Negative electrode active material for secondary battery, method for producing the same, and secondary battery

Abstract

This negative electrode active substance for a secondary battery comprises silicate complex particles containing silicon particles and a lithium silicate phase. The silicon particles are dispersed in the lithium silicate phase, and the porosity of the silicate complex particles is 2% or less. In addition, in a diffraction pattern obtained using an X-Ray diffraction method (XRD), the ratio, I1 / I1, of the integrated intensity I2 of peaks derived from the lithium silicate phase occurring within the range 2θ=23-25º relative to the integrated intensity I1 of peaks derived from the Si (111) face of the silicon particles occurring within the range 2θ=27-30º is 0.3 or less.

Description

[Technical Field]

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

[0002] In recent years, secondary batteries such as 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 a growing expectation 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 in that their initial charge-discharge efficiency (particularly the ratio of the initial discharge capacity to the initial charge capacity) is low due to their large irreversible capacity. Therefore, various techniques have been proposed to preliminarily 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 dispersed within the lithium silicate phase (Patent Document 1). The silicon particles contribute to the charge-discharge reaction (reversible absorption and release of lithium).

[0004] The composite particles are produced, for example, by sintering a mixture of glassy lithium silicate powder and silicon particles in a high-temperature and high-pressure atmosphere. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-153520 Summary of the Invention

[0006] It is known that the silicon particles of the composite particles expand and contract significantly during charge and discharge due to the absorption and release of lithium. This causes significant stress in the lithium silicate phase surrounding the silicon particles, resulting in cracks and fractures in the composite particles. This weakens the bonding strength between the composite particles and the surrounding binder, and fractured composite particles, in particular, lose their conductive paths to the surrounding particles and become isolated. This also promotes side reactions between the electrolyte and the silicon particles. This results in a deterioration of the charge and discharge cycle characteristics.

[0007] In view of the above, one aspect of the present disclosure relates to a negative electrode active material for a secondary battery, comprising silicate composite particles containing silicon particles and a lithium silicate phase, the silicon particles being dispersed in the lithium silicate phase, the porosity of the silicate composite particles being 2% or less, and a diffraction pattern obtained by X-ray diffraction (XRD) in which the ratio I2 / I1 of the integrated intensity I1 of the peak originating from the Si(111) plane of the silicon particles and appearing in the range of 2θ=27° to 30° and the integrated intensity I2 of the peak originating from the lithium silicate phase and appearing in the range of 2θ=23° to 25° is 0.3 or less.

[0008] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode includes a current collector and a negative electrode active material layer, and the negative electrode active material layer includes the above-described negative electrode active material for secondary batteries.

[0009] Yet another aspect of the present disclosure relates to a method for producing a negative electrode active material for a secondary battery, the method comprising the steps of: preparing silicate composite particles comprising silicon particles and an amorphous lithium silicate phase, with the silicon particles dispersed in the lithium silicate phase; molding a powder of the silicate composite particles; heating the molded silicate composite particles to a temperature of 600°C or higher and 1000°C or lower for a period of 40 seconds or higher and 200 seconds or lower; and applying pressure to the silicate composite particles in the heated state.

[0010] By using the negative electrode active material of the present disclosure, a negative electrode for a secondary battery and a secondary battery having excellent charge-discharge cycle characteristics can be obtained. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating a negative electrode active material (LSX particles) for a secondary battery according to one embodiment of the present disclosure. [Figure 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. [Figure 3] FIG. 3 is a schematic diagram showing a schematic configuration of an apparatus used in producing a negative electrode active material for a secondary battery according to one embodiment of the present disclosure. [Figure 4] FIG. 4 shows an example of a diffraction pattern of an LSX particle by XRD. DETAILED DESCRIPTION OF THE INVENTION

[0012] The negative electrode active material for a secondary battery according to an embodiment of the present disclosure includes silicate composite particles (hereinafter also referred to as LSX particles) containing silicon particles and a lithium silicate phase containing Li, O, and Si elements, and the porosity of the silicate composite particles is 2% or less.

[0013] LSX particles have a sea-island structure in which silicon particles (islands) are dispersed within a lithium silicate phase (sea). High capacity can be achieved by controlling the amount of silicon particles dispersed in the lithium silicate phase. Furthermore, because the silicon particles are dispersed within the lithium silicate phase, the stress associated with the expansion and contraction of the silicon particles during charge and discharge is alleviated by the lithium silicate phase, reducing the expansion and contraction of the LSX particles. This reduces cracking and fractures in the LSX particles, making it easy to achieve both high battery capacity and improved cycle performance.

[0014] Conventionally, composite particles have been produced by mixing lithium silicate powder and silicon particles, pulverizing the mixture using a ball mill or the like to form a composite, and then sintering the pulverized mixture in a high-temperature, high-pressure atmosphere. The lithium silicate powder may be amorphous or crystalline. However, even when crystalline lithium silicate powder is used, the application of shear force caused by stirring the mixture can cause the crystalline lithium silicate to change to an amorphous state. High-temperature sintering restores the crystallinity of the lithium silicate, resulting in composite particles in which silicon particles are dispersed in a crystalline lithium silicate phase.

[0015] However, the higher the crystallinity of the lithium silicate phase, the more likely it is that cracks will occur due to the expansion and contraction of LSX particles during charge and discharge. When cracks occur, new surfaces are generated, which can increase side reactions with the electrolyte. Furthermore, cracks can isolate silicon particles, preventing them from contributing to charge and discharge. This can result in a deterioration in charge and discharge cycle performance. From this perspective, it is preferable for the lithium silicate phase to have low crystallinity, and the lithium silicate phase may be amorphous.

[0016] For example, in the above-mentioned conventional method, if the sintering process of the mixture after pulverization is omitted, LSX particles can be obtained in which silicon particles are dispersed in a low-crystalline (amorphous) lithium silicate phase. In this case, the amorphous nature of the lithium silicate phase suppresses the expansion and contraction of the LSX particles. However, if the sintering is insufficient, although the crystallinity of the lithium silicate phase can be reduced, the porosity inside the LSX particles tends to increase. As a result, the surface area reacting with the electrolyte increases, and side reactions tend to increase. As a result, it may be difficult to obtain poor charge-discharge cycle performance.

[0017] In contrast, in this embodiment, for example, a heat treatment is performed at a high temperature (e.g., 600°C to 1000°C) for a relatively short time (e.g., 40 seconds to 200 seconds), and the heated mixture is then pressurized to obtain a sintered body with low porosity and high hardness while suppressing crystallization of the lithium silicate phase. This suppresses cracking and fracture of the LSX particles during charge and discharge. Furthermore, the porosity of the LSX particles is 2% or less, and the surface area of ​​the LSX particles is also reduced. Therefore, side reactions are not increased, and deterioration of cycle characteristics is suppressed. The pressurization treatment may be performed simultaneously with the heat treatment, or may be performed while the high temperature after heating is maintained.

[0018] The porosity (%) of LSX particles may be 1.7% or less, or 0.6% or less. Here, "porosity" refers to the ratio of the area occupied by voids to the total area of ​​the base particle in the particle cross section, and can be determined by SEM observation of the particle cross section. The specific method for measuring porosity is as follows. (1) Using an ion milling device (e.g., IM4000) manufactured by Hitachi High-Technologies Corporation, the cross section of the base particle is exposed. (2) The exposed particle cross-section is observed using an SEM, the ratio of the void area to the total area of ​​the particle cross-section is measured, and the porosity (void area × 100 / total area of ​​particle cross-section) is calculated. The porosity is the average value for 10 particles.

[0019] The BET specific surface area of ​​LSX particles is, for example, 0.8 m 2 / g or less. The smaller the BET specific surface area, the more likely it is that side reactions will be reduced. The BET specific surface area is measured by the BET method using LSX particles separated from the negative electrode active material layer after removing the negative electrode from a discharged battery using the method described below. The BET method is an analytical method in which an inert gas with a known molecular size is adsorbed onto the surface of a measurement sample, and the surface area is determined from the amount of adsorption and the area occupied by the inert gas. Nitrogen gas is used as the inert gas.

[0020] The Vickers hardness of LSX particles is, for example, 800 HV or more. The higher the Vickers hardness, the harder the LSX particles are, and the more likely they are to be prevented from cracking or breaking during charging and discharging. The Vickers hardness is calculated from the test force and indentation depth when a triangular pyramidal indenter is applied to a sample prepared by embedding LSX particles separated from the negative electrode active material layer in a resin using the method described below and polishing the sample.

[0021] These LSX particles contain a large amount of amorphous lithium silicate phase, which tends to increase the charge capacity during the initial charge / discharge cycle. Furthermore, the amorphous lithium silicate phase is highly flexible and easily follows the expansion and contraction of silicon, reducing the frequency with which silicon particles become isolated and unable to charge / discharge. As a result, the capacity loss during subsequent charge / discharge cycles is significantly suppressed. Therefore, the LSX particles of this embodiment are advantageous when considering multiple charge / discharge cycles.

[0022] LSX particles may also contain a crystalline silicon oxide phase. LSX particles are produced by pulverizing and compounding silicon and lithium silicate. During the lithium silicate production process, if the silicon raw material is in excess relative to the lithium raw material, silicon oxide may be produced. While much of the silicon oxide becomes amorphous during the pulverization process, fine crystalline silicon oxide may precipitate within the lithium silicate phase during the subsequent heating process. Crystalline silicon oxide is stable and does not react with lithium ions during charging, preventing irreversible reactions. Because it is so fine, it is unlikely to interfere with the expansion and contraction of the silicon particles.

[0023] On the other hand, if LSX particles contain a large amount of lithium silicate crystals such as Li2Si2O5, the expansion and contraction of the particles can easily cause cracking. Furthermore, the lithium silicate phase is more flexible when it is amorphous than when it is crystalline, and therefore more easily follows the expansion and contraction of silicon particles. Therefore, the presence of two phases, a crystalline silicon oxide phase and an amorphous lithium silicate phase, can further alleviate the stress associated with the expansion and contraction of silicon particles. This allows for the realization of a negative electrode active material with good cycle characteristics and low irreversible capacity.

[0024] The silicon oxide phase may be present dispersedly within the lithium silicate phase within the LSX particles, i.e., the silicon oxide phase and silicon particles may be present dispersedly within the amorphous lithium silicate phase.

[0025] The diffraction pattern of the LSX particles of this embodiment obtained by X-ray diffraction (XRD) may show peaks derived from silicon particles and the lithium silicate phase. However, because the LSX particles contain a large amount of amorphous lithium silicate phase, the peak intensity derived from the lithium silicate phase is smaller than that of conventional LSX particles. Specifically, a peak (peak position) derived from the Si(111) plane of the silicon particles may appear in the range of 2θ = 27° to 30°. Furthermore, a peak (peak position) derived from the lithium silicate phase (Li2Si2O5) may appear in the range of 2θ = 23° to 25°. The peak derived from the Si(111) plane of the silicon particles is separated from the XRD diffraction pattern, and the integrated intensity of the separated peak is integrated over the range of 2θ = 23° to 30°. This is defined as I1. Similarly, the peak derived from the lithium silicate phase is separated from the XRD diffraction pattern, and the integrated intensity of the separated peak is integrated over the range of 2θ = 23° to 30°. This is defined as I2. Note that if multiple peaks exist within the range, the sum of the integrated intensities of the respective peaks is calculated. The ratio I2 / I1 is 0.3 or less. The ratio I2 / I1 may be 0.2 or less, or may be 0.1 or less. The X-rays used in the XRD method are Cu Kα rays.

[0026] Examples of XRD diffraction patterns of the LSX particles of this embodiment and conventional LSX particles are shown in Figure 4. In Figure 4, the diffraction pattern of LSX particle X1 shown by the solid line relates to the LSX particles of this embodiment, and the diffraction pattern of LSX particle Y1 shown by the dashed line relates to the conventional LSX particles.

[0027] For both LSX particles X1 and Y1, near 2θ = 28°, there exists the peak position of the peak derived from the Si(111) plane of silicon particles. Also, in the range of 2θ = 23° to 25°, there exist the peak positions of multiple peaks derived from the lithium silicate phase (Li2Si2O5). Further, near 2θ = 26°, a peak derived from the SiO2(011) plane of the silicon oxide phase is observed. Also, near 2θ = 19°, a peak derived from lithium silicate Li2SiO3 is observed, and in the range of 2θ = 20° to 21°, a peak derived from SiO2 is observed.

[0028] However, when comparing LSX particles X1 and Y1, the integrated intensity I2 of the peak derived from the lithium silicate phase in LSX particle X1 is smaller than that in LSX particle Y1.

[0029] In LSX particles, the lithium silicate phase contains, for example, an oxide phase represented by the formula: Li 2z SiO 2+z (0 < z < 1). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z = 1 / 2 is preferred.

[0030] The lithium silicate phase may contain other elements in addition to Li, Si, and O (oxygen). The lithium silicate phase may contain, for example, at least one element selected from the group consisting of alkali metal elements (excluding Li) and Group II elements.

[0031] By including an alkali metal element other than Li in the lithium silicate phase, it becomes difficult to crystallize, the viscosity in the softened state is low, and the fluidity is high. Therefore, even in heat treatment at a low temperature, it is easy to fill the gaps between silicon particles and easy to generate dense composite particles. Since alkali metal elements are inexpensive, they may be Na and / or K. The atomic ratio of an alkali element X other than Li (for example, K) to Li contained in the lithium silicate phase: X / Li may be, for example, 0.1 to 7.1, may be 0.4 or more and 5 or less, or may be 0.7 or more and 2 or less.

[0032] The lithium silicate phase may contain a Group II element. Generally, silicate phases exhibit alkaline properties, but the Group II element has the effect of suppressing the elution of alkali metals from the silicate phase. Therefore, the viscosity of the slurry containing the negative electrode active material is easily stabilized when it is prepared. Therefore, the need for a treatment (e.g., an acid treatment) to neutralize the alkaline components of the LSX particles is reduced.

[0033] The Group II element may be at least one selected from the group consisting of Be, Mg, Ca, Sr, Ba, and Ra. Among these, Ca is preferred because it can improve the Vickers hardness of the lithium silicate phase and further improve cycle characteristics. The content of the Group II element is, for example, 20 mol % or less, or may be 15 mol % or less, or may be 10 mol % or less, based on the total amount of elements other than O contained in the lithium silicate phase.

[0034] The lithium silicate phase may contain an element M other than alkali metal elements and Group II elements. The element M may be at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, La, V, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Specifically, for example, B has a low melting point and is advantageous for improving fluidity during sintering. Ca reduces ionic conductivity but has the effect of increasing the hardness of the lithium silicate phase. Al, Zr, Nb, Ta, and La can improve hardness while maintaining ionic conductivity. The content of the element M is, for example, 10 mol % or less, and may be 5 mol % or less, based on the total amount of elements other than O contained in the lithium silicate phase.

[0035] The contents of B, Na, K and Al contained in the lithium silicate phase are determined by quantitative analysis in accordance with JIS R3105 (1995) (method of analysis of borosilicate glass), and the Ca content is determined by quantitative analysis in accordance with JIS R3101 (1995) (method of analysis of soda-lime glass).

[0036] Other contained elements are determined by the following method. First, a sample of lithium silicate phase or LSX particles containing the same is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and carbon in the solution residue is removed by filtration. Then, the obtained filtrate is analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) to measure the spectral intensity of each element. Subsequently, a calibration curve is created using a commercially available standard solution of the element, and the content of each element contained in the lithium silicate phase is calculated.

[0037] In the silicate composite particles, there exist a lithium silicate phase, a silicon oxide phase, and silicon particles, and these can be distinguished and quantified by using Si-NMR. The Si content obtained by ICP-AES as described above is the sum of the amount of Si constituting the silicon particles, the amount of Si in the lithium silicate phase, and the amount of Si in the silicon oxide phase. On the other hand, the amount of Si constituting the silicon particles can be quantified separately by using Si-NMR. Therefore, the amount of Si in the lithium silicate phase can be quantified by subtracting the amount of Si constituting the silicon particles and the amount of Si in the silicon oxide phase from the Si content obtained by ICP-AES. For the standard substance required for quantification, a mixture containing a silicate phase and silicon particles with a known Si content in a predetermined ratio may be used.

[0038] The following shows the desirable measurement conditions for Si-NMR.

[0039] <Si-NMR Measurement Conditions> Measuring device: 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 3,000 sec Observation width: 100 kHz <​​Signal acquisition time: 0.05 sec Accumulation count: 560 Sample amount: 207.6 mg To increase capacity and improve cycle characteristics, the content of silicon particles in the LSX particles may be, for example, 30% by mass or more and 80% by mass or less. By setting the content of silicon particles to 30% by mass or more, the proportion of the lithium silicate phase decreases, making it easier to improve initial charge / discharge efficiency. By setting the content of silicon particles to 80% by mass or less, it is easier to reduce the degree of expansion and contraction of the LSX particles during charge / discharge. The content of silicon particles in the LSX particles is preferably 40% by mass or more, and more preferably 50% by mass or more.

[0040] The silicon particles dispersed within the lithium silicate phase have a particulate phase of simple silicon (Si) and are composed of single or multiple crystallites. The crystallite size of the silicon particles is preferably 30 nm or less. When the crystallite size of the silicon particles is 30 nm or less, the amount of volume change due to the expansion and contraction of the silicon particles during charge and discharge can be reduced, further improving cycle characteristics. For example, when the silicon particles contract, voids are formed around the silicon particles, reducing the contact points of the particles with their surroundings, thereby suppressing particle isolation and preventing a decrease in charge and discharge efficiency due to particle isolation. The lower limit of the crystallite size of the silicon particles is not particularly limited, but is, for example, 5 nm.

[0041] The crystallite size of the silicon particles is more preferably 2 nm or more and 30 nm or less, and even more preferably 2 nm or more and 20 nm or less. When the crystallite size of the silicon particles is 20 nm or less, the expansion and contraction of the silicon particles can be made uniform, and microcracks in the particles due to the expansion and contraction of the silicon particles during charging and discharging can be reduced, thereby improving cycle characteristics. The crystallite size of the silicon particles 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 of the silicon particles.

[0042] At least a portion of the surface of the LSX particles may be coated with a conductive material. Because the lithium silicate phase has poor electronic conductivity, the conductivity of the LSX particles tends to be low. However, by coating the surface of the base particles with a conductive material to form a conductive layer, the conductivity of the LSX particles can be dramatically improved. A carbon material is preferred as the conductive material. The carbon material preferably contains at least one material selected from the group consisting of carbon compounds and carbonaceous materials.

[0043] The thickness of the conductive layer is preferably thin enough not to substantially affect the average particle size of the LSX particles. Considering the need to ensure 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 LSX particles using a SEM or TEM (transmission electron microscope).

[0044] Examples of carbon compounds include compounds containing carbon and hydrogen, and compounds containing carbon, hydrogen, and oxygen. Examples of carbonaceous materials that can be used include amorphous carbon with low crystallinity and graphite with high crystallinity. Examples of amorphous carbon include carbon black, coal, coke, charcoal, and activated carbon. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. Among these, amorphous carbon is preferred because of its low hardness and its strong buffering effect against silicon particles, which change in volume during charging and discharging. The amorphous carbon may be either easily graphitized carbon (soft carbon) or difficult-to-graphitize carbon (hard carbon). Examples of carbon black include acetylene black and ketjen black.

[0045] LSX particles can be extracted from batteries using the following method. First, the battery is disassembled and the negative electrode is removed. The negative electrode is then washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove the electrolyte. Next, the negative electrode mixture is 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 alkali metals such as Na and Li that may be contained in the binder. Next, the sample powder is washed with ion-exchanged water, filtered, and dried at 200°C for 1 hour. The carbon components are then removed by heating to 900°C in an oxygen atmosphere, allowing the LSX particles to be isolated.

[0046] Cross-sectional observation of LSX particles can be performed, for example, by the following method. First, the battery is disassembled, the negative electrode is removed, and a cross-section of the negative electrode mixture layer is obtained using a cross-section polisher (CP). The cross-section of the negative electrode mixture layer is observed using a scanning electron microscope (SEM). Ten LSX particles with a maximum particle diameter of 5 μm or more are randomly selected from the cross-sectional backscattered electron image of the negative electrode mixture layer, and each is subjected to element (e.g., carbon) mapping analysis using energy dispersive X-ray (EDX). The area containing 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 containing the specified element for the 10 obtained particles are then averaged.

[0047] During the charge / discharge process, a coating is formed on the surface of the LSX particles due to the decomposition of the electrolyte. As described below, LSX particles may also have a conductive layer covering the surface of the composite particles. Therefore, EDX mapping analysis is performed within a range of 1 μm inside the periphery of the cross-section of the LSX particle, so as not to include the thin coating or conductive layer in the measurement range. EDX mapping analysis also allows us to confirm the distribution of carbon materials inside the LSX particles. Since it becomes difficult to distinguish them from electrolyte decomposition products at the end of the cycle, it is preferable to measure samples before or at the beginning of the cycle.

[0048] Desirable measurement conditions for cross-sectional SEM-EDX analysis are shown below.

[0049] <SEM-EDX Measurement Conditions> Processing equipment: JEOL, SM-09010 (Cross Section Polisher) Processing conditions: Acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 ~2×10 -3 Pa Measurement device: Electron microscope SU-70 made 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 In addition, the quantification of each element in the silicate composite particles contained in the negative electrode active material layer in the discharge state is possible not only by SEM-EDX analysis but also by Auger electron spectroscopy (AES), laser ablation ICP mass spectrometry (LA-ICP-MS), X-ray photoelectron spectroscopy (XPS), etc.

[0050] Next, an example of the manufacturing method of LSX particles will be described in detail.

[0051] Step (i) The raw material for lithium silicate is a raw material mixture containing a Si-containing raw material and a Li raw material in a predetermined ratio. The raw material mixture may also contain the above-mentioned alkali metal element, Group II element, and / or element M. A mixture of the above raw materials in predetermined amounts is melted, and the molten liquid is passed through a metal roll to form flakes, producing lithium silicate. The flaked silicate is then crystallized by heat treatment in an air atmosphere at a temperature above the glass transition point and below the melting point. Note that the flaked silicate can also be used without crystallization. Alternatively, a mixture of predetermined amounts can be fired at a temperature below the melting point without melting, to produce silicate through a solid-state reaction.

[0052] Silicon oxide can be used as the Si raw material. Li raw materials can be, for example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. These can be used alone or in combination of two or more. Raw materials for alkali metal elements, Group II elements, and element M can include oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, etc. of each element.

[0053] The Si raw material that has not reacted with the Li raw material may remain in the lithium silicate, and the remaining Si raw material is dispersed in the lithium silicate as a silicon oxide phase.

[0054] Process (ii) Next, raw silicon is mixed with the lithium silicate to form a composite. For example, composite particles are produced through the following steps (a) to (c).

[0055] Process (a) First, raw silicon powder and lithium silicate powder are mixed in a mass ratio of, for example, 20:80 to 95:5. The raw silicon may be coarse silicon particles with an average particle size of several μm to several tens of μm.

[0056] Process (b) Next, using a grinding device such as a ball mill, the mixture of raw silicon and lithium silicate is ground and composited while being reduced to fine particles. At this time, an organic solvent may be added to the mixture and wet-ground. A predetermined amount of organic solvent may be added to the grinding vessel all at once at the beginning of grinding, or a predetermined amount of organic solvent may be added to the grinding vessel 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] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0058] The raw silicon and lithium silicate may be separately microparticulated and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared without using a pulverizer and then mixed. The nanoparticles may be prepared 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).

[0059] Process (c) Next, the mixture is heated to 600°C to 1000°C in an inert gas atmosphere (e.g., argon, nitrogen, etc.), and then pressurized and sintered. The heating time may be a short time of 200 seconds or less. Pressurization may be performed simultaneously with (or in parallel with) heating, or may be performed after heating. Pressurization may be performed while the high-temperature state caused by heating is maintained. Sintering can be performed using a sintering device that can apply pressure in an inert atmosphere, such as a hot press. During sintering, the 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 as the sea portion and the silicon particles as the island portion.

[0060] The resulting sintered body is pulverized to obtain silicate composite particles. By appropriately selecting the pulverization conditions, silicate composite particles having a predetermined average particle size can be obtained.

[0061] The pressure treatment may be carried out by rolling the powder of the mixture that has been molded into a sheet shape. Rolling is usually carried out by passing a sheet of the mixture through the gap between two rotating rolls. By passing the sheet of the mixture through the gap between heated rolls, the mixture can be heated and pressurized simultaneously. The sheet of the mixture may be heated before and / or after passing through the rolls. By carrying out the heat treatment while rolling the mixture molded into a sheet shape, the negative electrode active material for secondary batteries (LSX particles) can be continuously produced with high productivity. The pressure applied during rolling is, for example, 200 MPa to 600 MPa.

[0062] Process (iii) Next, at least a portion of the surface of the composite particle may be coated with a conductive material to form a conductive layer. The conductive material is preferably electrochemically stable, and a conductive carbon material is preferred. Examples of methods for coating the surface of the composite particle with a conductive carbon material include a CVD method using hydrocarbon gases such as acetylene and methane as raw materials, and a method in which coal pitch, petroleum pitch, phenolic resin, etc. is mixed with the composite particle and heated to carbonize it. Carbon black may also be attached to the surface of the composite particle. The coating with the conductive carbon material can be performed, for example, by heating a mixture of the particles from which the fine particles have been removed and the conductive carbon material in an inert atmosphere (e.g., an argon, nitrogen, etc.) at 700°C to 950°C.

[0063] Process (iv) The composite particles (including those having a conductive layer on the surface) may be washed with an acid. For example, by washing the composite particles with an acidic aqueous solution, trace amounts of alkaline components present on the surface of the composite particles, which may be generated when the raw silicon and lithium silicate are combined, can be dissolved and removed. The acidic aqueous solution may be an aqueous solution of an inorganic acid such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, or carbonic acid, or an aqueous solution of an organic acid such as citric acid or acetic acid.

[0064] FIG. 1 shows a schematic cross section of an LSX particle 20 as an example of a negative electrode material.

[0065] The base particle 23 includes a lithium silicate phase 21 and silicon particles 22 dispersed within the lithium silicate phase 21. The base particle 23 has a sea-island structure in which fine silicon particles are dispersed within the matrix of the lithium silicate phase 21. The surface of the base particle 23 is coated with a conductive layer 26, and an LSX particle 20 is formed.

[0066] A silicon oxide phase may also be dispersed in the lithium silicate phase 21. The SiO2 content in the base particle 23 measured by Si-NMR is, for example, preferably 30 mass % or less, and more preferably less than 7 mass %.

[0067] The base particles 23 may contain other components in addition to the lithium silicate phase 21, silicon particles 22, silicon oxide phase, and carbon material. For example, from the viewpoint of improving the strength of the base particles 23, the base particles 23 may contain a reinforcing material such as an oxide such as ZrO or a carbide up to 10% by weight of the base particles 23.

[0068] The average particle size of the silicon particles 22 before the first charge is 500 nm or less, preferably 200 nm or less, and more preferably 50 nm or less. By appropriately miniaturizing the silicon particles 22 in this manner, volume change during charge and discharge is reduced, improving structural stability. Furthermore, uniform expansion and contraction of the silicon particles is achieved, suppressing particle cracking and improving cycle characteristics. The average particle size of the silicon particles 22 is measured by observing the cross section of the negative electrode material using a SEM or TEM. Specifically, it is determined by averaging the maximum diameters of 100 randomly selected silicon particles 22.

[0069] A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode. The negative electrode includes a current collector and a negative electrode active material layer containing the negative electrode active material for a secondary battery. The negative electrode, positive electrode, electrolyte, and separator included in the secondary battery according to an embodiment of the present disclosure will be described below.

[0070] [Negative electrode] The negative electrode comprises, for example, a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector and containing a negative electrode active material. 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.

[0071] The negative electrode mixture contains, as the negative electrode active material, the above-mentioned negative electrode active material for secondary batteries containing LSX particles as an essential component, and may contain, as optional components, a binder, a conductive agent, a thickener, etc. The silicon particles in the LSX particles can absorb many lithium ions, thereby contributing to increasing the capacity of the negative electrode.

[0072] The negative electrode active material may further contain other active materials that electrochemically absorb and release lithium ions. Examples of such other active materials include carbon-based active materials. Because LSX particles expand and contract in volume with charge and discharge, a high proportion of LSX particles in the negative electrode active material can easily cause 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 LSX particles and a carbon-based active material makes it possible to achieve excellent cycle characteristics while still providing the high capacity of silicon particles to the negative electrode. The proportion of LSX particles in the total of the LSX particles and the carbon-based active material is preferably 0.5 to 15% by mass, more preferably 1 to 5% by mass. This facilitates achieving both high capacity and improved cycle characteristics.

[0073] Examples of carbon-based active 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, 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.

[0074] 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, more preferably 5 to 20 μm, from the viewpoint of balancing the strength and weight of the negative electrode.

[0075] 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.

[0076] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and a mixed solvent of these.

[0077] [Positive electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode mixture layer formed 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 a 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.

[0078] 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.

[0079] The positive electrode active material may be a lithium composite metal oxide.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 Mn 2-b M b O 4、 LiMePO 4、 Examples include 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 (for example, 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, is the value immediately after preparation of the active material and increases or decreases with charge and discharge.

[0080] 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.

[0081] 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.

[0082] [Electrolyte] The electrolyte contains a solvent and a lithium salt dissolved in the solvent. The concentration of the lithium salt in the electrolyte is, for example, 0.5 to 2 mol / L. The electrolyte may contain known additives.

[0083] The solvent used may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that may be used include cyclic carbonates, chain carbonates, and cyclic 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). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0084] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0085] [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, woven fabric, nonwoven fabric, or the like. The separator can be made of polyolefins such as polypropylene and polyethylene.

[0086] 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 an electrolyte are housed in an outer casing. 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.

[0087] FIG. 2 is a schematic perspective view of a partially cutaway prismatic secondary battery according to an embodiment of the present disclosure.

[0088] The battery includes a bottomed, rectangular battery case 4, an electrode group 1 and an electrolyte housed in the battery case 4, and a sealing plate 5 that seals the opening of 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, positive electrode, and separator are wound around a flat, plate-shaped winding core, and the electrode group 1 is formed by removing the winding core. The sealing plate 5 includes a liquid inlet closed with a sealing plug 8 and a negative electrode terminal 6 insulated from the sealing plate 5 by a gasket 7.

[0089] One end of a negative electrode lead 3 is attached to the negative electrode current collector of the negative electrode by welding or the like. One end of a positive electrode lead 2 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6. The other end of the positive electrode lead 2 is electrically connected to a sealing plate 5. A resin frame is disposed on top of the electrode group 1, separating the electrode group 1 from the sealing plate 5 and separating the negative electrode lead 3 from the battery case 4.

[0090] A method for producing a negative electrode active material for a secondary battery according to an embodiment of the present disclosure includes, for example, the steps of preparing silicate composite particles comprising silicon particles and an amorphous lithium silicate phase, with the silicon particles dispersed in the lithium silicate phase, pressure-molding the silicate composite particle powder, heating the pressure-molded silicate composite particles to a temperature of 600°C to 1000°C for 40 to 200 seconds, and pressurizing the silicate composite particles in the heated state. The step of pressing the silicate composite particles may be a step of rolling the silicate composite particles in the heated state. The above-described steps (i) to (ii)(b) correspond to the step of preparing the silicate composite particles.

[0091] 3 is a schematic diagram showing the overall configuration of a manufacturing apparatus 30 used to manufacture a negative electrode active material for a secondary battery according to one embodiment of the present disclosure. The manufacturing apparatus 30 includes a conveying section 31, a pressurizing section 32, and a preheating section 35.

[0092] The powder of silicate composite particles (LSX particles) is molded in advance, for example, into a sheet. The molding process can be performed by, for example, compressing the powder into a sheet using a pair of rolls or by pressing using a mold. The molded LSX particles are sent to a pressurizing unit 32 via a conveying unit 31.

[0093] The conveying section 31 has a function of supplying the molded LSX particles 36 to the pressurizing section 32. The conveying section 31 can be configured, for example, with a belt conveyor. The belt conveyor can be a steel belt, a mesh belt, or the like.

[0094] The LSX particles 36 on the transport section 31 are heated by the preheating section 35 before being transported to the pressure section 32. During heating, the temperature of the LSX particles needs to be maintained at 600°C or higher and 1000°C or lower. The heating time needs to be 40 seconds or higher and 200 seconds or lower. The heating time is the time it takes for the LSX particles 36 to pass through the preheating section 35.

[0095] The pressurizing unit 32 includes a pair of rolls, an upper roll 33a and a lower roll 33b. The upper roll 33a and the lower roll 33b are provided with internal heating units 34a and 34b, respectively. Heaters 34a and 34b may be formed using a sheath heater, a ceramic heater, a halogen lamp heater, a carbon heater, a flash lamp heater, an induction heater, or the like. Heaters 34a and 34b may be provided inside the upper roll 33a and the lower roll 33b, respectively, and / or may be disposed on the outer periphery of the upper roll 33a and the lower roll 33b. The pressure applied during pressurization is, for example, 200 MPa to 600 MPa, or 300 MPa to 500 MPa. The LSX particles supplied from the conveying unit 31 are heated and pressurized by the pressurizing unit 32, and sintering proceeds, resulting in a sintered body 37.

[0096] <Example> 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.

[0097] Example 1 [Preparation of LSX particles] Lithium carbonate and silicon dioxide were mixed in a molar ratio of Li2CO3:SiO2 = 34:66, the mixture was melted in an inert gas atmosphere at 1500°C for 5 hours, the molten liquid was passed through a metal roller to form flakes, and then heat-treated at 750°C for 5 hours to obtain a lithium silicate composite oxide existing as a mixed phase of amorphous and crystalline. The obtained lithium silicate composite oxide was pulverized to an average particle size of 10 μm.

[0098] A lithium silicate composite oxide with an average particle size of 10 μm was mixed with raw silicon (3N, average particle size of 10 μm) in a mass ratio of 70:30. The mixture was placed in a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5), 24 SUS 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.

[0099] The pulverized material was then pressure-molded into a sheet having a thickness of 4.2 mm at room temperature to obtain a sheet-like molded body. The molded body was placed in the apparatus shown in Figure 3 to obtain a sintered body. A pressure of 400 MPa was applied to the molded body via the pressure unit 32. The heating temperature was set to 800°C. The conveying speed was set so that the heating time was 100 seconds.

[0100] The sintered body was then crushed and passed through a 40 μm mesh. The mixture was then mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation). The mixture was then fired in an inert atmosphere at 800°C for 5 hours to coat the LSX particle surfaces with conductive carbon, forming a conductive layer. The amount of the conductive layer coating was 5 mass% based on the total mass of the LSX particles and conductive layer. LSX particles with an average particle size of 5 μm and a conductive layer were then obtained using a sieve. XRD analysis of the LSX particles was performed, and peaks derived from Si, SiO2, Li2Si2O5, and Li2SiO3 were measured.

[0101] [Preparation of negative electrode] LSX particles and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. The negative electrode active material, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), and lithium polyacrylate were mixed in a mass ratio of 96.5:1:1.5:1, water was added, and the mixture was stirred using a mixer (TK Hibismix, manufactured by Primix Corporation) to prepare a negative electrode slurry. Next, a 1 m thick film was applied to the surface of copper foil. 2 The negative electrode slurry was applied 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 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.

[0102] [Preparation of positive electrode] Lithium cobalt oxide, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 95:2.5:2.5, N-methyl-2-pyrrolidone (NMP) was added, and the mixture was stirred using a mixer (TK Hibismix, manufactured by Primix Corporation) 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 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.

[0103] [Preparation of electrolyte] A non-aqueous electrolyte solution was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7.

[0104] [Secondary battery production] A tab was attached to each electrode, and the positive and negative electrodes were spirally wound with the separator interposed therebetween so that the tabs were positioned at the outermost periphery to produce an electrode assembly. The electrode assembly was inserted into an exterior case made of aluminum laminate film and vacuum dried at 105°C for 2 hours. After that, a nonaqueous electrolyte solution was poured into the exterior case, and the opening of the exterior case was sealed to obtain secondary battery A1.

[0105] <Example 2> When the compact was placed in the apparatus shown in FIG. 3 to obtain a sintered body, the conveying speed was set so that the heating temperature of the compact was 800° C. and the heating time was 140 seconds.

[0106] Other than this, LSX particles were prepared in the same manner as in Example 1, and a negative electrode comprising the LSX particles was fabricated to obtain a secondary battery A2.

[0107] <Comparative Example 1> After pulverizing the mixture of lithium silicate composite oxide and raw material silicon, the pulverized product was fired in an inert atmosphere at 600°C for 5 hours while applying pressure to the mixture using a hot press, thereby obtaining a sintered body of the mixture.

[0108] The sintered body was then crushed and passed through a 40 μm mesh, after which it was mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation). The mixture was fired in an inert atmosphere at 800°C for 5 hours to coat the surfaces of the LSX particles with conductive carbon, forming a conductive layer. The amount of the conductive layer coating was 5 mass% of the total mass of the LSX particles and the conductive layer. LSX particles (secondary particles) with an average particle size of 5 μm and equipped with a conductive layer were then obtained using a sieve.

[0109] Using the above LSX particles, a negative electrode was produced in the same manner as in Example 1, to obtain a secondary battery B1.

[0110] <Comparative Example 2> After pulverizing the mixture of lithium silicate composite oxide and raw silicon, the pulverized material was fired at 700°C for 2 hours under vacuum by spark plasma sintering while applying pressure to the mixture, to obtain a sintered body of the mixture.

[0111] The sintered body was then crushed and passed through a 40 μm mesh, after which it was mixed with coal pitch (MCP250, manufactured by JFE Chemical Corporation). The mixture was fired in an inert atmosphere at 800°C for 5 hours to coat the surfaces of the LSX particles with conductive carbon, forming a conductive layer. The amount of the conductive layer coating was 5 mass% of the total mass of the LSX particles and the conductive layer. LSX particles with an average particle size of 5 μm and a conductive layer were then obtained using a sieve.

[0112] Using the above LSX particles, a negative electrode was produced in the same manner as in Example 1, to obtain a secondary battery B2.

[0113] The following evaluations were carried out for each of the batteries of the Examples and Comparative Examples.

[0114] [Charge / discharge cycle test] Each battery was repeatedly charged and discharged under the following conditions.

[0115] <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).

[0116] <Discharge> At 25°C, constant current discharge was carried out at a current of 1 It (800 mA) until the voltage reached 2.75V.

[0117] The rest period between charge and discharge was 10 minutes. The ratio of the discharge capacity at the 500th cycle to the discharge capacity at the 1st cycle was defined as the cycle retention rate.

[0118] The evaluation results for batteries A1, A2, B1, and B2 are shown in Table 1. Table 1 also shows the porosity, BET specific surface area, and Vickers hardness of the LSX particles used as the negative electrode active material for each battery. Table 1 also shows the ratio I2 / I1 of the integrated intensity I1 of the peak derived from the lithium silicate phase to the integrated intensity I2 of the peak derived from the Si(111) plane of the silicon particles, determined by XRD analysis of the LSX particles.

[0119] [Table 1]

[0120] As can be seen from Table 1, batteries A1 and A2, which used LSX particles with a porosity of 2% or less and an I2 / I1 ratio of 0.3 or less as the negative electrode active material, exhibited higher cycle retention rates than battery B1, which used LSX particles with a porosity of more than 2% and an I2 / I1 ratio of greater than 0.3 as the negative electrode active material, and battery B2, which used LSX particles with a porosity of 2% or less but an I2 / I1 ratio of greater than 0.3 as the negative electrode active material. [Industrial Applicability]

[0121] 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]

[0122] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal 20 LSX particles 21 Lithium silicate phase 22 Silicon particles 23 Mother particle 26 Conductive layer 30 Manufacturing equipment 31 Conveyor 32 Pressure section 33a Upper Roll 33b Lower Roll 34a,34b Heating part 35 Preheating section

Claims

1. preparing silicate composite particles comprising silicon particles and an amorphous lithium silicate phase, the silicon particles being dispersed in the lithium silicate phase; A step of molding the powder of the silicate composite particles; a step of heating the molded silicate composite particles to a temperature of 600° C. or higher and 1000° C. or lower for a period of 40 seconds or higher and 200 seconds or lower; and pressing the silicate composite particles in the heated state.

2. 2 . The method for producing a negative electrode active material for a secondary battery according to claim 1 , wherein the step of pressurizing the silicate composite particles is a step of rolling the silicate composite particles in the heated state.

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