Negative electrode active material for secondary batteries, and secondary battery

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 RN 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

[0001] The present 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.

[0002] A negative electrode active material capable of absorbing and releasing lithium ions is used for the negative electrode of a secondary battery, such as a lithium-ion secondary battery. 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 investigated as a negative electrode active material (see, for example, Patent Document 1).

[0003] International Publication No. 2020 / 110917

[0004] Silicon-containing materials are promising as high-capacity anode materials for secondary batteries, but they tend to expand and contract significantly during charging and discharging, which can lead to side reactions.

[0005] Silicon-containing materials having a silicate phase and a silicon phase dispersed in the silicate phase are prone to cracking due to the expansion and contraction of the silicon phase during charge and discharge. Meanwhile, a side reaction can occur in which the electrolyte reacts with the silicate phase to produce hydrogen fluoride (HF). The generated HF corrodes and deactivates the silicon phase through cracks in the silicate phase. As a result, the capacity retention rate during charge and discharge cycles is likely to decrease.

[0006] In view of the above, one aspect of the present disclosure provides a silicon-containing particle, the silicon-containing particle including a silicate phase and a silicon phase dispersed within the silicate phase, wherein when a cross section of the silicon-containing particle is measured by X-ray photon spectroscopy (XPS), an N atom ratio R of the silicon-containing particle is quantified based on an N1s spectrum. Ｎ The present invention relates to a negative electrode active material for a secondary battery, in which the content of Zn is 1% or more and 4% or less.

[0007] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains the above-described negative electrode active material for a secondary battery.

[0008] According to the present disclosure, it is possible to suppress a decrease in the capacity retention rate that accompanies charge and discharge of a secondary battery.

[0009] 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.

[0010] 1 is a cross-sectional view schematically illustrating an example of a negative electrode active material (silicon-containing material) according to an embodiment of the present disclosure; 2 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away;

[0011] The following describes embodiments of the present disclosure using examples, 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 term "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 lower and upper limits 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 materials may be used in combination.

[0012] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0013] (Negative electrode active material for secondary battery) The negative electrode active material for secondary battery according to one embodiment of the present disclosure includes silicon-containing particles. The silicon-containing particles include a silicate phase and a silicon phase dispersed within the silicate phase. Hereinafter, the silicon-containing particles are also referred to as "composite particles."

[0014] The silicate phase may form an amorphous phase. The silicate phase may be a lithium silicate phase.

[0015] The silicon-containing particles contain nitrogen (N) atoms. The nitrogen atoms may be unevenly distributed on the surface of the silicon phase (the interface between the silicon phase and the silicate phase) inside the silicon-containing particles.

[0016] Silicon-containing particles are prepared, for example, by mixing silicate particles that form a silicate phase with raw silicon nanoparticles that form a silicon phase, as described below, and then compositing them together. In this case, the surfaces of the silicon nanoparticles tend to be lipophilic. On the other hand, because silicate is hydrophilic, it is difficult to bring the silicate particles and silicon nanoparticles into close contact during compositing, and voids tend to form at the interface between the silicate phase and the silicon phase inside the silicon-containing particles.

[0017] The nitrogen atoms form Si-N bonds on the surface of the silicon phase, which changes the surface of the silicon phase to hydrophilicity. 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 a decrease in the cycle retention rate can be suppressed.

[0018] The presence of nitrogen atoms in silicon-containing particles can be detected by X-ray photoelectron spectroscopy (XPS). The nitrogen content can also be quantitatively evaluated. When a cross section of a silicon-containing particle is measured by X-ray photoelectron spectroscopy (XPS), the N atom ratio R of the silicon-containing particle is quantified based on the N1s spectrum. Ｎ is 1% or more and 4% or less. Ｎ may be 2% or more and 4% or less.

[0019] X-ray photon spectroscopy is a technique for analyzing the composition and chemical bonding state of elements constituting 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. Analysis of the N1s spectrum can evaluate the bonding state of nitrogen atoms and the nitrogen content contained in silicon-containing particles. Relative sensitivity factors (RSFs) can be used as a method for quantitatively evaluating the content of each element contained in silicon-containing particles.

[0020] The N atom ratio R Ｎ An example of the conditions for XPS measurement to determine the binding energy is shown below. The C1s spectrum (248.5 eV) of graphite can be used to calibrate the binding energy. Measurement equipment: ESCA5600 manufactured by ULVAC-PHI, Inc. X-ray source used: Al-Kα radiation, Mg-Kα radiation Analysis area: 800 μmΦ Quantitative conversion method: Quantitative values ​​are calculated based on the value obtained by multiplying the peak area of ​​each element by the relative sensitivity coefficient.

[0021] When analyzing 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 a cross section of the negative electrode. The taken-out negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove non-aqueous electrolyte components, 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 each is subjected to XPS analysis. The N atomic ratio to all atoms is calculated for each composite particle, and the average N atomic ratio of the 10 composite particles is calculated to obtain the N atomic ratio R Ｎ Let's say.

[0022] The N atomic ratio R in this range ＮThe porosity of the silicon-containing particles having the above formula (I) 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 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.

[0023] The porosity of the 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 defined as the total volume occupied by pores, and the porosity is calculated.

[0024] The pore size distribution is measured by collecting 0.20 g to 0.25 g of a silicon-containing particle sample, placing the sample in a measurement cell composed of a glass tube for specific surface area measurement, and then drying and degassing the measurement cell. The 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 then washed with a solvent such as dimethyl carbonate. The negative electrode mixture layer is then extracted 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 measurement device. For example, a Shimadzu Tristar II 3020 automatic specific surface area / pore distribution measurement device is used. From the adsorption amount measurement results, the volumetric pore size distribution is determined using the BET multipoint method.

[0025] 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 as described above. In the cross-sectional image of the silicon-containing particles, 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.

[0026] 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 nitrogen-containing functional groups are added to the surface. Examples of nitrogen-containing functional groups include amino groups (-NH ２ ), imino group (=NH, -NH-), amide group (-CONH-), nitro group (NO ２ Among these, amino and nitro groups are preferred.

[0027] By mixing the surface-modified silicon nanoparticles with silicate particles and forming a composite, silicon-containing particles having nitrogen atoms at the interface between the silicon phase and the silicate phase and the silicon phase dispersed in the silicate phase can be obtained. However, the method for producing silicon-containing particles is not limited to the above method.

[0028] The presence of nitrogen atoms at the interface between the silicon phase and the silicate phase inside the silicon-containing particle can be confirmed, for example, by measuring the absorption spectrum due to molecular vibration using infrared spectroscopy and detecting -N bonds derived from nitrogen functional groups on the Si surface.

[0029] (Composite Particles) The composite particles have a structure in which a silicon phase is dispersed within an ion-conducting phase (matrix). The ion-conducting phase alleviates the stress associated with the expansion and contraction of the silicon phase during charge and discharge, suppressing cracking and fracture of the composite particles. This makes it possible to achieve both high capacity due to the inclusion of silicon and improved cycle characteristics. The ion-conducting phase contains a silicate phase. The ion-conducting phase may be composed of a single phase consisting of only a silicate phase, or may be composed of multiple phases including a silicate phase and an ion-conducting layer other than the silicate phase. The ion-conducting layer other than the silicate phase may contain either a silicon oxide phase or a carbon phase.

[0030] The silicate phase is composed of a compound containing a metal element, silicon (Si), and oxygen (O). Examples of the metal element include alkali metal elements such as lithium and Group 2 elements of the long periodic table. The silicate phase preferably contains at least lithium silicate. In this case, lithium ions can easily enter and exit 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-conducting phase may contain, for example, a silicate phase as a main component and a small amount of silicon oxide phase. Here, the "main component" refers to a component that accounts for 50% by mass or more of the total mass of the silicon compound phase, and may also account for 70% by mass or more. The silicate phase (lithium silicate phase) is composed of Li ２ Si ２ O ５ , Li ２ SiO ３ , and Li ４ SiO ４ The composition may include at least one selected from the group consisting of:

[0031] The atomic ratio of O to Si in the 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 the lithium silicate: Li / Si is, for example, greater than 0 and less than 4.

[0032] The composition of lithium silicate is represented by the formula: Li ２ｚ SiO ２＋ｚ (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z preferably satisfies the relationship 0<z≦1, and more preferably z=1 / 2. For example, when z=1, lithium silicate is Li ２ SiO ３ When z = 1 / 2, Li ２ Si ２ O ５ Lithium silicate can be expressed as Li ２ Si ２It is preferable that the main component is O5, and Li ２ Si ２ It is desirable that O5 be the main component of the entire silicate phase. Here, the term "main component" refers to a component that accounts for 50% by mass or more of the mass of the entire lithium silicate or the entire silicate phase, and may account for 70% by mass or more.

[0033] The silicate phase may further 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.

[0034] The silicate phase may contain, as an element M other than Li, Si and O, at least one element selected from the group consisting of alkali metal elements (excluding lithium) and Group II elements.

[0035] By including an alkali metal element other than Li in the silicate phase, crystallization becomes difficult, the viscosity in the softened state is low, and the fluidity is high. Therefore, in the heat treatment step, it is easy to fill the gaps between silicon particles, and it is easy to produce dense composite particles. As the alkali metal element, Na and / or K are preferable because they are inexpensive.

[0036] In addition, while silicate phases generally exhibit alkalinity, Group II elements have the effect of suppressing the elution of alkali metals from the silicate phase. Therefore, when a Group II element is contained in the silicate phase, the viscosity of the slurry containing the negative electrode active material is easily stabilized when it is prepared. This also reduces the need for a treatment (e.g., acid treatment) to neutralize the alkaline component of the composite particles. The Group II element may be either Ca or Mg. Among these, Ca is preferred because it can improve the Vickers hardness of the silicate phase and further improve the cycle characteristics.

[0037] The silicate phase may further contain 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) as element M. From the viewpoint of resistance to non-aqueous electrolytes and structural stability of the silicate phase, element M preferably contains at least one element selected from the group consisting of Zr, Ti, P, Al, and B.

[0038] The silicate phase may further contain a rare earth element as element M. The inclusion of a rare earth element in the silicate phase can improve the charge / discharge efficiency at the initial stage of the charge / discharge cycle. The rare earth element may be any of scandium (Sc), yttrium (Y), and a lanthanoid element. Note that lanthanum (La), yttrium (Y), and erbium (Er) are rare earth elements. The silicate phase may contain at least one rare earth element 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 proportion of La in the total rare earth elements is preferably 90 atomic % or more and 100 atomic % or less.

[0039] As the 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. Furthermore, Zr, Ti, P, Al, and B have the effect of increasing resistance to non-aqueous electrolytes and structural stability of the silicate phase.

[0040] 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).

[0041] The element M may form a compound. The compound may be, for example, a silicate of the element M or an oxide of the element M depending on the type of the element M. In the silicate phase, the content of the 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.

[0042] The average particle size of the fine silicon phase (before the first charge) dispersed within the ion-conducting phase may be 500 nm or less, 200 nm or more, or 50 nm or less. By appropriately miniaturizing the silicon phase in this manner, volume change during charge and discharge is reduced, improving structural stability. Furthermore, uniform expansion and contraction of the silicon phase is achieved, suppressing particle cracking and improving cycle performance. 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 100 randomly selected silicon phases.

[0043] 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 volume change due to expansion and contraction of the silicon phase (silicon particles) during 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 using the Scherrer equation from the half-width of the diffraction peak assigned to the Si (111) plane in the X-ray diffraction (XRD) pattern.

[0044] 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, relative to the entire composite particles.

[0045] 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. Within 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 integrated value is 50% in the particle size distribution measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by Horiba Ltd. can be used as the measuring device.

[0046] (Coating Layer) The negative electrode active material for a secondary battery may further include a coating layer that coats at least a portion of the surface of the silicon-containing particle. The coating layer is preferably a conductive layer having electrical conductivity. By coating at least a portion of the surface of the silicon-containing particle with a conductive layer, the electronic conductivity of the silicon-containing particle is improved. The coating layer also functions to prevent electrolyte from penetrating into the pores (voids) of the silicon-containing particle and to suppress side reactions. In order to suppress side reactions and maintain high cycle characteristics, the content of the carbon material is preferably 3% by mass or more and 5% by mass or less based on the entire silicon-containing particle including the coating layer.

[0047] The coating layer contains, for example, a conductive carbon material. The thickness of the coating layer is preferably thin enough not to 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.

[0048] The coating layer or conductive layer is formed by mixing a carbonaceous raw material with composite particles and firing the mixture to carbonize the conductive carbonaceous raw material. Examples of the carbonaceous raw material include coal pitch, coal tar pitch, petroleum pitch, and phenolic resin. The mixture of the carbonaceous raw material and composite particles is fired, for example, in an inert atmosphere (e.g., an argon or nitrogen atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, a highly conductive layer is easily formed in a silicate phase with low crystallinity. The firing temperature is preferably 550°C or higher and 900°C or lower, 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.

[0049] (Method for producing silicon-containing particles) An example of a method for producing composite particles containing a lithium silicate phase as the silicate phase will be described below in detail, as an example of silicon-containing particles. The composite particles are produced, for example, by a production method including the following steps 1 to 4: (Step 1) A step of obtaining a compound that forms an ion-conducting phase. (Step 2) After step 1, a step of compounding the compound that forms the ion-conducting phase with raw material silicon to disperse the silicon phase within the ion-conducting phase to obtain a composite intermediate. (Step 3) A step of heat-treating the composite intermediate to obtain a sintered body containing the ion-conducting phase and the silicon phase dispersed within the ion-conducting phase. (Step 4) A step of pulverizing the sintered body to obtain composite particles containing the ion-conducting phase and the silicon phase dispersed within the ion-conducting phase. Step 2 includes a step of covering the surfaces of raw material silicon particles with a nitrogen-containing compound prior to compounding.

[0050] <First Step> In the first step, a compound that forms 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, and more preferably 800°C or higher and 1100°C or lower.

[0051] The raw material mixture is melted, and the melt 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. The flaked silicate can also be used without crystallization. It is also possible to produce silicate by firing a predetermined amount of the mixture at a temperature below the melting point without melting it, resulting in a solid-state reaction.

[0052] 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, which is one of the lithium, silicon, and oxygen ultraviolet elements 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 silicon oxide crystals.

[0053] Examples of the lithium compound include lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. The lithium compounds may be used alone or in combination of two or more.

[0054] The compound containing the element M may be an oxide, hydroxide, hydride, halide, carbonate, oxalate, nitrate, sulfate, or the like of the element M. The compound containing the element M may be used alone or in combination of two or more.

[0055] <Second Step> In the second step, for example, a mixture of a compound that forms an ion-conductive phase (e.g., a carbon source or raw silicate) and raw silicon is pulverized while applying shear force to obtain a finely divided composite intermediate. Here, the surfaces of the raw silicon particles are covered with a nitrogen-containing compound in a process described below. For example, the compound that forms an ion-conductive 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 agitated and pulverized using a pulverizer such as a ball mill. An organic solvent may be added to the mixture, followed by wet pulverization. At this time, the raw silicon is pulverized to produce a silicon phase. The silicon phase is dispersed within a matrix of the compound that forms the ion-conductive phase.

[0056] Examples of organic solvents that can be used include alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, and metal alkoxides. A predetermined amount of organic solvent may be added to the grinding vessel all at once at the beginning of grinding, or may be added intermittently 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.

[0057] (Step of covering the surface of raw silicon particles with a nitrogen-containing compound) Prior to the composite formation, a step of covering the surface of raw silicon particles with a nitrogen-containing compound is carried out. For example, by exposing the raw silicon particles to plasma of a nitrogen-containing compound, 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 an amino group (-NH ２ ), imino group (=NH, -NH-), amide group (-CONH-), nitro group (NO ２ ) can be mentioned.

[0058] The raw silicon may be coarse silicon particles having an average particle size of several μm to several tens of μm. The silicon phase finally obtained is preferably controlled so that the crystallite size calculated from the half-width of the diffraction peak attributable to the Si(111) plane in the X-ray diffraction pattern using the Scherrer equation is 10 nm or less.

[0059] In the composite process, the compound forming 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).

[0060] <Step 3> In step 3, the microparticulated composite intermediate is fired while applying pressure to the composite intermediate using a hot press or the like to obtain a sintered body. The firing of the composite intermediate is carried out, for example, in an inert atmosphere (e.g., an argon, nitrogen, or the like). The firing temperature is preferably 450°C or higher and 1000°C or lower. When the firing temperature is within this temperature range, it is easy to disperse a fine silicon phase within the 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, with the silicate phase forming a sea portion and the silicon particles forming islands. The firing temperature is preferably 550°C or higher and 900°C or lower, 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.

[0061] <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 within the silicate phase. By appropriately selecting the pulverization conditions, composite particles with a predetermined average particle size can be obtained.

[0062] 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 on the negative electrode removed from a fully discharged battery by disassembling it. The removed negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the non-aqueous electrolyte component, and then dried. A cross-section of the negative electrode mixture layer may be obtained using a cross-section polisher (CP).

[0063] <EDX> From the cross-sectional image of the backscattered electron image of the negative electrode mixture layer taken 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 particle using 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 20,000 times. The measured values ​​of the area ratio of a specific element contained in the 10 particles are averaged. The content of the target element is calculated from the obtained average value.

[0064] Desirable measurement conditions for cross-sectional SEM-EDX analysis are shown below. <SEM-EDX measurement conditions> Processing equipment: SM-09010 (Cross Section Polisher) manufactured by JEOL Processing conditions: Acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 ~2 x 10 -3 Pa Measuring device: Electron microscope HITACHI SU-70 Acceleration voltage during analysis: 10 kV Field: Free mode Probe current mode: Medium Probe current range: High Anode Ap.: 3 OBJ Ap.: 2 Analysis area: 1 μm square Analysis software: EDAX Genesis CPS: 20500 Lsec: 50 Time constant: 3.2

[0065] <AES> From the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, ten composite particles having a maximum particle diameter of 5 μm or more are randomly selected, and each is subjected to qualitative and quantitative elemental analysis using an Auger electron spectroscopy (AES) analyzer (e.g., JAMP-9510F manufactured by JEOL Ltd.). 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 ten particles is averaged to calculate the content.

[0066] During the charge / discharge process, a coating is formed on the surface of the composite particle due to decomposition of the non-aqueous electrolyte, etc. Furthermore, as described below, the composite particle may further include a conductive layer covering the surface of the composite particle. Therefore, mapping analysis using EDX or AES is performed within 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. Mapping analysis also makes it possible to confirm the distribution of the carbon material inside the composite particle. Since it becomes difficult to distinguish from decomposition products of the non-aqueous electrolyte at the end of the cycle, it is preferable to measure a sample before or at the beginning of the cycle.

[0067] <ICP> A sample of 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 spectroscopy (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.

[0068] 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).

[0069] In silicate phase-containing composite particles, a silicate phase and a silicon phase exist, and these 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. Note that the standard substance required for quantification can be a mixture containing a silicate phase and a silicon phase in a predetermined ratio with known Si contents.

[0070] Desirable Si-NMR measurement conditions are shown below. <Si-NMR measurement conditions> Measurement device: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupled) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: near -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0071] 1 is a cross-sectional view schematically illustrating an example of a negative electrode active material (composite particle). Note that in FIG. 1, voids or pores present in the composite particle 23 are omitted.

[0072] The negative electrode active material 20 includes composite particles 23 (mother particles). The composite particles 23 include 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 a matrix of the ion-conducting phase 21. The surfaces of the composite particles 23 are covered with a conductive coating layer 26.

[0073] (Secondary Battery) A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains the above-described negative electrode active material for a secondary battery. The negative electrode of the secondary battery and other components will be described below.

[0074] (Negative Electrode) The negative electrode includes, for example, a negative electrode mixture layer containing the above-described 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 dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0075] The negative electrode mixture contains the above-mentioned negative electrode active material for a secondary battery as an essential component, and may contain optional components such as a binder, a conductive agent, and a thickener. The silicon phase in the composite particles can absorb many lithium ions, thereby contributing to a high 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.

[0076] The negative electrode active material may further contain another active material that electrochemically absorbs and releases lithium ions. A preferred example of such another active material is a carbon-based active material. Because the composite particles expand and contract in volume with charge and discharge, a large proportion of the composite particles in the negative electrode active material is likely to result in poor contact between the negative electrode active material and the negative electrode current collector with charge and discharge. On the other hand, the combined use of the composite particles and a carbon-based active material makes it possible to achieve excellent cycle characteristics while imparting high capacity to the negative electrode due to the silicon phase. The proportion of the composite particles in the total of the composite particles and the carbon-based active material is preferably 0.5 to 15 mass%, more preferably 1 to 5 mass%. This makes it easier to achieve both high capacity and improved cycle characteristics.

[0077] Examples of carbon-based active materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among 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. One type of carbon-based active material may be used alone, or two or more types may be used in combination.

[0078] As the negative electrode current collector, a non-porous conductive substrate (such as a 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 alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm, from the viewpoint of balancing the strength and weight of the negative electrode.

[0079] 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 fiber, 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) and polyvinyl alcohol. These may be used alone or in combination of two or more.

[0080] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and a mixed solvent thereof.

[0081] (Positive Electrode) The positive electrode comprises, 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 dried coating 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.

[0082] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, and the like as optional components.

[0083] As the positive electrode active material, a lithium transition metal composite oxide can be used. Examples of the lithium transition metal composite oxide include Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn2O4, Li a Mn2-b M b Examples of the lithium-ion battery include LiMePO4, 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., at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 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.

[0084] The binder and conductive agent may be the same as those exemplified for the negative electrode. As the conductive agent, graphite such as natural graphite or artificial graphite may be used.

[0085] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0086] (Electrolyte) 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 known additives.

[0087] The gel electrolyte contains a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0088] As the solid electrolyte, for example, a material known in all-solid-state lithium ion secondary batteries (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) can be used.

[0089] 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 contain various additives. The electrolyte is usually used in its liquid state, but its fluidity may be limited by a gelling agent or the like.

[0090] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. 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 carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0091] Other examples of the non-aqueous solvent include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0092] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts such as fluorine-containing acids (LiPF, LiBF, LiSbF, LiAsF, LiCF, SO, LiCF, CO, etc.), lithium salts of fluorine-containing acid imides (LiN(CF, SO), LiN(CF) SO (CF, SO), LiN(CF, SO) (CF, SO), LiN(CF, SO), etc.), and lithium halides (LiCl, LiBr, LiI, etc.). One type of lithium salt may be used alone, or two or more types may be used in combination.

[0093] The concentration of the lithium salt in the electrolyte may be 1 mol / L or more and 2 mol / L or less, or may be 1 mol / L or more and 1.5 mol / L 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.

[0094] (Separator) It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator can be made of, for example, a polyolefin such as polypropylene or polyethylene.

[0095] An 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 nonaqueous electrolyte are housed in an exterior body. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0096] Hereinafter, the structure of a prismatic non-aqueous 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.

[0097] The battery includes a bottomed prismatic battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes 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 plate-shaped winding core and then removing the winding core.

[0098] One end of a negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a 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 a positive electrode lead 2 is attached to the positive electrode current 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. That is, 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 also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. A non-aqueous electrolyte injection hole provided in the sealing plate 5 is closed with a seal 8.

[0099] (Additional Notes) The above embodiments disclose the following techniques: (Technology 1) A silicon-containing particle includes a silicate phase and a silicon phase dispersed in the silicate phase, and when a cross section of the silicon-containing particle is measured by X-ray photon spectroscopy (XPS), an N atom ratio R of the silicon-containing particle is quantified based on an N1s spectrum. Ｎ (Technology 2) The negative electrode active material for a secondary battery according to Technology 1, wherein the porosity of the silicon-containing particles is 5% or more and 8% or less. (Technology 3) The negative electrode active material for a secondary battery according to Technology 1 or 2, further comprising a coating layer that coats at least a part of the surface of the silicon-containing particles, the coating layer comprising a carbon material, and the content of the carbon material is 3% by mass or more and 5% by mass or less with respect to the whole of the silicon-containing particles including the coating layer. (Technology 4) The silicate phase is Li ２ Si ２ O ５ , Li ２ SiO ３ , and Li ４ SiO ４The negative electrode active material for a secondary battery according to any one of Techniques 1 to 3, comprising at least one selected from the group consisting of: (Technology 5) The negative electrode active material for a secondary battery according to any one of Techniques 1 to 4, 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 total mass of the silicon-containing particles. (Technology 6) The negative electrode active material for a secondary battery according to any one of Techniques 1 to 5, wherein the average particle size D50 of the silicon-containing particles is 6 μm or more and 8 μm or less. (Technology 7) The negative electrode active material for a secondary battery according to any one of Techniques 1 to 6, wherein the crystallite size of the silicon phase is 5 nm to 10 nm. (Technology 8) The negative electrode active material for a secondary battery according to any one of Techniques 1 to 7, wherein the surface of the silicon phase has a functional group containing N, the functional group including at least one selected from the group consisting of an amino group and a nitro group. (Technology 9) A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains the negative electrode active material for secondary batteries according to any one of Technologies 1 to 8.

[0100] Examples Examples of the present disclosure will be specifically described below, but the present disclosure is not limited to the following examples.

[0101] Example 1 (Preparation of Composite Particles) <First Step> Silicon dioxide and Li ２ CO ３ The mixture was mixed and fired in air at 950°C for 10 hours to obtain silicate, which was then pulverized to an average particle size of 10 µm.

[0102] <Second Step> Raw silicon (3N, average particle size 10 μm) was prepared. The prepared raw silicon was placed in a plasma processing device, and ammonia (NH ３ The raw silicon particles were exposed to plasma of the HCl gas for 30 minutes, thereby adding amino groups to the surface of the raw silicon particles.

[0103] The plasma-treated raw silicon was mixed with silicate to obtain a mixture, in which the mass ratio of silicate to raw silicon was 50:50.

[0104] The mixture was charged 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 pot was closed with a lid, and the mixture was milled at 200 to 300 rpm for 25 hours in an inert atmosphere.

[0105] <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.

[0106] <Fourth Step> Next, the obtained sintered body was pulverized 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 determined by the above-mentioned method was Li ２ Si ２ O ５ It was.

[0107] Next, the composite particles and coal tar pitch were mixed in a mass ratio of 95:5, and then calcined at 800°C in an argon atmosphere to form a conductive coating layer covering at least a portion of the surface of the composite particles, thereby obtaining silicon-containing particles. By calcining, the coal tar pitch was converted into amorphous carbon.

[0108] The mass ratio of the coating layer to the entire silicon-containing particle including the coating layer was calculated from the difference in mass between the composite particle before and after the coating layer was formed, and was found to be 3 mass %.

[0109] The silicon-containing particles were subjected to XPS analysis to determine the N atom ratio R Ｎ The result was 1%.

[0110] The porosity of the silicon-containing particles was determined by the method described above and was found to be 8%.

[0111] (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 a negative electrode mixture containing the negative electrode active material, a 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, a 1 m thick layer was applied to the surface of a copper foil. ２The negative electrode slurry was applied to both sides of the copper foil so that the mass of the negative electrode mixture per sheet was 190 g, and the coating was dried and then rolled to form a sheet with a density of 1.5 g / cm ３ A negative electrode having the negative electrode mixture layer formed thereon was fabricated.

[0112] (Preparation of Positive Electrode) NMP was added to a positive electrode mixture containing lithium cobalt oxide, acetylene black, and PVDF in a mass ratio of 95:2.5:2.5, and the mixture was stirred to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, and the coating was dried and then rolled to form a positive electrode slurry having a density of 3.6 g / cm on both sides of the aluminum foil. ３ A positive electrode having the positive electrode mixture layer formed thereon was fabricated.

[0113] (Preparation of non-aqueous electrolyte) LiPF was dissolved in a mixed solvent containing EC and DEC in a volume ratio of 3:7. ６ was dissolved at a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte (electrolytic solution).

[0114] (Fabrication of Secondary Battery) A positive electrode and a negative electrode, each having a tab attached thereto, were wound with a separator interposed therebetween to fabricate an electrode assembly with the tabs positioned at the outermost periphery. The electrode assembly 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 poured into the exterior body, and the opening of the exterior body was sealed to obtain a battery A1 of Example 1.

[0115] Examples 2 to 7, Comparative Examples 1 to 4 In Examples 2 and 3, the time for exposing raw material silicon to plasma in producing composite particles was changed to 60 minutes in Example 2 and 90 minutes in Example 3.

[0116] In Examples 4 and 5, the mass ratio of silicate to raw material silicon in the mixture of silicate and raw material silicon in the preparation of composite particles was changed to 45: 55. In addition, the time for exposing the raw material silicon to plasma in the preparation of composite particles was set to 65 minutes in Example 4 and 95 minutes in Example 5.

[0117] In Examples 6 and 7, in the production of composite particles, the mass ratio of silicate to raw material silicon in the mixture of silicate and raw material silicon was changed to 40: 60. In addition, in the production of composite particles, the time for exposing the raw material silicon to plasma was set to 70 minutes in Example 6 and 100 minutes in Example 7.

[0118] In Comparative Examples 1 to 4, the raw material silicon was not subjected to plasma treatment in the production of the composite particles. In Comparative Examples 2 to 4, the mass ratio of silicate to raw material silicon in the mixture of silicate and raw material silicon in the production of the composite particles was changed from that in Example 1, to silicate:raw material silicon = 45:55 in Comparative Example 2, 40:60 in Comparative Example 3, and 35:65 in Comparative Example 4.

[0119] Except for this, composite particles were prepared in the same manner as in Example 1, and negative electrodes and secondary batteries using the composite particles as active materials were prepared, thereby obtaining batteries A2 to A7 and B1 to B4 according to Examples 2 to 7 and Comparative Examples 1 to 4, respectively.

[0120] In addition, in the same manner as in Example 1, the N atomic ratio R of the silicon-containing particles used in each secondary battery was Ｎ and porosity were calculated. N atom ratio R Ｎ The results of the porosity are shown in Table 1.

[0121] (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).

[0122] <Discharge> Constant current discharge was carried out at a current of 1 It (800 mA) until the voltage reached 2.75 V.

[0123] The rest period between charging and discharging was 10 minutes. The charging and discharging were carried out in an environment of 25°C.

[0124] For each battery, the discharge capacity C1 at the first cycle and the discharge capacity C300 at the 300th cycle were determined. The value calculated by multiplying C300 by C300 was used as the capacity retention rate X (%). Table 1 shows the evaluation results for the capacity retention rates X of batteries A1 to A7 and B1 to B4.

[0125]

[0126] As shown in Table 1, the N atomic ratio R Ｎ In the batteries A1 to A7, in which silicon-containing particles having a porosity 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 300 cycles was reduced to 55% or less. Note that in the batteries B1 to B4, the N atomic ratio R Ｎ The reason why the value was 0.5% is thought to be due to a nitrogen-derived material contained in a small amount in the raw silicon prepared.

[0127] The secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0128] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0129] 1: Electrode group, 2: Positive electrode lead, 3: Negative electrode lead, 4: Battery case, 5: Sealing plate, 6: Negative electrode terminal, 7: Gasket, 8: Seal plug, 20: Negative electrode active material, 21: Ion conductive phase, 22: Silicon phase, 23: Composite particle, 26: Coating layer

Claims

1. A silicon-containing particle, comprising a silicate phase and a silicon phase dispersed in the silicate phase, and when a cross section of the silicon-containing particle is measured by X-ray photon spectroscopy (XPS), the N atom ratio R of the silicon-containing particle is quantified based on the N1s spectrum. Ｎ The negative electrode active material for a secondary battery has a content of 1% or more and 4% or less.

2. 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.

3. The negative electrode active material for a secondary battery according to claim 1, further comprising a coating layer covering at least a portion of a surface of the silicon-containing particle, the coating layer comprising a carbon material, and a content of the carbon material relative to the entire silicon-containing particle including the coating layer is 3 mass % or more and 5 mass % or less.

4. The silicate phase is Li ２ S ２ O ５ , Li ２ SiO ３ , and Li ４ SiO ４ The negative electrode active material for a secondary battery according to any one of claims 1 to 3, comprising at least one selected from the group consisting of:

5. The negative electrode active material for a secondary battery 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.

6. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein the silicon-containing particles have an average particle size D50 of 6 µm or more and 8 µm or less.

7. 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.

8. The negative electrode active material for a secondary battery according to any one of claims 1 to 3, wherein a surface of the silicon phase has a functional group containing N, the functional group including at least one type selected from the group consisting of an amino group and a nitro group.

9. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode contains the negative electrode active material for secondary batteries according to any one of claims 1 to 3.

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