Negative electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

This negative electrode material for nonaqueous electrolyte secondary batteries is provided with composite particles containing a lithium silicate phase, a silicon phase dispersed in the lithium silicate phase, and a silicon dioxide crystal phase dispersed in the lithium silicate phase. The abovementioned silicon dioxide crystal phase contains β-cristobalite and quartz.

Description

[Technical Field]

[0001] The present disclosure primarily relates to improvements in negative electrode active materials for non-aqueous electrolyte secondary batteries. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries have been expected to be used in small consumer applications, power storage devices, and power sources for electric vehicles due to their high voltage and high energy density. As higher energy densities are required for batteries, there is hope for the use of materials containing silicon, which alloys with lithium, as negative electrode active materials with high theoretical capacity densities.

[0003] However, silicon-containing materials have a problem of low initial charge / discharge efficiency due to their large irreversible capacity. Therefore, various techniques have been proposed to pre-introduce lithium equivalent to the irreversible capacity into silicon-containing materials. Specifically, it has been proposed to use composite particles containing a lithium silicate phase and silicon particles (fine silicon phases) dispersed within the lithium silicate phase. The silicon particles contribute to the charge / discharge reaction (reversible lithium absorption and desorption).

[0004] Patent Document 1 proposes dispersing at least one element Q selected from the group consisting of rare earth elements and alkaline earth elements within the lithium silicate phase of the composite particles. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 179969 Brochure Summary of the Invention

[0006] In the composite particles, the silicon phase expands and contracts significantly due to the absorption and desorption of lithium during charging and discharging, which generates large stress in the silicate phase surrounding the silicon phase and makes the silicate phase prone to cracking. Therefore, side reactions occur due to contact between the newly exposed surfaces of the silicate phase and the non-aqueous electrolyte, which can lead to deterioration in cycle performance. In Patent Document 1, the silicate phase is hardened to some extent by the addition of the element Q, but the suppression of cracking of the silicate phase is still insufficient.

[0007] Therefore, a problem arises in that the cycle characteristics of secondary batteries using composite particles having a silicon phase as the negative electrode active material are reduced.

[0008] In view of the above, one aspect of the present disclosure relates to a negative electrode active material for a non-aqueous electrolyte secondary battery, comprising composite particles including a lithium silicate phase, a silicon phase dispersed in the lithium silicate phase, and a silicon dioxide crystalline phase dispersed in the lithium silicate phase, wherein the silicon dioxide crystalline phase includes β-cristobalite and quartz.

[0009] Another aspect of the present disclosure relates to a non-aqueous electrolyte 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. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to suppress a decrease in the cycle characteristics of a non-aqueous electrolyte secondary battery. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing an example of an XRD pattern of a composite particle. [Figure 2] FIG. 2 is a cross-sectional view schematically illustrating a negative electrode active material (composite particle) according to one embodiment of the present disclosure. [Figure 3] FIG. 3 is a schematic perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure, with a portion cut away. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Non-aqueous electrolyte secondary battery negative electrode active material] A negative electrode active material for a non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes composite particles (hereinafter also referred to as composite particles) including a lithium silicate phase, a silicon phase dispersed within the lithium silicate phase, and a silicon dioxide crystalline phase dispersed within the lithium silicate phase. The silicon dioxide crystalline phase includes β-cristobalite and quartz. β-cristobalite has a cubic crystal structure, and β-cristobalite has a lower Vickers hardness than quartz.

[0013] The dispersion of silicon dioxide within the silicate phase increases the strength of the silicate phase to a certain extent. Furthermore, the inclusion of β-cristobalite, which has a relatively low hardness, as a crystalline phase of silicon dioxide along with quartz improves the flexibility of the silicate phase, alleviating the stress that occurs in the silicate phase around the silicon phase when the silicon phase expands. As a result, cracking of the silicate phase is suppressed, and the deterioration of cycle performance due to cracking of the silicate phase is sufficiently suppressed.

[0014] Furthermore, when both β-cristobalite and quartz are present as crystalline phases of silicon dioxide, the silicate phase can follow both the expansion and contraction of the silicon phase, and the silicate phase can fully maintain its function as a lithium ion conductive phase during charge and discharge. This allows the silicon phase to fully contribute to the charge and discharge reaction, resulting in high capacity and excellent cycle characteristics.

[0015] When almost all of the silicon dioxide dispersed in the silicate phase is β-cristobalite, the hardness of the silicate phase decreases, causing the silicate phase to expand significantly as the silicon phase expands during charging, making it difficult for the silicate phase to follow the contraction of the silicon phase during discharging. This results in gaps forming between the silicon phase and the silicate phase, isolating the silicon phase and preventing it from fully contributing to the charge-discharge reaction, resulting in reduced cycle performance.

[0016] If almost all of the silicon dioxide dispersed in the silicate phase is quartz, the flexibility of the silicate phase decreases and it becomes difficult for the silicate phase to follow the expansion of the silicon phase during charging, which increases the stress generated in the silicate phase and causes cracks in the silicate phase, resulting in a decrease in cycle characteristics.

[0017] In the XRD pattern of the composite particles obtained by X-ray diffraction (XRD) measurement, a peak derived from β-cristobalite appears around 2θ=21.6°, and a peak derived from quartz appears around 2θ=26.3°. Cu Kα rays are used as the X-rays for XRD measurement. In this specification, "near x°" means, for example, within the range of x±1°.

[0018] Intensity I of the peak derived from quartz appearing around 2θ=26.3° in the XRD pattern of the composite particles B Intensity of the peak originating from β-cristobalite appearing around 2θ=21.6°, I A Ratio of:I A / I B is preferably 0.1 or more, more preferably 0.3 or more. A / I B When the intensity ratio is within the above range, quartz and β-cristobalite are present in a well-balanced manner within the silicate phase, and the cycle characteristics are likely to be improved. A / I B The upper limit is, for example, 2.0.

[0019] Here, Fig. 1 shows an example of an XRD pattern of a composite particle. In Fig. 1, a1 shows the XRD pattern of the negative electrode active material (composite particle) of this embodiment, and b1 shows the XRD pattern of a conventional negative electrode active material (composite particle). Note that composite particle a1 corresponds to Example 1 (Battery A1) described below, and composite particle b1 corresponds to Comparative Example 1 (Battery B1) described below.

[0020] In both a1 and b1, a peak originating from the Si(111) plane of the silicon phase is observed near 2θ=28°. In both a1 and b1, a peak originating from the lithium silicate phase Li2Si2O5 is observed near 2θ=24°.

[0021] In a1, a peak derived from quartz is observed around 2θ=26.3°, and a peak derived from β-cristobalite is observed around 2θ=21.6°. A / I B The peak intensity ratio of b1 is 0.3. On the other hand, in b1, a peak derived from quartz is observed near 2θ=26.3°, but no peak derived from β-cristobalite is observed near 2θ=21.6°.

[0022] In the composite particles, multiple primary particles containing a lithium silicate phase and a silicon phase are bonded to form secondary particles. Looking at the secondary particles as a whole, the composite particles have a structure in which fine silicon phases are dispersed within the lithium silicate phase. High capacity can be achieved by controlling the amount of silicon phase dispersed in the lithium silicate phase. The lithium silicate phase alleviates the expansion and contraction of the silicon phase. Therefore, it is easy to achieve both high battery capacity and improved cycle characteristics.

[0023] The composite particles have a structure in which a fine silicon dioxide phase is dispersed within a lithium silicate phase. One silicon dioxide phase may contain both β-cristobalite and quartz, or the β-cristobalite phase and the quartz phase may each be formed independently within the lithium silicate phase.

[0024] The average particle size of the composite particles (secondary particles) is, for example, 1 μm or more and 25 μm or less, or may be 4 μm or more and 15 μm or less. This particle size range facilitates alleviating stress caused by volumetric changes in the composite particles during charging and discharging, making it easier to achieve good cycle characteristics. The surface area of ​​the composite particles is also appropriately sized, suppressing capacity loss due to side reactions with the non-aqueous electrolyte. The average particle size of the composite particles refers to the particle size (volume-average particle size) at which the volume-integrated value is 50% in the particle size distribution measured by laser diffraction scattering. For example, a measuring device such as the HORIBA LA-750 can be used. When the surface of the composite particles is covered with a conductive layer, the thickness of the conductive layer is small enough to not substantially affect the average particle size of the composite particles, so the average particle size of the composite particles having the conductive layer can be considered the average particle size of the composite particles.

[0025] The composite particles can be extracted from a battery using the following method. First, a fully discharged battery is disassembled to remove the negative electrode. The negative electrode is then washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the nonaqueous electrolyte components. As described below, the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer supported on its surface. The negative electrode mixture layer is then peeled off from the copper foil and crushed in a mortar to obtain a sample powder. The sample powder is then dried in a dry atmosphere for 1 hour and immersed in gently boiling 6M hydrochloric acid for 10 minutes to remove elements other than those derived from the composite particles. Next, the sample powder is washed with ion-exchanged water, filtered, and dried at 200°C for 1 hour. The conductive layer is then removed by heating to 900°C in an oxygen atmosphere, allowing the composite particles to be isolated. Note that a fully discharged state refers to a state in which the depth of discharge (DOD) is 90% or more (state of charge (SOC) is 10% or less).

[0026] (lithium silicate phase) The lithium silicate phase may be amorphous or highly crystalline. The lower the crystallinity of the lithium silicate phase, the higher its flexibility, and it is more likely to follow the expansion and contraction of the silicon phase, making it less likely to crack. Even when the lithium silicate phase has a high crystallinity, the flexibility of the lithium silicate phase is improved and it becomes less likely to crack if the SiO2 crystal contains β-cristobalite.

[0027] Lithium silicate is a silicate containing lithium (Li), silicon (Si), and oxygen (O). 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 (where z is 0 < z < 2 in the formula described later), it 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. Also, the atomic ratio of Li to Si in lithium silicate, Li / Si, is, for example, greater than 0 and less than 4.

[0028] The composition of lithium silicate can be 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 z = 1 / 2 is more preferable. Lithium silicate satisfying z = 1 / 2 can be represented by Li2Si2O5. It is desirable that lithium silicate contains Li2Si2O5 as a main component, and it is desirable that Li2Si2O5 is the main component of the entire silicate phase. Here, the "main component" refers to a component that occupies 50 mass% or more of the mass of the entire lithium silicate or the entire silicate phase, and may occupy 70 mass% or more of the component.

[0029] The silicate phase may further contain another element M in addition to Li, Si, and O. When the silicate phase contains the element M, the chemical stability and lithium ion conductivity of the silicate phase are improved, or side reactions due to the contact between the silicate phase and the non-aqueous electrolyte are suppressed.

[0030] The element M may be at least one element selected from the group consisting of lanthanoid elements such as sodium (Na), potassium (K), magnesium (Mg), barium (Ba), zirconium (Zr), niobium (Nb), and lanthanum (La), tantalum (Ta), vanadium (V), titanium (Ti), phosphorus (P), bismuth (Bi), zinc (Zn), tin (Sn), lead (Pb), antimony (Sb), cobalt (Co), fluorine (F), tungsten (W), aluminum (Al), and boron (B). From the viewpoints of resistance to non-aqueous electrolytes and structural stability of the silicate phase, the element M preferably includes at least one element selected from the group consisting of Zr, Ti, P, Al, and B.

[0031] The lanthanoid element can improve the charge-discharge efficiency at the initial stage of the charge-discharge cycle. From the viewpoint of improving lithium ion conductivity, it is more preferable that the lanthanoid element contains La. The proportion of La in the total lanthanoid elements is preferably 90 atomic % or more and 100 atomic % or less.

[0032] 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).

[0033] 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.

[0034] 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.

[0035] The contents of Li, Si, and element M in the silicate phase can be measured, for example, by analyzing a cross section of the negative electrode mixture layer.

[0036] First, a fully discharged battery was disassembled, the negative electrode was removed, the negative electrode was washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the non-aqueous electrolyte component, and then dried. After that, a cross-section of the negative electrode mixture layer was obtained using a cross-section polisher (CP), and the cross-section of the negative electrode mixture layer was then observed using a scanning electron microscope (SEM).

[0037] The content of each element can be determined by any of the following methods: The composition of the silicate phase is calculated from the content of each element.

[0038] <edx> From the cross-sectional image of the reflection electron image of the negative electrode active material layer, 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 of the target element is calculated using image analysis software. An observation magnification of 2000 to 20000 times is desirable. The measured values of the area of the predetermined element contained in the 10 particles are averaged. The content of the target element is calculated from the obtained average value.

[0039] The measurement conditions for the desirable cross-sectional SEM-EDX analysis are shown below.

[0040] <SEM-EDX Measurement Conditions> Processing device: 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 Measurement device: electron microscope SU-70 manufactured by HITACHI 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 <aes> From the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, ten composite particles with 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 given element contained in the ten particles is calculated by averaging the content.

[0041] The EDX analysis and AES analysis are carried out within a range of 1 μm or more inside from the peripheral edge of the cross section of the composite particle.

[0042] <icp> A sample of composite particles is completely dissolved in a heated acid solution (a mixture of hydrofluoric acid, nitric acid, and sulfuric acid), and the residual carbon in the solution is filtered off. The filtrate is then analyzed by inductively coupled plasma optical emission spectroscopy (ICP) to measure the spectral intensity of each element. A calibration curve is then created using commercially available elemental standard solutions, and the content of each element in the composite particles is calculated.

[0043] Alternatively, the quantitative determination of each element can be performed using an electron microanalyzer (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), or the like.

[0044] The contents of B, Na, K and Al contained in the composite particles can be quantitatively analyzed in accordance with JIS R3105 (1995) (method of analysis of borosilicate glass).

[0045] The carbon content of the composite particles can be measured using a carbon / sulfur analyzer (e.g., EMIA-520 manufactured by Horiba, Ltd.). The sample is weighed onto a magnetic board, a combustion improver is added, and the board is inserted into a combustion furnace (carrier gas: oxygen) heated to 1350°C. The amount of carbon dioxide gas generated during combustion is detected by infrared absorption. A calibration curve is created using, for example, carbon steel (carbon content: 0.49%) manufactured by the Bureau of Analyzed Samples, Ltd., and the carbon content of the sample is calculated (high-frequency induction heating furnace combustion - infrared absorption method).

[0046] The oxygen content of the composite particles can be measured using an oxygen, nitrogen, and hydrogen analyzer (e.g., EGMA-830 manufactured by Horiba, Ltd.). The sample is placed in a nickel capsule and placed in a carbon crucible heated at 5.75 kW along with Sn and Ni pellets as fluxes. The released carbon monoxide gas is detected. A calibration curve is created using a standard sample of Y2O3, and the oxygen content of the sample is calculated (inert gas fusion - non-dispersive infrared absorption method).

[0047] In the composite particles, there exist a silicate phase, a silicon phase, and a SiO2 phase. The Si content obtained by the above method is the total of the Si amount constituting the silicon phase, the Si amount in the silicate phase, and the Si amount in the SiO2 phase. On the other hand, the Si amount constituting the silicon phase can be separately quantified using Si-NMR. The Si amount in the SiO2 phase can also be separately quantified using Si-NMR. Therefore, by using Si-NMR, the Si amount constituting the silicon phase, the Si amount in the SiO2 phase, and the Si amount in the silicate phase can be distinguished and quantified. For the standard substance required for quantification, a mixture containing a silicate phase, a silicon phase, and a SiO2 phase with known Si contents in a predetermined ratio may be used.

[0048] The content of the SiO2 phase in the composite particles measured by Si-NMR is, for example, 50% by mass or less, and may be 10% by mass or more and 40% by mass or less.

[0049] The desirable measurement conditions of Si-NMR are shown below.

[0050] <Si-NMR Measurement Conditions> Measuring device: A solid nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian Probe: Varian 7mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupling) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: Around -100 ppm Signal acquisition time: 0.05 sec Number of integrations: 560 Sample amount: 207.6 mg (Silicon phase) The silicon phase is a phase of simple silicon (Si), which repeatedly absorbs and releases lithium ions as the battery is charged and discharged. Capacity is generated by a Faraday reaction involving the silicon phase. Because the silicon phase has a large capacity, it also expands and contracts greatly during charging and discharging. However, because the silicon phase is dispersed within the silicate phase, the stress caused by the expansion and contraction of the silicon phase is alleviated.

[0051] The silicon phase can be composed of multiple crystallites. The crystallite size of the silicon phase is preferably 30 nm or less. When the crystallite size of the silicon phase is 30 nm or less, the volume change due to expansion and contraction of the silicon phase during charge and discharge can be reduced, further improving cycle characteristics. For example, isolation of the silicon phase due to the formation of voids around the silicon phase during contraction is suppressed, and a decrease in charge and discharge efficiency is suppressed. The lower limit of the crystallite size of the silicon phase is not particularly limited, but is, for example, 1 nm or more.

[0052] The crystallite size of the silicon phase is more preferably 20 nm or less, which makes it easier to make the expansion and contraction of the silicon phase uniform, ease the stress generated in the composite particles, and improve the cycle characteristics.

[0053] The crystallite size of the silicon phase is more preferably 10 nm or less, and particularly preferably 2 nm or more and 10 nm or less. When silicon microparticles with an average particle size of 200 nm or less are used as the raw silicon in the second step described below, the specific surface area of ​​the composite obtained in the second step becomes large, and heat is easily transferred to the silicon phase and SiO2 phase dispersed in the lithium silicate phase during heating in the third step (or fifth step) described below. As a result, a crystalline silicon phase with a crystallite size of 10 nm or less is formed, and both quartz and β-cristobalite are easily formed as SiO2 crystalline phases.

[0054] The crystallite size of the silicon phase is calculated using the Scherrer equation from the full width at half maximum of the peak corresponding to the Si(111) plane of the silicon phase in the XRD pattern of the composite particles. The above XRD pattern is obtained by XRD measurement using Cu Kα radiation.

[0055] The silicon phase of the composite particles contained in the battery before the first charge is, for example, particulate. The average particle size of the particulate silicon phase is preferably 500 nm or less, more preferably 200 nm or less, even more preferably 100 nm or less, and particularly preferably 50 nm or less. When silicon microparticles with an average particle size of 200 nm or less are used as raw silicon in the second step described below, the average particle size of the final silicon phase may be 100 nm or less. After the first charge, the average particle size of the silicon phase is preferably 400 nm or less, more preferably 100 nm or less. By miniaturizing the silicon phase, the volume change of the composite particles during charge and discharge is reduced, further improving the structural stability of the composite particles. The average particle size of the silicon phase is measured using a cross-sectional image of the composite particles obtained by SEM. Specifically, the average particle size of the silicon phase is determined by averaging the maximum diameters of 100 arbitrary silicon phases.

[0056] From the viewpoint of increasing capacity, the content of the silicon phase in the composite particles is preferably 30% by mass or more, more preferably 35% by mass or more, and even more preferably 55% by mass or more. In this case, lithium ion diffusibility is good, and excellent load characteristics are obtained. On the other hand, from the viewpoint of improving cycle characteristics, the content of the silicon phase in the composite particles is preferably 95% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. In this case, the surface of the silicon phase that is exposed without being covered with the silicate phase is reduced, and side reactions between the non-aqueous electrolyte and the silicon phase are easily suppressed.

[0057] (Conductive layer) A conductive layer containing a conductive material may be formed on at least a portion of the surface of the composite particle (secondary particle). This can dramatically increase the conductivity of the composite particle. The thickness of the conductive layer is preferably thin enough that it does not substantially affect the average particle size of the composite particle. In consideration of ensuring conductivity and the diffusibility of lithium ions, the thickness of the conductive layer is preferably 1 to 200 nm, more preferably 5 to 100 nm. The thickness of the conductive layer can be measured by observing the cross section of the composite particle using an SEM or TEM.

[0058] The conductive material is preferably a conductive carbon material. Examples of conductive carbon materials that can be used include amorphous carbon, graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Among these, amorphous carbon is preferred because it is easy to form a thin conductive layer that covers the surface of the composite particles. Examples of amorphous carbon include carbon black, burned pitch, coke, and activated carbon. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon.

[0059] Here, FIG. 2 is a cross-sectional view schematically showing a negative electrode active material (composite particles) according to one embodiment of the present disclosure.

[0060] The composite particle 20 includes a mother particle 23 composed of secondary particles formed by agglomeration of a plurality of primary particles 24. The mother particle 23 (primary particle 24) includes a lithium silicate phase 21, and a silicon phase 22 and an SiO2 crystalline phase 28 dispersed within the lithium silicate phase 21. The mother particle 23 has a sea-island structure in which fine silicon phases and SiO2 phases are dispersed within the lithium silicate phase 21 matrix.

[0061] At least a portion of the surface of the base particle 23 can be coated with a conductive layer 26. The lithium silicate phase 21 may contain element M. As charging and discharging are repeated, adjacent particulate silicon phases 22 can be connected to each other to form a network-like silicon phase.

[0062] [Method of manufacturing composite particles] 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 raw material, lithium silicate (hereinafter also referred to as raw silicate). The raw silicate contains SiO2. (Second step) A step of mixing raw silicate and raw silicon, and pulverizing the mixture (composite formation) using a ball mill or the like. (Step 3) The composite (pulverized material) is compressed and heated to obtain a sintered body. (Fourth step) A step of pulverizing the sintered body to obtain composite particles.

[0063] In the first step, a mixture of SiO2 and Li compounds is used as the raw material, and SiO2 that did not react with the Li compounds during the production process of the raw silicate remains in the raw silicate. When the amount of SiO2 used is large relative to the Li compounds, SiO2 is likely to remain.

[0064] Although most of the SiO2 remaining in the raw silicate becomes amorphous in the second step, fine SiO2 crystals can be precipitated by heating in the subsequent third step (or the fifth step described below). In other words, a fine SiO2 crystalline phase can be dispersed within the lithium silicate phase. The SiO2 crystalline phase is stable and does not react with lithium ions during charging, so it does not become the key to irreversible reactions. Furthermore, because it is so fine, it does not interfere with the expansion and contraction of the silicon phase.

[0065] By appropriately adjusting the particle size (average particle size) of the raw silicon used in the second step and / or the heating temperature and degree of compression in the third step, it is possible to precipitate a mixture of β-cristobalite and quartz as SiO2 crystals. It is also possible to control the balance between β-cristobalite and quartz. The particle size of the raw silicon can adjust the ease of heat transfer to the SiO2 phase dispersed within the lithium silicate phase during heating in the third step. For example, silicon microparticles, as described below, are used as the raw silicon. The heating temperature of the composite is, for example, 450°C or higher and 1000°C or lower. The pressure applied to the composite is, for example, 100 MPa or higher and 400 MPa or lower. The heating and compression time of the composite is, for example, 1 hour or higher and 10 hours or lower.

[0066] The above manufacturing method may further include a fifth step of forming a conductive layer on at least a portion of the surface of the composite particle. The heat treatment in the fifth step may be used to mix β-cristobalite and quartz, or to adjust the balance between β-cristobalite and quartz.

[0067] Each step of the method for producing composite particles will be described in detail below.

[0068] (1st step) The first step includes, for example, step 1a of mixing silicon dioxide, a lithium compound, and, if necessary, a compound containing element M to obtain a mixture, and step 1b of calcining the mixture to obtain a starting silicate. The calcination in step 1b is carried out, for example, in an oxidizing atmosphere. The calcination 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.

[0069] 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.

[0070] 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.

[0071] (2nd process) The second step includes, for example, a step of pulverizing a mixture of raw silicate and raw silicon while applying shear force to the mixture to obtain a pulverized product (composite product). Here, for example, the raw silicate and raw silicon are mixed in a predetermined mass ratio (for example, 20:80 to 95:5), and the mixture is pulverized while being stirred using a pulverizer such as a ball mill.

[0072] It is preferable to use silicon microparticles as the raw silicon. The average particle size of the silicon microparticles is, for example, 500 nm or less, or may be 200 nm or less, or 150 nm or less. The lower limit of the average particle size of the silicon microparticles is, for example, 10 nm. When using the above silicon microparticles (especially microparticles with an average particle size of 200 nm or less), the specific surface area of ​​the composite obtained in the second step is large, and heat is easily transferred to the SiO2 phase dispersed in the lithium silicate phase during heating in the third step (or fifth step). This makes it easier to improve the crystallinity of the SiO2 phase and heat it to a temperature at which β-cristobalite can be formed together with quartz. Furthermore, when using the above silicon microparticles, the crystallite size of the silicon phase in the finally obtained composite particles can be 10 nm or less (or 2 nm or more, 10 nm or less). The average particle size of the raw silicon means the particle size (volume-average particle size) at which the volume-integrated value is 50% in the particle size distribution measured by laser diffraction scattering.

[0073] (3rd step) The sintering of the composite is carried out for the purpose of producing dense composite particles and appropriately reducing the surface area of ​​the composite particles. In the third step, for example, the pulverized product (composite) is heated while being compressed using a hot press or the like to obtain a sintered body. Alternatively, the pulverized product may be molded into a plate shape and conveyed between a pair of heated rolling rolls and rolled to obtain a sintered body. The third step is carried out, for example, in an inert atmosphere (e.g., an atmosphere of argon, nitrogen, etc.).

[0074] The heating temperature in the third step may be 450°C or higher and 1000°C or lower. Within this temperature range, it is easy to disperse small silicon particles and SiO2 particles within the silicate phase with low crystallinity. The raw silicate is stable within this temperature range and hardly reacts with silicon. The heating time is, for example, 1 hour or higher and 10 hours or lower.

[0075] (4th step) In the fourth step, the sintered body is pulverized to have a desired particle size distribution to obtain composite particles (secondary particles) containing a silicate phase and a silicon phase and an SiO2 phase dispersed within the silicate phase. The composite particles are pulverized to have an average particle size of, for example, 1 to 25 μm.

[0076] (5th step) A conductive layer may be formed by coating at least a portion of the surface of the composite particles (secondary particles) with a conductive material. The conductive material is preferably electrochemically stable, and a conductive carbon material is preferred. Examples of methods for coating the surface of the composite particles with a conductive carbon material include a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material, and a method in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with the composite particles and heated to carbonize them. Carbon black may also be attached to the surface of the composite particles. In the fifth step, for example, a conductive layer may be formed on the surface of the composite particles by heating the mixture of the composite particles and the conductive carbon material in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere) at a temperature of 700°C or higher and 950°C or lower.

[0077] [Nonaqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode contains the above-described negative electrode active material for a non-aqueous electrolyte secondary battery.

[0078] The nonaqueous electrolyte secondary battery will be described in detail below.

[0079] [Negative electrode] The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. 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 the applied layer. 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.

[0080] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, a thickener, etc. The negative electrode active material contains at least the above composite particles.

[0081] The negative electrode active material preferably further contains a carbon material that electrochemically absorbs and releases lithium ions. Because the composite particles expand and contract in volume with charge and discharge, a high ratio of the composite particles to the negative electrode active material is likely to result in poor contact between the negative electrode active material and the negative electrode current collector during charge and discharge. On the other hand, the combined use of the composite particles and a carbon material makes it possible to achieve excellent cycle characteristics while imparting the high capacity of the silicon particles to the negative electrode. From the viewpoint of increasing capacity and improving cycle characteristics, the ratio of the carbon material to the total of the composite particles and the carbon material is preferably 98% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 75% by mass or more and 95% by mass or less.

[0082] Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and non-graphitizable carbon (hard carbon). Among these, graphite is preferred because it has 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, graphitized mesophase carbon particles, and the like. One type of carbon material may be used alone, or two or more types may be used in combination.

[0083] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of materials 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.

[0084] Examples of binders include resin materials, such as fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; polyimide and polyamideimide; acrylic resins such as polyacrylic acid, polymethyl acrylate, and ethylene-acrylic acid copolymer; vinyl resins such as polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials such as styrene-butadiene copolymer rubber (SBR). One type of binder may be used alone, or two or more types may be used in combination.

[0085] Examples of conductive agents include carbons such as acetylene black, conductive fibers such as carbon fiber and metal fiber, carbon fluoride, metal powders such as aluminum, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and organic conductive materials such as phenylene derivatives. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0086] Examples of thickeners include carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), saponified polymers having vinyl acetate units such as polyvinyl alcohol, polyethers (polyalkylene oxides such as polyethylene oxide, etc.), etc. One type of thickener may be used alone, or two or more types may be used in combination.

[0087] The dispersion medium is not particularly limited, but examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0088] [Positive electrode] The positive electrode may include 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 the applied layer. 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. The positive electrode mixture contains a positive electrode active material as an essential component and may contain a binder, a conductive agent, etc. as optional components. NMP, etc., is used as the dispersion medium for the positive electrode slurry.

[0089] As the positive electrode active material, for example, a lithium-containing composite oxide can be used. a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b Me 1-b Oc, Li a Ni 1-b Me b O c , Li a Mn2O4, Li a Mn 2-b Me b O4, LiMePO4, Li2MePO4F (Me 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). Here, a = 0 to 1.2, b = 0 to 0.9, and c = 2.0 to 2.3. The value of a indicating the molar ratio of lithium increases or decreases during charge and discharge.

[0090] Among them, Li a Ni b Me 1-b Lithium nickel composite oxide represented by O2 (Me is at least one selected from the group consisting of Mn, Co, and Al, 0 < a ≤ 1.2, and 0.3 ≤ b ≤ 1) is preferred. From the perspective of increasing the capacity, it is more preferable to satisfy 0.85 ≤ b < 1. From the perspective of the stability of the crystal structure, Li a Ni b Co c Al d O2 (0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c < 0.15, 0 < d ≤ 0.1, b + c + d = 1) is even more preferred.

[0091] 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 and artificial graphite may be used.

[0092] The shape and thickness of the positive electrode current collector can be selected respectively from the shapes and ranges corresponding to the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, titanium, etc.

[0093] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0094] 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) and ethylene carbonate (EC). 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 formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0095] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10 Examples of the lithium salt include lithium carboxylates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of the borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO2)2), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonyl nonafluorobutanesulfonyl imide (LiN(CF3SO2)(C4F9SO2)), and lithium bispentafluoroethanesulfonyl imide (LiN(C2F5SO2)2). Among these, LiPF6 is preferred. The lithium salts may be used alone or in combination of two or more.

[0096] [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 may be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator may be made of polyolefins such as polypropylene and polyethylene.

[0097] An example of the structure of a nonaqueous electrolyte 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 is housed in an outer casing together with a nonaqueous electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the nonaqueous electrolyte secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0098] Hereinafter, the structure of a prismatic nonaqueous electrolyte secondary battery will be described as an example of the nonaqueous electrolyte secondary battery according to the present disclosure with reference to FIG.

[0099] The battery includes a bottomed, rectangular battery case 4, an electrode group 1, and a nonaqueous electrolyte 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 therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal 8 after injection.

[0100] The present disclosure will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0101] Example 1 [Preparation of composite particles] (1st step) Lithium oxide, silicon dioxide, aluminum oxide, and lanthanum oxide were mixed in a molar ratio of Li2O:SiO2:Al2O3:La2O3 = 21:75:3:1, and the mixture was melted in an inert atmosphere at 1500°C for 5 hours. The melt was passed through a metal roller to form flakes, yielding a lithium silicate composite oxide containing Li, Si, Al, and La. The resulting lithium silicate composite oxide was pulverized to an average particle size of 10 μm, yielding the raw silicate.

[0102] (2nd process) The raw silicate (average particle size: 10 μm) and raw silicon were mixed in a mass ratio of 40:60. The raw silicon was silicon fine powder (3N, average particle size: 100 nm). The mixture was loaded into a 500 mL pot (made of stainless steel, volume: 500 mL) of a planetary ball mill (Fritsch, P-5). 24 stainless steel balls (diameter: 20 mm) were placed inside, the lid was closed, and the mixture was milled at 200 rpm for 25 hours in an inert atmosphere.

[0103] (3rd step) The pulverized material (composite) was then heated and compressed in an inert atmosphere using a hot press to obtain a sintered body. The heating temperature was 600°C, the pressure applied to the pulverized material was 190 Ma, and the heating (compression) time was 4 hours.

[0104] (4th step) Thereafter, the sintered body was crushed and passed through a 40 μm mesh to obtain composite particles.

[0105] (5th step) The composite particles were mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation). The mixture was fired at 800°C for 5 hours in an inert atmosphere, and the surfaces of the composite particles were coated with conductive carbon to form a conductive layer. The amount of the conductive layer was 5% by mass based on the total mass of the composite particles and the conductive layer. Then, composite particles a1 (secondary particles) with an average particle size of 5 μm and equipped with a conductive layer were obtained using a sieve.

[0106] XRD measurement was performed on composite particle a1. The XRD pattern of composite particle a1 is shown in Figure 1. In the XRD pattern, peaks due to Si, SiO2, and Li2Si2O5 were confirmed. For SiO2, a peak due to quartz was observed around 2θ = 26.3°, and a peak due to β-cristobalite was observed around 2θ = 21.6°. The intensity of the peaks due to quartz mentioned above, I B The intensity of the peaks derived from the above β-cristobalite, I A Ratio of:I A / I B was 0.3. The crystallite diameter of the silicon phase was determined by the above-mentioned method using the XRD pattern of the composite particle a1. The crystallite diameter of the silicon phase was 8 nm.

[0107] [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. A negative electrode mixture containing the negative electrode active material, a sodium salt of CMC, and SBR in a mass ratio of 97.5:1:1.5 was mixed with water and stirred to prepare a negative electrode slurry. The negative electrode slurry was then applied to the surface of copper foil, dried, and rolled to form a copper foil with a density of 1.5 g / cm on both sides. 3 A negative electrode having the negative electrode mixture layer formed thereon was fabricated.

[0108] [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 and stirred to prepare a positive electrode slurry. The positive electrode slurry was then applied to the surface of an aluminum foil, the coating was dried, and the aluminum foil was rolled to form a positive electrode slurry having a density of 3.6 g / cm on both sides. 3 A positive electrode having the positive electrode mixture layer formed thereon was fabricated.

[0109] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte 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.

[0110] [Fabrication of non-aqueous electrolyte secondary battery] The positive and negative electrodes, each with a tab attached, were wound with a separator interposed therebetween to prepare an electrode assembly with the tabs positioned at the outermost periphery. The electrode assembly was inserted into an exterior case 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 case, and the opening of the exterior case was sealed to obtain a battery A1 of Example 1.

[0111] <Comparative Example 1> Composite particles b1 were produced in the same manner as in Example 1, except that in the second step, coarse silicon powder (3N, average particle size 10 μm) was used as raw silicon.

[0112] XRD measurements were performed on composite particle b1. The XRD pattern of composite particle b1 is shown in Figure 1. In the XRD pattern, peaks derived from Si, SiO2, and Li2Si2O5 were confirmed. For SiO2, a peak derived from quartz was observed, but no peak derived from β-cristobalite was observed.

[0113] Using the composite particles b1 obtained above, a negative electrode was produced in the same manner as in Example 1, to obtain a nonaqueous electrolyte secondary battery B1.

[0114] The following evaluations were carried out for each of the batteries of the Examples and Comparative Examples.

[0115] [Thickness change rate of negative electrode after first charge] At 25°C, constant current charging was performed at a current of 1 It (800 mA) until the voltage reached 4.2 V, and then constant voltage charging was performed at a voltage of 4.2 V until the current reached 1 / 20 It (40 mA).

[0116] After the initial charge, the battery was disassembled, the negative electrode was removed, washed with ethyl methyl carbonate or dimethyl carbonate, dried, and the electrolyte was removed. The thickness of the negative electrode was then measured at 10 random points, and the average value was calculated as the negative electrode thickness T1 after the initial charge. The thickness of the negative electrode (the negative electrode used in the battery fabrication) was also calculated in the same manner as above, and this was calculated as the negative electrode thickness T0 before the initial charge.

[0117] The ratio of the thickness T1 to the thickness T0 of the negative electrode before the first charge (T1 / T0×100) was determined as the thickness change rate of the negative electrode after the first charge.

[0118] [Thickness change rate of negative electrode after first discharge] A separate battery was prepared and charged at 25°C at a constant current of 1 It (800 mA) until the voltage reached 4.2 V, followed by constant voltage charging at 4.2 V until the current reached 1 / 20 It (40 mA). After a 10-minute rest period, the battery was discharged at a constant current of 1 It (800 mA) until the voltage reached 2.75 V.

[0119] The battery after the first discharge was disassembled, and the thickness T2 of the negative electrode after the first discharge was determined in the same manner as above. The ratio of the thickness T2 of the negative electrode after the first discharge to the thickness T0 of the negative electrode before the first charge (T2 / T0 × 100) was determined as the thickness change rate of the negative electrode after the first discharge.

[0120] [Charge / discharge cycle test] Charge and discharge were repeated under the following conditions.

[0121] <Charging> At 25°C, constant current charging was performed at a current of 1 It (800 mA) until the voltage reached 4.2 V, and then constant voltage charging was performed at a voltage of 4.2 V until the current reached 1 / 20 It (40 mA).

[0122] <Discharge> At 25°C, constant current discharge was carried out at a current of 1 It (800 mA) until the voltage reached 2.75V.

[0123] The rest period between charge and discharge was 10 minutes. The ratio of the discharge capacity at the 300th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate.

[0124] The evaluation results for batteries A1 and B1 are shown in Table 1.

[0125] [Table 1]

[0126] In Battery A1, the addition of β-cristobalite as a crystalline phase of silicon dioxide along with quartz alleviated the stress generated in the silicate phase due to the expansion of the silicon phase, resulting in a 5% reduction in the thickness change rate of the anode after the first charge compared to Battery B1. As a result, Battery A1 had an 8% increase in capacity retention compared to Battery B1, significantly improving its cycle characteristics. [Industrial Applicability]

[0127] The present disclosure can provide a nonaqueous electrolyte secondary battery having high capacity and good charge-discharge cycle characteristics. The nonaqueous electrolyte secondary battery of the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0128] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal 20 Composite particles 21 Lithium silicate phase 22 Silicon phase 23 Mother particle 24 Primary particles 26 Conductive layer 28 Crystalline phase of SiO2< / icp> < / aes> < / edx>

Claims

1. a lithium silicate phase; a silicon phase dispersed within the lithium silicate phase; a crystalline phase of silicon dioxide dispersed within the lithium silicate phase; Composite particles comprising: the crystalline phase of the silicon dioxide includes β-cristobalite and quartz; The negative electrode active material for a non-aqueous electrolyte secondary battery, wherein the crystallite size of the silicon phase is 1 nm or more and 8 nm or less.

2. In the X-ray diffraction pattern of the composite particles obtained by X-ray diffraction measurement, A peak derived from the β-cristobalite appears around 2θ=21.6°, 2. The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein a peak derived from said quartz appears around 2θ=26.3°.

3. In the X-ray diffraction pattern of the composite particles, the intensity I of the peak derived from the quartz Ｂ The intensity I of the peak derived from the β-cristobalite Ａ Ratio of: I Ａ / I Ｂ The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 2 , wherein is 0.1 or more.

4. The lithium silicate phase is Li ２ Si ２ O ５ The negative electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, comprising:

5. 5. The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium silicate phase contains at least one element selected from the group consisting of sodium, potassium, magnesium, barium, zirconium, niobium, a lanthanoid element, tantalum, vanadium, titanium, phosphorus, bismuth, zinc, tin, lead, antimony, cobalt, fluorine, tungsten, aluminum, and boron.

6. A positive electrode, a negative electrode, and a non-aqueous electrolyte, A non-aqueous electrolyte secondary battery, wherein the negative electrode comprises the negative electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5.

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