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

Abstract

A negative electrode active substance for a secondary battery according to the present invention contains composite particles having a matrix and a silicon phase dispersed in the matrix. The composite particles are coated by a coating layer. The coating layer is a mixture of a carbon material and a fluorine-containing material.

Description

[Technical Field]

[0001] This invention relates to a negative electrode active material for secondary batteries, a method for producing the same, and a secondary battery using the negative electrode active material for secondary batteries. [Background technology]

[0002] In recent years, secondary batteries, such as non-aqueous electrolyte secondary batteries, have been attracting attention for their high voltage and high energy density, making them promising for small-scale consumer applications, power storage devices, and electric vehicle power sources. Amidst the growing demand for higher energy density in secondary batteries, materials containing silicon alloyed with lithium are expected to be used as anode active materials with high theoretical capacity density.

[0003] However, silicon-containing materials have a large irreversible capacity, which leads to a problem of low initial charge-discharge efficiency (especially the ratio of the initial discharge capacity to the initial charge capacity). Therefore, various techniques have been proposed to introduce lithium equivalent to the irreversible capacity into the silicon-containing material beforehand. 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 intercalation and release of lithium).

[0004] Patent Document 2 proposes a negative electrode material for a non-aqueous electrolyte secondary battery having negative electrode active material particles, wherein the negative electrode active material particles have a silicon compound represented by SiOx (0.5 ≤ x ≤ 1.6), the silicon compound contains a Li compound on its surface or inside, and the negative electrode active material particles have a coating layer made of an organic polymer that covers the surface of the silicon compound. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2015-153520 [Patent Document 2] International Publication No. 2015 / 107581 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, the composite particles described in Patent Document 1 are known to exhibit significant expansion and contraction of silicon particles during charging and discharging due to the intercalation and release of lithium. As a result, large stresses are generated in the lithium silicate phase surrounding the silicon particles due to the expansion and contraction of the silicon particles, causing cracks and fractures in the composite particles. Consequently, the bonding force between the composite particles and the surrounding binder weakens, and fractured composite particles, in particular, lose their conductive pathways with surrounding particles and become isolated. Furthermore, the lithium silicate phase tends to be gradually eroded by side reactions within the battery, leading to a decrease in charge-discharge cycle characteristics.

[0007] By coating the surface of the composite particles with the organic polymer described in Patent Document 2, it is possible to suppress the degradation of cycle characteristics to some extent. However, because the coating layer made of organic polymer acts as a resistor, the electronic conductivity and lithium-ion conductivity decrease, and the desired effect of suppressing the degradation of cycle characteristics may not be obtained. [Means for solving the problem]

[0008] In view of the above, one aspect of the present invention relates to a negative electrode active material for a secondary battery, comprising composite particles having a matrix and a silicon phase dispersed in the matrix, wherein the composite particles are coated with a coating layer, and the coating layer is a mixture of a carbon material and a fluorine-containing material.

[0009] Another aspect of the present invention relates to a method for manufacturing a negative electrode active material for a secondary battery, the method including: obtaining composite particles having a matrix and a silicon phase dispersed in the matrix; coating the composite particles with a carbon material to form a coating layer; mixing the composite particles having the coating layer with a powder of a fluorine-containing organic polymer to obtain a mixture; and heat-treating the mixture at a temperature not lower than the melting point of the fluorine-containing organic polymer to allow the fluorine-containing organic polymer to penetrate into the coating layer.

[0010] Yet another aspect of the present invention 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, in which the negative electrode includes a current collector and a negative electrode active material layer, and the negative electrode active material layer includes the negative electrode active material for the secondary battery described above.

Advantages of the Invention

[0011] By using the negative electrode active material of the present disclosure, a secondary battery having excellent charge / discharge cycle characteristics can be realized. The novel features of the present invention are set forth in the appended claims, but the invention, both as to its construction and its content, will be better understood from the following detailed description, when read in conjunction with the accompanying drawings, in connection with the other objects and features of the invention.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a cross-sectional view schematically showing a negative electrode active material (silicate composite particles) for a secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic perspective view of a part of a secondary battery according to an embodiment of the present invention, with a part cut away.

Embodiments of the Invention

[0013] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. In addition, components other than those characteristic of this disclosure may be replaced with components of known secondary batteries. In this specification, when "range of numerical value A to numerical value B" is used, the range includes numerical values ​​A and B. For example, "A to B mol%" is synonymous with "A mol% or more and B mol% or less". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits may be arbitrarily combined as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0014] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0015] Secondary batteries include, at a minimum, non-aqueous electrolyte secondary batteries such as lithium-ion batteries, and all-solid-state batteries.

[0016] (Negative electrode active material for secondary batteries) The negative electrode active material for a secondary battery according to the embodiments of this disclosure includes composite particles having a matrix and a silicon phase dispersed in the matrix. The composite particles are coated with a coating layer, which is a mixture of a carbon material and a fluorine-containing material.

[0017] This composite particle has a sea-island structure in which the matrix acts as the ocean and the silicon phase (silicon particles), which act as islands, are dispersed within the matrix. High capacity can be achieved by controlling the amount of silicon particles dispersed within the matrix, and because the silicon phase is dispersed within the matrix, the stress associated with the expansion and contraction of the silicon phase during charging and discharging is mitigated by the matrix, thereby reducing the expansion and contraction of the composite particle. As a result, cracks and fractures in the composite particle can be reduced, making it easy to achieve both high battery capacity and improved cycle characteristics.

[0018] The coating layer on the surface of the composite particles (i.e., the matrix surface) suppresses cracking and fracture of the composite particles, protects them from the electrolyte, and inhibits side reactions. This reduces erosion of the matrix due to side reactions with the electrolyte, improving charge-discharge cycle characteristics.

[0019] Among the materials constituting the coating layer, the carbon material is electrically conductive. Because the matrix portion of the composite particles has poor electronic conductivity, the conductivity of the composite particles tends to be low. However, by coating the surface of the composite particles with a conductive carbon material, the conductivity of the silicate composite particles can be dramatically increased. The carbon material preferably contains at least one selected from the group consisting of carbon compounds and carbonaceous materials. The carbonaceous material may be crystalline carbon or amorphous carbon.

[0020] In contrast, among the materials constituting the coating layer, the fluorine-containing material has resistance to the electrolyte. By making the coating layer a mixture of carbon material and fluorine-containing material, both conductivity and resistance to the electrolyte can be achieved, improving the charge-discharge cycle characteristics.

[0021] A coating layer composed of a mixture of carbon material and fluorine-containing material is formed, for example, by coating the surface of composite particles with a conductive layer made of carbon material, then attaching a fluorine-containing organic polymer such as polyvinylidene fluoride (PVdF), and then heat-treating it at a temperature above the melting point of the fluorine-containing organic polymer. Through heat treatment at a temperature above the melting point of the fluorine-containing organic polymer, the fluorine-containing organic polymer penetrates into the conductive layer of the carbon material, forming a coating layer that is a mixture of carbon material and fluorine-containing material.

[0022] In this case, the concentration of the fluorine-containing organic polymer in the coating layer may have a distribution that is high on the surface side of the coating layer and decreases as it goes inward in the thickness direction (towards the matrix side). In other words, the fluorine (F) atoms in the coating layer may be distributed such that they are high on the surface side of the coating layer and decrease as they go inward in the thickness direction (towards the matrix side). It is sufficient that the fluorine-containing organic polymer penetrates to a depth of at least half the thickness of the coating layer and a mixture is formed.

[0023] It is preferable that the fluorine (F) content A on the surface of the coating layer and the fluorine content B at a depth of half the thickness of the coating layer satisfy the condition B > 0.1A. In this case, a high level of both conductivity and resistance to the electrolyte can be achieved, and the effect of improving charge-discharge cycle characteristics is enhanced. The fluorine content is determined by observing a cross-section of the negative electrode active material layer (negative electrode mixture layer) with a SEM and performing elemental analysis such as EDS on the location of the composite particle coating layer obtained from the cross-sectional image.

[0024] Fluorine-containing organic polymers react with alkaline substances at temperatures above their melting point, potentially causing a reaction in which fluorine atoms are removed from the polymer. During this reaction, carbon-carbon double bonds are formed within the polymer, and lithium fluoride (LiF) is produced. As a result, a coating layer is formed containing the fluorine-containing organic polymer with carbon-carbon double bonds and lithium fluoride as fluorine-containing materials. The generated LiF coats the surface of the composite particles (the matrix surface), protecting the matrix from the electrolyte. The coating layer containing lithium fluoride stabilizes the matrix surface and suppresses side reactions.

[0025] For example, if the fluorine-containing organic polymer is polyvinylidene fluoride, the polyvinylidene fluoride reacts with lithium silicate according to the following reaction equation, causing the fluorine atom to be removed from the polyvinylidene fluoride and producing lithium fluoride. At this time, the polyvinylidene fluoride undergoes polyene formation, and some of the carbon-carbon single bonds are changed to carbon-carbon double bonds, forming a -CH=CF- structure.

[0026] [ka]

[0027] Fluorine-containing organic polymers may include polyvinylidene fluoride, as well as polymers containing vinylidene fluoride units. Examples of polymers containing vinylidene fluoride units include copolymers of vinylidene fluoride with other monomers. Examples of other monomers include hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). Examples of polymers containing vinylidene fluoride units include polyvinylidene fluoride (PVdF) and its modified forms, vinylidene fluoride-hexafluoropropylene copolymers, and vinylidene fluoride-chlorotrifluoroethylene copolymers. In polymers containing vinylidene fluoride units, the content of vinylidene fluoride units may be, for example, 30 mol% or more, and may be 50 mol% or more.

[0028] The thickness of the coating layer is preferably thin enough not to substantially affect the average particle size of the composite particles. Considering the effect of protecting the composite particles from the electrolyte, ensuring conductivity, and diffusing lithium ions, the thickness of the coating layer is preferably between 1 nm and 100 nm. A coating layer thickness of 1 nm or more provides sufficient protection for the composite particles. A coating layer thickness of 100 nm or less suppresses the increase in resistance due to the coating layer, allowing for the maintenance of high cycle characteristics. The thickness of the conductive layer can be measured by cross-sectional observation of the silicate composite particles using SEM or TEM (transmission electron microscope).

[0029] In order to suppress an increase in resistance in the coating layer and maintain high conductivity in the coating layer, it is preferable that the mass of carbon material in the coating layer is greater than the mass of fluorine-containing material in the coating layer.

[0030] The mass of the fluorine-containing material can be determined, for example, by washing the composite particles with a solvent such as N-methyl-2-pyrrolidone to dissolve the fluorine-containing material in the coating layer, and then measuring the weight difference of the solvent before and after dissolution. Subsequently, the composite particles are immersed in hydrofluoric acid to dissolve the silicon and silicate components. The remaining material that does not dissolve in hydrofluoric acid is the carbon material. Therefore, the mass of the carbon material can be determined by measuring the weight of the remaining material that does not dissolve in hydrofluoric acid.

[0031] A method for producing a negative electrode active material for a secondary battery according to the embodiments of this disclosure includes, for example, the steps of: obtaining composite particles having a matrix and a silicon phase dispersed in the matrix; coating the composite particles with a carbon material to form a coating layer; mixing the composite particles with the coating layer with a powder of a fluorine-containing organic polymer to obtain a mixture; and heat-treating the mixture at a temperature above the melting point of the fluorine-containing organic polymer to allow the fluorine-containing organic polymer to permeate into the coating layer. Details of the production method will be described later.

[0032] (composite particles) The matrix constituting the composite particles may be at least one of a silicon compound phase and a carbon phase. The silicon compound phase includes at least one of a silicon oxide phase and a silicate phase. The silicon oxide phase is composed of a compound of Si and O. The compound of Si and O may be SiO2. That is, the main component (e.g., 95 to 100% by mass) of the silicon oxide phase may be silicon dioxide. The silicate phase is composed of a compound containing a metal element, silicon (Si), and oxygen (O). The metal element is not particularly limited, but by containing at least lithium, for example, the entry and exit of lithium ions into the silicate phase become easy. That is, the silicate phase preferably contains at least lithium silicate. Among the silicon oxide phase and the silicate phase, those having the silicate phase as the main component are preferred in terms of having a small irreversible capacity. Here, the "main component" refers to a component that occupies 50% by mass or more of the total mass of the silicon compound phase, and may occupy 70% by mass or more of the component.

[0033] 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, more than 2 and less than 4. When the O / Si ratio is more than 2 and less than 4 (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 more than 2 and less than 3. The atomic ratio of Li to Si in lithium silicate: Li / Si is, for example, more than 0 and less than 4.

[0034] 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 of 0 < z < 1, and z = 1 / 2 is more preferable. Lithium silicate satisfying z = 1 / 2 can be represented by Li2Si2O5.

[0035] The silicate phase may contain another element M in addition to Li, Si, and O. The inclusion of element M in the silicate phase improves its chemical stability and lithium ion conductivity, or suppresses side reactions caused by contact between the silicate phase and the non-aqueous electrolyte.

[0036] As element M, for example, at least one selected from the group consisting of sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), barium (Ba), zirconium (Zr), niobium (Nb), 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), boron (B), and rare earth elements may be used. From the viewpoint of resistance to non-aqueous electrolytes and structural stability of the silicate phase, it is preferable that element M includes at least one selected from the group consisting of Zr, Ti, P, Al, and B.

[0037] Rare earth elements can improve the initial charge-discharge efficiency of the charge-discharge cycle. The rare earth elements may be scandium (Sc), yttrium (Y), or lanthanide elements. Preferably, the rare earth elements include at least one selected from the group consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd). From the viewpoint of improving lithium-ion conductivity, it is more preferable that the rare earth elements include La. The proportion of La in the total rare earth elements is preferably 90 atomic% or more and 100 atomic% or less.

[0038] By including alkali metal elements other than Li in the silicate phase, crystallization becomes less likely, resulting in lower viscosity in the softened state and higher fluidity. Therefore, during the heat treatment process, it is easier to fill the gaps between silicon particles and to easily produce dense composite particles. Since alkali metal elements are inexpensive, Na and / or K are preferred.

[0039] The silicate phase may contain Group II elements such as Ca and Mg. Generally, the silicate phase is alkaline, but Group II elements have the effect of suppressing the elution of alkali metals from the silicate phase. Therefore, the slurry viscosity is easily stabilized when preparing a slurry containing the negative electrode active material. Consequently, the need for treatment (e.g., acid treatment) to neutralize the alkaline components of the silicate composite particles is also reduced. Among these, Ca is preferred because it can improve the Vickers hardness of the silicate phase and further improve the cycle characteristics.

[0040] Other elements M besides alkali metals and Group II elements, such as B, have a low melting point and are advantageous for improving fluidity during sintering. Al, Zr, and La can improve hardness while maintaining ionic conductivity. Additionally, Zr, Ti, P, Al, and B enhance resistance to non-aqueous electrolytes and improve the structural stability of the silicate phase.

[0041] The silicate phase may also contain trace amounts of elements such as iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), and molybdenum (Mo).

[0042] Element M may form a compound. Depending on the type of element M, the compound may be, for example, a silicate of element M or an oxide of element M.

[0043] In the silicate phase, the content of element M is, for example, between 1 mol% and 40 mol% relative to the total amount of elements other than oxygen.

[0044] The content of Li, Si, and element M in the silicate phase can be measured, for example, by analyzing the cross-section of the negative electrode mixture layer.

[0045] First, a fully discharged battery is disassembled, the negative electrode is removed, and the negative electrode is washed with anhydrous ethyl methyl carbonate or dimethyl carbonate to remove non-aqueous electrolyte components. After drying, a cross-section of the negative electrode mixture layer is obtained using a cross-section polisher (CP). Next, the cross-section of the negative electrode mixture layer is observed using a scanning electron microscope (SEM).

[0046] The content of each element can then be determined by one of the following methods. Furthermore, the composition of the silicate phase can be calculated from the content of each element.

[0047] <edx> From the cross-sectional image of the reflected electron image of the negative electrode mixture layer, randomly select 10 silicate composite particles with a maximum particle diameter of 5 μm or more, and perform elemental mapping analysis on each of them by energy-dispersive X-ray (EDX). Calculate the area ratio of the target element using image analysis software. The observation magnification is preferably 2000 to 20000 times. Average the measured values of the area ratio of the predetermined element contained in 10 particles. The content of the target element is calculated from the obtained average value.

[0048] The following shows the measurement conditions for the desirable cross-sectional SEM-EDX analysis. <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

[0049] <aes> Ten silicate composite particles with a maximum diameter of 5 μm or more are randomly selected from the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, and qualitative and quantitative elemental analysis is performed on each particle using an Auger electron spectroscopy (AES) analyzer (e.g., JEOL Ltd., JAMP-9510F). The measurement conditions can be, for example, an acceleration voltage of 10 kV, a beam current of 10 nA, and an analysis area of ​​20 μmφ. The content of a predetermined element is calculated by averaging the content of each of the ten particles.

[0050] During the charging and discharging process, a film is formed on the surface of the silicate composite particles due to the decomposition of the electrolyte. Furthermore, as described later, silicate composite particles may also have a conductive layer that coats the surface of the composite particles. Therefore, mapping analysis using EDX or AES is performed on a range 1 μm inward from the peripheral edge of the cross-section of the silicate composite particle, so that the measurement range does not include thin films or conductive layers. Mapping analysis can also confirm the distribution of carbon material inside the composite particles. Since it becomes difficult to distinguish between electrolyte decomposition products and other substances at the end of the cycle, it is preferable to measure samples before or at the beginning of the cycle.

[0051] <icp> A sample of the silicate composite particles is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon in the solution residue is removed by filtration. Then, the resulting filtrate is analyzed by inductively coupled plasma optical emission spectrometry (ICP) 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 Si-containing particles is calculated.

[0052] In addition, the contents of B, Na, K, and Al contained in the silicate phase can be quantitatively analyzed in accordance with JIS R3105 (1995) (Method for Analysis of Borosilicate Glass).

[0053] In the silicate composite particles, there are a silicate phase and silicon particles, and these can be distinguished and quantified by using Si-NMR. The Si content obtained by the above method is the sum of the amount of Si constituting the silicon particles, the amount of Si in the silicate phase (and the amount of Si constituting silicon oxide), and the amount of Si in the silicate composite particles. The amount of Si element contained in the silicate composite particles is distributed to the silicon particles, the silicon oxide phase, and the silicate phase by using the results of quantitative analysis by Si-NMR. For the standard substance required for quantification, a mixture containing silicon particles, a silicon oxide phase, and a silicate phase with a known Si content ratio in a predetermined ratio may be used.

[0054] The desirable Si-NMR measurement conditions are shown below. <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 3000 sec Observation width: 100 kHz Observation center: Around -100 ppm Signal acquisition time: 0.05 sec Total count: 560 Sample amount: 207.6 mg

[0055] For increased capacity and improved cycle characteristics, the silicon particle content in the silicate composite particles may be, for example, 30% by mass or more and 80% by mass or less. A silicon particle content of 30% by mass or more reduces the proportion of the silicate phase, making it easier to improve initial charge-discharge efficiency. A silicon particle content of 80% by mass or less makes it easier to reduce the degree of expansion and contraction of the silicate composite particles during charge and discharge. The silicon particle content in the silicate composite particles is preferably 40% by mass or more, and more preferably 50% by mass or more.

[0056] The silicon particles dispersed within the silicate phase have a particulate phase of elemental silicon (Si) and consist of one or more crystallites. The crystallite size of the silicon particles is preferably 50 nm or less. When the crystallite size of the silicon particles is 50 nm or less, the volume change due to the expansion and contraction of the silicon particles during charging and discharging can be reduced, further improving the cycle characteristics. For example, when silicon particles contract, voids are formed around them, reducing the contact points with the surrounding area and thus suppressing the isolation of the particles, thereby suppressing the decrease in charge-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, 2 nm.

[0057] The crystallite size of the silicon particles is more preferably 5 nm or more and 30 nm or less, and even more preferably 5 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 the fine cracks in the particles caused by the expansion and contraction of the silicon particles during charging and discharging can be reduced, thereby improving the cycle characteristics. The crystallite size of the silicon particles is calculated from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane of the X-ray diffraction (XRD) pattern of the silicon particles using Scherrer's formula.

[0058] Silicate composite particles can be extracted from batteries by 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 the copper foil and ground in a mortar to obtain a sample powder. The sample powder is then dried in a dry atmosphere for 1 hour and immersed in weakly boiled 6M hydrochloric acid for 10 minutes to remove alkali metals such as Na and Li that may be present in the binder. Next, the sample powder is washed with deionized water, filtered, and dried at 200°C for 1 hour. After that, the silicate composite particles can be isolated by heating to 900°C in an oxygen atmosphere to remove the carbon component.

[0059] Next, we will describe in detail an example of a method for producing silicate composite particles. Process (i) (Process for obtaining lithium silicate) The raw materials for lithium silicate are a mixture containing a Si-containing raw material and a Li-containing raw material in predetermined proportions. The raw material mixture may also contain the aforementioned element M. A mixture of predetermined amounts of the above raw materials is dissolved, and the melt is passed through a metal roll to flake it and produce lithium silicate. The flake silicate is then crystallized by heat treatment in an atmospheric atmosphere at a temperature above the glass transition point and below the melting point. It is also possible to use the flake silicate without crystallization. Alternatively, silicate can be produced by solid-phase reaction by firing a mixture of predetermined amounts at a temperature below the melting point without dissolving it.

[0060] Silicon oxide can be used as the Si raw material. For Li raw materials, for example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc., can be used. These may be used individually or in combination of two or more. Raw materials for element M include oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, etc., of each element. Within the lithium silicate, residual Si raw materials that did not react with Li raw materials may remain. These residual Si raw materials are dispersed within the lithium silicate as fine crystals of silicon oxide.

[0061] Process (ii) (Process for obtaining silicate composite particles) Next, the lithium silicate is compounded with silicon raw material. For example, composite particles are produced through the following steps (a) to (c).

[0062] Process (a) First, the raw silicon powder and lithium silicate powder are mixed in a mass ratio of, for example, 20:80 to 95:5. For the raw silicon, coarse silicon particles with an average particle size of several micrometers to several tens of micrometers should be used.

[0063] Process (b) Next, the mixture of raw silicon and lithium silicate is pulverized and compounded using a pulverizing device such as a ball mill, while simultaneously reducing it to fine particles. At this time, an organic solvent may be added to the mixture for wet pulverization. A predetermined amount of the organic solvent may be added to the pulverizing container all at once at the beginning of pulverization, or a predetermined amount of the organic solvent may be added to the pulverizing container intermittently in multiple portions during the pulverization process. The organic solvent serves to prevent the material to be pulverized from adhering to the inner wall of the pulverizing container.

[0064] Suitable organic solvents include alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, and metal alkoxides.

[0065] For the raw material silicon, coarse silicon particles with an average particle size of several micrometers to several tens of micrometers can be used. Preferably, the silicon particles obtained in the final product are controlled so that the crystallite size, calculated by Scherrer's formula from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane of the X-ray diffraction pattern, is between 5 nm and 50 nm.

[0066] Alternatively, the raw materials, silicon and lithium silicate, may be separately atomized into fine particles before being mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared without using a pulverizing device and then mixed. Known methods such as gas-phase methods (e.g., plasma methods) or liquid-phase methods (e.g., liquid-phase reduction methods) can be used to prepare the nanoparticles.

[0067] Process (c) Next, the pulverized material is fired under pressure using a hot press or the like to obtain a sintered body. The firing is carried out, for example, in an inert atmosphere (e.g., an atmosphere of argon, nitrogen, etc.). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, it is easy to disperse fine silicon particles in the silicate phase with low crystallinity. During sintering, the lithium silicate softens and flows to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body can be obtained in which the silicate phase forms the sea and the silicon particles form the islands. The firing temperature is preferably 550°C or higher and 900°C or lower, and more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or more and 10 hours or less.

[0068] By crushing the resulting sintered body, silicate composite particles can be obtained. By appropriately selecting the crushing conditions, silicate composite particles with a predetermined average particle size can be obtained. Steps (i) and (ii) yield composite particles having a silicate phase as a matrix and a silicon phase dispersed within the matrix.

[0069] Process (iii) (Process of coating the surface of silicate composite particles with carbon material) Next, a conductive layer is formed by coating at least a portion of the surface of the silicate composite particles with a carbon material. Examples of carbon materials include coal pitch or coal tar pitch, petroleum pitch, and phenolic resin. The conductive material raw materials and the composite particles are mixed, and the mixture is fired to carbonize the conductive material raw materials, thereby forming a coating layer that covers at least a portion of the surface of the composite particles.

[0070] The firing of the mixture of carbon material raw materials and composite particles is carried out, for example, in an inert atmosphere (e.g., an atmosphere of argon, nitrogen, etc.). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, it is easy to form a highly conductive layer on the silicate phase with low crystallinity. The firing temperature is preferably 550°C or higher and 900°C or lower, and more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or more and 10 hours or less.

[0071] A coating layer may be formed on the composite particles by other methods. For example, a coating layer may be formed by reacting a hydrocarbon gas on the surface of the Si-containing particles using a gas-phase method such as CVD. Acetylene, methane, etc., can be used as the hydrocarbon gas. Alternatively, carbon black may be mixed with the composite particles to deposit a precursor on the surface of the composite particles, and then the precursor may be calcined together with the composite particles to form a coating layer.

[0072] Process (iv) Next, the composite particles are mixed with a powder of a fluorine-containing organic polymer to obtain a mixture. Then, the mixture is heat-treated at a temperature above the melting point of the fluorine-containing organic polymer and below its decomposition temperature to diffuse and permeate the fluorine-containing organic polymer into the interior of the coating layer. This forms a coating layer composed of a mixture of carbon material and fluorine-containing material. When polyvinylidene fluoride (PVdF) is used as the fluorine-containing organic polymer, the heat treatment temperature should be above the melting point of PVdF (150°C to 170°C) and below its decomposition temperature of 340°C, preferably 200°C to 250°C. The heat treatment can be carried out under an inert gas atmosphere. The heat treatment time should be, for example, about 1 to 3 hours. By crushing the mixture after heat treatment, composite particles are obtained in which a coating layer composed of a mixture of carbon material and fluorine-containing material is formed. Due to the heat treatment, polyvinylidene fluoride is polyene-formed, and at least a portion of the -CH2-CF2- structure may change to a -CH=CF- structure.

[0073] If the particle size of the fluorine-containing organic polymer in the mixture is too large, the surface of the composite particles (coating layer) cannot be uniformly covered with the liquefied fluorine-containing organic polymer by heat treatment, and the fluorine-containing organic polymer may form aggregates. To ensure that the surface of the composite particles (coating layer) is uniformly covered with the liquefied fluorine-containing organic polymer, it is preferable that the particle size of the fluorine-containing organic polymer be smaller than the particle size of the composite particles. Here, particle size refers to the particle size at which the volume integrated value in the particle size distribution measured by laser diffraction scattering method becomes 50% (volume-average particle size). The volume-average particle size of the fluorine-containing organic polymer is, for example, 1 to 100 μm, and more preferably 1 to 10 μm.

[0074] In the coating layer, in order to suppress the decrease in conductivity due to the addition of fluorine-containing organic polymers, it is preferable that the mass ratio of the fluorine-containing organic polymer mixed to the composite particles (which do not contain carbon material) when obtaining the mixture is smaller than the mass ratio of the carbon material to the composite particles when coating the composite particles with carbon material.

[0075] A process of washing the composite particles with acid may be performed. For example, washing the composite particles with an acidic aqueous solution can dissolve and remove 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 compounded. As the acidic aqueous solution, aqueous solutions of inorganic acids such as hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, phosphoric acid, and carbonic acid, or aqueous solutions of organic acids such as citric acid and acetic acid can be used.

[0076] The composite particles are SiO X The composite particles may also contain silicon oxide represented by the equation (0.5 ≤ X < 1.6). Such composite particles have a silicon oxide phase in their matrix. The average particle size of such composite particles may be in the range of 1 μm to 25 μm (for example, in the range of 4 μm to 15 μm).

[0077] Furthermore, the composite particles may also consist of a carbon phase and a silicon phase (silicon particles) dispersed within the carbon phase. Since the carbon phase is conductive, even if voids are formed around such composite particles, contact between the composite particles and their surroundings is easily maintained. As a result, capacity degradation due to repeated charge-discharge cycles is easily suppressed. The carbon phase may consist of amorphous carbon. Amorphous carbon may be hard carbon, soft carbon, or something else. Amorphous carbon is generally defined as having an average interplanar spacing d of the (002) plane measured by X-ray diffraction. 002 This refers to carbon materials with a wavelength exceeding 0.34 nm.

[0078] The average particle size of the composite particles having a carbon phase may be 3 μm or more and 18 μm or less, 6 μm or more and 15 μm or less, or 8 μm or more and 12 μm or less. The content of the silicon phase (silicon particles) in the composite particles having a carbon phase may be 30% by mass or more and 80% by mass or less, or 40% by mass or more and 70% by mass or less. Within this range, a sufficiently high capacity of the negative electrode can be achieved, and the cycle characteristics are also easily improved.

[0079] The above SiO X For composite particles or composite particles with a carbon phase as a matrix, the cycle characteristics when used as a negative electrode active material can be improved by forming a coating layer composed of a mixture of carbon material and fluorine-containing material. The formation of the coating layer can be carried out in the same manner as in steps (iii) and (iv) above.

[0080] The average particle size of composite particles may be measured by observing the cross-section of the negative electrode active material layer of the negative electrode, which has been disassembled and removed from a secondary battery, using a SEM or TEM. In this case, the average particle size can be determined by arithmetic mean of the maximum diameters of any 100 particles. Alternatively, the median diameter (D50) at which the cumulative volume in the volume-based particle size distribution reaches 50% can be used as the average particle size of composite particles before the negative electrode active material layer is formed. The median diameter can be determined, for example, using a laser diffraction / scattering particle size distribution analyzer.

[0081] Figure 1 schematically shows a cross-section of silicate composite particles 20 as an example of a negative electrode active material. The mother particle 23 comprises a lithium silicate phase 21 and silicon phase (silicon particles) 22 dispersed within the silicate phase 21. The mother particle 23 has a sea-island structure in which fine silicon phase is dispersed in the matrix of lithium silicate phase 21. The surface of the mother particle 23 is covered with a coating layer 26 to form silicate composite particles 20.

[0082] The mother particle 23 may contain other elements besides the lithium silicate phase 21, silicon phase 22, silicon oxide phase, and coating layer.

[0083] The average particle size of the silicon particles 22 is 500 nm or less, preferably 200 nm or less, and more preferably 50 nm or less, before the first charge. By moderately refining the silicon particles 22 in this way, the volume change during charging and discharging is reduced, and structural stability is improved. In addition, the expansion and contraction of the silicon particles are made uniform, and particle cracking is suppressed, 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 SEM or TEM. Specifically, it is determined by averaging the maximum diameters of any 100 silicon particles 22.

[0084] A secondary battery according to the embodiments of this disclosure comprises 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 secondary batteries. The secondary battery may be a non-aqueous electrolyte secondary battery. The negative electrode, positive electrode, electrolyte, and separator comprising the secondary battery according to the embodiments of the present invention will be described below.

[0085] [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 coating the surface of the negative electrode current collector with a negative electrode slurry, which is obtained by dispersing the negative electrode mixture in a dispersion medium, and drying it. The dried coating may be rolled if necessary. The negative electrode mixture layer may be formed on one surface of the negative electrode current collector or on both surfaces.

[0086] The negative electrode mixture contains, as an essential component, a negative electrode active material for secondary batteries containing the above-mentioned silicate composite particles as the negative electrode active material, and may contain optional components such as binders, conductive agents, and thickeners. Since the silicon particles in the silicate composite particles can absorb a large amount of lithium ions, they contribute to increasing the capacity of the negative electrode.

[0087] The negative electrode active material may further contain other active material materials that electrochemically intercalate and release lithium ions. For example, carbon-based active materials are preferred as other active material materials. Since silicate composite particles expand and contract in volume during charging and discharging, if their proportion in the negative electrode active material is large, poor contact between the negative electrode active material and the negative electrode current collector is likely to occur during charging and discharging. On the other hand, by using silicate composite particles and carbon-based active materials in combination, it becomes possible to achieve excellent cycle characteristics while imparting the high capacity of silicon particles to the negative electrode. The proportion of silicate composite particles in the total of silicate composite particles and carbon-based active material is preferably, for example, 0.5 to 15 mass%, and more preferably 1 to 5 mass%. This makes it easier to achieve both high capacity and improved cycle characteristics.

[0088] Examples of carbon-based active materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). Among these, graphite is preferred because it has excellent charge-discharge stability and low irreversible capacity. Graphite refers to materials having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, and graphitized mesophase carbon particles. Carbon-based active materials may be used individually or in combination of two or more.

[0089] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) or porous conductive substrates (such as mesh, net, or perforated sheet) are used. 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 from the viewpoint of balancing the strength of the negative electrode with weight reduction, 1 to 50 μm is preferred, and 5 to 20 μm is more desirable.

[0090] Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, vinyl resins, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives. These may be used individually or in combination of two or more. Examples of conductive agents include carbon black, conductive fibers, fluorinated carbon, and organic conductive materials. These may be used individually or in combination of two or more. Examples of thickeners include carboxymethylcellulose (CMC) and polyvinyl alcohol. These may be used individually or in combination of two or more.

[0091] Examples of dispersion media include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), or mixtures thereof.

[0092] [Positive electrode] The positive electrode comprises, 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 coating the surface of the positive electrode current collector with a positive electrode slurry, which is obtained by dispersing the positive electrode mixture in a dispersion medium, and drying it. The dried coating may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces.

[0093] The positive electrode mixture may contain a positive electrode active material as an essential component and optionally include binders, conductive agents, etc.

[0094] A lithium composite metal oxide can be used as the positive electrode active material. For example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn2O4, Li a Mn 2-b M b O 4、 LiMePO 4、 Li2MePO4F can be mentioned. 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, contains at least one selected from the group consisting of Mn, Fe, Co, Ni). Here, 0 ≦ a ≦ 1.2, 0 ≦ b ≦ 0.9, 2. ≦ c ≦ 2.3. Note that the a value indicating the molar ratio of lithium increases or decreases by charge and discharge.

[0095] As the binder and the 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.

[0096] The shape and thickness of the positive electrode current collector can be selected respectively from the shape and range according to the negative electrode current collector. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, titanium, etc. can be exemplified.

[0097] [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.

[0098] The solvent used may be an aqueous or non-aqueous solvent. Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, and cyclic carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Examples of linear carbonate esters 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). The non-aqueous solvent may be used alone or in combination of two or more types.

[0099] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts of fluorine-containing acids (such as LiPF6, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0100] [Separator] Typically, it is desirable to interpose a separator between the positive and negative electrodes. The separator should have high ion permeability, appropriate mechanical strength, and insulating properties. Microporous thin films, woven fabrics, nonwoven fabrics, etc., can be used as separators. For example, polyolefins such as polypropylene and polyethylene can be used as the material for the separator.

[0101] One example of a secondary battery structure is a structure in which an electrode group, in which a positive electrode and a negative electrode are wound around each other with a separator, and an electrolyte are housed in an outer casing. Alternatively, other forms of electrode groups may be used instead of wound electrode groups, such as a laminated electrode group in which the positive electrode and negative electrode are stacked with a separator. Secondary batteries may take any form, such as cylindrical, prismatic, coin-type, button-type, or laminated type.

[0102] Figure 2 is a schematic perspective view showing a portion of a rectangular secondary battery according to one embodiment of the present invention.

[0103] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 and an electrolyte (not shown) housed within the battery case 4, and a sealing plate 5 that seals the opening of the battery case 4. The electrode group 1 has a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them. The negative electrode, positive electrode, and separator are wound around a flat core, and the electrode group 1 is formed by removing the core. The sealing plate 5 has an electrolyte inlet sealed with a plug 8 and a negative electrode terminal 6 insulated from the sealing plate 5 by a gasket 7.

[0104] One end of the negative electrode lead 3 is attached to the negative electrode current collector of the negative electrode by welding or other means. One end of the positive electrode lead 2 is attached to the positive electrode current collector of the positive electrode by welding or other means. The other end of the negative electrode lead 3 is electrically connected to the negative electrode terminal 6. The other end of the positive electrode lead 2 is electrically connected to the sealing plate 5. Above the electrode group 1, a resin frame is positioned to isolate the electrode group 1 from the sealing plate 5, and also to isolate the negative electrode lead 3 from the battery case 4. [Examples]

[0105] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0106] <Example 1> [Preparation of silicate composite particles] Lithium carbonate (Li2CO3) was used as the Li raw material, and silicon dioxide (SiO2) was used as the Si raw material. These were mixed in a predetermined ratio, and the mixture was calcined at 800°C for 10 hours in an inert gas atmosphere to obtain lithium silicate. The obtained lithium silicate was pulverized to an average particle size of 10 μm.

[0107] Lithium silicate with an average particle size of 10 μm and raw silicon (3N, average particle size 10 μm) were mixed in a mass ratio of 42:58.

[0108] The mixture was filled into a 500mL pot (made of stainless steel) of a planetary ball mill (Fritsch, P-5), 24 stainless steel balls (20mm in diameter) were placed inside, the lid was closed, and the mixture was ground at 200rpm for 25 hours in an inert atmosphere.

[0109] Next, the powdered mixture was taken out in an inert atmosphere and fired at 600°C for 4 hours under pressure from a hot press in an inert atmosphere to obtain a sintered body of the mixture. The obtained sintered body was crushed and passed through a 40 μm mesh to obtain silicate composite particles.

[0110] 100 parts by mass of silicate composite particles and 3 parts by mass of coal tar pitch were mixed, and then fired at 800°C in an argon atmosphere to form a conductive layer (coating layer) covering at least a portion of the surface of the composite particles. During firing, the coal tar pitch was converted into amorphous carbon. The mass ratio of the conductive layer to the total mass of the silicate composite particles and the conductive layer was 3% by mass.

[0111] Next, silicate composite particles and polyvinylidene fluoride (PVdF) (average particle size (D50) 5 μm) were mixed in a ratio of 1 part by mass of PVdF to 100 parts by mass of silicate composite particles before the formation of the conductive layer, and the mixture was heat-treated at 200°C for 2 hours in an inert gas atmosphere. After that, silicate composite particles with an average particle size of 10 μm, having a coating layer composed of a mixture of carbon material and fluorine-containing material, were obtained using a sieve.

[0112] SEM-EDX analysis was performed on silicate composite particles to measure the distribution of fluorine atoms in the coating layer. As a result, in a region where a coating layer with a thickness of 50 nm was formed, if the fluorine content at the surface of the coating layer is denoted as A, the fluorine content B at a depth of 25 nm (half the thickness of the coating layer) from the surface was approximately 0.5 times A.

[0113] [Fabrication of the negative electrode] Silicate composite particles and graphite were mixed in a mass ratio of 5:95 and used as the negative electrode active material. The negative electrode slurry was prepared by adding water to a negative electrode mixture containing the negative electrode active material, carboxymethylcellulose sodium (CMC-Na), and styrene-butadiene rubber (SBR) in a mass ratio of 97.5:1:1.5, and stirring. Next, 1 m of this slurry was applied to the surface of the copper foil. 2 The negative electrode slurry is applied so that the mass of the negative electrode mixture per unit is 190g. After the coating is dried, it is rolled out to create a copper foil with a density of 1.5g / cm³ on both sides. 3 A negative electrode was fabricated with a negative electrode mixture layer formed thereon.

[0114] [Fabrication of the positive electrode] LiRing 0.88 Co 0.09 Al 0.03 A positive electrode slurry was prepared by adding N-methyl-2-pyrrolidone (NMP) to a positive electrode mixture containing O2, acetylene black, and polyvinylidene fluoride (PVdF) in a mass ratio of 95:2.5:2.5 and stirring. Next, the positive electrode slurry was applied to the surface of aluminum foil, the coating was dried, and then it was rolled to coat both sides of the aluminum foil with a density of 3.6 g / cm³. 3 A positive electrode was fabricated with a positive electrode mixture layer formed thereon.

[0115] [Preparation of electrolyte solution] A non-aqueous electrolyte 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.

[0116] [Manufacturing of secondary batteries] An electrode group was fabricated by attaching tabs to each electrode and spirally winding the positive and negative electrodes via a separator so that the tabs were located on the outermost periphery. The electrode group was inserted into an aluminum laminate film enclosure, vacuum-dried at 105°C for 2 hours, then a non-aqueous electrolyte was injected, and the opening of the enclosure was sealed to obtain a secondary battery (non-aqueous electrolyte secondary battery) A1.

[0117] <Examples 2-4> In the preparation of silicate composite particles, the mixing ratio of PVdF when mixing silicate composite particles with PVdF was changed to 2 parts by mass, 5 parts by mass, or 10 parts by mass of PVdF per 100 parts by mass of silicate composite particles before coating layer formation. Remaining batteries (non-aqueous electrolyte secondary batteries) A2 to A4 were obtained in the same manner as in Example 1.

[0118] <Comparative Example 1> In the preparation of the silicate composite particles, the silicate composite particles and PVdF were not mixed, and the silicate composite particles after the formation of an amorphous carbon conductive layer (coating layer) were used as the negative electrode active material. Aside from this, the same procedure as in Example 1 was followed to obtain Comparative Example B1, a secondary battery (non-aqueous electrolyte secondary battery).

[0119] The following evaluations were performed on secondary batteries A1-A4 and B1.

[0120] (1) Charge-discharge cycle test Each battery was repeatedly charged and discharged under the following conditions. <Charging> At 25°C, constant current charging was performed with a current of 1 It (800mA) until the voltage reached 4.2V, and then constant voltage charging was performed with a voltage of 4.2V until the current was reduced to 1 / 20 It (40mA).

[0121] <Discharge> At 25°C, constant current discharge was performed with a current of 1 It (800mA) until the voltage reached 2.75V.

[0122] The rest period between charging and discharging was set to 10 minutes. For each battery, the charging capacity C0 and discharging capacity C1 at the first cycle were determined, and the ratio expressed as 100 × C1 / C0 (%) was defined as the initial efficiency. Furthermore, for each battery, the discharging capacity C at 300 cycles was determined. 300 Find 100 × C 300 The percentage expressed as / C1(%) was calculated as the capacity retention rate.

[0123] The evaluation results for each secondary battery A1-A4 and B1 are shown in Table 1. Table 1 also shows the composition of the coating layer of the silicate composite particles used as the negative electrode active material for each battery (content of carbon material and fluorine-containing material in the coating layer). In Table 1, the content of carbon material and fluorine-containing material is expressed in parts by mass per 100 parts by mass of silicate composite particles without the coating layer.

[0124] [Table 1]

[0125] As shown in Table 1, secondary batteries A1 to A4, in which the coating layer, a mixture of carbon material and fluorine-containing material, was formed on silicate composite particles, showed significantly higher capacity retention compared to secondary battery B1, in which the coating layer was made of carbon material only on silicate composite particles.

[0126] In secondary batteries A3 and A4, where the fluorine-containing material content in the coating layer was higher than the carbon material content, the capacity retention rate was significantly higher than that of battery B1, but lower than that of batteries A1 and A2. Furthermore, the initial efficiency was slightly lower compared to battery B1. This is thought to be due to a decrease in the conductivity of the coating layer caused by the increased fluorine-containing material content. [Industrial applicability]

[0127] The present invention can provide a non-aqueous electrolyte secondary battery having good charge-discharge cycle characteristics. The non-aqueous electrolyte secondary battery of the present invention 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 electrode lead, 3: Negative electrode lead, 4: Battery case, 5: Sealing plate, 6: Negative electrode terminal, 7: Gasket, 8: Sealing plug, 20: Silicate composite particles, 21: Lithium silicate phase, 22: Silicon phase, 23: Mother particles, 26: Coating layer< / icp> < / aes> < / edx>

Claims

1. Composite particles comprising a matrix and a silicon phase dispersed within the matrix, The composite particles are covered by a coating layer, The coating layer is a mixture of a carbon material and a fluorine-containing material. A negative electrode active material for a secondary battery, wherein the fluorine content A on the surface of the coating layer and the fluorine content B at a depth of half the thickness of the coating layer satisfy B > 0.1A.

2. The fluorine-containing material comprises a fluorine-containing organic polymer and lithium fluoride (LiF), The fluorine-containing organic polymer has a carbon-carbon double bond in part of its molecular structure, as described in claim 1, for a negative electrode active material for a secondary battery.

3. The anode active material for a secondary battery according to claim 2, wherein the fluorine-containing organic polymer has a -CH=CF- structure in at least a part of its molecular structure.

4. The fluorine-containing organic polymer includes polyvinylidene fluoride (PVdF), The negative electrode active material for a secondary battery according to claim 3, wherein some of the carbon-carbon single bonds in the polyvinylidene fluoride are changed to carbon-carbon double bonds.

5. The negative electrode active material for a secondary battery according to any one of claims 1 to 4, wherein the thickness of the coating layer is 1 nm or more and 100 nm or less.

6. The negative electrode active material for a secondary battery according to any one of claims 1 to 4, wherein the mass of the carbon material in the coating layer is greater than the mass of the fluorine-containing material in the coating layer.

7. The negative electrode active material for a secondary battery according to any one of claims 1 to 4, wherein the matrix is ​​at least one of a silicon compound phase and a carbon phase.

8. The silicon compound phase mainly consists of a silicate phase containing lithium (Li), silicon (Si), and oxygen (O). The silicate phase contains elements M other than lithium (Li), silicon (Si), and oxygen (O), The negative electrode active material for a secondary battery according to claim 7, wherein element M comprises at least one selected from the group consisting of sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), barium (Ba), zirconium (Zr), niobium (Nb), 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), boron (B), and rare earth elements.

9. A step to obtain composite particles having a matrix and a silicon phase dispersed in the matrix, The process involves coating the composite particles with a carbon material to form a coating layer, The process involves mixing the composite particles having the coating layer formed with a fluorine-containing organic polymer powder to obtain a mixture. A method for producing a negative electrode active material for a secondary battery, comprising the steps of heat-treating the mixture at a temperature above the melting point of the fluorine-containing organic polymer to permeate the coating layer with the fluorine-containing organic polymer.

10. The method for producing a negative electrode active material for a secondary battery according to claim 9, wherein the fluorine-containing organic polymer includes polyvinylidene fluoride (PVdF).

11. A method for producing a negative electrode active material for a secondary battery according to claim 9, wherein in the step of obtaining the mixture, the particle size of the fluorine-containing organic polymer is smaller than the particle size of the composite particles.

12. A method for producing a negative electrode active material for a secondary battery according to any one of claims 9 to 11, wherein the mass ratio of the fluorine-containing organic polymer to the composite particles in the step of obtaining the mixture is smaller than the mass ratio of the carbon material to the composite particles in the step of forming the coating layer.

13. The device comprises 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. A secondary battery wherein the negative electrode active material layer includes the negative electrode active material for secondary batteries described in any one of claims 1 to 4.

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