claims arbitrarily selected from the multiple claims set forth in the appended claims. In other words, the features of two or more claims arbitrarily selected from the multiple claims set forth in the appended claims may be combined, provided that no technical contradiction arises.

(Negative electrode active material for secondary batteries)
The negative electrode active material for a secondary battery according to an embodiment of the present disclosure includes silicon-containing particles. The silicon-containing particles include a silicate phase and a silicon phase dispersed in the silicate phase. Hereinafter, the silicon-containing particles are also referred to as "composite particles."

The silicate phase may form an amorphous phase. The silicate phase may be a lithium silicate phase.

Silicon-containing particles contain nitrogen (N) atoms. Nitrogen atoms may be concentrated on the surface of the silicon phase (the interface between the silicon phase and the silicate phase) inside the silicon-containing particles.

Silicon-containing particles are produced, for example, as described below, by mixing silicate particles that form a silicate phase with nanoparticles of raw silicon that form a silicon phase, and compounding them. In this case, the surface of the silicon nanoparticles tends to become lipophilic. On the other hand, since silicate is hydrophilic, it is difficult to bring the silicate particles and silicon nanoparticles into close contact during compounding, and voids tend to form at the interface between the silicate phase and silicon phase inside the silicon-containing particles.

The nitrogen atoms form Si-N bonds on the surface of the silicon phase. This makes the surface of the silicon phase hydrophilic. The nitrogen atoms (or functional groups containing nitrogen atoms) form hydrogen bonds with -O- groups or -OH groups derived from the silicate phase, improving adhesion at the interface between the silicon phase and the silicate phase and reducing the porosity at the interface. As a result, erosion of the silicon phase is suppressed, and the decrease in cycle retention rate can be suppressed.

The inclusion of nitrogen atoms in silicon-containing particles can be detected by X-ray photoelectron spectroscopy (XPS). Quantitative evaluation of the nitrogen content is also possible. When the cross section of the silicon-containing particle is measured by X-ray photoelectron spectroscopy (XPS), the N atomic ratio R _N of the silicon-containing particle quantified based on the N1s spectrum is 1% or more and 4% or less. Preferably, the N atomic ratio R _N may be 2% or more and 4% or less.

X-ray photon spectroscopy is a technique for analyzing the composition and chemical bonding state of the elements that make up a sample surface by irradiating the sample surface with X-rays and measuring the kinetic energy of photoelectrons emitted from the sample surface due to the ionization of atoms. The area of each peak in the XPS spectrum is proportional to the number of atoms of the corresponding element. By analyzing the N1s spectrum, the bonding state of nitrogen atoms and the nitrogen content contained in silicon-containing particles can be evaluated. Relative sensitivity factors (RSFs) can be used as a method for quantitatively evaluating the content of each element contained in silicon-containing particles.

An example of the conditions for XPS measurement to obtain the N atomic ratio R _N is shown below. For calibration of the binding energy, the C1s spectrum (248.5 eV) of graphite can be used.
Measuring device: ULVAC-PHI ESCA5600
X-ray source used: Al-Kα ray, Mg-Kα ray Analysis area: 800μmΦ
Quantitative conversion method: Quantitative values are calculated based on the peak area of each element multiplied by the relative sensitivity coefficient.

When analyzing the composite particles contained in the negative electrode of a secondary battery, the cross section of the silicon-containing particles can be obtained by disassembling a fully discharged battery and taking out the cross section of the negative electrode. The taken out negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the non-aqueous electrolyte component, and then dried. The cross section of the negative electrode may be obtained using a cross section polisher (CP). The cross section of the composite particle is observed using a scanning electron microscope (SEM), and 10 composite particles with a maximum diameter of 5 μm or more are randomly selected from the backscattered electron image, and XPS analysis is performed on each of them. The N atomic ratio to all atoms is derived for each composite particle, and the average of the N atomic ratios in the 10 composite particles is calculated, and the N atomic ratio R _N is obtained.

The porosity of the silicon-containing particles having such an N atomic ratio R _N may be 8% or less. By reducing the porosity of the silicon-containing particles to 8% or less, erosion of the silicon phase is suppressed, and a decrease in cycle retention rate can be suppressed. The porosity may be 5% or more and 8% or less, 6% or more and 8% or less, or 7% or more and 8% or less.

The porosity of silicon-containing particles can be determined by measuring the pore size distribution of the silicon-containing particles using a pore size analysis method such as the BET method. In the pore size distribution, the pore volume V1 obtained by integrating the pore sizes in the range of 2 nm to 100 nm is taken as the total volume occupied by pores, and the porosity is calculated.

The pore size distribution is measured after taking 0.20 g to 0.25 g of a sample of silicon-containing particles, placing the sample in a measurement cell consisting of a glass tube for measuring the specific surface area, and drying and degassing the measurement cell. Drying and degassing is performed for at least one hour at a pressure of 6.67 Pa and a temperature of 250°C ± 5°C. The secondary battery is disassembled to remove the negative electrode, which is washed with a solvent such as dimethyl carbonate. The negative electrode mixture layer is taken from the dried negative electrode, and the silicon-containing particles are separated and used as the sample. The mass of the sample in the measurement cell is then measured to the nearest 0.1 mg. The amount of nitrogen adsorption of the sample at a temperature of -196°C is then measured using a specific surface area measuring device. For example, an automatic specific surface area/pore distribution measuring device "Tristar II 3020" manufactured by Shimadzu Corporation is used as the measuring device. From the measurement results of the amount of adsorption, the volumetric pore size distribution is obtained by the BET multipoint method.

When silicon-containing particles are contained in the negative electrode of a secondary battery, the porosity may be determined by image analysis of the cross section of the negative electrode described above. In the cross-sectional image of the silicon-containing particle, the area occupied by voids and the area other than the voids are binarized, and the ratio of the area occupied by voids to the total area is determined. The average ratio for 10 silicon-containing particles is taken as the porosity.

Silicon-containing particles having nitrogen atoms at the interface between the silicon phase and the silicate phase are produced, for example, by preparing silicon nanoparticles that will become the silicon phase, and then exposing the silicon nanoparticles to plasma of a compound having a nitrogen-containing functional group. The plasma treatment modifies the surface of the silicon nanoparticles, and a nitrogen-containing functional group is added to the surface. Examples of the nitrogen-containing functional group include an amino group (-NH ₂ ), an imino group (=NH, -NH-), an amide group (-CONH-), and a nitro group (NO ₂ ). Among these, the amino group and the nitro group are preferred.

By mixing the surface-modified silicon nanoparticles with silicate particles and forming a composite, silicon-containing particles are obtained that have nitrogen atoms at the interface between the silicon phase and the silicate phase and in which the silicon phase is dispersed in the silicate phase. However, the method for producing silicon-containing particles is not limited to the above method.

The presence of nitrogen atoms at the interface between the silicon phase and the silicate phase inside silicon-containing particles can be confirmed, for example, by measuring the absorption spectrum due to molecular vibrations using infrared spectroscopy and detecting -N bonds derived from nitrogen functional groups on the Si surface.

(Composite particles)
The composite particles have a structure in which the silicon phase is dispersed in the ion conductive phase (matrix). The stress accompanying the expansion and contraction of the silicon phase during charging and discharging is alleviated by the ion conductive phase, and cracks and breaks in the composite particles are suppressed. Therefore, it is possible to achieve both high capacity due to the inclusion of silicon and improved cycle characteristics. The ion conductive phase includes a silicate phase. The ion conductive phase may be composed of a single phase of only the silicate phase, or may be composed of multiple phases including the silicate phase and an ion conductive layer other than the silicate phase. The ion conductive layer other than the silicate phase may include either a silicon oxide phase or a carbon phase.

The silicate phase is composed of a compound containing a metal element, silicon (Si), and oxygen (O). Examples of the metal element include an alkali metal element such as lithium and an element of Group 2 of the long periodic table. The silicate phase preferably contains at least lithium silicate. In this case, the lithium ions can easily enter and leave the silicate phase. The lithium silicate phase has a smaller irreversible capacity than the silicon oxide phase. The composite particles may be silicate phase-containing composite particles. The ion conductive phase may contain, for example, a silicate phase as a main component and a small amount of a silicon oxide phase. Here, the "main component" refers to a component that occupies 50% by mass or more of the total mass of the silicon compound phase, and may occupy 70% by mass or more of the component. The silicate phase (lithium silicate phase) may contain at least one selected from the group consisting of Li ₂ Si ₂ O ₅ , Li ₂ SiO ₃ , and Li ₄ SiO ₄ .

The atomic ratio of O to Si in lithium silicate: O/Si is, for example, greater than 2 and less than 4. When the O/Si ratio is greater than 2 and less than 4 (z in the formula described below is 0<z<2), this is advantageous in terms of the stability of the silicate phase and lithium ion conductivity. Preferably, the O/Si ratio is greater than 2 and less than 3. The atomic ratio of Li to Si in lithium silicate: Li/Si is, for example, greater than 0 and less than 4.

The composition of lithium silicate can be expressed by the formula: Li _2z SiO _{2 + z} (0<z<2). From the viewpoint of stability, ease of preparation, lithium ion conductivity, etc., z preferably satisfies the relationship of 0<z≦1, and more preferably z=1/2. For example, lithium silicate can be expressed by Li ₂ SiO ₃ when z=1, and can be expressed by Li ₂ Si ₂ O ₅ when z=1/2. Lithium silicate desirably contains Li ₂ Si ₂ O ₅ as a main component, and Li ₂ Si ₂ O ₅ is desirably the main component of the entire silicate phase. Here, the "main component" refers to a component that occupies 50% by mass or more of the mass of the entire lithium silicate or the entire silicate phase, and may occupy 70% by mass or more.

The silicate phase may contain another element M in addition to Li, Si, and O. When the silicate phase contains another element, the chemical stability and lithium ion conductivity of the silicate phase are improved, or side reactions due to contact between the silicate phase and the non-aqueous electrolyte are suppressed.

The silicate phase may contain at least one element selected from the group consisting of alkali metal elements (excluding lithium) and Group II elements as an element M other than Li, Si, and O.

By including an alkali metal element other than Li in the silicate phase, crystallization becomes more difficult, the viscosity of the softened state becomes lower, and fluidity becomes higher. Therefore, in the heat treatment process, it becomes easier to fill the gaps between silicon particles, and dense composite particles can be easily produced. As the alkali metal element, Na and/or K are preferable because they are inexpensive.

Generally, the silicate phase is alkaline, but the Group II element has the effect of suppressing the elution of alkali metals from the silicate phase. Therefore, if the silicate phase contains a Group II element, the slurry viscosity is easily stabilized when preparing a slurry containing a negative electrode active material. This also reduces the need for treatment (e.g., acid treatment) to neutralize the alkaline components of the composite particles. The Group II element may be either Ca or Mg. Among these, Ca is preferable because it can improve the Vickers hardness of the silicate phase and further improve the cycle characteristics.

The silicate phase may further contain, as element M, at least one element selected from the group consisting of boron (B), aluminum (Al), zirconium (Zr), niobium (Nb), tantalum (Ta), vanadium (V), lanthanum (La), yttrium (Y), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), erbium (Er), fluorine (F), and tungsten (W). From the viewpoint of resistance to non-aqueous electrolytes and structural stability of the silicate phase, it is preferable that element M contains at least one element selected from the group consisting of Zr, Ti, P, Al, and B.

The silicate phase may further contain a rare earth element as element M. The inclusion of a rare earth element in the silicate phase may improve the charge/discharge efficiency at the beginning of the charge/discharge cycle. The rare earth element may be any of scandium (Sc), yttrium (Y) and lanthanoid elements. The above-mentioned lanthanum (La), yttrium (Y) and erbium (Er) are rare earth elements. As the rare earth element, the silicate phase may contain at least one selected from the group consisting of cerium (Ce), praseodymium (Pr) and neodymium (Nd). From the viewpoint of improving lithium ion conductivity, it is more preferable that the rare earth element contains La. The ratio of La to the total rare earth elements is preferably 90 atomic % or more and 100 atomic % or less.

As an element M other than alkali metal elements and Group II elements, for example, B has a low melting point and is advantageous for improving fluidity during sintering. Al, Zr, and La can improve hardness while maintaining ionic conductivity. In addition, Zr, Ti, P, Al, and B have the effect of increasing resistance to non-aqueous electrolytes and the structural stability of the silicate phase.

The silicate phase may further contain trace amounts of elements such as iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

The element M may form a compound. Depending on the type of element M, the compound may be, for example, a silicate of element M or an oxide of element M. In the silicate phase, the content of element M is, for example, 1 mol % or more and 40 mol % or less with respect to the total amount of elements other than oxygen.

The average particle size of the fine silicon phase (before the first charge) dispersed within the ion conductive phase may be 500 nm or less, 200 nm or more, or 50 nm or less. By appropriately miniaturizing the silicon phase in this way, the volume change during charging and discharging is reduced, improving structural stability. In addition, the expansion and contraction of the silicon phase is made uniform, suppressing particle cracking, improving cycle characteristics. The average particle size of the silicon phase is measured by observing the cross section of the composite particle using a SEM or TEM. Specifically, it is determined by averaging the maximum diameters of any 100 silicon phases.

The silicon phase dispersed within the ion-conducting phase is a particulate phase of simple silicon (Si) and is composed of a single or multiple crystallites. From the viewpoint of reducing the amount of volume change due to expansion and contraction of the silicon phase (silicon particles) accompanying charging and discharging and facilitating improved cycle characteristics, the crystallite size of the silicon phase may be 50 nm or less, preferably 20 nm or less, and more preferably 10 nm or less. The crystallite size of the silicon phase may be, for example, 5 nm or more. The crystallite size of the silicon phase is calculated by the Scherrer formula from the half-width of the diffraction peak assigned to the Si (111) plane in the X-ray diffraction (XRD) pattern.

From the viewpoint of increasing capacity and improving cycle characteristics, the content of the silicon phase in the composite particles may be 45% by mass or more and 70% by mass or less, 50% by mass or more and 70% by mass or less, or 50% by mass or more and 60% by mass or less, based on the entire composite particle.

The average particle size of the composite particles is, for example, 1 μm or more and 25 μm or less, and may be 4 μm or more and 15 μm or less, or 6 μm or more and 8 μm or less. In the above range, good battery performance is likely to be obtained. The average particle size of the composite particles is the particle size (volume average particle size) at which the volume accumulated value is 50% in the particle size distribution measured by the laser diffraction scattering method. For example, the "LA-750" manufactured by Horiba Ltd. can be used as the measuring device.

(Covering layer)
The negative electrode active material for secondary batteries may further include a coating layer that covers at least a part of the surface of the silicon-containing particle. The coating layer is preferably a conductive layer having electrical conductivity. At least a part of the surface of the silicon-containing particle is coated with a conductive layer, thereby improving the electronic conductivity of the silicon-containing particle. In addition, the coating layer also has a function of suppressing the electrolyte from penetrating into the pores (voids) of the silicon-containing particle and suppressing side reactions. In terms of suppressing side reactions and maintaining high cycle characteristics, the content of the carbon material is preferably 3% by mass or more and 5% by mass or less with respect to the entire silicon-containing particle including the coating layer.

The coating layer contains, for example, a conductive carbon material. The thickness of the coating layer is preferably thin enough that it does not substantially affect the average particle size of the composite particles. From the viewpoint of ensuring conductivity, the thickness of the coating layer is preferably 1 nm or more.

The coating layer or conductive layer is formed by mixing the raw carbon material with the composite particles, and then firing the mixture to carbonize the raw conductive carbon material. Examples of the raw carbon material include coal pitch or coal tar pitch, petroleum pitch, and phenolic resin. The mixture of the raw carbon material and the composite particles is fired, for example, in an inert atmosphere (for example, an argon or nitrogen atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. In the above temperature range, it is easy to form a highly conductive conductive layer in a silicate phase with low crystallinity. The firing temperature is preferably 550°C or higher and 900°C or lower, and more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or higher and 10 hours or lower.

(Method for producing silicon-containing particles)
An example of a method for producing composite particles containing a lithium silicate phase as a silicate phase as an example of silicon-containing particles will be described in detail below. The composite particles are produced, for example, by a production method including the following first to fourth steps.
(First step) A step of obtaining a compound that forms an ion-conducting phase.
(Second step) After the first step, a compound that forms an ion-conducting phase is compounded with raw material silicon to disperse a silicon phase in the ion-conducting phase, thereby obtaining a composite intermediate.
(Third step) A step of subjecting the composite intermediate to a heat treatment to obtain a sintered body containing an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase.
(Fourth step) A step of pulverizing the sintered body to obtain composite particles containing an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase.
The second step includes a step of covering the surfaces of the raw silicon particles with a nitrogen-containing compound prior to the composite formation.

<First step>
In the first step, a compound forming an ion-conducting phase is prepared or synthesized. When a silicate phase is used as the ion-conducting phase, the first step includes, for example, step 1a of mixing a raw material containing Si and a Li raw material in a predetermined ratio to obtain a raw material mixture, and step 1b of firing the raw material mixture to obtain a raw silicate. The firing in step 1b is performed, for example, in an oxidizing atmosphere. The firing temperature in step 1b is preferably 400° C. or higher and 1200° C. or lower, more preferably 800° C. or higher and 1100° C. or lower.

The raw material mixture is melted, and the molten liquid is passed through a metal roll to form flakes, producing lithium silicate. The flaked silicate is then crystallized by heat treatment in an air atmosphere at a temperature above the glass transition point and below the melting point. Note that the flaked silicate can also be used without being crystallized. It is also possible to produce silicate by firing a specified amount of the mixture at a temperature below the melting point, without melting it, through a solid-phase reaction.

Silicon oxide can be used as the Si raw material. For example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. can be used as the Li raw material. The raw material mixture may contain another element M, such as lithium, silicon, and oxygen ultraviolet, as described above. The element M can be added to the raw material mixture in the form of a compound containing the element M. In the lithium silicate, Si raw material that has not reacted with the Li raw material may remain. The remaining Si raw material is dispersed in the lithium silicate as fine crystals of silicon oxide.

Examples of lithium compounds include lithium carbonate, lithium oxide, lithium hydroxide, and lithium hydride. One type of lithium compound may be used alone, or two or more types may be used in combination.

The compound containing element M may be an oxide, hydroxide, hydride, halide, carbonate, oxalate, nitrate, sulfate, or the like of element M. The compound containing element M may be used alone or in combination of two or more kinds.

<Second step>
In the second step, for example, a mixture of a compound (e.g., a carbon source or raw silicate) that forms an ion-conducting phase and raw silicon is pulverized while applying a shear force to the mixture to obtain a finely divided composite intermediate. Here, the surface of the raw silicon particles is covered with a compound containing nitrogen by a step described later. For example, the compound that forms an ion-conducting phase and raw silicon are mixed in a predetermined mass ratio (e.g., a mass ratio of 20:80 to 95:5), and the mixture is stirred and pulverized using a pulverizing device such as a ball mill. An organic solvent may be added to the mixture to perform wet pulverization. At this time, the raw silicon is pulverized to generate a silicon phase. The silicon phase is dispersed in the matrix of the compound that forms the ion-conducting phase.

As the organic solvent, alcohol, ether, fatty acid, alkane, cycloalkane, silicate ester, metal alkoxide, etc. can be used. A predetermined amount of organic solvent may be added to the grinding vessel all at once at the beginning of grinding, or a predetermined amount of organic solvent may be added intermittently to the grinding vessel in multiple batches during the grinding process. The organic solvent serves to prevent the material to be ground from adhering to the inner wall of the grinding vessel.

(A process for covering the surface of raw silicon particles with a nitrogen-containing compound)
Prior to the composite formation, a process is carried out in which the surface of the raw silicon particles is covered with a nitrogen-containing compound. For example, the raw silicon particles are exposed to plasma of the nitrogen-containing compound, so that the surface of the raw silicon particles is covered with a nitrogen-containing compound (functional group). Examples of the nitrogen-containing compound (functional group) include compounds having an amino group (-NH ₂ ), an imino group (=NH, -NH-), an amide group (-CONH-), and a nitro group (NO ₂ ).

The raw silicon material can be coarse silicon particles with an average particle size of several μm to several tens of μm. It is preferable to control the crystallite size of the silicon phase that is ultimately obtained so that it is 10 nm or less, calculated using the Scherrer formula from the half-width of the diffraction peak assigned to the Si (111) plane in the X-ray diffraction pattern.

In the composite process, the compound that forms the ion-conducting phase and the raw silicon may be separately microparticulated and then mixed. Alternatively, silicon nanoparticles and raw silicate nanoparticles may be synthesized and then mixed without using a grinding device. The nanoparticles may be produced by known methods such as a gas phase method (e.g., a plasma method) or a liquid phase method (e.g., a liquid phase reduction method).

<Third step>
In the third step, for example, the composite intermediate is sintered while applying pressure to the finely divided composite intermediate by a hot press or the like to obtain a sintered body. The composite intermediate is sintered, for example, in an inert atmosphere (for example, an atmosphere of argon, nitrogen, etc.). The sintering temperature is preferably 450°C or higher and 1000°C or lower. When the sintering temperature is within the above temperature range, it is easy to disperse a fine silicon phase in a silicate phase with low crystallinity. During sintering, the lithium silicate softens and flows to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body can be obtained in which the silicate phase is the sea portion and the silicon particles are the island portion. The sintering temperature is preferably 550°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower. The sintering time is, for example, 1 hour or higher and 10 hours or lower.

<Fourth step>
In the fourth step, the sintered body is pulverized to have a desired particle size distribution to obtain composite particles containing a silicate phase and a silicon phase dispersed in the silicate phase. By appropriately selecting the pulverization conditions, composite particles having a desired average particle size can be obtained.

The composition of the composite particles can be determined, for example, by the following analytical method. When analyzing the composite particles contained in the negative electrode of a secondary battery, the analysis can be performed by disassembling a fully discharged battery and removing the negative electrode. The removed negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the non-aqueous electrolyte components, and then dried. A cross-section of the negative electrode mixture layer may be obtained using a cross-section polisher (CP).

<EDX>
From the cross-sectional image of the backscattered electron image of the negative electrode mixture layer by SEM, 10 composite particles with a maximum particle diameter of 5 μm or more are randomly selected, and elemental mapping analysis is performed on each of them by energy dispersive X-ray (EDX). The area ratio of the target element is calculated using image analysis software. The observation magnification is preferably 2000 to 20000 times. The measured values of the area ratio of a predetermined element contained in the 10 particles are averaged. The content of the target element is calculated from the obtained average value.

Desirable measurement conditions for cross-sectional SEM-EDX analysis are shown below.
<SEM-EDX measurement conditions>
Processing equipment: JEOL, SM-09010 (Cross Section Polisher)
Processing conditions: Acceleration voltage 6 kV
Current value: 140 μA
Vacuum degree: 1×10 ^-3 ~2×10 ^-3 Pa
Measuring device: HITACHI SU-70 electron microscope
Acceleration voltage during analysis: 10 kV
Field: Free mode Probe current mode: Medium
Probe current range: High
Anode Ap.: 3
OBJ Apr.: 2
Analysis area: 1 μm square Analysis software: EDAX Genesis
CPS: 20500
Lsec: 50
Time constant: 3.2

<AES>
From the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, 10 composite particles having a maximum particle diameter of 5 μm or more are randomly selected, and each is subjected to a qualitative and quantitative analysis of elements using an Auger electron spectroscopy (AES) analyzer (e.g., JAMP-9510F manufactured by JEOL Ltd.). The measurement conditions may be, for example, an acceleration voltage of 10 kV, a beam current of 10 nA, and an analysis area of 20 μmφ. The content of a predetermined element contained in the 10 particles is averaged to calculate the content.

In the course of charging and discharging, a coating is formed on the surface of the composite particle due to the decomposition of the non-aqueous electrolyte. As described below, the composite particle may further include a conductive layer that covers the surface of the composite particle. Therefore, mapping analysis using EDX or AES is performed on a range 1 μm inside from the peripheral edge of the cross section of the composite particle so that the thin coating or conductive layer is not included in the measurement range. The mapping analysis also makes it possible to confirm the distribution state of the carbon material inside the composite particle. It is preferable to measure samples before or at the beginning of the cycle, as it becomes difficult to distinguish from decomposition products of the non-aqueous electrolyte at the end of the cycle.

<ICP>
A sample of the composite particles is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is filtered off. The filtrate is then analyzed by inductively coupled plasma emission spectrometry (ICP) to measure the spectral intensity of each element. A calibration curve is then created using commercially available standard solutions of the elements, and the content of each element contained in the composite particles is calculated.

The contents of Na, K, Al, and B contained in the composite particles can be quantitatively analyzed in accordance with JIS R3105 (1995) (method of analysis of borosilicate glass).

　Silicate phase-containing composite particles contain a silicate phase and a silicon phase, which can be distinguished and quantified using Si-NMR. The Si content obtained by the above method is the sum of the amount of Si constituting the silicon phase and the amount of Si in the silicate phase. The amount of Si element contained in the composite particles is distributed between the silicate phase and the silicon phase using the results of quantitative analysis by Si-NMR. The standard substance required for quantification can be a mixture containing a silicate phase and a silicon phase in a specified ratio with a known Si content.

Desirable conditions for Si-NMR measurement are shown below.
<Si-NMR measurement conditions>
Measurement equipment: Solid-state nuclear magnetic resonance spectrometer (INOVA-400), manufactured by Varian
Probe: Varian 7mm CPMAS-2
MAS: 4.2kHz
MAS speed: 4kHz
Pulse: DD (45° pulse + signal acquisition time 1H decoupled)
Repeat time: 1200 sec to 3000 sec
Observation width: 100kHz
Observation center: Around -100 ppm Signal acquisition time: 0.05 sec
Accumulation count: 560
Sample amount: 207.6 mg

FIG. 1 is a cross-sectional view showing a schematic example of a negative electrode active material (composite particle). Note that FIG. 1 omits the depiction of voids or pores present in the composite particle 23.

The negative electrode active material 20 comprises composite particles 23 (mother particles). The composite particles 23 comprise an ion-conducting phase 21 and a silicon phase (silicon particles) 22 dispersed within the ion-conducting phase 21. The composite particles 23 have a sea-island structure in which fine silicon phases 22 are dispersed within the matrix of the ion-conducting phase 21. The surface of the composite particles 23 is covered with a conductive coating layer 26.

(Secondary battery)
The secondary battery according to the embodiment of the present disclosure includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains the above-mentioned negative electrode active material for a secondary battery. The negative electrode of the secondary battery and other components will be described below.

(Negative electrode)
The negative electrode includes, for example, a negative electrode mixture layer containing the above-mentioned negative electrode active material for secondary batteries, and a negative electrode current collector supporting the negative electrode mixture layer. The negative electrode mixture layer can be formed by applying a negative electrode slurry in which the negative electrode mixture is dispersed in a dispersion medium to the surface of the negative electrode current collector and drying it. The coating film after drying may be rolled as necessary. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector, or on both surfaces.

The negative electrode mixture contains the above-mentioned negative electrode active material for secondary batteries as an essential component, and may contain binders, conductive agents, thickeners, etc. as optional components. The silicon phase in the composite particles can absorb many lithium ions, which contributes to increasing the capacity of the negative electrode. The content of the composite particles in the negative electrode mixture layer may be 1 mass% or more and 50 mass% or less with respect to the entire negative electrode mixture layer.

The negative electrode active material may further contain other active material materials that electrochemically absorb and release lithium ions. For example, a carbon-based active material is preferable as the other active material material. Since the composite particles expand and contract in volume with charging and discharging, if the ratio of the composite particles in the negative electrode active material increases, poor contact between the negative electrode active material and the negative electrode current collector is likely to occur with charging and discharging. On the other hand, by using the composite particles in combination with a carbon-based active material, it is possible to achieve excellent cycle characteristics while imparting the high capacity of the silicon phase to the negative electrode. The ratio of the composite particles to the total of the composite particles and the carbon-based active material is preferably, for example, 0.5 to 15 mass%, more preferably 1 to 5 mass%. This makes it easier to achieve both high capacity and improved cycle characteristics.

Examples of carbon-based active materials include graphite, easily graphitized carbon (soft carbon), and non-graphitizable carbon (hard carbon). Of these, graphite is preferred because of its excellent charge/discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, and graphitized mesophase carbon particles. Carbon-based active materials may be used alone or in combination of two or more types.

As the negative electrode current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. There are no particular limitations on the thickness of the negative electrode current collector, but from the viewpoint of the balance between the strength and weight reduction of the negative electrode, a thickness of 1 to 50 μm is preferable, and 5 to 20 μm is more preferable.

Examples of binders include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives. These may be used alone or in combination of two or more. Examples of conductive agents include carbon black, conductive fibers, carbon fluoride, and organic conductive materials. These may be used alone or in combination of two or more. Examples of thickeners include carboxymethyl cellulose (CMC), polyvinyl alcohol, and the like. These may be used alone or in combination of two or more.

Examples of dispersion media include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and mixtures of these.

(Positive electrode)
The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry in which the positive electrode mixture is dispersed in a dispersion medium to the surface of the positive electrode current collector and drying it. The coating film after drying may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector.

The positive electrode mixture contains a positive electrode active material as an essential component, and can contain optional components such as a binder and a conductive agent.

The positive electrode active material may be a lithium transition metal composite oxide. Examples _{of the lithium transition metal composite oxide include LiaCoO2 , LiaNiO2 , LiaMnO2} , _{LiaCobNi1 - bO2} , _LiaCobM1 - _bOc , _LiaNi1 - _bMbOc , _LiaMn2O4 , _{LiaMn2 - bMbO4} , _LiMePO4 , and _Li2MePO4F . Here, _M is at least one selected from _the group consisting of Na _, Mg, Sc, Y, _Mn , Fe, _{Co ,} Ni, Cu, _Zn , Al, Cr, _Pb , Sb _, and _B. Me contains at least a transition element (e.g., contains at least one selected from the group consisting of Mn, Fe, Co, and Ni), where 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

As the binder and conductive agent, the same ones as those exemplified for the negative electrode can be used. As the conductive agent, graphite such as natural graphite or artificial graphite can be used.

The shape and thickness of the positive electrode current collector can be selected from the same shape and range as the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

(Electrolytes)
The electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte is, for example, an electrolytic solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolytic solution is, for example, 0.5 mol/L or more and 2 mol/L or less. The electrolytic solution may contain a known additive.

The gel electrolyte contains a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

As the solid electrolyte, for example, a material known in all-solid-state lithium-ion secondary batteries (e.g., oxide-based solid electrolyte, sulfide-based solid electrolyte, halide-based solid electrolyte, etc.) is used.

For example, a liquid non-aqueous electrolyte is prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that ionizes in the electrolyte, and may include, for example, a lithium salt. The electrolyte may include various additives. The electrolyte is usually used in liquid form, but may also have its fluidity restricted by a gelling agent or the like.

Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylates, and chain carboxylates. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylates include γ-butyrolactone (GBL), and γ-valerolactone (GVL). Examples of chain carboxylates include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). The non-aqueous solvents may be used alone or in combination of two or more.

Other non-aqueous solvents include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

Examples of the lithium salt include lithium salts of chlorine-containing acids ( _LiClO4 , _LiAlCl4 , _{LiB10Cl10 ,} etc.), lithium salts of fluorine-containing acids ( _LiPF6 , _LiBF4 , _LiSbF6 , LiAsF6, _LiCF3SO3 , _{LiCF3CO2 , etc.} ), lithium salts of fluorine- _containing acid imides ₍ LiN( _{CF3SO2 ) 2} , LiN( _CF3SO2 )( _{C4F9SO2 )} , LiN( _{C2F5SO2 ) 2} , etc.), and _lithium halides (LiCl, LiBr _, LiI, etc.). The lithium salts may be used alone or in combination of two or more.

The concentration of the lithium salt in the electrolyte may be 1 mol/liter or more and 2 mol/liter or less, or may be 1 mol/liter or more and 1.5 mol/liter or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

(Separator)
Usually, it is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and has appropriate mechanical strength and insulation properties. As the separator, a microporous thin film, a woven fabric, a nonwoven fabric, etc. can be used. As the material of the separator, for example, a polyolefin such as polypropylene or polyethylene can be used.

One example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween, and a non-aqueous electrolyte are housed in an exterior body. Alternatively, instead of a wound type electrode group, other types of electrode groups may be used, such as a stacked type electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any type, such as a cylindrical type, a square type, a coin type, a button type, a laminate type, etc.

Below, the structure of a rectangular nonaqueous electrolyte secondary battery as an example of a secondary battery according to the present disclosure will be described with reference to FIG. 2. FIG. 2 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure with a portion cut away.

The battery comprises a bottomed rectangular battery case 4, and an electrode group 1 and a non-aqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 has a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat winding core and removing the winding core.

One end of the negative electrode lead 3 is attached to the negative electrode collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to the negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate (not shown). The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of the positive electrode lead 2 is attached to the positive electrode collector by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. In other words, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5, and separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting portion is laser welded. In this way, the opening of the battery case 4 is sealed by the sealing plate 5. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is blocked by a plug 8.

(Additional Note)
The above description of the embodiments discloses the following techniques.
(Technique 1)
Silicon-containing particles are included,
The silicon-containing particles include a silicate phase and a silicon phase dispersed within the silicate phase;
a N atom ratio R _N of the silicon-containing particle, which is quantified based on an N1s spectrum when a cross section of the silicon-containing particle is measured by X-ray photon spectroscopy (XPS), is 1% or more and 4% or less.
(Technique 2)
2. The negative electrode active material for a secondary battery according to claim 1, wherein the silicon-containing particles have a porosity of 5% or more and 8% or less.
(Technique 3)
The silicon-containing particle further includes a coating layer that covers at least a portion of the surface of the silicon-containing particle;
the coating layer includes a carbon material,
3. The negative electrode active material for a secondary battery according to claim 1, wherein the content of the carbon material is 3% by mass or more and 5% by mass or less with respect to the entirety of the silicon-containing particle including the coating layer.
(Technique 4)
The negative electrode active material for a secondary battery according to any one of Techniques 1 to 3, wherein the silicate phase contains at least one selected from the group consisting of Li ₂ Si ₂ O ₅ , Li ₂ SiO ₃ , and Li ₄ SiO ₄ .
(Technique 5)
The content of the silicon phase in the silicon-containing particles is 50 mass% or more and 70 mass% or less with respect to the whole of the silicon-containing particles.
(Technique 6)
The negative electrode active material for a secondary battery according to any one of Techniques 1 to 5, wherein the silicon-containing particles have an average particle size D50 of 6 μm or more and 8 μm or less.
(Technique 7)
The negative electrode active material for a secondary battery according to any one of the first to sixth aspects, wherein the crystallite size of the silicon phase is 5 nm to 10 nm.
(Technique 8)
The silicon phase has a functional group containing N on the surface thereof,
The negative electrode active material for a secondary battery according to any one of claims 1 to 7, wherein the functional group includes at least one selected from the group consisting of an amino group and a nitro group.
(Technique 9)
A positive electrode, a negative electrode, and an electrolyte,
The negative electrode of the secondary battery includes the negative electrode active material for the secondary battery according to any one of the first to eighth aspects.

[Example]
Examples of the present disclosure will be specifically described below, but the present disclosure is not limited to the following examples.

Example 1
(Preparation of Composite Particles)
<First step>
_Silicon dioxide and _Li2CO3 were mixed, and the mixture was calcined in air at 950°C for 10 hours to obtain silicate. The obtained silicate was pulverized to an average particle size of 10 µm.

<Second step>
Raw silicon (3N, average particle size 10 μm) was prepared. The prepared raw silicon was placed in a plasma processing device and exposed to ammonia (NH ₃ ) gas plasma for 30 minutes. As a result, amino groups were added to the particle surfaces of the raw silicon.

The raw silicon after plasma treatment was mixed with silicate to obtain a mixture. In the mixture, the mass ratio of silicate to raw silicon was 50:50.

The mixture was loaded into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was ground at 200-300 rpm for 25 hours in an inert atmosphere.

<Third step>
Next, the powder mixture was taken out in an inert atmosphere, and sintered in an inert atmosphere while applying pressure using a hot press machine to obtain a sintered body of the mixture.

<Fourth step>
Next, the obtained sintered body was crushed and passed through a 40 μm mesh to obtain composite particles having an average particle size (median size) of 6 μm. The composition of the main component of the silicate phase in the composite particles obtained by the above-mentioned method was Li ₂ Si ₂ O ₅ .

The composite particles and coal tar pitch were then mixed in a mass ratio of 95:5 and then fired at 800°C in an argon atmosphere to form a conductive coating layer that covered at least a portion of the surface of the composite particles, thereby obtaining silicon-containing particles. The firing converted the coal tar pitch into amorphous carbon.

The mass ratio of the coating layer to the entire silicon-containing particle including the coating layer was calculated from the mass difference between the composite particle before and after the coating layer was formed, and was found to be 3 mass%.

The silicon-containing particles were subjected to XPS analysis, and the N atom ratio R 2 _N was determined to be 1%.

The porosity of the silicon-containing particles was also determined using the method previously described and was found to be 8%.

(Preparation of negative electrode)
The composite particles and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. Water was added to the negative electrode mixture containing the negative electrode active material, the Na salt of CMC, and SBR in a mass ratio of 97.5:1:1.5, and the mixture was stirred to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to the surface of the copper foil so that the mass of the negative electrode mixture per 1 ^m2 was 190 g, and the coating was dried and then rolled to prepare a negative electrode in which a negative electrode mixture layer with a density of 1.5 g/ ^cm3 was formed on both sides of the copper foil.

(Preparation of Positive Electrode)
A positive electrode mixture containing lithium cobalt oxide, acetylene black, and PVDF in a mass ratio of 95:2.5:2.5 was mixed with NMP to prepare a positive electrode slurry. The positive electrode slurry was then applied to the surface of an aluminum foil, and the coating was dried and rolled to prepare a positive electrode having a positive electrode mixture layer with a density of 3.6 g/ ^cm3 formed on both sides of the aluminum foil.

(Preparation of non-aqueous electrolyte)
A non-aqueous electrolyte (electrolytic solution) was prepared by dissolving _LiPF6 at a concentration of 1.0 mol/L in a mixed solvent containing EC and DEC in a volume ratio of 3:7.

(Preparation of secondary battery)
The positive and negative electrodes, each having a tab attached thereto, were wound with a separator interposed therebetween to prepare an electrode group in which the tabs were located at the outermost periphery. The electrode group was inserted into an exterior body made of an aluminum laminate film, and vacuum dried at 105° C. for 2 hours. After that, a nonaqueous electrolyte was injected, and the opening of the exterior body was sealed to obtain a battery A1 of Example 1.

Examples 2 to 7 and Comparative Examples 1 to 4
In Examples 2 and 3, the time for which the raw material silicon was exposed to plasma in the production of the composite particles was changed to 60 minutes in Example 2 and 90 minutes in Example 3.

In Examples 4 and 5, the mass ratio of silicate to raw silicon in the mixture of silicate and raw silicon in the production of composite particles was changed to 45:55. In addition, the time for exposing the raw silicon to plasma in the production of composite particles was 65 minutes in Example 4 and 95 minutes in Example 5.

In Examples 6 and 7, the mass ratio of silicate to raw silicon in the mixture of silicate and raw silicon in the production of composite particles was changed to 40:60. In addition, the time for exposing the raw silicon to plasma in the production of composite particles was 70 minutes in Example 6 and 100 minutes in Example 7.

In Comparative Examples 1 to 4, the raw silicon was not plasma-treated in the production of the composite particles. In Comparative Examples 2 to 4, the mass ratio of silicate to raw silicon in the mixture of silicate and raw silicon in the production of the composite particles was changed from that in Example 1, with silicate:raw silicon = 45:55 in Comparative Example 2, 40:60 in Comparative Example 3, and 35:65 in Comparative Example 4.

Otherwise, the composite particles were produced in the same manner as in Example 1, and negative electrodes and secondary batteries using the composite particles as the active material were produced to obtain batteries A2 to A7 and B1 to B4 according to Examples 2 to 7 and Comparative Examples 1 to 4, respectively.

The N atomic ratio R _{1 N} and the porosity of the silicon-containing particles used in each secondary battery were determined in the same manner as in Example 1. The results of the N atomic ratio R _{1 N} and the porosity are shown in Table 1.

(evaluation)
<Charging>
Each battery was charged at a constant current of 1 It (800 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 1/20 It (40 mA).

<Discharge>
A constant current discharge was performed at a current of 1 It (800 mA) until the voltage reached 2.75 V.

The rest period between charging and discharging was 10 minutes.
The charging and discharging were carried out in an environment of 25°C.

For each battery, the discharge capacity C1 at the first cycle and the discharge capacity C300 at the 300th cycle were determined. The value expressed as (C300/C1) x 100 was evaluated as the capacity retention rate X (%). Table 1 shows the evaluation results of the capacity retention rate X for batteries A1 to A7 and B1 to B4.

As shown in Table 1, in the batteries A1 to A7 in which silicon-containing particles having an N atomic ratio R _N of 1% or more were used as the negative electrode active material, the porosity of the silicon-containing particles was reduced to 8% or less, and a high capacity was maintained even after 300 cycles. In contrast, in the batteries B1 to B4 of the comparative example, the capacity retention rate at the time of 300 cycles was reduced to 55% or less. Note that the N atomic ratio R _N of 0.5% in the batteries B1 to B4, even though no plasma treatment was performed, is thought to be due to a nitrogen-derived material contained in a small amount in the raw silicon prepared.

The secondary battery disclosed herein is useful as a main power source for mobile communication devices, portable electronic devices, etc.

Although the present invention has been described with respect to the presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various modifications and alterations will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to embrace all such modifications and alterations without departing from the true spirit and scope of the invention.

1: Electrode group, 2: Positive electrode lead, 3: Negative electrode lead, 4: Battery case, 5: Sealing plate, 6: Negative electrode terminal, 7: Gasket, 8: Plug, 20: Negative electrode active material, 21: Ion conductive phase, 22: Silicon phase, 23: Composite particle, 26: Coating layer

Claims (9)

Silicon-containing particles are included,
The silicon-containing particles include a silicate phase and a silicon phase dispersed within the silicate phase;
a N atom ratio R _N of the silicon-containing particle, which is quantified based on an N1s spectrum when a cross section of the silicon-containing particle is measured by X-ray photon spectroscopy (XPS), is 1% or more and 4% or less. The negative electrode active material for a secondary battery according to claim 1, wherein the porosity of the silicon-containing particles is 5% or more and 8% or less. The silicon-containing particle further includes a coating layer that covers at least a portion of the surface of the silicon-containing particle;
The coating layer includes a carbon material,
2 . The negative electrode active material for a secondary battery according to claim 1 , wherein the content of the carbon material is 3% by mass or more and 5% by mass or less with respect to the entirety of the silicon-containing particle including the coating layer. 4. The negative electrode active material for a secondary battery according to claim 1, wherein the silicate phase contains at least one selected from the group consisting of Li ₂ Si ₂ O ₅ , Li ₂ SiO ₃ , and Li ₄ SiO ₄ . The negative electrode active material for secondary batteries according to any one of claims 1 to 3, wherein the content of the silicon phase in the silicon-containing particles is 50 mass% or more and 70 mass% or less with respect to the entire silicon-containing particles. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein the average particle size D50 of the silicon-containing particles is 6 μm or more and 8 μm or less. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein the crystallite size of the silicon phase is 5 nm to 10 nm. The silicon phase has a functional group containing N on the surface thereof,
4. The negative electrode active material for a secondary battery according to claim 1, wherein the functional group includes at least one selected from the group consisting of an amino group and a nitro group. A positive electrode, a negative electrode, and an electrolyte,
The negative electrode of a secondary battery comprises the negative electrode active material for a secondary battery according to any one of claims 1 to 3.

Images