Composite particles, negative electrode mixture layer, and lithium-ion secondary battery

Abstract

One embodiment of the present application is a composite particle containing silicon and carbon, having the following features. The cross-sectional diameter and the silicon content of the composite particle are measured by cross-sectional SEM-EDS, and when composite particles having a cross-sectional diameter of less than 1 / 2 of the number average of the cross-sectional diameters are referred to as small-diameter composite particles, the ratio of the number of the small-diameter composite particles to the number of the measured composite particles is 5% or more and 50% or less, the ratio of the average silicon content (mass%) of the particles other than the small-diameter composite particles to the average silicon content (mass%) of the small-diameter composite particles is 0.90 or less, and the silicon content of the entire composite particle is 45 mass% or more.

Description

Technical Field

[0001] One embodiment of the present invention relates to a composite particle comprising silicon and carbon, a negative electrode compound layer comprising the composite particle, and a lithium-ion secondary battery comprising the negative electrode compound layer. Background Technology

[0002] In lithium-ion secondary batteries used in smartphones, tablets and other IT devices, vacuum cleaners, power tools, electric bicycles, drones, and automobiles, anode active materials with both high capacity and high output power are considered essential. Silicon (theoretical specific capacity: 4200 mAh / g), which has a higher theoretical specific capacity than currently used graphite (theoretical specific capacity: 372 mAh / g), is attracting attention as an anode active material.

[0003] However, the volume of silicon (Si) expands / contracts by approximately 3 to 4 times with electrochemical lithiation / delithiation. This is known to cause silicon particles to break or peel off from the electrode, resulting in very low charge-discharge cycle characteristics (hereinafter referred to as cycle performance) for lithium-ion secondary batteries using silicon. Therefore, instead of simply replacing graphite with silicon, research is actively underway to develop structures that reduce the overall expansion / contraction of silicon as a negative electrode material. Among these efforts, extensive attempts have been made to combine silicon with carbonaceous materials.

[0004] One of the key characteristics of lithium-ion rechargeable batteries is their high volumetric energy density. To achieve this, both the positive and negative electrodes must possess high electrode density and high capacity. Therefore, the negative and positive electrode materials that constitute these batteries also require high powder compressibility and specific capacity.

[0005] In this specification, "electrode density" refers to the apparent density of the electrode mixture layer. Additionally, "negative electrode material" refers to the negative electrode active material. The definition of "positive electrode material" is the same as that of the negative electrode material, referring to the positive electrode active material. Furthermore, in this specification, the positive electrode material and / or negative electrode material are sometimes referred to as "electrode material".

[0006] In order to obtain electrode materials with the desired powder properties and electrochemical characteristics, the following operation has been widely carried out: mixing different types of electrode materials or electrode materials with different powder properties and electrochemical characteristics.

[0007] For example, Japanese Patent Publication No. 2020-537324 (Patent Document 1) discloses a negative electrode having a specific surface area (BET) of 0.1 to 1.2 m². 2 / g of artificial graphite, and one or more substances selected from natural graphite and softened carbon having a larger specific surface area than the aforementioned artificial graphite as negative electrode active materials.

[0008] In Japanese Patent Publication No. 2019-522872 (Patent Document 2), an electrode for a metal ion battery is disclosed, which is an electrode for a metal ion battery comprising an active layer in contact with a current collector, the active layer comprising D having a D in the range of 0.5 to 40 μm 50 porous particles (i) having a particle diameter and an intraparticle porosity of less than 30%, and D 50 a plurality of carbon particles (ii) having a particle diameter in the range of 1 to 100 μm, the porous particles (i) being a plurality of porous particles (i) comprising an electroactive substance selected from silicon, silicon oxide represented by SiO x (wherein 0<x≤1.5), germanium, tin, aluminum and mixtures thereof, the plurality of carbon particles (ii) are selected from one or more of graphite, soft carbon and hard carbon, the active layer comprises at least 50% by weight of the carbon particles (ii), and the ratio of the D50 particle diameter of the carbon particles (ii) to the D50 particle diameter of the porous particles (i) is in the range of 1.5 to 30.

[0009] In U.S. Patent No. 10424786 (Patent Document 3), a particulate material is disclosed, wherein the porous carbon framework has a bimodal or multimodal pore size distribution.

[0010] Prior Art Literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Publication No. 2020-537324

[0013] Patent Document 2: Japanese Patent Publication No. 2019-522872

[0014] Patent Document 3: U.S. Patent No. 10424786 Summary of the Invention

[0015] Problems to be Solved by the Invention

[0016] However, in the negative electrode disclosed in Patent Document 1, only graphite is included as the active material and silicon is not included, so only a capacity depending on graphite can be expected.

[0017] In the electrode for a metal ion battery disclosed in Patent Document 2, only the size ratio of the porous particles (i) comprising the electroactive substance to the carbon particles (ii) is controlled, and only the electrode density and capacity obtained through the optimization thereof can be expected.

[0018] The particulate material disclosed in Patent Document 3 mainly focuses on the improvement of rate characteristics, and cannot necessarily improve electrode density and specific capacity.

[0019] Furthermore, regarding these materials, if the silicon content is increased, although the initial specific capacity in the battery is high, the expansion / contraction during charging and discharging is large, thus it is believed that the electrode deteriorates severely and the cycle characteristics become worse.

[0020] The problem to be solved by the present invention is to provide a negative electrode material for lithium-ion secondary batteries with high compression density, high silicon concentration and good cycle characteristics.

[0021] Methods for solving problems

[0022] In order to solve the above-mentioned problems, the inventors of this application conducted in-depth research and found that by making the proportion of composite particles with a specific particle size in the whole, the average silicon content, and the silicon content of the composite particles as a whole within a specific range, the above-mentioned problems can be fully solved, thereby completing the present invention.

[0023] One embodiment of the present invention is configured as follows.

[0024] [1] A composite particle comprising silicon and carbon,

[0025] The cross-sectional diameter and silicon content of the composite particles were determined by cross-sectional SEM-EDS. Composite particles with a cross-sectional diameter less than half the average cross-sectional diameter were classified as small-diameter composite particles.

[0026] The proportion of the small-diameter composite particles in the total number of composite particles measured is more than 5% and less than 50%.

[0027] The ratio of the average silicon content (mass%) of the composite particles other than the small-diameter composite particles to the average silicon content (mass%) of the small-diameter composite particles is 0.90 or less.

[0028] The silicon content of the composite particles as a whole is over 45% by mass.

[0029] [2] According to the composite particles described in [1], the carbon comprises porous carbon material.

[0030] [3] The composite particles according to [1] or [2] have a pore volume of 0.10 cm³. 3 / g or less.

[0031] [4] The composite particles according to any one of [1] to [3] have an oxygen content (mass%) to a silicon content (mass%) ratio of 0.001 or more and 0.300 or less.

[0032] [5] The composite particles according to any one of [1] to [4], in the Raman spectrum, at 450 to 495 cm⁻¹ -1 There are peaks between them.

[0033] [6] The composite particles according to any one of [1] to [5] have an XRD pattern obtained by powder XRD measurement using Cu-Kα rays, in which the ratio of (peak height of SiC111 surface) to (peak height of Si111 surface) is 0.010 or less.

[0034] [7] When the composite particles according to any one of [1] to [6] are analyzed by SEM-EDS, one or more elements selected from Al, Ti, V, Cr, Mn, Fe, Co, Ni, Y, Zr, Mo, Nb, La, Ce, Ta, and W are detected.

[0035] [8] The composite particles according to any one of [1] to [7], comprising 5 to 50% by mass of composite particles (S) and 50 to 95% by mass of composite particles (L),

[0036] The composite particles (S) are formed by the action of silane gas on D. V50 The micrometer size is 1.0–10.0 μm and the pore volume is 0.8–2.2 cm³. 3 / g of porous carbon (S) is obtained by filling the fine pores of the porous carbon (S) with silicon.

[0037] The composite particles (L) are formed by the action of silane gas on D. V50 Larger than the porous carbon (S) with a pore volume of 0.2–0.8 cm³. 3 / g porous carbon (L) and silicon is filled into the pores of the porous carbon (L) to obtain (the total mass of the composite particles (S) and the composite particles (L) is recorded as 100% by mass).

[0038] [9] A negative electrode compound layer comprising any one of the composite particles described in [1] to [8].

[0039]

[10] A lithium-ion secondary battery comprising the negative electrode compound layer described in [9].

[0040] Invention Effects

[0041] The composite particles of one embodiment of the present invention have high compressibility and high specific capacity, thus enabling the provision of a negative electrode composite layer with high volumetric energy density and good cycle characteristics, and a lithium-ion secondary battery. Detailed Implementation

[0042] The specific embodiments of the present invention will be described below.

[0043] (1) Composite particles

[0044] The composite particle (hereinafter also referred to as "this composite particle") of one embodiment of the present invention is a composite particle comprising silicon and carbon. The carbon is not particularly limited, but preferably comprises a porous carbon material. By comprising the carbon with a porous carbon material, it is possible to absorb the volume changes caused by the expansion / contraction of silicon during lithiation / delithiation. Silicon is more preferably disposed within the pores of the porous carbon material, but silicon may also be present on the particle surface of the porous carbon material.

[0045] In one embodiment of the present invention, the composite particles can be controlled using the cross-sectional diameter and silicon content determined by cross-sectional SEM-EDS. Next, an example of a method for cross-sectional SEM-EDS measurement will be described.

[0046] First, cross-sectional SEM-EDS is performed on the composite particle swarm collected from a homogeneous state. For example, a spoonful of powder can be scraped from a large amount of well-mixed composite particle powder using a micro-scraper and placed on a carbon tape. The cross-section can then be exposed using a cross-section polisher, for example, or the well-mixed powder can be embedded in resin and ground to generate a cross-section.

[0047] Even when the aforementioned composite particles are mixed with other particles of significantly different shapes and compositions, they can be distinguished from the composite particles using the aforementioned SEM and EDS (SEM-EDS), thus enabling the investigation of the cross-sectional diameter and silicon content of the composite particles.

[0048] Therefore, even if the aforementioned composite particles exist in the electrode composite layer of a lithium-ion secondary battery, their cross-sectional diameter and silicon content can be investigated by SEM-EDS. However, particles that break during electrode pressure are excluded from the measurement. Whether breakage occurred during electrode pressure can be easily determined based on the shape of adjacent composite particles.

[0049] For example, 100 composite particles were randomly selected for cross-sectional SEM-EDS measurement. Here, the "cross-sectional diameter" in the cross-sectional SEM image of the composite particles is the diameter of the cross-section of a single particle obtained by cross-sectional SEM observation of the composite particles, for example, the equivalent circle diameter calculated from the cross-sectional area of ​​the particle using image analysis software such as ImageJ.

[0050] The average cross-sectional diameter of the composite particles is the sum of the measured cross-sectional diameters. That is, the average cross-sectional diameter is the value obtained by dividing the total number of cross-sectional diameters by the number of composite particles measured. Composite particles with a cross-sectional diameter less than half of this average cross-sectional diameter are called "small-diameter composite particles." Furthermore, composite particles other than the aforementioned small-diameter composite particles are called "large-diameter composite particles."

[0051] In one embodiment of the present invention, the proportion of small-diameter composite particles in the measured total number of composite particles is 5% or more. By making the proportion 5% or more, composite particles with a size similar to small-diameter composite particles can efficiently enter the gaps between particles other than small-diameter composite particles, thereby increasing the compressive density of the powder made from these composite particles. From this viewpoint, the proportion is preferably 10% or more, and more preferably 20% or more.

[0052] The number of the aforementioned small-diameter composite particles accounts for 50% or less of the total number of composite particles measured. By keeping this ratio below 50%, fewer voids are generated between particles of similar size to small-diameter composite particles in this composite particle, thereby increasing the compressive density of the powder. From this viewpoint, the ratio is preferably 45% or less, and more preferably 35% or less.

[0053] When analyzing the silicon content in composite particles using EDS, for the cross-sectional SEM images of the composite particles whose cross-sectional diameters were measured as described above, EDS was used to analyze the silicon and carbon in the central part of the particles, and the proportion of these elements was calculated in mass percent. The silicon content obtained from each composite particle that was measured was averaged by the number of particles, and the resulting value (average value) was taken as the average silicon content of the composite particle. That is, the average silicon content is the value obtained by dividing the sum of the silicon contents of each composite particle by the number of composite particles measured.

[0054] The value obtained by averaging only small-diameter composite particles in the above method is called the "average silicon content (x) of small-diameter composite particles". Similarly, the value obtained by averaging only large-diameter composite particles is called the "average silicon content (y) of large-diameter composite particles".

[0055] In the composite particles of one embodiment of the present invention, the value of (y) / (x) is 0.90 or less. That is, in the composite particles of one embodiment of the present invention, the silicon content of the small-diameter composite particles is higher than that of the large-diameter composite particles. This is based on the principle that small-diameter composite particles are less prone to breakage due to expansion / contraction compared to large-diameter composite particles. Therefore, compared to setting the same silicon content for all composite particles, the silicon content of the composite particles as a whole can be increased, and thus the specific capacity of the composite particles can be increased. The value of (y) / (x) is preferably 0.85 or less, more preferably 0.80 or less.

[0056] In one embodiment of the present invention, the silicon content of the composite particles is 45% by mass or more. By achieving a silicon content of 45% by mass or more, the composite particles can possess a sufficiently high specific capacity. From this viewpoint, the silicon content is preferably 48% by mass or more, and more preferably 50% by mass or more.

[0057] The silicon content is preferably 85% by mass or less. By keeping the silicon content at 85% by mass or less, good cycle characteristics can be obtained in the battery. From this point of view, the silicon content is more preferably 75% by mass or less, and even more preferably 65% ​​by mass or less.

[0058] The silicon content in composite particles can be quantitatively analyzed using methods such as X-ray fluorescence (XRF) and inductively coupled plasma atomic emission spectrometry (ICP-AES). This silicon content is sometimes specifically referred to as "quantitative silicon content."

[0059] In one embodiment of the present invention, the pore volume of the composite particles is preferably 0.10 cm³. 3 / g or less. By making the pore volume 0.10cm³ 3 With a concentration below / g, the specific surface area decreases, and electrolyte decomposition occurs only to an acceptable extent, resulting in excellent initial coulombic efficiency in the battery. Furthermore, when silicon is present within the pores of the composite particles, the pore volume is reduced to 0.10 cm³. 3 With a particle size below 0.05 g, silicon has fewer opportunities to come into contact with oxygen and moisture in the air, thus becoming less susceptible to oxidation. From this perspective, the aforementioned pore volume is more preferably 0.05 cm³. 3 / g or less.

[0060] The pore volume can be obtained by analyzing the adsorption isotherms obtained from nitrogen adsorption experiments using known methods. Details will be discussed later.

[0061] In the composite particles of one embodiment of the present invention, the ratio of oxygen content to silicon content is preferably 0.001 or higher. Here, the units for oxygen content and silicon content are mass percent. By making the above ratio 0.001 or higher, the outermost surface of silicon is stabilized by an oxidized film, thus resulting in excellent initial coulombic efficiency in the battery. From this viewpoint, the above ratio of oxygen content to silicon content is more preferably 0.002 or higher, and even more preferably 0.005 or higher.

[0062] In the composite particles of one embodiment of the present invention, the ratio of oxygen content to silicon content is preferably 0.300 or less. If the ratio is 0.300 or less, the irreversible capacity generated by silicon oxide is small. From this viewpoint, the ratio of oxygen content to silicon content is more preferably 0.200 or less, and even more preferably 0.100 or less.

[0063] The oxygen content in the composite particles can be determined, for example, using an oxygen analysis apparatus in which the sample is heated with carbon at high temperature in an inert gas, and the resulting CO and CO2 are quantified using an infrared detector. The silicon content can be determined using XRF and ICP-AES as described above.

[0064] Regarding the composite particles of one embodiment of the present invention, the Raman spectrum preferably ranges from 450 to 495 cm⁻¹. -1 The presence of peaks within this range indicates that the composite particles contain amorphous silicon. For the composite particles of one embodiment of the present invention, the peaks in the Raman spectrum are more preferably in the range of 450–495 cm⁻¹. -1 Peaks exist within the range of 500–530 cm⁻¹. -1 There are no peaks within the range of 500–530 cm⁻¹. -1 Since no peaks are found within the specified range, the composite particles can be considered to not contain crystalline silicon. It is assumed that the expansion / contraction of amorphous silicon during lithiation is isotropic compared to crystalline silicon, thus enabling the production of composite particles and electrode layers that are more resistant to mechanical degradation.

[0065] In the composite particles of one embodiment of the present invention, the ratio of (peak height of the 111 facet of SiC) to (peak height of the 111 facet of Si) in the XRD pattern obtained by powder X-ray diffraction (powder XRD measurement) using Cu-Kα rays is preferably 0.010 or less. By making the above ratio 0.010 or less, the silicon carbide (SiC) content of the composite particles is lower, and therefore the specific capacity is higher. From this viewpoint, the above ratio is more preferably 0.005 or less, further preferably 0.001 or less, and most preferably 0.000. The above ratio is also expressed as (I... SiC111 ) / (I Si111 ).

[0066] In one embodiment of the present invention, the composite particles may have a coating on their surface. Specific coatings include carbon coating, inorganic oxide coating, and polymer coating. Methods for carbon coating include chemical vapor deposition (CVD), physical vapor deposition (PVD), etc. Methods for inorganic oxide coating include CVD, PVD, atomic layer deposition (ALD), wet coating, etc. Wet coating methods include dissolving or dispersing an inorganic oxide precursor (a metal carboxylate or alkoxide) in a solvent, coating the composite particles with the resulting liquid, and removing the solvent by heat treatment or the like. Types of polymer coating include coating using a polymer solution, coating using a polymer precursor containing monomers and polymerizing it under the influence of temperature, light, etc., or combinations thereof.

[0067] The inorganic oxide is preferably one or more inorganic oxides selected from oxides of Al, Ti, V, Cr, Mn, Fe, Co, Ni, Y, Zr, Mo, Nb, La, Ce, Ta, W, and Li-containing oxides.

[0068] The coating can be a single coating or a combination of multiple coatings.

[0069] To avoid the reaction of silicon and carbon in the composite particles to form silicon carbide, heating is preferably performed at a temperature below 800°C, even if heating is required during coating.

[0070] It should be noted that, in order to manufacture such composite particles, in the composite particle manufacturing method described later, after manufacturing composite particles (S) and composite particles (L) obtained by filling porous carbon with silicon (steps 1 and 2), it is necessary to perform the above-mentioned treatment on both composite particles (S) and composite particles (L), or composite particles (S) and composite particles (L). When performing the same coating on both composite particles (S) and composite particles (L), the composite particles (S) and composite particles (L) can be coated separately and then mixed (step 3), or the coating can be performed after mixing (step 3).

[0071] As an effect of the coating, examples include, for instance, suppression of time-induced oxidation of silicon in the composite particles, increase in initial coulombic efficiency, and improvement in cycling characteristics, as shown below.

[0072] When the composite particles are exposed to air or an oxygen-containing gas atmosphere, the silicon gradually oxidizes over time. Because of the coating on the surface of the composite particles, the intrusion of air or oxygen-containing gas into the interior of the composite particles can be suppressed.

[0073] Furthermore, within a lithium-ion battery, after the initial insertion of lithium ions into the composite particles, if an electrolyte decomposition product film (SEI <Solid Electrolyte Interface> film) forms on the surface of the composite particles or at the entry point for lithium ions into the composite particles, lithium ions may remain trapped in the pores of the composite particles, thus reducing the initial coulombic efficiency. During subsequent lithium ion insertions, the presence of the SEI film significantly reduces the percentage of lithium ions captured by the composite particles. As described above, by having a coating on the surface of the composite particles, insertion of lithium ions into the pores that are easily blocked by the SEI film can be prevented, thereby increasing the initial coulombic efficiency.

[0074] It is also believed that if repeated charging and discharging occurs in a lithium-ion battery, the silicon in the composite particles reacts with fluorine, a component element of the electrolyte, and dissolves out as silicon fluoride compounds. If silicon dissolves, the specific capacity of the composite particles decreases. If a coating is present on the surface of the composite particles, the dissolution of the silicon-containing compounds can be suppressed, thus preventing the decrease in the specific capacity of the composite particles and also suppressing side reactions with the electrolyte. Furthermore, the coating reduces resistance, thereby improving the rate characteristics.

[0075] When the particles in this composite have a coating, they can have the same coating, but the presence or absence and type of coating can vary depending on the particle size. For example, the following combinations are possible.

[0076] Method (i) involves small-diameter composite particles having a coating, while large-diameter composite particles do not have a coating.

[0077] Method (ii): Small-diameter composite particles have no coating, while large-diameter composite particles have a coating.

[0078] Method (iii) involves small-diameter composite particles and large-diameter composite particles having coatings, but the types of coatings differ.

[0079] The surface coating of composite particles can be analyzed by performing particle surface analysis. Examples of methods include SEM-EDS, Auger electron spectroscopy, micro-infrared spectroscopy, and micro-Raman spectroscopy. Since the surface of the composite particles is being analyzed, the spatial resolution of the analytical device is preferably less than half the average particle size of the composite particles.

[0080] In one embodiment of the present invention, the composite particles have a high silicon content, and therefore it is preferable that the small-diameter composite particles have a coating.

[0081] The thickness of the coating is not limited as long as it does not impair battery performance. However, if it is too thick, the quantitative silicon content in the composite particles will decrease. Therefore, a thinner coating is preferred, preferably 0.1 to 30 nm.

[0082] The coating thickness can be measured using transmission electron microscopy (TEM) or cross-sectional SEM. Alternatively, it can be calculated based on the elemental content of the coating obtained from elemental analysis of the composite particles and the equivalent surface area of ​​the sphere calculated from the average cross-sectional diameter of the composite particles obtained from SEM. The calculation methods are detailed below. For example, when the coating can be confirmed in small-diameter composite particles by SEM-EDS, the coating thickness of the small-diameter composite particles is calculated as follows: The mass concentration of small-diameter composite particles within the composite particles can be calculated based on the average composition of the small-diameter composite particles obtained from the average Si concentration, carbon concentration, and oxygen concentration, and the proportion of small-diameter composite particles in the composite particles. The ratio of small-diameter composite particles to coating components can be calculated based on the elemental content and mass concentration of the coating obtained from elemental analysis of the composite particles. Alternatively, the spherical equivalent surface area can be calculated from the average cross-sectional diameter of the small-diameter composite particles within the composite particle group, and the average mass of the small-diameter composite particles can be calculated based on their average composition. Using these values, along with the previously calculated ratio of small-diameter composite particles to coating components, the mass of elements from the coating present on the surface of the small-diameter composite particles can be calculated. If this value, along with the previously calculated spherical equivalent surface area and the density of the coating type, is used, the coating thickness can be calculated. The density of the coating type can be appropriately selected from literature values.

[0083] Regarding the composite particles of one embodiment of the present invention, when the small-diameter composite particles are analyzed by SEM-EDS, it is preferable to detect one or more elements selected from Al, Ti, V, Cr, Mn, Fe, Co, Ni, Y, Zr, Mo, Nb, La, Ce, Ta, and W.

[0084] The detection of the aforementioned elements in the small-diameter composite particles refers to the coating of these elements on the surface of the composite particles with oxides of these elements. By having a coating composed of oxides of one or more of these elements on the surface of the composite particles, cycling characteristics can be improved.

[0085] (2) Methods for manufacturing composite particles

[0086] The method for manufacturing the composite particles according to one embodiment of the present invention is not particularly limited, and examples of manufacturing methods including the following steps can be given.

[0087] Step 1: Apply silane gas to D V50 The micrometer size is 1.0–10.0 μm and the pore volume is 0.8–2.2 cm³. 3 The process of producing composite particles (S) filled with silicon in the pores of the porous carbon (S) by using / g of porous carbon (S).

[0088] Step 2: Apply silane gas to D V50 Larger than porous carbon (S) with a micropore volume of 0.2–0.8 cm³. 3 The process of producing porous carbon (L) of / g, thereby manufacturing composite particles (L) filled with silicon in fine pores.

[0089] Step 3: A step of mixing composite particles (S) and composite particles (L) in a ratio of 5 to 50% by mass (the total mass of composite particles (S) and composite particles (L) is recorded as 100% by mass).

[0090] (Process 1)

[0091] The so-called D V50 This represents the 50% particle size in the cumulative particle size distribution on a volumetric basis. It can be determined using a particle size analyzer employing laser diffraction.

[0092] In step 1, silane gas is applied to D. V50 The micrometer size is 1.0–10.0 μm, for example, 1.0–4.0 μm, and the pore volume is 0.8–2.2 cm³. 3 / g porous carbon (S). From this, D can be obtained. V50 Smaller composite particles with a higher silicon content. For example, porous carbon (S) is placed in a tubular furnace and heated to 350°C–450°C under an inert atmosphere such as argon or nitrogen. By setting the temperature to this range, silicon can be precipitated only within the pores and on the surface of the porous carbon (S). Next, the gas is switched to a reaction gas containing silane (SiH4). The concentration of silane in the reaction gas can be 100% by volume, but for example, 0.01–99.9% by volume of silane can be used by mixing nitrogen and argon. The reaction pressure can be atmospheric pressure (101 ± 10 kPaA). As a result, the silane gas is adsorbed into the pores of the porous carbon (S) and further thermally decomposed. As a result, silicon is precipitated within the pores of the porous carbon (S). The silane gas is converted into silicon and hydrogen through the reaction shown in the following formula.

[0093] SiH4→Si+2H2

[0094] Therefore, the composition of the exhaust gas can be analyzed and the concentrations of silane and hydrogen can be investigated to determine the endpoint of the reaction, i.e., the time point at which silicon fills the pores of porous carbon (S) to its limit. At this point, the silane gas can be stopped before the silicon completely fills the pores of porous carbon (S), thereby modulating a Si-C composite with a low silicon content.

[0095] After the reaction, the gas is switched from silane to a non-reactive gas. As a subsequent step, the silicon can be exposed to air or oxygen at a time point when the temperature is lowered to 100–50°C in the non-reactive gas atmosphere, thereby forming an oxide film on the silicon surface. Alternatively, as another subsequent step, after the thermal decomposition of the silane gas, the gas can be switched from silane to hydrocarbon gas, and a carbon film can be formed on the surface of the composite particles using CVD. These subsequent steps can be performed individually or in combination. When combined, any step can be performed first.

[0096] The Si-C composite obtained from step 1 is called "composite particle (S)".

[0097] (Process 2)

[0098] In step 2, silane gas is applied to D. V50 Larger than porous carbon (S) with a micropore volume of 0.2–0.8 cm³. 3 / g of porous carbon. For example, in the D of porous carbon (S) V50 When the value is below 4.0 μm, use D V50 Porous carbon (L) with a diameter greater than 4.0 μm.

[0099] Therefore, we can obtain D. V50 Larger composite particles with lower silicon content. The reaction conditions are the same as in step 1, except for the porous carbon used as the raw material.

[0100] The Si-C composite obtained from step 2 is called "composite particle (L)".

[0101] (Process 3)

[0102] The present invention relates to a process of preparing the composite particles by mixing composite particles (S) and composite particles (L) in an appropriate ratio. Here, composite particles (S) are added in a manner that constitutes 5 to 50% by mass of the whole, with the remainder being composite particles (L). That is, the ratio of composite particles (S) to the total mass of composite particles (S) and composite particles (L) is 5 to 50% by mass, and the ratio of composite particles (L) is 50 to 95% by mass.

[0103] The aforementioned “small-diameter composite particles” are not limited to the case where they are composed entirely of composite particles (S). Similarly, the aforementioned “large-diameter composite particles” are not limited to the case where they are composed entirely of composite particles (L). In the composite particles described in the aforementioned configuration example [1] of one embodiment of the present invention, in order to manufacture composite particles having the desired ratio of small-diameter composite particles, average cross-sectional diameter, average silicon content, (average silicon content (y) of large-diameter composite particles) / (average silicon content (x) of small-diameter composite particles), and silicon content obtained by quantitative analysis of the composite particles as a whole, it is necessary to analyze the composite particles obtained by combining composite particles (S) and composite particles (L) using cross-sectional SEM-EDS, and to confirm whether the desired composite particles are obtained while making improvements.

[0104] It should be noted that, here, examples with different D are shown. V50 It can be modulated and mixed with two types of composite particles with silicon content, but it can also be combined with more than three types of composite particles.

[0105] Regarding D V50 The micrometer size is 1–10 μm and the pore volume is 0.8–2.2 cm³. 3 / g porous carbon (S), and D V50 Larger than porous carbon (S) with a micropore volume of 0.2–0.8 cm³. 3 / g of porous carbon (L) can be obtained by purchasing and using commercially available activated carbon and carbon molecular sieves, or by synthesizing polymers such as phenolic resin and melamine resin and then thermally decomposing and activating them. Regarding D... V50 This allows for the appropriate pulverization, sieving, and air classification of purchased porous carbon, or materials produced before, during, or after the manufacturing process of porous carbon, thereby adjusting the D... V50 .

[0106] Among the above, the D of the complex particle (S) and the complex particle (L) V50 The boundary value is arbitrary and can be set to any value between 1 and 10 μm, such as 4 μm. For example, the D of the composite particle (S) can be adjusted. V50 The size is below 4 μm, and the D of the composite particles (L) is... V50 Larger than 4 μm can also make the D of the complex particle (S) V50 The size is below 10 μm, and the D of the composite particles (L) is... V50 Greater than 10μm.

[0107] (3) Negative electrode materials for lithium-ion secondary batteries

[0108] The composite particles of one embodiment of the present invention can be widely used as electrode materials for metal-ion secondary batteries, and are particularly suitable as negative electrode materials for lithium-ion secondary batteries. The composite particles can be used alone or together with other negative electrode materials for purposes such as adjusting battery capacity and absorbing volume changes caused by the expansion / contraction of the composite particles. As other negative electrode materials, negative electrode materials commonly used in lithium-ion secondary batteries can be used. When using other negative electrode materials, the composite particles are usually mixed with the other negative electrode materials.

[0109] Other examples of anode materials include graphite, hard carbon, soft carbon, and lithium titanate (Li4Ti5O). 12 These negative electrode materials include alloys such as silicon and tin, as well as their composite materials. These negative electrode materials are typically in particulate form. One or more negative electrode materials, other than composite particles, can be used. Graphite and hard carbon are particularly preferred. From the perspective of adjusting capacity and reducing the overall volume change of the negative electrode mixture layer, the negative electrode material of the present invention comprising composite particles and graphite particles is one of the preferred methods. When using multiple materials as negative electrode materials, they can be used after pre-mixing, or they can be added sequentially when preparing the slurry for forming the negative electrode mixture (described later).

[0110] Commercially available mixers and agitators can be used as devices for mixing composite particles with other materials. Specific examples include mortars, sputter mixers, V-type mixers, W-type mixers, single-paddle mixers, and Nota mixers.

[0111] (4) Negative electrode mixture layer

[0112] The negative electrode compound layer of one embodiment of the present invention comprises the aforementioned negative electrode material. That is, the negative electrode compound layer of one embodiment of the present invention comprises particles of this composite. The negative electrode compound layer of one embodiment of the present invention can be used as a negative electrode compound layer for lithium-ion secondary batteries. The negative electrode compound layer is typically composed of a negative electrode material, a binder, and a conductive additive as an arbitrary component.

[0113] The manufacturing method of the negative electrode flux layer can be, for example, a known method as shown below. A slurry for forming the negative electrode flux layer is prepared using a negative electrode material, a binder, a conductive additive as an arbitrary component, and a solvent. The slurry is applied to a current collector such as copper foil and allowed to dry. It is then further vacuum dried to remove the solvent. The resulting product is sometimes called a negative electrode sheet. The negative electrode sheet consists of a negative electrode flux layer and a current collector. After the negative electrode sheet is cut or punched into the necessary shape and size, it is pressurized to increase the density of the electrode flux layer (sometimes called electrode density). Increasing the electrode density increases the energy density of the battery. The electrode density varies from 1.1 to 1.9 g / cm³ depending on the composition of the electrode flux layer. 3 The pressing method is not particularly limited, as long as it can achieve the desired electrode density; examples include uniaxial pressing and rolling. Here, a process of pressing after shaping is shown, but shaping can also be performed after pressing. In this invention, the article in this state is referred to as a negative electrode. A negative electrode also includes a negative electrode in which current collector tabs are further mounted on the current collector as needed.

[0114] As a binder, any binder commonly used in the negative electrode binder layer of lithium-ion secondary batteries can be freely selected and used. Examples include polyethylene, polypropylene, ethylene-propylene terpolymer, butadiene rubber, styrene-butadiene rubber, butyl rubber, acrylic rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyethylene oxide, polyepoxychlorohydrin, polyphosphazene, polyacrylonitrile, carboxymethyl cellulose and its salts, polyacrylic acid, and polyacrylamide. One binder can be used alone, or two or more can be used. The amount of binder is preferably 0.5 to 30 parts by weight relative to 100 parts by weight of the negative electrode material.

[0115] Conductive additives only need to play a role in imparting conductivity and dimensional stability to the electrode (absorbing the volume changes of the complex particles during lithium insertion / extraction), and there are no particular limitations. Examples include carbon nanotubes, carbon nanofibers, vapor-deposited carbon fibers (e.g., "VGCF-H" manufactured by Showa Denko Co., Ltd.), conductive carbon black (e.g., "Denka Black" manufactured by Denka Co., Ltd., "SUPER C65" manufactured by Imerys Graffito & Carbon Co., Ltd., "SUPER C45" manufactured by Imerys Graffito & Carbon Co., Ltd.), and conductive graphite (e.g., "KS6L" manufactured by Imerys Graffito & Carbon Co., Ltd., "SFG6L" manufactured by Imerys Graffito & Carbon Co., Ltd.). Furthermore, two or more of the aforementioned conductive additives may be used.

[0116] Preferably, the composite material includes carbon nanotubes, carbon nanofibers, and vapor-phase carbon fibers, wherein the fiber length of these conductive additives is preferably a fraction of the D-axis of the composite particles. V50More than half the length of the fiber. At this fiber length, these conductive additives bridge the negative electrode active material containing composite particles, improving cycle performance. If single-walled or multi-walled carbon nanotubes or carbon nanofibers with a fiber diameter of 15 nm or less are used, the number of bridges increases further with the same amount of addition compared to using thicker materials. Furthermore, this is even more preferable from the viewpoint of greater flexibility, thereby increasing electrode density.

[0117] The amount of conductive additive is preferably 1 to 30 parts by mass relative to 100 parts by mass of negative electrode material.

[0118] There are no particular limitations on the solvent used as a solvent in the slurry for coating the modulating electrode; examples include N-methyl-2-pyrrolidone, dimethylformamide, isopropanol, and water. When using water as a binder, it is preferable to also use a thickener. The amount of solvent is adjusted to achieve a viscosity that facilitates coating the current collector.

[0119] (5) Lithium-ion secondary batteries

[0120] The lithium-ion secondary battery of the present invention includes the aforementioned negative electrode mixture layer. The aforementioned lithium-ion secondary battery typically includes a negative electrode composed of the aforementioned negative electrode mixture layer and a current collector, a positive electrode composed of a positive electrode mixture layer and a current collector, at least one of a non-aqueous electrolyte and a non-aqueous polymer electrolyte present therebetween, a separator, and a battery casing housing them. The aforementioned lithium-ion secondary battery only needs to include the aforementioned negative electrode mixture layer; other configurations, including those previously known, can be used without particular limitation.

[0121] The positive electrode layer typically consists of positive electrode material, conductive additives, and binders. The positive electrode in the aforementioned lithium-ion secondary battery can adopt the general structure of a typical lithium-ion secondary battery.

[0122] As a positive electrode material, any material capable of repeated electrochemical lithium insertion / desorption, and whose redox potentials are sufficiently high compared to those of the negative electrode reaction, is suitable; there are no particular limitations. Suitable materials include, for example, LiCoO2, LiNiO2, LiMn2O4, and LiCo. 1 / 3 Mn 1 / 3 Ni 1 / 3 O2, carbon-coated LiFePO4, and mixtures thereof.

[0123] The substances listed in the negative electrode section can be used as conductive additives, binders, and solvents for slurry preparation. Aluminum foil is suitable as a current collector.

[0124] Non-aqueous electrolytes and non-aqueous polymer electrolytes used in lithium-ion batteries can be substances known in lithium-ion secondary batteries. As non-aqueous electrolytes, electrolytes obtained by dissolving lithium salts such as LiClO4, LiPF6, LiAsF6, LiBF4, LiSO3CF3, and CH3SO3Li in solvents such as ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propylene carbonate, butyl carbonate, acetonitrile, propionitrile, dimethoxyethane, tetrahydrofuran, and γ-butyrolactone are examples of non-aqueous solvents.

[0125] Examples of non-aqueous polymer electrolytes include, for example, gel-like polymer electrolytes containing polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polymethyl methacrylate; and solid polymer electrolytes containing polymers with oxyethylidene bonds.

[0126] In addition, a small amount of additives commonly used in lithium-ion battery electrolytes can be added to the aforementioned non-aqueous electrolyte. Examples of such additives include, for instance, vinylene carbonate (VC), biphenyl, propanesulfonate lactone (PS), fluoroethylene carbonate (FEC), and ethylene sultone (ES). VC and FEC are preferred. The amount added is preferably 0.01 to 20% by mass relative to 100% by mass of the aforementioned non-aqueous electrolyte.

[0127] As the separator, it can be freely selected from materials that can be used in conventional lithium-ion secondary batteries, including combinations thereof, such as microporous membranes made of polyethylene or polypropylene. In addition, separators obtained by incorporating particles such as SiO2 and Al2O3 as fillers, or separators obtained by attaching particles such as SiO2 and Al2O3 to the surface, can also be used.

[0128] As for the battery casing, there are no special restrictions as long as it can accommodate the positive and negative electrodes, as well as the separator and electrolyte. In addition to commercially available battery packs, 18650 cylindrical batteries, button batteries and other products that have been standardized in the industry, there are also products packaged using aluminum packaging materials, which can be freely designed and used.

[0129] Each electrode can be packaged after being stacked. Alternatively, individual cells can be connected in series for use as batteries or modules.

[0130] The lithium-ion secondary battery involved in this invention can be used as a power source for electronic devices such as smartphones, tablets, and portable information terminals; a power source for electric motors in power tools, vacuum cleaners, electric bicycles, drones, and electric vehicles; and for storing electricity obtained through fuel cells, solar power generation, wind power generation, etc.

[0131] Example

[0132] The following describes one embodiment of the present invention in detail using examples. It should be noted that the following examples do not limit the present invention.

[0133] The evaluation methods for the composite particles, the battery manufacturing methods, the battery characteristic measurement methods, and the raw materials used in each example and comparative example are described below.

[0134] <1> Evaluation of porous carbon and complex particles

[0135] (D V50 (Particle size distribution determination)

[0136] As a laser diffraction particle size distribution measuring device, the LMS-2000e manufactured by Seishin Corporation was used. A 5 mg sample was placed in a container, and 10 g of water containing 0.04% by mass surfactant was added. After ultrasonic treatment for 5 minutes, the sample was measured to determine the 50% particle size (D) in the volume-based cumulative particle size distribution of porous carbon and composite particles. V50 ).

[0137] (Pore volume, nitrogen adsorption test)

[0138] Nitrogen adsorption experiments were conducted under the following conditions to determine the pore volume.

[0139] Within the sample unit (9mm × 135mm), the total surface area of ​​the sample is 2–60 m². 2 The sample was loaded into the container and dried at 300°C under vacuum for 1 hour. The weight of the sample was then measured using the following method.

[0140] ·Device: NOVA4200e (registered trademark) made by Quantachrome Corporation

[0141] • Gas to be measured: Nitrogen

[0142] • Relative pressure setting for the measurement range: 0.005~0.995

[0143] Based on the adsorption isotherm data at two points before and after a relative pressure of 0.99, the adsorption amount at a relative pressure of 0.99 was calculated using linear fitting, and the micropore volume was determined. At this point, the liquid nitrogen density was set to 0.808 g / cm³. 3 The volume of 1 mol of nitrogen in its standard state is 22.4133 L, and the atomic weight of nitrogen is 14.0067, which were used for calculation.

[0144] (Cross-sectional diameter of composite particles / average silicon content, cross-sectional SEM-EDS)

[0145] A small scraper was used to sample the powder from the thoroughly mixed composite particles, and the sample was loaded onto a carbon tape for direct observation of the particles. For cross-section observation, products obtained by cross-section processing using CrossSection Polystyrene (registered trademark) manufactured by Nippon Electronics Co., Ltd. were observed. The following methods were used for observation and measurement.

[0146] SEM: Scanning electron microscope device: Regulus (registered trademark) 8220 (manufactured by Hitachi Hitek Co., Ltd.)

[0147] EDS: XFlash (registered trademark) 5060 FlatQUAD (made by Blu-ray Co., Ltd.)

[0148] Accelerating voltage: 1~20kV

[0149] Magnification: 500–5,000x (selected appropriately based on particle size)

[0150] (particle size)

[0151] The particle size is determined by measuring the length of the cross-sectional SEM image. For spherical particles, the maximum diameter is used as the particle size. For particles other than spheres, the equivalent circle diameter is calculated from the cross-sectional area and used as the particle size.

[0152] The cross-sectional area was calculated using image analysis software (ImageJ). Methods for determining the cross-sectional diameter and for classifying small-diameter complex particles and large-diameter complex particles are as described in the specific implementation section.

[0153] (Si and C (carbon) content obtained by EDS)

[0154] Using an EDS (XFlash 5060 FlatQUAD), the Si and C content at the center of each particle in the cross-sectional sample were analyzed with an accelerating voltage of 5 kV. The Si content (mass%) and C content (mass%) were calculated from their respective proportions. The method for determining the average silicon (Si) content is as described in the Specific Embodiments section.

[0155] (metal component)

[0156] The surface metallic composition of the particle samples was analyzed using an EDS (XFlash 6-60, manufactured by Blu-ray Co., Ltd.) at an accelerating voltage of 20 kV.

[0157] (Silicon content of the composite particles as a whole, XRF)

[0158] The sample is filled into a sample cup and the following method is used for determination: the basic parameter method (FP method) is employed, and the silicon content (the content of Si element) is calculated in mass % (%).

[0159] • Fluorescence X-ray device: Rigaku NEX CG

[0160] • Tube voltage: 50kV

[0161] Tube current: 1.00mA

[0162] • Sample cup: Φ3212mL CH1530

[0163] Sample weight: 2-4g

[0164] Sample height: 5–18 mm

[0165] It should be noted that the FP method is implemented using the analysis software provided with the device.

[0166] (Oxygen content)

[0167] Approximately 20 mg of sample was weighed into a nickel capsule, and the oxygen content was determined using the following method.

[0168] • Apparatus: Horiba Manufacturing Co., Ltd. Oxygen / Nitrogen Analyzer EMGA-920

[0169] Carrier gas: Argon

[0170] (Raman spectroscopy analysis)

[0171] The sample was loaded onto a glass slide using a small spatula, spreading it evenly without exposing the substrate. The area over which the sample was spread was larger than the measurement area described later. This was to ensure that only the complex particles were distributed within the measurement area. The sample was then measured using the following method.

[0172] • Micro Raman spectroscopy apparatus: LabRAM HR Evolution manufactured by Horiba Manufacturing Co., Ltd.

[0173] • Excitation wavelength: 532nm

[0174] • Exposure time: 10 seconds

[0175] • Total number of times: 2

[0176] • Diffraction grating: 300 lines / mm (600nm)

[0177] • Measurement range: 60μm (vertical) × 60μm (horizontal)

[0178] • Number of points: Measurements were performed at 30 points with a longitudinal feed of 12.0 μm and a transverse feed of 15.0 μm. The averaged spectra were obtained, and the Raman spectra from 450 to 495 cm⁻¹ were analyzed. -1 The Si peak was observed and its value was recorded.

[0179] (I SiC111 / I Si111 (Powder XRD determination)

[0180] The sample was filled into a glass specimen plate (window length × width: 18 mm × 20 mm, depth: 0.2 mm), and powder XRD was performed using the following method.

[0181] XRD device: SmartLab manufactured by Rigaku Co., Ltd. (registered trademark)

[0182] X-ray source: Cu-Kα rays

[0183] Kβ ray removal method: Ni filter

[0184] X-ray output power: 45kV, 200mA

[0185] Measurement range: 10.0~80.0°

[0186] Scanning speed: 10.0° / min

[0187] For the obtained XRD pattern, the analysis software (PDXL2, manufactured by Rigaku Co., Ltd.) was used to remove the background and Kα2 components, and after smoothing, peak shape fitting was performed to determine the peak position, height (intensity) and half-width.

[0188] The peak height of SiC's 111 facet is calculated as (peak height of Si's 111 facet) / (peak height of Si's 111 facet). It should be noted that the Si 111 facet uses diffraction peaks near 2θ = 28°, while the SiC 111 facet uses diffraction peaks near 2θ = 35°.

[0189] (Compacted density of powder)

[0190] Device: Tetsuno Universal Material Testing Machine (manufactured by Eretron Co., Ltd.)

[0191] Approximately 0.3 g of composite particles were weighed and placed into a 7 mm diameter die. The top cover was then attached, and the aforementioned device was used to apply a pressure of 196 MPa for molding. The compressive density was calculated from the weight of the composite particles, the displacement, and the shape of the die. It should be noted that, prior to the test, the die was not filled with the composite sample, and pressure was applied under the same conditions to measure the displacement. This displacement was then used as the deformation of the die itself to correct for the displacement of the powder during the compression test.

[0192] <2> Test battery

[0193] (Modulation of the negative electrode material)

[0194] The anode material was obtained by mixing composite particles and graphite particles with a silicon content of 5.7% by mass in the total amount of anode material. D was used as the graphite particles. V50 Artificial graphite with a thickness of 14 μm, an initial discharge specific capacity of 360 mAh / g, and an initial coulombic efficiency of 92%.

[0195] [2-1] Fabrication of the negative electrode

[0196] Styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were used as binders. Specifically, an SBR aqueous dispersion containing SBR particles with a solid content of 40% by mass and a CMC aqueous solution obtained by dissolving CMC powder with a solid content of 2% by mass were used.

[0197] As a mixed conductive additive, carbon black (SUPER C 45, manufactured by Imelis Graffit & Carbon Co., Ltd.) and fumed carbon fiber (VGCF-H, manufactured by Showa Denko Co., Ltd.) are mixed in a mass ratio of 3:2 to form a mixed conductive additive.

[0198] In the examples and comparative examples described below, the negative electrode material (a mixture of composite particles and artificial graphite) was prepared in the manner of 90 parts by mass, mixed conductive additive in the form of 5 parts by mass, CMC solid component in the form of 2.5 parts by mass, and SBR solid component in the form of 2.5 parts by mass. The negative electrode material, mixed conductive additive, CMC aqueous solution, and SBR aqueous dispersion were mixed, and an appropriate amount of water was added to adjust the viscosity. The mixture was then kneaded using a rotary / revolutionary mixer (Sinkee Co., Ltd.) to obtain a slurry for forming the negative electrode composite layer. The slurry concentration was 45-55% by mass.

[0199] Using a scraper with a gap of 150 μm, the above-mentioned negative electrode mixture layer forming slurry was uniformly coated onto a 20 μm thick copper foil serving as the current collector foil. After drying using a heating plate, it was vacuum dried at 70°C for 12 hours, forming a negative electrode mixture layer on the current collector foil. This is referred to as a negative electrode sheet (a sheet composed of a negative electrode mixture layer and a current collector foil).

[0200] [2-2] Fabrication of a button cell (lithium counter electrode unit)

[0201] The negative electrode sheet is punched to a diameter of 16mm and then pressurized using a uniaxial press to achieve a negative electrode binder layer density of 1.4g / cm³. 3 The negative electrode is obtained by adjusting the method.

[0202] The electrode density (negative electrode density) of the negative electrode is calculated as follows. The mass and thickness of the negative electrode obtained by the above method are measured. The mass and thickness of the current collector foil, which are separately measured beforehand and cut to 16 mm φ, are subtracted from these values ​​to obtain the mass and thickness of the negative electrode mixture layer. The electrode density (negative electrode density) is then calculated from these values.

[0203] A separator (polypropylene microporous membrane) impregnated with electrolyte was laminated within an insulating sealing gasket (approximately 18 mm inner diameter) made of polypropylene, using the aforementioned negative electrode and a 1.7 mm thick lithium metal foil punched to a diameter of 17.5 mm. The negative electrode and lithium metal foil were then stacked with the negative electrode layer facing each other, sandwiching the separator between them. This was then placed in a 2320 button cell and sealed using a seam sealing machine to produce a test battery (lithium counter electrode unit).

[0204] It should be noted that the electrolyte in the lithium counter electrode unit is the following liquid: 1 part by mass of vinylene carbonate (VC) and 10 parts by mass of fluoroethylene carbonate (FEC) are mixed in 100 parts by mass of a solvent obtained by mixing ethylene carbonate, methyl ethyl carbonate and diethyl carbonate in a volume ratio of 3:5:2, and then lithium hexafluorophosphate (LiPF6) is dissolved in the solvent to obtain a concentration of 1 mol / L.

[0205] [2-3] Initial charge specific capacity, initial discharge specific capacity

[0206] Experiments were conducted using a lithium-ion counter electrode unit. Constant current (CC) charging was performed from OCV (Open Circuit Voltage) to 0.005V, equivalent to a current value of 0.1C. Constant voltage (CV) charging was then switched at the point where the voltage reached 0.005V. The cutoff condition was set at the point where the current value decayed to the equivalent of 0.005C. The specific capacity at this point was taken as the initial charging specific capacity. Next, a constant current discharge was performed with an upper limit voltage of 1.5V, equivalent to a current value of 0.1C. The specific capacity at this point was taken as the initial discharge specific capacity.

[0207] The experiment was conducted in a constant temperature bath set at 25°C. Here, the specific capacity is the value obtained by dividing the capacity by the mass of the negative electrode active material. Furthermore, in this experiment, the "current equivalent to 1C" refers to the current required to discharge the negative electrode's capacity over one hour. The negative electrode's capacity can be estimated from the mass of Si and carbon (including graphite) in the negative electrode active material and the theoretical specific capacities (4200 mAh / g and 372 mAh / g, respectively).

[0208] [2-4] Initial Coulomb efficiency

[0209] The initial coulombic efficiency (%) is the value obtained by dividing the initial discharge specific capacity by the initial charge specific capacity, expressed as a percentage, i.e., (initial discharge specific capacity) / (initial charge specific capacity) × 100.

[0210] <3> Tri-layer laminated half-cell

[0211] [3-1] Fabrication of a Tri-Laminated Half-Cell

[0212] For the negative electrode sheet obtained in [2-1], a roller press was used to achieve a negative electrode binder layer density of 1.3–1.6 g / cm³. 3 The method was adjusted so that the area of ​​the negative electrode coating layer was 4.0 cm². 2 (2.0cm × 2.0cm), the uncoated portion of the negative electrode mixture layer (tab portion) is 0.5cm. 2 The working electrode (using a negative electrode sheet) is formed by punching in a 1.0cm × 0.5cm pattern.

[0213] The Li roll was cut to obtain an area of ​​7.5 cm². 2 A Li sheet (3.0cm × 2.5cm) for the counter electrode and an area of ​​3.75cm². 2A 1.5cm × 2.5cm Li sheet is used as a reference electrode. 5mm wide Ni tabs are prepared for the counter and reference electrodes, and a 5mm × 20mm Ni mesh is installed, overlapping the front 5mm portion of the tab. The 5mm width of the Ni tab is aligned with the 5mm width of the Ni mesh. Ni tabs for the working electrode are also installed on the Cu foil tab portion of the negative electrode sheet. The Ni mesh at the front end of the counter electrode Ni tab is attached to the corner of the Li sheet, perpendicular to the 3.0cm edge of the counter electrode. The Ni mesh at the front end of the reference electrode Ni tab is attached to the center of the 1.5cm edge of the Li sheet, perpendicular to the 1.5cm edge of the reference electrode. A polypropylene microporous membrane is sandwiched between the working and counter electrodes, and the reference electrode is liquid-bonded via the vicinity of the working electrode and through the polypropylene microporous membrane in a manner that prevents short circuits. In this state, two rectangular aluminum laminate packaging materials are used to clamp the three sides together with all the front ends of the Ni electrodes protruding outwards. Then, electrolyte is injected through the opening. Finally, the opening is sealed by heat fusion to produce a tri-laminated half-cell for evaluation.

[0214] The electrolyte used was the same as the electrolyte used in [2-2] above.

[0215] [3-2] Determination of C-rate

[0216] Based on the initial discharge specific capacity and the mass of active material on the negative electrode calculated in [2-3], the C-rate of each battery is calculated.

[0217] [3-3] Charge-discharge cycle test using a tri-layer laminated half-cell

[0218] The triode laminated half-cell obtained in [3-1] was placed in a charge-discharge device and aged for 6 cycles under the following conditions. In the first cycle of aging, the voltage was increased from the rest potential to 0.005 V vs. Li / Li. + A constant current (CC) charge of 0.05C was performed. Discharging was carried out at a constant current (CC) of 0.05C up to 1.5V vs. Li / Li. + During aging cycles 2 through 6, the cells were charged at a constant current (CC) of 0.2C to 0.005V vs. Li / Li. + At a value of 0.005 V vs. Li / Li + The charging process was switched to constant voltage (CV) charging at specific time points, with a cutoff current set at 0.025C. Discharging was performed at a constant current (CC) of 0.2C up to 1.5V vs. Li / Li. + .

[0219] After the above aging process, charge-discharge cycle tests were conducted using the following method.

[0220] Charging is performed at a constant current of 1C (CC) up to 0.005V vs. Li / Li. + Then, it switched to constant voltage (CV) charging, with the cutoff current set at 0.025C. Discharging was carried out at a constant current (CC) of 1C up to 1.5V vs. Li / Li. + .

[0221] This charge-discharge operation was considered as one cycle and was performed for 20 cycles. In the 21st cycle, a low-rate test was conducted with the charge-discharge rate changed to 0.1C. The discharge capacity of the 50th cycle after the start of the test at 1C was taken as the Li removal capacity of the 50th cycle.

[0222] The discharge (Li removal) capacity retention rate for the 50th cycle was defined and calculated using the following formula.

[0223] Capacity retention (%) after 50 discharge cycles (Li removal)

[0224] = {(Li removal capacity in the 50th cycle after the start of the 1C test) / (Li removal capacity in the 1st cycle after the start of the 1C test)} × 100

[0225] (Porous carbon)

[0226] The following porous carbon was used as a porous carbon.

[0227] Porous carbon S1(D) V50 =1.6μm, pore volume =1.10cm 3 / g)

[0228] Porous carbon S2(D) V50 =4.4μm, pore volume =1.10cm³ 3 / g)

[0229] Porous carbon L1(D) V50 =19.8μm, pore volume =0.70cm³ 3 / g)

[0230] Porous carbon L2(D) V50 =9.5μm, pore volume =0.66cm 3 / g)

[0231] Porous carbon C1(D) V50 =6.5μm, pore volume =0.63cm 3 / g)

[0232] Porous carbon C2(D)V50 =6.5μm, pore volume =0.80cm³ 3 / g)

[0233] (Synthesis of composite particles (S1): used in Examples 1, 2, 4, 5, and 7)

[0234] 1.9 g of porous carbon S1 was added to a tubular furnace, and the temperature was increased while argon gas was flowing through it at a rate of 1 L / min. When the temperature reached 400 °C and was maintained at a constant level, the gas flow was switched to 100% vol% silane gas at a rate of 65 mL / min. The pressure inside the furnace during the reaction was 101 kPaA. After 1.9 hours, the gas flow was switched back to argon gas, and the furnace was cooled. This yielded the composite particles (S1).

[0235] (Synthesis of composite particles (S2): used in Example 3)

[0236] 1.7 g of porous carbon S2 was added to a tubular furnace, and the temperature was increased while argon gas was flowing through it at a rate of 1 L / min. When the temperature reached 400 °C and was maintained at a constant level, the gas flow was switched to 100% silane gas at a rate of 65 mL / min. The pressure inside the furnace during the reaction was 101 kPaA. After 1.9 hours, the gas flow was switched back to argon gas, and the furnace was cooled. This yielded composite particles (S2).

[0237] (Synthesis of composite particles (L1): used in Examples 1, 3, 4, 5, and 9)

[0238] 12.9 g of porous carbon L1 was added to a tubular furnace, and the temperature was increased while argon gas was flowing through it at a rate of 1 L / min. When the temperature reached 400 °C and was maintained at a constant level, the gas flow was switched to 100% vol% silane gas at a rate of 65 mL / min. The pressure inside the furnace during the reaction was 101 kPaA. After 2.6 hours, the gas flow was switched back to argon gas, and the furnace was cooled. This yielded the composite particles (L1).

[0239] (Synthesis of composite particles (L2): used in Examples 2 and 6)

[0240] 20g of porous carbon L2 was added to a tubular furnace, and the temperature was increased while argon gas was flowing through it at a rate of 1L / min. When the temperature reached 400℃ and was maintained at a constant level, the gas flow was switched to 130mL / min of 100% silane gas. The pressure inside the furnace during the reaction was 101kPaA. After 2 hours, the gas flow was switched back to argon gas, and the furnace was cooled. This yielded composite particles (L2).

[0241] (Synthesis of coated composite particles)

[0242] (Synthesis of composite particles (S3): used in Examples 6 and 8)

[0243] Composite particles (S1) were fed into a drum sputtering apparatus (manufactured by Toyoshima Corporation), and Y2O3 was deposited on the particle surface under the following conditions to obtain composite particles (S3).

[0244] ●Splashing conditions

[0245] Target: Y2O3

[0246] Target density: 4.81

[0247] • Film deposition rate: 0.84 nm / h

[0248] Film formation time: 3 hours

[0249] (Synthesis of composite particles (L3): used in Examples 7 and 8)

[0250] Composite particles (L1) were fed into a drum sputtering apparatus (manufactured by Toyoshima Corporation), and Nb2O5 was deposited on the particle surface under the following conditions to obtain composite particles (L3).

[0251] ●Splashing conditions

[0252] Target: Nb2O5

[0253] Target density: 4.52

[0254] • Film formation rate: 0.21 nm / h

[0255] Film formation time: 14h

[0256] (Synthesis of composite particles (S4): used in Example 9)

[0257] 1.9 g of porous carbon S1 was added to a tubular furnace, and the temperature was increased while argon gas was flowing through it at a rate of 1 L / min. When the temperature reached 400 °C and was maintained at a constant level, the gas was switched to 100% vol% silane gas at a flow rate of 65 mL / min. The pressure inside the furnace during the reaction was 101 kPaA. After 1.8 hours, the gas was switched from silane to argon and maintained for 1 hour. Then, the gas was switched from argon to a 20% acetylene argon dilution gas, and the temperature was increased to 650 °C. The temperature was maintained at 650 °C for 30 minutes, then the 20% acetylene argon dilution gas was switched back to argon, and the temperature was lowered. This yielded composite particles (S4).

[0258] (Synthesis of composite particles (C1): used for Comparative Example 1)

[0259] 5.0 g of porous carbon C1 was added to a tubular furnace, and the temperature was increased while argon gas was flowing through it at a rate of 1 L / min. When the temperature reached 400 °C, the gas was switched to 17.4 vol% argon-diluted silane gas at a flow rate of 230 mL / min. The pressure inside the furnace during the reaction was 101 kPaA. After 2 hours, the gas was switched back to argon gas, and the furnace was cooled. This yielded composite particles (C1).

[0260] (Synthesis of composite particle (C2): used for Comparative Example 2)

[0261] 4.1 g of porous carbon (C2) was added to a tubular furnace, and the temperature was increased while argon gas was flowing through it at a rate of 1 L / min. When the temperature reached 400 °C, the gas was switched to 17.4 vol% argon-diluted silane gas at a flow rate of 230 mL / min. The pressure inside the furnace during the reaction was 101 kPaA. After 3.25 hours, the gas was switched back to argon gas, and the furnace was cooled. This yielded composite particles (C2).

[0262] [Examples 1-9]

[0263] As composite particles, composite particles (S) and composite particles (L) were uniformly mixed using a mortar and pestle in such a manner that they constitute the composite particles described in Table 1, to obtain composite particles 1 to 9. The composite particles 1 to 9 were evaluated as described above. The evaluation results are shown in Table 1.

[0264] (Comparative Examples 1-3)

[0265] As composite particles, Comparative Example 1 used only composite particle (C1), Comparative Example 2 used only composite particle (C2), and Comparative Example 3 used only composite particle (L1), and the above evaluation was performed. The evaluation results are shown in Table 1.

[0266] Table 1

[0267]

[0268] As can be seen from Examples 1 to 9, the composite particles that satisfy the requirements of the above-described configuration example [1] of one embodiment of the present invention (i.e., the ratio of the number of small-diameter composite particles in cross-sectional SEM-EDS to the number of measured composite particles is 5 to 45%, the value of (average silicon content (y) of large-diameter composite particles) / (average silicon content (x) of small-diameter composite particles) is 0.90 or less, and the silicon content of the composite particles as a whole obtained by XRF is 45% by mass or more) also has a silicon content of 1.20 g / cm³. 3 The above-mentioned powder has a compressive density and a capacity retention rate of over 80%.

[0269] Furthermore, as demonstrated in Examples 6-8, by coating the composite particles with metal oxides, the capacity retention rate in cycle tests of a triode-laminated half-cell is improved compared to the uncoated case. This is because side reactions can be suppressed through coating.

[0270] As demonstrated in Example 9, by coating the composite particles with carbon, the capacity retention rate in a triode-laminated half-cell during cycling tests was improved compared to the uncoated case. This is because the resistance can be reduced through coating.

[0271] The composite particles of Comparative Examples 1 to 3 are composite particles that do not meet the requirements of the above-described configuration example [1] of one embodiment of the present invention.

[0272] As in Comparative Example 1, when the silicon content is around 40% by mass, minimal electrode degradation due to the expansion / contraction of the Si-C composite is observed, resulting in high capacity retention. However, the powder's compressive density is only 1.15 g / cm³. 3 The lower density suggests that only electrodes with low volumetric energy density can be obtained in actual batteries.

[0273] Furthermore, as in Comparative Example 2, simply increasing the silicon loading resulted in a lower capacity retention rate, which is attributed to significant electrode degradation caused by its expansion / contraction. Additionally, the powder's compressibility was low, which is believed to be due to the inefficient entry of small-diameter composite particles into the gaps between large-diameter composite particles.

[0274] In Comparative Example 3, although the proportion of small-diameter composite particles exceeded 5%, it is clear that merely satisfying this requirement does not yield the effects of the present invention.

[0275] Furthermore, in Si-C composites prepared by means of non-component composite particles with different particle sizes and silicon content, such as Comparative Examples 1 to 3, the value of (y) / (x) only varies slightly at around 1.00, and does not become below 0.90.

[0276] As can be seen from the above results, it is important that the requirements of the ratio of small-diameter composite particles being 5% or more and 45% or less, and the requirement of (y) / (x) being 0.90 or less, are simultaneously met. Moreover, according to the comparison between Examples 1 to 9 and Comparative Examples 1 to 3, even if the silicon content of the composite particles as a whole is 45% by mass or more, composite particles that provide batteries with good cycle characteristics can be provided.

Claims

1. A composite particle comprising silicon and carbon. The cross-sectional diameter and silicon content of the composite particles were determined by cross-sectional SEM-EDS. Composite particles with a cross-sectional diameter less than half the average cross-sectional diameter were classified as small-diameter composite particles. The proportion of small-diameter complex particles in the total number of the complex particles measured is more than 5% and less than 50%. The ratio of the average silicon content of particles other than small-diameter composite particles to the average silicon content of small-diameter composite particles is less than 0.90, and the unit of silicon content is mass%. The silicon content of the composite particles as a whole is over 45% by mass. The small-diameter composite particles are formed by the action of silane gas on D. V50 The micrometer size is 1.0~10.0 μm and the pore volume is 0.8~2.2 cm³. 3 / g of porous carbon is obtained by filling the pores of the porous carbon with silicon.

2. The composite particle according to claim 1, wherein the carbon comprises a porous carbon material.

3. The composite particles according to claim 1 or 2, wherein the pore volume is 0.10 cm³. 3 / g or less.

4. The composite particles according to claim 1 or 2, wherein the ratio of oxygen content to silicon content is 0.001 or more and 0.300 or less, and the units of oxygen content and silicon content are mass.

5. The composite particles according to claim 1 or 2, in the Raman spectrum, in the range of 450~495 cm⁻¹ -1 There are peaks between them.

6. The composite particles according to claim 1 or 2, wherein in the XRD pattern obtained by powder XRD using Cu-Kα rays, the ratio of (peak height of SiC111 surface) to (peak height of Si111 surface) is less than 0.

010.

7. The composite particles according to claim 1 or 2, wherein when the small-particle-size composite particles are analyzed by SEM-EDS, one or more elements selected from Al, Ti, V, Cr, Mn, Fe, Co, Ni, Y, Zr, Mo, Nb, La, Ce, Ta, and W are detected.

8. The composite particles according to claim 1 or 2, comprising 5-50% by mass of composite particles S and 50-95% by mass of composite particles L. The composite particle S is used to act silane gas on D. V50 The micrometer size is 1.0~10.0 μm and the pore volume is 0.8~2.2 cm³. 3 / g of porous carbon S is obtained by filling the fine pores of the porous carbon S with silicon. The composite particle L is used to act silane gas on D. V50 The porous carbon S is larger than that described above, and the pore volume is 0.2~0.8 cm³. 3 / g of porous carbon L is obtained by filling the fine pores of the porous carbon L with silicon. in, The total mass of composite particle S and composite particle L is denoted as 100 mass.

9. A negative electrode compound layer comprising the composite particles according to any one of claims 1 to 8.

10. A lithium-ion secondary battery comprising the negative electrode binder layer as described in claim 9.

Images