Composite particles, method for producing same and use thereof

Abstract

The present invention provides composite particles containing a carbon material and silicon while reducing electrical resistivity. The present invention comprises composite particles containing a carbon material and silicon wherein: the true density of He, i.e. The true density as determined by dry density measurement using helium, is from 1.85 g / cm3 to 2.10 g / cm3; a silicon content ratio of 30% by mass to 80% by mass; the oxygen content ratio is 4.0 mass% or less, and the BET specific surface area is 0.5 m2 / g to 30.0 m2 / g.

Description

Technical Field

[0001] The present invention relates to composite particles and to a method for producing the same and to their use. Background Art

[0002] Lithium-ion rechargeable batteries used in IT devices such as smartphones, tablet PCs, vacuum cleaners, power tools, electric bicycles, drones, and automobiles require anode active materials that combine high capacity and high output. Silicon (theoretical capacity: 4,200 mAh / g) has a higher theoretical capacity than currently used graphite (theoretical capacity: 372 mAh / g), and is attracting attention as anode active materials.

[0003] However, silicon (Si) expands and contracts in response to electrochemical lithium insertion and desorption, with the volume during expansion reaching approximately three to four times its volume during contraction. As a result, silicon particles self-destruct and fall off the electrode, leading to very poor cycling characteristics in lithium-ion rechargeable batteries using Si. Consequently, extensive research is underway not only to use Si as a graphite replacement but also to use Si as anode active materials with reduced overall structural expansion and contraction. This research has led to numerous attempts to form composite materials with carbon materials.

[0004] As an example of a negative electrode with high capacity and long life, Patent Document 1 discloses a silicon-carbon composite material obtained by contacting a porous carbon material with silane gas at high temperature to generate silicon in the pores of the porous carbon material. The porous carbon material taught in Patent Document 1 has pores of 5 nm to 1,000 nm and is reacted with silane gas to fill the pores with silicon.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: JP 2018-534720 A (Translation of PCT Patent) Summary of the Invention

[0008] Problems to be solved by the present invention

[0009] The porous carbon material described in Patent Document 1 has the problem of small pore size and excessive silicon precipitation on the surface of the composite material during silicon filling, resulting in high resistivity. When the resistivity is high, the coulombic efficiency of the lithium-ion secondary battery may decrease, resulting in reduced cycle characteristics.

[0010] It is therefore an object of the present invention to provide a composite particle comprising a carbon material and silicon having reduced resistivity.

[0011] Methods used to solve problems

[0012] The present invention has the following aspects, for example. [1]

[0014] A composite particle comprising

[0015] Carbon materials and silicon, where the He true density obtained by dry density measurement using helium is 1.85 g / cm 3 or above and 2.10 g / cm 3 or below, The silicon content is 30% by mass or more and 80% by mass or less, And the oxygen content is 4.0 mass % or less. [2]

[0017] The composite particles according to [1], wherein the BA true density, i.e., the true density obtained by wet density measurement using 1-butanol, is 1.55 g / cm 3 or above and 1.90 g / cm 3 or below, And the He true density is greater than the BA true density. [3]

[0019] The composite particles according to [1] or [2], wherein in the Raman spectrum, at 450 cm -1 or above and 495 cm -1 There are peaks at or below. [4]

[0021] The composite particles according to [3], wherein in the Raman spectrum, when the intensity of the peak is expressed as I Si And the intensity of the G band (1,580 cm -1 The peak intensity near G When I Si / I G 1.3 or below. [5]

[0023] The composite particles according to [4], wherein in the Raman spectrum, when the intensity of the D band (1,350 cm -1 The peak intensity near D When I D / I G 0.2 or more and 1.4 or less. [6]

[0025] The composite particles according to [1] to [5], wherein the carbon material is porous carbon, and silicon is present in pores of the porous carbon.

[0026] [7] The composite particles according to any one of [1] to [6], wherein in the XRD pattern obtained by powder XRD using Cu-Kα radiation, (peak intensity of SiC(111) surface) / (peak intensity of Si(111) surface) is 0.01 or less. [8]

[0028] The composite particles according to any one of [1] to [7], wherein the 50% particle diameter D in the cumulative particle size distribution based on volume is V50 1.0 µm or more and 40.0 µm or less, and 90% particle size D V90 50.0 µm or less. [9]

[0030] The composite particles according to any one of [1] to [8], wherein the BET specific surface area is 0.5 m 2 / g or more and 30.0 m 2 / g or less.

[10]

[0032] The composite particles according to any one of [1] to [9], wherein the atomic ratios of Si, O, and C according to the narrow spectrum of X-ray photoelectron spectroscopy are respectively represented by A Si 、A O and A C , and in the Si type ratios analyzed from Si 2p spectra, the ratios of SiO2 and SiO are expressed as B and B, respectively. SiO2 and B SiO , A Si is 0.05 or more, and A C / (A C + A Si × (B SiO2 +B SiO )) is 0.55 or above.

[11]

[0034] A method for producing composite particles, the method comprising the step (A) of bringing a gas including a silicon-containing gas in an amount satisfying the following condition (2) into contact with porous carbon satisfying the following condition (1) at 300° C. or higher and 500° C. or lower to precipitate silicon in pores and on the surface of the porous carbon.

[0035] Condition (1): In the nitrogen adsorption test, the pore volume at a relative pressure P / P0 of 0.01 is expressed as V 0.01 , and the pore volume when the relative pressure P / P0 is 0.99 is expressed as V 0.99 , where V 0.99 Between 0.4 cm3 / g and 1.5 cm 3 / g, V 0.01 / V 0.99 0.4 or above and a true density between 1.90 g / cm² as determined by dry density measurement using helium 3 and 2.30 g / cm 3 between.

[0036] Condition (2): The volume of silicon when all silicon-containing gases are converted into silicon is the V of the porous carbon. 0.99 1.4 times or less of the

[12]

[0038] The method for producing composite particles according to

[11] , further comprising the step (B) of bringing a gas including a hydrocarbon having an unsaturated bond into contact with the particles obtained in the step (A) at 500°C or lower.

[13]

[0040] The method for producing composite particles according to

[11] or

[12] , further comprising the step (C) of oxidizing the particles obtained in the step (A).

[14]

[0042] The method for producing composite particles according to

[12] , further comprising the step (C) of oxidizing the particles obtained in the step (B).

[15]

[0044] The method for producing composite particles according to any one of

[11] to

[14] , wherein the composite particles according to any one of [1] to

[10] are produced.

[16]

[0046] A negative electrode active material comprising the composite particles according to any one of [1] to

[10] .

[17]

[0048] A negative electrode mixture comprising the negative electrode active material according to

[16] .

[18]

[0050] A lithium-ion rechargeable battery comprising the negative electrode mixture according to

[17] .

[0051] Effects of the present invention

[0052] According to the present invention, it is possible to reduce the resistivity of composite particles including a carbon material and silicon. DETAILED DESCRIPTION

[0053] Next, the present invention will be described in detail. Unless otherwise specified, a "lithium ion rechargeable battery" may be simply referred to as a "battery."

[0054] [1] Composite particles

[0055] The composite particles according to the present invention are composite particles including a carbon material and silicon.

[0056] The composite particles may have a layer containing carbon on their surface (hereinafter also referred to as a "coating layer containing carbon" or "coating"). That is, the composite particles may be particulate matter (hereinafter also referred to as "coated composite particles") having a coating layer containing carbon on their surface and particulate matter containing a carbon material and silicon within the coating layer. Particulate matter containing a carbon material and silicon is also referred to as "Si / C particles."

[0057] In the present invention, Si / C particles and coated composite particles may be collectively referred to as "composite particles" without distinction.

[0058] "Composite particles comprising a carbon material and silicon" are particulate matter containing silicon (Si) on the surface and within the carbon material. The carbon material is preferably porous carbon, and the silicon is preferably contained at least within the pores of the porous carbon. "Porous carbon" is a carbon material having pores. Porous carbon produced using known techniques can be used.

[0059] In composite particles, fine Si domains are preferably uniformly formed within the carbon material. "Si domains" refer to areas where silicon is present. Composite particles with this structure allow expansion and contraction associated with charge and discharge to occur isotropically, thereby improving the durability of charge and discharge cycles. This structure can be confirmed by cross-sectional SEM-EDS observation of the composite particles using a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). When the distributions of silicon and carbon overlap within the composite particles, fine Si domains at or below the spatial resolution of SEM-EDS can be interpreted as being uniformly dispersed.

[0060] Composite particles can be obtained by contacting a silicon source, such as silane (SiH4), with particulate porous carbon to precipitate silicon (typically amorphous) within the pores of the porous carbon and optionally forming an additional coating. In this case, the use of porous carbon with fine pores allows for the uniform formation of fine Si domains within the particles.

[0061] The silicon content of the composite particles is 30% by mass or more. Here, the "silicon content" of the composite particles refers to the content of silicon element as silicon in the compound included in silicon metal and the composite particles.

[0062] When the silicon content is 30% by mass or more, the amount of silicon in the composite particles is sufficient, and the discharge capacity is increased. From the same perspective, the silicon content is more preferably 35% by mass or more and even more preferably 40% by mass or more. On the other hand, if the silicon content is too low, less than 30% by mass, the amount of silicon in the composite particles is insufficient, and the discharge capacity is low.

[0063] The silicon content of the composite particles is 80% by mass or less. When the silicon content is 80% by mass or less, the amount of silicon inside the composite particles is not excessive, allowing the carbon to absorb volume changes caused by expansion and contraction. From the same perspective, the silicon content is more preferably 75% by mass or less and even more preferably 70% by mass or less. On the other hand, if the silicon content is too high, exceeding 80% by mass, the amount of silicon in the composite particles becomes excessive, thereby preventing the carbon from absorbing volume changes caused by expansion and contraction.

[0064] The silicon content in composite particles can be quantified using conventional analytical methods. For example, the silicon content in composite particles can be quantified by fluorescent X-ray analysis or analysis using methods such as the fundamental parameter method (FP method). Alternatively, the silicon content in composite particles can be quantified using inductively coupled plasma atomic emission spectrometry (ICP-AES), which is performed by burning the composite particles to remove carbon and completely dissolving the ash in an acid or alkali.

[0065] The oxygen content of the composite particles is 4.0% by mass or less. When the oxygen content exceeds 4.0% by mass, this means that when the composite particles are used as the active material of the negative electrode of a lithium ion rechargeable battery, the irreversible capacity of the negative electrode is large. From the perspective of the irreversible capacity of the negative electrode and the resistivity of the composite particles, the oxygen content is more preferably 2.0% by mass or less, even more preferably 1.5% by mass or less, particularly preferably 1.0% by mass or less, and most preferably 0.7% by mass or less. The lower limit of the oxygen content is preferably 0.2% by mass.

[0066] The oxygen content in the composite particles can be measured, for example, by means of a simultaneous oxygen-nitrogen measurement device.

[0067] In the present invention, unless otherwise specified, the oxygen content of composite particles refers to the oxygen content of the composite particles within two days of production or stored under a non-oxidizing atmosphere. When measurement within two days of production is not possible due to process conditions, for example, the particles can be stored under an inert atmosphere (e.g., argon) and measured at a later time, and the value can be considered equivalent to the value measured within two days of production. This is because oxidation does not occur if stored under an inert atmosphere.

[0068] For example, by storing the composite particles under an inert atmosphere, the oxygen content can be maintained within the above range.

[0069] The He true density of the composite particles, calculated by dry density measurement using helium, is 1.85 g / cm 3 or above.

[0070] The true density of He is 1.85 g / cm 3 or more, which indicates that the pores of the carbon material in the composite particles are fully filled with silicon, the coating layer is thin, etc. When the He true density is 1.85 g / cm 3 When the specific capacity of the composite particles is increased or more, the resistivity may be decreased.

[0071] On the other hand, when the He true density is less than 1.85 g / cm 3 When , this means that the amount of silicon filling the pores of the carbon material in the composite particles is low, the coating is a thick layer of a low-density organic material such as a tar component or a polymer, excess silicon is precipitated on the surface of the composite particles, etc.

[0072] The He true density of the composite particles is 2.10 g / cm 3 or less, preferably 2.05 g / cm 3 or less and more preferably 2.00 g / cm 3 or below.

[0073] Since the density of carbon and silicon is usually 2.26 g / cm 3 and 2.33 g / cm 3 , so the true density of He is 2.10 g / cm 3 A He true density of 1.5 or less is believed to indicate the presence of pores in the composite particles that prevent helium from penetrating from the outside. Therefore, since the effect of relieving expansion and contraction stress during charge and discharge can be expected, when the He true density is within the above range, cycle characteristics may be improved.

[0074] On the other hand, when the He true density exceeds 2.10 g / cm 3 This is believed to indicate that the carbon material in the composite particles is not amorphous, and its structure is not isotropic, including a large amount of silicon carbide. Silicon carbide has a higher density than carbon or silicon, so the inclusion of silicon carbide in the composite particles increases the true density. Furthermore, since the inclusion of silicon carbide reduces the specific volume, this is not preferable. Silicon carbide is produced when the composite particles are produced at high temperatures of 500°C or higher, as described later.

[0075] The true density obtained by dry density measurement can be measured using the gas displacement method. This method involves placing a sample and helium gas in a container whose volume has been previously measured, while maintaining a constant temperature. The true density is calculated based on the volume of helium gas displaced by the sample and the mass of the sample. For example, the AccuPyc (registered trademark) II 1340 gas pycnometer manufactured by Micromeritics can be used as an apparatus for the gas displacement method.

[0076] The He true density can be changed by adjusting the reaction conditions (gas composition ratio, gas flow rate, temperature program, reaction time, etc.) in step (A) of the method for producing composite particles described later.

[0077] When the He true density, silicon content, and oxygen content of the composite particles are within the above ranges, excess silicon is not precipitated on the surfaces of the composite particles, and thus the resistivity of the powder is low.

[0078] For the composite particles, the BA true density calculated by wet density measurement using 1-butanol is preferably 1.55 g / cm 3 or more, more preferably 1.57 g / cm 3 or more and even more preferably 1.60 g / cm 3 The BA true density being within this range means that silicon is not excessively precipitated on the surfaces of the composite particles.

[0079] The BA true density of the composite particles is preferably 1.90 g / cm 3 or less, more preferably 1.80 g / cm 3 or less and even more preferably 1.70 g / cm 3 The BA true density being within this range means that the carbon material in the composite particles is amorphous and the structure of the carbon material is more isotropic.

[0080] On the other hand, the density of silicon carbide is higher than that of carbon or Si, so when silicon carbide is included in the composite particles, the true density increases.

[0081] The true density obtained by wet density measurement can be measured by the liquid phase displacement method. The liquid phase displacement method is a method in which a sample and a solvent are placed in a container of pre-measured volume, and the true density is calculated based on the volume of solvent displaced by the sample and the mass of the sample. Typically, ethanol, 1-butanol, isopropanol, etc. can be used as solvents. The composite particles are preferably defined by their true density, as measured by a wet density measurement using 1-butanol as the solvent. For example, the Auto True Denser MAT-7000 manufactured by Seishin Enterprise can be used as a wet density measuring device.

[0082] The BA true density can be changed by adjusting the reaction conditions (gas composition ratio, gas flow rate, temperature program, reaction time, etc.) in step (A) of the method for producing composite particles described later.

[0083] In the composite particles, preferably, the He true density is greater than the BA true density. The molecular size of 1-butanol is larger than that of helium, and therefore the composite particles having a He true density greater than BA true density include closed pores that are permeable to helium but not to 1-butanol.

[0084] During silicon filling in the method for producing composite particles described later, when the surface of the composite particles is completely covered with silicon and there are no pores on the surface, or only small pores such as those that do not allow helium to penetrate, the He true density and the BA true density are equal.

[0085] When the He true density is low and close to the BA true density, it can be inferred that silicon covers the surface of the pores through which the helium on the surface of the composite particles can pass.

[0086] In the Raman spectrum of composite particles, the Raman shift is usually 450 cm -1 Up to 495 cm -1 、1,350 cm -1 Nearby and 1,580 cm -1 There is at least one peak near the surface of crystalline silicon (such as silicon wafers, micro-silicon and heat-treated SiO x ) peak at 520 cm -1 The peak of amorphous silicon at the Raman shift is lower than this peak, indicating that when the peak is at 450 cm -1 Up to 495 cm -1 When a peak appears at , the composite particles contain amorphous silicon. When silicon is in an amorphous state, expansion and contraction during charge and discharge are relatively isotropic, and thus cycle characteristics may be improved.

[0087] When 450 cm -1 Up to 495 cm-1 The peak intensity at is denoted as I Si , and the intensity of the G band (1,580 cm -1 The peak intensity near G When I Si / I G The ratio is preferably 1.3 or less, more preferably 0.94 or less, further preferably 0.64 or less and most preferably 0.54 or less.

[0088] The presence of a silicon peak in the Raman spectrum indicates the presence of silicon on the surface of the composite particles and / or in pores near the surface of the composite particles. XPS, described later, can obtain information from the surface of a substance to a depth of several nanometers, but Raman spectroscopy is known to provide information from approximately 1 µm to submicron depth in carbon materials (hereinafter, the location of information obtained in Raman spectroscopy is also referred to as "near the surface").

[0089] When the composite particles I Si / I G When the composite particles are within the above range, the surface of the composite particles is not thickly covered with the components including silicon, and the powder resistivity of the composite particles tends to be low. Si / I G Within the above range, this indicates that the surface of the composite particle has a configuration including silicon and carbon.

[0090] On the other hand, when the proportion of silicon is very high, I Si / I G Will be much larger than the above range. Si / I G Within the above range, the surface of the composite particle also expands and contracts to the same extent as the interior of the composite particle during charge and discharge, and thus it is possible to avoid concentration of expansion and contraction stress on the surface of the composite particle, thereby improving cycle characteristics.

[0091] I Si / I G The lower limit of is preferably 0.01 and more preferably 0.02. Si / I G When it is less than 0.01, the thickness of the coating increases, which becomes a factor that increases the resistivity.

[0092] Note that "peak intensity" is defined as the height from the baseline to the peak apex after baseline correction.

[0093] I can be changed, for example, by adjusting the reaction conditions (gas composition ratio, gas flow rate, temperature program, reaction time, etc.) in step (A) of the method for producing composite particles described later. Si / IG value.

[0094] R value of composite particles (I D / I G ) is the intensity of the G band I G and is represented by I D The intensity of the D band (1,350 cm -1 The ratio of the peak intensity near the carbon nanoparticles to the peak intensity near the carbon nanoparticles (R value) is preferably between 0.2 and 1.4, and more preferably between 0.26 and 1.34. If the R value is 0.2 or higher, the negative electrode using the composite particles has a sufficiently low reaction resistance, thereby improving the coulombic efficiency of the battery. On the other hand, if the R value is 1.4 or lower, it means that there are few defects in the carbon material. An R value of 1.4 or lower can reduce the internal resistance of the battery and improve the rate characteristics. From the same point of view, the R value is further preferably 0.45 or higher and particularly preferably 0.65 or higher. In addition, the R value is further preferably 1.30 or lower and particularly preferably 1.20 or lower.

[0095] The half width of the peak of the Si (111) plane in the XRD pattern of the composite particles as measured by powder X-ray diffraction (powder XRD) using Cu-Kα radiation is preferably 3.0° or more. A half width of 3.0° or more means that the crystallite size of silicon in the composite particles is small, which suppresses the destruction of silicon accompanying charging and discharging. From the same point of view, the half width is preferably 4.0° or more and more preferably 5.0° or more. In addition, the half width is preferably 10.0° or less and more preferably 8.0° or less. Note that the peak of the Si (111) plane refers to a peak derived from Si, which appears near 28° in 2θ. In addition, the "peak intensity" when measuring the half width is defined as the height from the baseline to the peak apex after baseline correction.

[0096] For example, by using V in the method for producing composite particles described later 0.01 / V 0.99 The half width of the peak of Si(111) plane is increased by adding the multiporous carbon.

[0097] As measured by powder X-ray diffraction (powder XRD) using Cu-Kα radiation, the (peak intensity of SiC (111) plane) / (peak intensity of Si (111) plane) of the composite particles in the XRD pattern is preferably 0.01 or less. Therefore, the composite particles do not include SiC (silicon carbide) or the SiC content is extremely low, thereby improving the utilization of silicon as a battery active material and making the initial discharge capacity higher. Note that (peak intensity of SiC (111) plane) / (peak intensity of Si (111) plane) can also be expressed as I SiC(111) / I Si(111) ISiC(111) / I Si(111) The lower limit of 0.00, which means that the peak intensity of the SiC (111) plane is not observed, is more preferable. Note that the peak of the SiC (111) plane refers to a peak derived from SiC and appears around 35° in 2θ. In addition, the peak of the Si (111) plane refers to a peak derived from Si and appears around 28° in 2θ.

[0098] For example, (peak intensity of SiC (111) plane) / (peak intensity of Si (111) plane) can be reduced by lowering the reaction temperature in the method for producing composite particles described later.

[0099] The composite particles preferably do not contain graphite. The presence of graphite within the composite particles can be determined by XRD patterns obtained using Cu-Kα radiation via powder X-ray diffraction (powder XRD). When a large amount of graphite is present within the composite particles, a sharp peak originating from graphite is observed, with the peak occurring near 26° in 2θ. Although halos originating from carbon and silicon oxide are also observed in this area, their intensities are low, while graphite is observed as a very sharp, high-intensity peak. Therefore, when no graphite peak is observed, the composite particles are considered to contain substantially no graphite.

[0100] For example, composite particles that do not substantially include graphite can be produced by using hard carbon as the carbon material or by producing a composite material without adding graphite.

[0101] It is preferable that the composite particles uniformly form fine Si domains within the particles. When graphite is included inside the composite particles, expansion and contraction during charge and discharge become non-uniform within the particles, and cycle characteristics deteriorate.

[0102] As the 50% particle size in the volume-based cumulative particle size distribution of the composite particles, it is preferred that D V50 is 1.0 µm or more. This is because D V50 A thickness of 1.0 µm or more can reduce side reactions with electrolytes. V50 It is more preferably 2.0 μm or more, even more preferably 3.0 μm or more, and most preferably 3.5 μm or more. This is because it is easy to prepare a slurry having excellent handling characteristics and suitable viscosity and density for coating, and it is also easy to increase the density when used as an electrode.

[0103] Preferably, the D of the composite particles V50 40.0 µm or less. D V50A particle diameter of 40.0 µm or less shortens the diffusion length of lithium in each particle, thereby improving the rate characteristics of lithium-ion batteries and preventing streaks and / or abnormal non-uniformity when coating the current collector with slurry. From this perspective, D V50 It is more preferably 20.0 μm or less and even more preferably 15.0 μm or less.

[0104] As the 90% particle size in the volume-based cumulative particle size distribution of the composite particles, it is preferred that D V90 50.0 µm or less. D V90 A particle diameter of 50.0 µm or less shortens the diffusion length of lithium in each particle, thereby improving the rate characteristics of lithium-ion batteries and preventing streaks and / or abnormal non-uniformity when coating the current collector with slurry. From this perspective, D V90 It is more preferably 40.0 μm or less, even more preferably 30.0 μm or less and most preferably 20.0 μm or less.

[0105] For example, the cumulative particle size distribution based on these volume standards is measured by a laser diffraction particle size analyzer.

[0106] The BET specific surface area of ​​the composite particles is preferably 0.5 m 2 / g or more and 30 m 2 / g or less, more preferably 0.9m 2 / g or more and 20 m 2 / g or less, and even more preferably 1.5 m 2 / g or more and 7.0 m 2 / g or less, particularly preferably 2.0 m 2 / g or more and 5.0 m 2 / g or less, and most preferably 2.5 m 2 / g or above and 4.5 m 2 / g or less.

[0107] Composite particles with a BET specific surface area within the above range tend to have lower composite particle resistivity, and the surfaces of the composite particles are not thickly covered with components including silicon.

[0108] The BET specific surface area is 0.5 m 2 / g or more can properly adjust the slurry viscosity during electrode production, thereby producing a good electrode. From the same perspective, the BET specific surface area is more preferably 0.9 m 2 / g or above.

[0109] The BET specific surface area of ​​the composite particles is preferably 30.0 m 2 / g or less. At 30.0 m 2 / g or less, the side reaction with the electrolyte can be reduced. From the same point of view, the BET specific surface area is more preferably 25.0 m 2 / g or less.

[0110] The BET specific surface area is generally measured by a dedicated measuring device known in the relevant technical field and calculated from an adsorption isotherm using the BET method. Nitrogen is generally used as the adsorbed gas.

[0111] Based on the narrow spectrum of X-ray photoelectron spectroscopy (XPS), the atomic number ratios of Si, O, and C in the composite particles are expressed as A Si 、A O and A C , and in the Si type ratio of Si 2p spectrum state analysis, the ratios of SiO2 and SiO are expressed as B SiO2 and B SiO , And preferably, A Si is 0.05 or more, and A C / (A C + A Si × (B SiO2 + B SiO )) is 0.55 or above.

[0112] Note that A Si + A O + A C = 1.00.

[0113] XPS is a method for obtaining information on the type, amount, and chemical bonding state of elements present on the surface of a material, and is known to be able to obtain information at a depth of several nm from the surface of the material.

[0114] <1> A Si

[0115] A Si A value of 0.05 or more means that when a coating layer is present on the surface of the composite particle, the coating layer is thin. When the coating layer is thin, the resistivity of the composite particle is low. Si It is preferably 0.15 or more and more preferably 0.25 or more. Since the XPS analysis depth is very shallow, only a few nm, the fact that a peak derived from Si can be observed to some extent means that the coating is extremely thin (less than a few nm).

[0116] The electronic conductivity of the coating is low compared to that of the carbon coating. When the coating is too thick, the resistivity of the composite particles increases, and therefore the coating must be thin.

[0117] <2> A C / (A C + A Si × (B SiO2 + B SiO ))

[0118] A C / (A C + A Si × (B SiO2 + B SiO The value of )) is an indicator of the carbon concentration at a depth of several nm from the surface of the composite particle (the spatial resolution depth of XPS). Si is believed to exist on the surface of the composite particle in the form of oxides such as SiO2 and SiO, and furthermore, most of the surface of the composite particle is believed to be composed of carbon oxides and silicon oxides such as SiO2 and SiO. Note that it is also believed that a very small part of the surface of the composite particle may be composed of carbon oxides. When carbon oxides exist on the surface of the composite particle, the number of carbon atoms derived from the carbon oxides is also included in A. C middle.

[0119] However, A C It includes information not only on the carbon on the surface of the composite particles but also on carbon at a depth of several nm from the surface, and thus, this index does not reflect the carbon concentration of the coating alone.

[0120] A C / (A C + A Si × (B SiO2 + B SiO A larger )) indicates a higher carbon concentration on the surface of the composite particles. As the carbon concentration increases, the oxidation inhibition ability also increases. In other words, the composite particles become less susceptible to oxidation.

[0121] A C / (A C + A Si × (B SiO2 + B SiO )) of 0.55 or more means that the carbon concentration of the surface increases and the oxidation inhibition ability also improves. C / (A C + A Si × (B SiO2 + B SiO )) is preferably 0.55 or higher, more preferably 0.70 or higher and even more preferably 0.80 or higher. In addition, A C / (A C + A Si× (B SiO2 + B SiO )) is preferably 0.98 or less. Exceeding 0.98 means that the silicon oxide content on the surface is too low, indicating that the oxidation inhibition ability is low.

[0122] Although the structure of the coating cannot be limited, A C / (A C + A Si × (B SiO2 + B SiO )) within the above range is considered to indicate that the coating is a thin film layer in which carbon, silicon and oxygen on the surface of the composite particles form a composite material.

[0123] The coating is preferably a thin film layer in which the surface carbon and silicon oxide form a composite material.

[0124] A C / (A C + A Si × (B SiO2 + B SiO The value of )) can be changed, for example, by adjusting the reaction temperature, reaction time, reaction pressure, or the type or concentration of hydrocarbon in step (B) in the method for producing composite particles described later.

[0125] When the composite particles have a coating, it is preferred that the coating include a hydrocarbon-derived compound. The presence of the hydrocarbon-derived compound in the coating can be determined by performing pyrolysis GC-MS measurement on the composite particles, since hydrocarbon-derived compounds are included in the gas generated by the composite particles at temperatures between 200° C. and 600° C.

[0126] The coating can be produced by contacting Si / C particles with a hydrocarbon having an unsaturated bond at low temperature and then oxidizing the resulting substance. Specific details will be described later.

[0127] In coated composite particles, the coating is preferably thin enough to be essentially unmeasurable when observed in cross section using an electron microscope. As described above, when the coating is thin, the resistivity of the composite particles is low. The resolution of a scanning electron microscope (SEM) is insufficient to resolve thicknesses of several nanometers, and therefore, the thickness of coatings thinner than this cannot be measured. A transmission electron microscope (TEM) has sufficient resolution and can observe thicknesses of several nanometers, but when preparing samples for observation of thin films including the coating of composite particles using a TEM, the coating of the composite particles is damaged and destroyed during processing, making it virtually impossible to observe the coating thickness using a TEM. "Essentially unmeasurable" refers to this state. However, even for thin films that are essentially unmeasurable when observed in cross section using an electron microscope, the presence of the coating can be confirmed using the aforementioned XPS.

[0128] As mentioned above, it is preferred that expansion and contraction during charge and discharge be uniform within the particles. Therefore, the shape of the composite particles is preferably such that the average aspect ratio is 1.25 or less, and it is more preferred that a portion of the particles have no corners. Even more preferably, the composite particles are spherical (where the cross-section of the composite particles is circular). The aspect ratio is the value obtained by dividing the major diameter by the minor diameter of the particle. An aspect ratio of 1.00 indicates that the major and minor diameters are equal, so an average aspect ratio closer to 1.00 is more preferred.

[0129] The roundness can be determined by calculating the average roundness based on the cross-sectional shape. An average roundness of 0.95 or more and 1.00 or less is preferred. The roundness is expressed by the following formula.

[0130] (roundness) = 4π × (S / L 2 )

[0131] Here, S is the cross-sectional area of ​​the particle [m 2 ], and L is the particle perimeter [m].

[0132] The above average aspect ratio and average circularity can be calculated from images obtained using a scanning electron microscope (SEM) using image analysis software. 20 composite particles randomly selected from the SEM images are analyzed and the average value (number average) of these 20 composite particles is used for determination. Image analysis software such as ImageJ is available.

[0133] In the method for producing composite particles described later, the average aspect ratio and the average circularity can be adjusted by using porous carbon that satisfies the above conditions.

[0134] [2] Method for producing composite particles

[0135] The method according to the invention for producing composite particles has the following steps (A). By means of the method according to the invention for producing composite particles, it is possible to obtain composite particles according to the invention, ie the composite particles described in [1].

[0136] Step (A): a step of bringing a gas including a silicon-containing gas in an amount satisfying the following condition (2) into contact with porous carbon satisfying the following condition (1) at 300° C. or higher and 500° C. or lower to precipitate silicon in the pores and on the surface of the porous carbon.

[0137] Condition (1): In the nitrogen adsorption test, the pore volume at a relative pressure P / P0 of 0.01 is expressed as V 0.01 , and the pore volume when the relative pressure P / P0 is 0.99 is expressed as V 0.99 , where V 0.99 Between 0.4 cm 3 / g and 1.5 cm 3 / g, V 0.01 / V 0.99 The density is 0.4 or more, preferably between 0.4 and 0.66, and the true density obtained by dry density measurement using helium is 1.90 g / cm 3 and 2.30 g / cm 3 between.

[0138] Condition (2): The volume of silicon when all silicon-containing gases are converted into silicon is the V of the porous carbon. 0.99 1.4 times or less of the

[0139] A carbon material having pores is referred to as "porous carbon" in the present invention. Since the composite particles preferably have a structure including silicon within the particles, the porous carbon preferably has a pore volume capable of carrying silicon inside. In addition, in a nitrogen adsorption test of a carbon material (porous carbon), the pore volume at a relative pressure P / P0 of 0.99 is expressed as V. 0.99 , and V 0.99 More preferably between 0.5 cm 3 / g and 1.5 cm 3 / g. In addition, from the perspective of reducing resistivity, it is preferred that V 0.99 Between 0.85cm 3 / g and 1.0 cm 3 / g.

[0140] Since silicon is preferably included as fine domains within the composite particles, the porous carbon even more preferably has many fine pores. Specifically, V 0.01 / V 0.99It is preferably 0.40 or more, more preferably 0.45 or more, and most preferably 0.50 or more.The nitrogen adsorption test can be performed using a known method.

[0141] Therefore, the porous carbon preferably has a pore volume that allows silicon to be carried therein, and the porous carbon has fine pores. Specifically, in a nitrogen adsorption test, the pore volume at a relative pressure P / P0 of 0.01 is expressed as V 0.01 , and the pore volume when the relative pressure P / P0 is 0.99 is expressed as V 0.99 , wherein preferably, V 0.99 Between 0.4 cm 3 / g and 1.5 cm 3 / g, V 0.01 / V 0.99 0.4 or above and a true density between 1.90 g / cm² as determined by dry density measurement using helium 3 and 2.30 g / cm 3 between.

[0142] The porous carbon may or may not include components other than carbon within a range that does not hinder performance. Preferably, carbon accounts for 90% by mass or more of the porous carbon, and 95% by mass or more is more preferred.

[0143] As the porous carbon, for example, activated carbon, activated carbon fibers, molecular sieve carbon, or inorganic template carbon can be used. In addition, porous carbon obtained by activating hard carbon with steam or carbon dioxide can also be used. It is preferred to select porous carbon that meets the above conditions for use. In the case of larger materials such as activated carbon fibers, the material can be crushed into particles before step (A), or the material can be crushed into particles after carrying silicon. In addition, the porous carbon can be an aggregate and a granular body of multiple particles. When the porous carbon is an aggregate and a granular body, the porous carbon can be crushed into particles before step (A), or the porous carbon can be crushed into particles after carrying silicon.

[0144] The hard carbon can be obtained by heat-treating a phenolic resin at 500° C. to 1,400° C., preferably 600° C. to 1,200° C., in an inert atmosphere.

[0145] Preferably, the porous carbon is adjusted to the desired shape or particle size distribution of the composite particles before performing step (A). This is because the shape and particle size distribution of the particles do not change significantly during step (A), and the shape and particle size distribution of the composite particles remain consistent with that of the porous carbon. Therefore, the porous carbon used in step (A) can be crushed or pulverized and sieved.

[0146] The porous carbon is preferably spherical. Spherical porous carbon obtained by carbonizing and activating a spherical phenolic resin is more preferred. Furthermore, spherical porous carbon that has not undergone a pulverization step is even more preferred because the spherical shape can be maintained.

[0147] Step (A) involves placing porous carbon in a reactor and contacting a silicon-containing gas with the porous carbon to deposit silicon in the pores and on the surface of the porous carbon, thereby obtaining composite particles (Si / C particles).

[0148] As the silicon-containing gas, silane gas is preferably used, and the silane gas can be mixed with an inert gas (such as helium or argon) or a reducing gas (such as hydrogen).

[0149] During silicon filling, silicon is deposited in the pores of the porous carbon and on the surface of the porous carbon. However, if an excessive amount of silicon-containing gas reacts with the porous carbon, silicon will be excessively deposited on the surface of the composite particles, resulting in a higher resistivity of the powder measured in the powder state, while the porous carbon is an aggregate of composite particles. Therefore, the amount of silicon-containing gas to be reacted should be such that the volume of silicon when all the silicon-containing gas is converted to silicon is equal to the volume V of the porous carbon. 0.99 Note that the density of silicon used to calculate the volume of silicon is 2.33 g / cm 3 In addition, the “volume of silicon” refers to the volume of silicon per unit mass of porous carbon (cm 3 / g).

[0150] When the amount of silicon-containing gas is too low, the deposition of silicon in the pores of porous carbon will be insufficient, resulting in an excessively large BET specific surface area, so the lower limit of the amount of silicon-containing gas to be reacted is preferably 0.5 times or more and more preferably 0.6 times or more.

[0151] Note that “conversion” means that the silicon-containing gas reacts with the porous carbon to deposit silicon on the surface and in the pores of the porous carbon.

[0152] The form of the reactor is not limited. As the reactor, furnaces with a powder stirring function such as fixed furnaces, fluidized bed furnaces and rotary kilns, as well as continuous furnaces such as roller kilns and pusher furnaces can be used.

[0153] The reaction temperature is not limited as long as the silicon-containing gas (such as silane gas) decomposes at the temperature and deposits silicon in the pores of the porous carbon, but the temperature is preferably between 300°C and 500°C. At temperatures below 300°C, the silane gas does not decompose sufficiently, resulting in insufficient silicon deposition. When the temperature exceeds 500°C, the silane gas decomposes within the pores of the porous carbon, and silicon deposits more significantly on the surface of the porous carbon (including the pore openings) than within the pores, resulting in insufficient deposition within the pores because the pore openings are blocked by the deposited silicon.

[0154] Even at 500°C and below, silane decomposes and deposits silicon on the surface of the porous carbon. Typically, the surface area of ​​the pores in porous carbon is much larger than the outer surface area, so the amount of silicon deposited within the pores of the porous carbon is significantly greater. Silicon is preferably present within the pores of the porous carbon rather than on the outer surface of the porous carbon because this improves the durability of the composite particles to withstand the stresses associated with the expansion and contraction of silicon during battery charging and discharging. At higher processing temperatures, deposition on the surface of the porous carbon becomes more pronounced, increasing the number of areas where the pore openings are blocked.

[0155] As silicon deposition within the pores of porous carbon increases, a reaction temperature of 450° C. or less is preferred, with a reaction temperature of 420° C. or less being even more preferred.

[0156] While observing the characteristics of the composite particles, conditions such as the gas composition ratio, gas flow rate, and temperature program are appropriately adjusted.

[0157] The method for producing composite particles preferably includes the following step (B) in addition to step (A).

[0158] Step (B): a step of contacting a gas including a hydrocarbon having an unsaturated bond with the particles obtained in step (A) at 500° C. or lower.

[0159] The composite particles obtained in step (B) are composite particles having a coating layer.

[0160] The step (B) involves placing the Si / C particles obtained in the step (A) in a reactor and bringing a gas including a hydrocarbon having an unsaturated bond into contact with the Si / C particles at 500° C. or lower.

[0161] The coating layer in the coated composite particles is relatively thin. Methods that deposit carbon on the surface, such as carbon CVD, are not suitable because they result in a thicker carbon coating. Preferably, the Si—H groups on the surface of the Si / C particles are reacted with a hydrocarbon containing an unsaturated bond to form a layer containing a hydrocarbon on the surface of the Si / C particles.

[0162] The layer containing hydrocarbons may include substances that react with each other among hydrocarbons. As the hydrocarbon gas having an unsaturated bond, a hydrocarbon gas having a double bond or a triple bond can be used. When the hydrocarbon is a compound that does not vaporize at normal pressure due to a low vapor pressure, the hydrocarbon can be used at a pressure lower than normal pressure.

[0163] The hydrocarbon having an unsaturated bond is preferably acetylene, ethylene, propylene, or 1,3-butadiene, all of which are gases at normal pressure, with acetylene or ethylene being more preferred. In this case, it is acceptable to use a variety of hydrocarbon types. Furthermore, it is permissible to mix and use an inert gas (such as helium or argon) or a reducing gas (such as hydrogen).

[0164] In step (B), treatment needs to be performed at a low temperature of 500°C or lower. From the perspective of preventing the formation of silicon carbide, the reaction temperature in step (B) is preferably 450°C or lower and more preferably 420°C or lower. Furthermore, from the perspective of eliminating the need for a heating process and decomposing Si-H groups accompanied by hydrogen generation, the reaction temperature in step (B) is preferably the temperature of step (A) or lower.

[0165] If the temperature exceeds 500°C, the number of decomposed Si-H groups increases, making it difficult for the desired reaction between the Si-H groups on the surface of the Si / C particles and the unsaturated bonds of the hydrocarbon to occur. Furthermore, due to the high reactivity of silicon in the Si / C particles, at temperatures exceeding 500°C, the porous carbon reacts with silicon to produce silicon carbide, which reduces the specific capacity of the composite particles.

[0166] The lower limit of the reaction temperature is not limited as long as it is a temperature at which hydrocarbons having unsaturated bonds react on the surfaces of Si / C particles, but since the reaction rate is low at a low reaction temperature, the temperature is preferably 100°C or higher and more preferably 150°C or higher.

[0167] The thickness of the hydrocarbon-containing layer can be a thickness corresponding to a molecular layer of hydrocarbons, or a thickness corresponding to multiple molecular layers. Furthermore, the hydrocarbons may have partially decomposed. Even if the hydrocarbons have partially decomposed into carbon, the hydrocarbon-containing layer is preferably a thin film because hydrocarbons are a material with a high resistivity, unlike a carbon coating. Therefore, the weight change before and after step (B) is preferably small. The mass increase of the Si / C particles having the hydrocarbon-containing layer obtained in step (B) relative to the mass of the Si / C particles before step (B) is more preferably 1.0 mass% or less and even more preferably 0.5 mass% or less.

[0168] It is preferred to perform steps (A) and (B) continuously. The surface silicon of the Si / C particles obtained in step (A) is highly reactive, so when exposed to the atmosphere, oxidation occurs, and the number of surface Si-H groups decreases. Therefore, it is preferred to perform steps (A) and (B) continuously without exposure to the atmosphere (air). It is preferred to use the same equipment for steps (A) and (B), as this facilitates continuous performance. As long as the Si / C particles are not exposed to the atmosphere, the time between steps (A) and (B) is not limited. For example, after step (A), it is permissible to store the particles under an inert atmosphere before performing step (B). Furthermore, if the Si / C particles obtained in step (A) are not exposed to the atmosphere, it is permissible to use different equipment for steps (A) and (B).

[0169] Furthermore, it is permissible to perform step (A2) shown below after step (B).

[0170] Step (A2): a process of contacting a gas including a silicon-containing gas at 300° C. to 500° C.

[0171] Furthermore, steps (B) and (A2) may be performed multiple times. For example, a method may include performing step (B) followed by step (A2), and then continuing to perform step (B). When step (B) is performed after step (A2), the "particles obtained in step (A)" become the particles obtained in step (A2).

[0172] The method for producing composite particles preferably comprises the following step (C) in addition to steps (A) and (B).

[0173] Step (C): a process of oxidizing the particles (Si / C particles) obtained in step (A) or the particles (coated composite particles) obtained in step (B).

[0174] In the method for producing composite particles, in which step (C) is performed on the particles obtained in step (A) without performing step (B), there is a problem that oxidation may occur when the composite particles are stored in the air because hydrocarbons having unsaturated bonds do not react, but the process is relatively simple.

[0175] For the coated composite particles obtained in step (B), this step (C) is a process of introducing oxygen into the coating layer (more specifically, the hydrocarbon-containing coating layer formed on the surface of the composite particles in step (B)). Although the structure of the coating layer is unclear, the inclusion of oxygen in the coating layer can improve oxidation suppression and reduce resistivity. Oxidation can be performed by contacting the coating layer with a gas containing oxygen (more specifically, an oxidizing gas). The oxygen concentration of the oxygen-containing gas is preferably 1 to 25 vol%, more preferably 1 to 20 vol%, and even more preferably 5 to 20 vol%. In such cases, argon or nitrogen is used to dilute the oxygen. Air can be used as the oxygen-containing gas, but for stable oxidation, it is preferable to adjust and maintain the humidity of the air.

[0176] After the particles obtained in step (A) or the coated composite particles obtained in step (B) are contacted with a gas containing oxygen, heat treatment is preferably performed under an inert atmosphere or at low pressure. The temperature during the heat treatment is preferably 400°C or lower. Heat treatment at 400°C or lower is believed to decompose unreacted Si—H groups and convert them into silicon, thereby stably achieving oxygen formation in the composite material in the coating layer. The heat treatment time is, for example, 0.1 to 100 hours.

[0177] The reaction temperature of step (C) is preferably between room temperature and 200° C. Exceeding 200° C. is not preferred, as it may result in decomposition of the coating or excessive oxidation of silicon.

[0178] The reaction time of step (C) is, for example, 0.1 to 120 hours.

[0179] In step (C), it is acceptable to change the oxygen concentration used for the oxidation treatment.

[0180] In step (C), a different apparatus from that used in step (B) may be used. Furthermore, it is acceptable to perform steps (B) and (C) multiple times. Examples of the method include performing step (C) followed by step (B), performing step (C) followed by step (B), and then continuing with step (C). When step (C) is performed followed by step (B), the "particles obtained in step (C)" correspond to the "particles obtained in step (A)" in step (B).

[0181] After the composite particles are produced through steps (A), (B) and (C), the particles may agglomerate. In this case, it is preferable to decompose the particles so that they return to the shape and particle size distribution of the porous carbon raw material.

[0182] [3] Negative electrode active materials

[0183] The negative electrode active material according to the present invention includes the composite particles according to the present invention. The composite particles can be used by mixing two or more types. The negative electrode active material may further include another component. Examples of other components include components commonly used as negative electrode active materials for lithium ion rechargeable batteries. For example, graphite, hard carbon, soft carbon, lithium titanate (Li4Ti5O 12 ), silicon, tin, and other alloy-based active materials, composite materials, etc. These components are generally used in the form of particles. A single component other than composite particles may be used, or two or more components may be used. Among them, graphite particles and hard carbon are particularly preferred.

[0184] When another component is included to form a negative electrode active material, the composite particles are generally adjusted to 1% to 50% by mass, preferably 2% to 25% by mass, in the negative electrode active material. By mixing other components, it is possible to produce a negative electrode active material that maintains the excellent properties of the composite particles while also having the excellent properties of another carbon material. As described later, when multiple materials are used as negative electrode active materials, such materials can be pre-mixed before use, or can be added sequentially when preparing the slurry for forming the negative electrode mixture.

[0185] As an apparatus for mixing the composite particles with other materials, commercially available mixers and stirrers can be used. Specific examples include stirrers such as mortar mixers, ribbon mixers, V-type mixers, W-type mixers, single-blade mixers, and Nauta mixers.

[0186] [4] Negative electrode composite layer

[0187] The negative electrode composite layer according to the present invention includes the negative electrode active material according to the present invention.

[0188] The negative electrode composite layer can be used as a negative electrode composite layer of a lithium ion rechargeable battery. The negative electrode composite layer is generally composed of a negative electrode active material, a binder, and a conductive auxiliary agent as an optional component.

[0189] The negative electrode composite layer can be produced using known methods, such as those described below. A slurry is prepared using the negative electrode active material, a binder, an optional conductive additive, and a solvent to form the negative electrode composite material. The slurry is applied to a current collector such as copper foil and dried. It is then further vacuum-dried to remove the solvent. The resulting product is referred to as a negative electrode sheet. A negative electrode sheet consists of a negative electrode composite layer and a current collector. The negative electrode sheet is cut or punched into the desired shape and size, then pressed to increase the density of the electrode composite layer (this can be referred to as the electrode density). When the electrode density increases, the energy density of the battery also increases. The pressing method is not particularly limited, as long as the process can achieve the desired electrode density, but methods such as uniaxial pressing and roller pressing are mentioned. The order of shaping and pressing is not limited. Pressing can be performed after shaping, or after pressing. In the present invention, an electrode with the desired shape and electrode density is referred to as a negative electrode. The negative electrode can further include a negative electrode in a state where the current collector is attached using a current collector tab, as needed.

[0190] The binder can be any binder commonly used in negative electrode composite layers of lithium-ion rechargeable batteries. Examples of binders include polyethylene, polypropylene, ethylene-propylene terpolymer, butadiene rubber, styrene-butadiene rubber (SBR), butyl rubber, acrylic rubber, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, carboxymethyl cellulose (CMC) and its salts, polyacrylic acid, and polyacrylamide. A single binder may be used, or two or more binders may be used. The amount of binder is preferably 0.5 to 30 parts by mass per 100 parts by mass of the negative electrode material.

[0191] There is no particular limitation on the conductive aid, as long as it is used to impart electronic conductivity and dimensional stability (the ability to absorb volume changes caused by the insertion and removal of lithium) to the electrode. Examples of conductive aids include carbon nanotubes, carbon nanofibers, vapor-phase carbon fibers (e.g., "VGCF (registered trademark)-H" produced by Showa Denko KK), conductive carbon black (e.g., "DenkaBlack (registered trademark)" produced by Denka Co., Ltd., "SUPERC65" produced by Imerys Graphite & Carbon, and "SUPERC45" produced by Imerys Graphite & Carbon), and conductive graphite (e.g., "KS6L" produced by Imerys Graphite & Carbon, and "SFG6L" produced by Imerys Graphite & Carbon). A variety of these conductive aids can be used.

[0192] The conductive additive preferably includes carbon nanotubes, carbon nanofibers or vapor-phase carbon fibers, and the fiber length of these conductive additives is D of the composite particles. V50 Preferably, at least 1 / 2 of the total length. At this length, these conductive additives can act as bridges between the negative electrode active materials (including composite particles), thereby improving cycle characteristics. Furthermore, compared to other conductive additives added at the same amount, single-walled or multi-walled conductive additives with a fiber diameter of 15 nm or less are preferred from the perspective of increasing the number of bridges. Furthermore, these conductive additives are more flexible and are preferred from the perspective of increasing electrode density.

[0193] The amount of the conductive aid is preferably 1 part by mass to 30 parts by mass relative to 100 parts by mass of the negative electrode material.

[0194] When preparing the slurry for electrode coating, the solvent used is not particularly limited, and examples include N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), isopropyl alcohol, tetrahydrofuran (THF), and water. When water is used as a solvent or binder, it is also preferred to use it in combination with a thickener. The amount of solvent can be adjusted to a viscosity that allows the slurry to easily coat the current collector.

[0195] [5] Lithium-ion rechargeable battery

[0196] The lithium-ion rechargeable battery according to the present invention includes the negative electrode composite layer according to the present invention. A lithium-ion rechargeable battery generally includes a negative electrode composed of the negative electrode composite layer and a current collector, a positive electrode composed of the positive electrode composite 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 case housing these components. The lithium-ion rechargeable battery may include the negative electrode composite layer and may employ another configuration, including conventionally known configurations, without particular limitation.

[0197] The positive electrode composite layer is generally composed of a positive electrode material, a conductive additive, and a binder. The positive electrode of a lithium-ion rechargeable battery can use the general configuration of a common lithium-ion rechargeable battery.

[0198] As the positive electrode active material, there is no particular limitation as long as the material allows reversible electrochemical lithium insertion / extraction and the standard redox potential of the material is higher than the standard redox potential of the negative electrode reaction. Examples of positive electrode active materials include: LiCoO2, LiNiO2, LiMn2O4, LiCo 1 / 3 Mn 1 / 3 Ni 1 / 3 O2、LiCo 0.6 Mn 0.2 Ni 0.2 O2、LiCo0.8 Mn 0.1 Ni 0.1 O2, carbon-coated LiFePO4 or suitable mixtures thereof.

[0199] The conductive additives, binders and solvents mentioned in the negative electrode composite layer section can be used for slurry preparation. Aluminum foil is preferably used as the current collector.

[0200] As the non-aqueous electrolyte and non-aqueous polymer electrolyte used in lithium-ion batteries, known electrolytes for lithium-ion rechargeable batteries can be used, and lithium salts such as LiClO4, LiPF6, LiAsF6, LiBF4, LiSO3CF3, and CH3SO3Li can be used, dissolved in the following solvents and polymers. Examples of the solvent include: non-aqueous solvents such as ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, butylene carbonate, acetonitrile, propionitrile, dimethoxyethane, tetrahydrofuran, and γ-butyrolactone; gel-like polymers containing polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polymethyl methacrylate; and polymers having ethylene oxide bonds.

[0201] In addition, a small amount of additives commonly used in lithium-ion battery electrolytes can be added to the non-aqueous electrolyte. Examples of such substances include ethylene carbonate (VC), biphenyl, propane sultone (PS), fluoroethylene carbonate (FEC), and ethylene sultone (ES), with VC and FEC being preferred. The amount added is preferably 0.01 to 20% by mass relative to 100% by mass of the non-aqueous electrolyte.

[0202] The separator can be any material used in typical lithium-ion rechargeable batteries, including combinations thereof, such as microporous membranes made of polyethylene or polypropylene. Such separators can also include particles such as SiO2 and Al2O3, which can be mixed in as fillers or adhered to the surface.

[0203] The battery case is not particularly limited as long as it can accommodate the positive and negative electrodes, separator, and electrolyte. In addition to commercially available battery packs, 18650 cylindrical battery cells, button-type battery cells, and other standard formats in the industry, battery packs packaged in aluminum packaging can also be freely designed and used.

[0204] Each electrode can be stacked and packaged for use. In addition, individual battery cells can be connected in series and used as batteries and modules.

[0205] Examples

[0206] Hereinafter, the present invention will be described in detail by way of examples and comparative examples, but the present invention is not limited to these examples.

[0207] The measurement of physical characteristics and battery evaluation were performed as follows.

[0208] [1] Measurement of physical properties

[0209] [1-1] Dry true density measurement

[0210] After vacuum drying the sample at 180°C for 12 hours, the sample was filled into a measuring cell to 40% to 60% under a dry argon atmosphere in a glove box, and the weight of the sample was measured after tapping the measuring cell more than 100 times. The sample was then removed and placed in the atmosphere, and a dry density measurement was performed using helium using a constant volume expansion method using the following method, and the He true density was calculated.

[0211] Apparatus: Micromeritics AccuPyc (registered trademark) II 1340 gas pycnometer

[0212] Measuring cell: Aluminum, depth 39.3 mm, inner diameter 18 mm

[0213] Carrier gas: Helium

[0214] Air pressure: 19.5 psiG (134.4 kPaG)

[0215] Number of purges during measurement: 200 times, temperature: 25°C ± 1°C

[0216] [1-2] Wet true density measurement

[0217] Wet density was measured by the liquid phase displacement method using the following apparatus, and the BA true density was calculated.

[0218] Device: Seishin Corporation Auto-TruDensitometer MAT-7000

[0219] Measuring cell volume: approximately 40 cm 3

[0220] Measurement sample volume: approximately 10 mL

[0221] Waiting time: 5 minutes

[0222] Vacuum time: 30 minutes

[0223] Solvent used: 1-butanol

[0224] [1-3] Silicon content ratio

[0225] The silicon content ratio of the sample was measured under the following conditions.

[0226] Fluorescent X-ray device: Rigaku Corporation, NEX CG

[0227] Tube voltage: 50 kV

[0228] Tube current: 1.00 mA

[0229] Sample cup: Φ 32 12 mL CH1530

[0230] Sample weight: 2 g to 3 g

[0231] Sample height: 5 mm to 18 mm

[0232] The sample cup is filled with the sample, measured with the above-mentioned method, and the silicon content ratio (in mass %) in the composite particles is calculated using fundamental parameters (FP method).

[0233] [1-4] Oxygen content ratio

[0234] A 20 mg sample was weighed in a nickel capsule, and the oxygen content ratio (in mass %) in the composite particles was calculated using an oxygen-nitrogen analyzer, EMGA (registered trademark)-920 (manufactured by Horiba, Ltd.). Argon was used as the carrier gas. By dividing the oxygen content ratio in the composite particles by the aforementioned silicon content ratio, the oxygen content ratio (in mass %) was obtained when the silicon content ratio in the composite particles was set to 100 mass %.

[0235] The measurement of the oxygen content ratio was performed within two days after the composite particles were produced.

[0236] [1-5] Powder resistivity

[0237] The measurement of the powder resistivity of the sample was performed under the following conditions.

[0238] 0.4 g of sample was placed in a powder resistivity measurement cell with a compression surface of 5 mm x 10 mm, and a small vibrator was applied to the powder resistivity measurement cell to make the powder inside the cell fully flat. The powder resistivity measurement device and the resistivity measurement device were connected, and the powder resistivity measurement cell was fixed to the compression device to measure the resistivity of the powder while compressing it. According to the compression device at 1 × 10 7 N / m 2 The volume of the compressed powder filled in the powder resistivity measurement cell is calculated based on the compression height under the pressure and the area of ​​the compression surface of the powder resistivity measurement cell, and the resistivity per unit volume (volume resistivity (Ω cm)) is calculated based on the calculated powder volume and resistivity.

[0239] [Installation]

[0240] Compression device: A&D Tensilon Universal Materials Testing Machine RTG-1310 Resistivity Measurement Device

[0241] Current generator: ADCMT DC voltage current source 6146

[0242] Voltmeter: Tsuruga Electric's 4257-01, 4257-03 [Conditions] Compression condition: 5 mm / min

[0243] [1-6] BET surface area and pore volume (nitrogen adsorption test)

[0244] Quantachrome NOVA (registered trademark) 4200e was used as a measuring device, and the sample was placed in a sample cell (9 mm × 135 mm) to obtain a 2 m 2 Up to 60 m 2 The total surface area of ​​the sample was measured, dried at 300° C. under vacuum conditions for 1 hour, the sample weight was measured, and the measurement was performed. Nitrogen was used as the gas in the measurement.

[0245] The minimum relative pressure during the measurement was set to 0.005, and the maximum relative pressure was set to 0.995. The BET specific surface area of ​​the porous carbon material was calculated using the BET multipoint method from the adsorption isotherm data at relative pressures close to 0.005 and below 0.08. The BET specific surface area of ​​the composite particles was calculated using the BET multipoint method from the adsorption isotherm data at three points near relative pressures of 0.1, 0.2, and 0.3. Pore volume V 0.99 The pore volume V at a relative pressure of 0.01 is determined by calculating the adsorption amount at a relative pressure of 0.99 using a linear approximation based on the adsorption isotherm data at two points near a relative pressure of 0.99 and using the standard molar volume of nitrogen, the density of liquid nitrogen, and the atomic weight of nitrogen. 0.01 With V 0.99 Similarly, the adsorption amount at a relative pressure of 0.01 was calculated by using a linear approximation based on the adsorption isotherm data at two points near a relative pressure of 0.01.

[0246] At this point, the density of liquid nitrogen is calculated to be 0.808 (g / cm 3 ), the standard molar volume of nitrogen is 22.4133 L, and the atomic weight of nitrogen is 14.0067.

[0247] [1-7] Particle size distribution measurement

[0248] A spoonful of the sample with a very small spatula and two drops of a 100-fold diluted nonionic surfactant solution (SARAYA Co., Ltd., Coconut Detergent High Power) at 32% by mass were added to 15 mL of water and ultrasonically dispersed for 3 minutes. The following method was used to measure this dispersion.

[0249] Equipment: Seishin's laser diffraction particle size distribution measuring instrument (LMS-2000e)

[0250] Analysis: Calculate the volume-based cumulative particle size distribution and determine the 10% particle size D V10 (µm), 50% particle size D V50 (µm) and 90% particle size D V90 (µm).

[0251] [1-8] Raman Si peak, I Si / I G , Raman R value (I D / I G )

[0252] The measurement was performed under the following conditions.

[0253] Raman microscope: HORIBA LabRAM (registered trademark) HR Evolution

[0254] Excitation wavelength: 532 nm

[0255] Exposure time: 10 seconds

[0256] Cumulative count: 2 times

[0257] Diffraction grating: 300 lines / mm (600 nm)

[0258] Measuring sample: Use a small spatula to place the composite particles on a glass slide, ensuring that the powder is homogeneous. Expand beyond the measurement range below.

[0259] Measuring range: vertical 80 µm × horizontal 100 µm. Only composite particles are spread within the measuring range.

[0260] Point number: 100-point measurements were performed with a vertical feed of 17.8 µm and a horizontal feed of 22.2 µm, and the average spectrum was obtained and analyzed as follows.

[0261] The Raman spectrum was observed at 450 cm -1 Up to 495 cm -1 The four peaks at.

[0262] The intensity of this Si peak is expressed as I Si , and 1,580 cm -1 The peak intensity near G ) is the ratio of (I Si / I G ).

[0263] [Example 1]

[0264] [Resin synthesis]

[0265] 180.0 g of pre-melted phenol (Kanto Chemical Co., Inc.), 149.5 g of polyvinyl alcohol (GM-14R (18% by mass solids aqueous solution), Mitsubishi Chemical Corporation), and 180 g of water were added to a 1L flask connected to a dropping funnel equipped with a cooling tube and a stirrer, and stirred at 50°C and 150 rpm. Separately, 0.971 g of a 74.1% by mass aqueous solution of tetrakishydroxymethylphosphonium sulfate (THPS, Tokyo Chemical Industry Co., Ltd.) was added to the raw phenol, and 5.4 g of triethylamine (Kanto Chemical Co., Ltd.) was added while stirring at 50°C. Then, 383.4 g of a 37% by mass aqueous formaldehyde solution (Kanto Chemical Co., Ltd.) was added over approximately 2 minutes, and the reaction solution was heated to reflux.

[0266] Two hours after the start of reflux, 9.0 g of triethylamine (Kanto Chemical Reagent) was added.

[0267] Three hours and 30 minutes after the start of reflux, 5.4 g of triethylamine (Kanto Chemical Reagent) was added.

[0268] Approximately 200 mL of water was added to the reaction solution to bring the total volume to 1 L. The solution was centrifuged at 6,000 rpm for 5 minutes using a high-speed refrigerated centrifuge (CR21-N, himac Eppendorf Group). A 300 mL portion of the supernatant was removed. Another 300 mL of water was added and the solution was centrifuged at 6,000 rpm for 5 minutes. A 400 mL portion of the supernatant was removed.

[0269] The remaining sediment was partially spread on a tray and dried at 120°C for 18 hours using a WFO-400 hot air dryer. 50 g of the dried material was pulverized for 30 seconds using a Wonder Blender (WB-1, imported, distributor: Osaka Chemical Co., Ltd.) to obtain a dry resin.

[0270] [Carbonization]

[0271] 90 g of the resulting dried resin was placed in a tube furnace and carbonized at 900°C for 1 hour in a nitrogen atmosphere. The resulting product was pulverized for 30 seconds using a Wonder Blender (WB-1, imported, distributor: Osaka Chemical Co., Ltd.) and sieved through a 45 µm sieve to remove coarse particles, yielding a carbide. The various physical properties of the resulting carbide are as follows: BET specific surface area = 544 m 2 / g,V 0.99 = 0.25 cm 3 / g,D V10 = 12 µm, D V50 = 20 µm, D V90 = 33 µm.

[0272] [activation]

[0273] 10 g of the obtained carbide was loaded into a tube furnace, activated at 950°C for 2.9 hours using carbon dioxide as the activation gas, and cooled to room temperature to obtain porous carbon. The various physical properties of the obtained porous carbon are as follows. BET specific surface area = 2,132 m 2 / g,V 0.99 = 0.96 cm 3 / g,V 0.01 / V 0.99 = 0.63, D V10 = 12 µm, D V50 = 21 µm, D V90 = 34 µm.

[0274] [Step (A)]

[0275] The porous carbon as listed in Table 1 was loaded into a tube furnace and operated under the conditions listed in Table 2.

[0276] When the concentration of the silicon-containing gas is not 100%, argon gas is introduced together with the silicon-containing gas.

[0277] The "silicon-containing gas concentration" in Table 2 refers to the volume ratio of the silicon-containing gas to the total 100 volume % of the silicon-containing gas and argon gas. The "silicon-containing gas flow rate" refers to the total flow rate of the silicon-containing gas and argon gas. The "amount of silane introduced" refers to the volume of the silicon-containing gas calculated by multiplying the silicon-containing gas concentration, the silicon-containing gas flow rate, and the silicon-containing gas introduction time. "SL" means 1 / 1,000 of the gas volume calculated by multiplying the gas flow rate (sccm) by the gas introduction time (minutes), and is expressed in volume units (standard liters). The "volume of silicon per unit mass of porous carbon" refers to the number of silicon atoms contained in the amount of silane introduced, and the volume of silicon of the same substance in the porous carbon per unit mass. Note that the density of silicon is set to 2.33 g / cm 3 In addition, the molar volume of silane under standard conditions is set to 22.4133 L.

[0278] [Step (B)]

[0279] The following steps (A) were performed. After cooling the temperature to 350° C., the tube furnace was evacuated and ethylene was introduced. The pressure inside the tube furnace was maintained at 106 kPa, and the reaction was carried out for one hour without gas flow.

[0280] [Step (C)]

[0281] Follow step (B). After cooling to 70°C, the tube furnace was evacuated and 5% oxygen / Ar was introduced at 1,000 sccm. After 30 minutes, the tube furnace was evacuated and 10% oxygen / Ar was introduced at 1,000 sccm. After 30 minutes, the gases were stopped, the mixture was cooled to room temperature, and composite particles were obtained.

[0282] [Example 2]

[0283] Similar to Example 1, the dry resin was replaced with a commercially available spherical phenolic resin (D V50 = 10 µm) and activated at 950 °C for 6 h.

[0284] [Example 3]

[0285] Similar to Example 1, the dry resin was replaced with a commercially available spherical phenolic resin (D V50 = 4 µm) and activated at 900 °C for 2.3 h.

[0286] [Example 4]

[0287] Similar to Example 1, the dry resin was replaced with a commercially available spherical phenolic resin (D V50 = 4 µm) and activated at 950 °C for 4 h.

[0288] [Example 5]

[0289] Similar to Example 1, 1,3-butadiene was used in place of ethylene as the gas including hydrocarbons having an unsaturated bond in step (B), and the dry resin was replaced with a commercially available spherical phenolic resin (D V50 = 6 µm) and activated at 1,000°C for 5.5 hours. [Comparative Example 1]

[0290] Similar to Example 1, the dry resin was replaced with a commercially available spherical phenolic resin (D V50 = 5.5 µm) and activated at 900 °C for 3 h.

[0291] [Comparative Example 2]

[0292] Similar to Example 1, the dry resin was replaced with a commercially available spherical phenolic resin (D V50 = 7 µm) and activated at 950 °C for 5 h.

[0293] [Comparative Example 3]

[0294] Similar to Example 1, the dry resin was replaced with a commercially available spherical phenolic resin (D V50 = 17 µm) and activated at 950 °C for 4.5 h.

[0295] [Table 1]

[0296] Table 1

[0297] [Table 2]

[0298] [Table 3]

[0299] The He true density of the composite particles produced in Examples 1 to 5 was greater than that of the composite particles produced in Comparative Examples 1 to 3, and the amount of silicon deposited on the surface of the composite particles was smaller. Therefore, the powder resistivity of the powder including only the composite particles of the Examples was lower by about 1 to 2 orders of magnitude than the powder resistivity of the powder including only the composite particles of the Comparative Examples.

[0300] Industrial Applicability

[0301] For example, the composite particles of the present invention can be suitably used as a negative electrode active material constituting a negative electrode composite layer of a lithium ion rechargeable battery. The lithium ion rechargeable battery of the present invention can be suitably used as a power source for electronic devices such as smartphones, mobile phone terminals, and tablet PCs, as well as a power source for electric motors such as vacuum cleaners, power tools, electric bicycles, drones, and electric vehicles, and for storing electricity obtained from fuel cells, solar power generation, and wind power generation.

Claims

1. A composite particle comprising a carbon material and silicon, wherein The true density of He obtained by dry density measurement using helium is 1.85 g / cm 3 or above and 2.10 g / cm 3 or below, The silicon content is 30% by mass or more and 80% by mass or less, the oxygen content is 4.0% by mass or less, And the BET specific surface area is 0.5 m 2 / g or more and 30.0 m 2 / g or less.

2. The composite particle according to claim 1, wherein the BET specific surface area is 1.5 m 2 / g or more and 30.0m 2 / g or less. The composite particles according to claim 1 , wherein the oxygen content is 0.7% by mass or less.

4. The composite particles according to claim 1, wherein the Raman spectrum at 450 cm -1 and 495 cm -1 There are peaks between them.

5. The composite particle according to claim 4, wherein in the Raman spectrum, when the intensity of the peak is expressed as 1 Si And the intensity of the G band (1,580 cm -1 The peak intensity near G When I Si / I G 0.54 or below.

6. The composite particles according to claim 1, wherein the BA true density, i.e., the true density obtained by wet density measurement using 1-butanol, is 1.55 g / cm 3 or above and 1.90 g / cm 3 or below, and the He true density is greater than the BA true density.

7. The composite particle according to claim 1, wherein the intensity of the D band (1,350 cm -1 The peak intensity near D , and I D / I G Between 0.2 and 1.

4. 8 . The composite particle according to claim 1 , wherein the carbon material is porous carbon, and silicon is present in pores of the porous carbon.

9. The composite particles according to claim 1, wherein in an XRD pattern obtained by powder XRD using Cu-Kα radiation, (peak intensity of SiC (111) surface) / (peak intensity of Si (111) surface) is 0.01 or less.

10. The composite particle according to claim 1, wherein the 50% particle diameter D in the volume-based cumulative particle size distribution is V50 Between 1.0 µm and 40.0 µm, and 90% of the diameter D V90 50.0 µm or less.

11. The composite particles according to claim 1, wherein the atomic ratios of Si, O, and C obtained from a narrow spectrum of X-ray photoelectron spectroscopy are expressed as A, B, C, and D, respectively. Si 、A O and A C , and the ratios of Si type, SiO2 and SiO according to Si 2p spectrum analysis are expressed as B SiO2 and B SiO , A Si is 0.05 or more, and A C / (A C + A Si × (B SiO2 + B SiO )) is 0.55 or above.

12. A method for producing composite particles, the method comprising the steps of (A): bringing a gas including a silicon-containing gas in an amount satisfying the following condition (2) into contact with porous carbon satisfying the following condition (1) at 300° C. or higher and 500° C. or lower to precipitate silicon in pores and on the surface of the porous carbon; Condition (1): In the nitrogen adsorption test, the pore volume at a relative pressure P / P0 of 0.01 is expressed as V 0.01 , and the pore volume when the relative pressure P / P0 is 0.99 is expressed as V 0.99 , where V 0.99 Between 0.4 cm 3 / g and 1.5 cm 3 / g, V 0.01 / V 0.99 Between 0.4 and 0.66, and the true density obtained by dry density measurement using helium is between 1.90 g / cm 3 and 2.30 g / cm 3 between; Condition (2): The volume of silicon when all silicon-containing gases are converted into silicon is the V of the porous carbon. 0.99 1.4 times or less of the 13 . The method for producing composite particles according to claim 12 , further comprising the step (B) of contacting a gas including a hydrocarbon having an unsaturated bond with the particles obtained in the step (A) at 500° C. or lower. 14 . The method for producing composite particles according to claim 12 , comprising a step (C) of oxidizing the particles obtained in the step (A). 15 . The method for producing composite particles according to claim 13 , comprising a step (C) of oxidizing the particles obtained in the step (B).

16. The method for producing composite particles according to claim 12, wherein the composite particles according to claim 1 are produced. 17 . A negative electrode active material comprising the composite particles according to claim 1 . 18 . A negative electrode composite layer comprising the negative electrode active material according to claim 17 .

19. A lithium ion rechargeable battery comprising the negative electrode composite layer according to claim 18.

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