Negative electrode active material and battery

Abstract

A negative electrode active material 10 according to the present disclosure comprises composite particles 1 each including a carbon phase 11 and silicon phases 12 dispersed in the carbon phase 11, and a coating layer 2 containing carbon and covering at least a portion of the surface of the composite particle 1, and satisfies the following (1) and (2).　(1) In the negative electrode active material 10, the ratio (B / A) of the true density (B) determined by a gas phase substitution method using helium with respect to the true density (A) determined by a liquid phase substitution method using 1-butanol is 1.04 or more.　(2) In a Raman spectrum of the carbon phase 11 observed by laser Raman spectroscopy using laser light having a wavelength of 532 nm, the ratio of the intensity of the D-band observed in the vicinity of 1360 cm-1 with respect to the intensity of the G-band observed in the vicinity of 1600 cm-1 is 1.1-1.5.

Description

Negative electrode active material and battery

[0001] The present disclosure relates to a negative electrode active material and a battery.

[0002] Secondary batteries, typified by lithium ion batteries, are widely used as power sources for electronic devices such as mobile terminals, power sources for vehicles such as electric vehicles, etc. In recent years, there has been a demand for higher capacity secondary batteries.

[0003] As a negative electrode active material for increasing the capacity of secondary batteries, Si-based materials containing silicon (Si) are promising candidates. However, negative electrode active materials containing silicon undergo a large volume increase upon charging, and electrodes containing silicon-containing negative electrode active materials swell upon charging. Previously, techniques have been proposed to suppress this electrode swelling. For example, silicon-carbon composite materials, in which silicon particles are dispersed in a carbon matrix, have been considered as negative electrode active materials based on Si-based materials. In such composite materials, the carbon matrix can absorb the expansion of silicon during charging, thereby mitigating the impact of silicon volume increase. For example, Patent Document 1 proposes a silicon-carbon composite negative electrode material that can reduce the impact of silicon volume increase and improve cycle characteristics.

[0004] Special Publication No. 2023-523107

[0005] In addition to the swelling problem, the Si-based negative electrode active material also faces the challenge of improving battery efficiency. To improve battery efficiency, a coating layer has been proposed to prevent side reactions between silicon and the electrolyte on the surface of the negative electrode active material.

[0006] However, when the coating layer is provided on the surface of the negative electrode active material in order to improve the battery efficiency, it becomes difficult to sufficiently reduce the swelling of the electrode. Thus, with conventional techniques, it has been difficult to achieve both improvement in battery efficiency and reduction in electrode swelling.

[0007] The present disclosure provides a technology that can improve the initial efficiency of a battery and reduce swelling of the electrode when using a negative electrode active material that uses a composite material containing silicon and a carbon material.

[0008] The negative electrode active material of the present disclosure comprises composite particles including a carbon phase and a silicon phase dispersed within the carbon phase, and a coating layer including carbon covering at least a portion of the surface of the composite particles, and simultaneously satisfies the following (1) and (2): (1) With respect to the negative electrode active material, the ratio (B / A) of the true density (B) determined by a gas-phase substitution method using helium to the true density (A) determined by a liquid-phase substitution method using 1-butanol is 1.04 or more; (2) In a Raman spectrum of the carbon phase observed by laser Raman spectroscopy using a laser beam with a wavelength of 532 nm, -1 The intensity of the G band observed near 1360 cm -1 The intensity ratio of the D band observed in the vicinity is 1.1 or more and 1.5 or less.

[0009] According to the technology of the present disclosure, for a negative electrode active material using a composite material containing silicon and a carbon material, it is possible to achieve both an improvement in the initial efficiency of a battery and a reduction in electrode swelling.

[0010] Fig. 1 is a cross-sectional view schematically showing the general configuration of an example of a negative electrode active material according to embodiment 1. Fig. 2 is a flowchart showing an example of a method for manufacturing the negative electrode active material according to embodiment 1. Fig. 3 is a cross-sectional view schematically showing the general configuration of an example of an electrode according to embodiment 2. Fig. 4 is a longitudinal cross-sectional view schematically showing an example of a battery according to embodiment 3.

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0012] 1 is a cross-sectional view schematically illustrating the general configuration of an example of an anode active material according to Embodiment 1. An anode active material 10 according to Embodiment 1 includes a composite particle 1 and a coating layer 2 that covers at least a portion of the surface of the composite particle 1. The composite particle 1 includes a carbon phase 11 and a silicon phase 12 dispersed within the carbon phase 11. The coating layer 2 includes carbon.

[0013] The negative electrode active material 10 further satisfies the following (1) and (2) simultaneously: (1) The ratio (B / A) of the true density (B) determined by a gas-phase substitution method using helium to the true density (A) determined by a liquid-phase substitution method using 1-butanol is 1.04 or more; (2) In the Raman spectrum of the carbon phase 11 observed by laser Raman spectroscopy using a laser beam with a wavelength of 532 nm, -1 The intensity of the G band observed near 1360 cm -1 The intensity ratio of the D band observed in the vicinity is 1.1 or more and 1.5 or less.

[0014] With the above-described configuration, the negative electrode active material 10 according to the first embodiment can improve the initial efficiency of the battery and reduce swelling of the electrode at the same time.

[0015] Specifically, the coating layer 2 in the negative electrode active material 10 can reduce side reactions between the silicon phase 12 and the electrolyte, thereby improving the initial efficiency of the battery. Furthermore, by satisfying the above condition (1), the negative electrode active material 10 can contain an appropriate proportion of closed pores that are difficult for liquid to penetrate and are large enough to effectively absorb the volume increase of the silicon phase 12 and reduce swelling of the negative electrode active material 10. Furthermore, by satisfying the above condition (2), the negative electrode active material 10 can realize a carbon phase in which the closed pores are easily formed and defects are less likely to occur. Therefore, the negative electrode active material 10 that simultaneously satisfies the above conditions (1) and (2) can sufficiently mitigate the volume increase of the silicon phase 12 during charging by the closed pores, thereby reducing swelling of the electrode. Furthermore, by satisfying the above condition (2), the negative electrode active material 10 can suppress the occurrence of defects such as cracks and fractures in the carbon phase 11 that cause side reactions with the electrolyte. Therefore, in addition to the effect of improving the initial efficiency due to the coating layer, the initial efficiency can be further improved.

[0016] The ratio (B / A) may be, for example, 1.50 or less, which can increase the energy density per volume.

[0017] The mass ratio of carbon to the total of carbon, oxygen, and silicon calculated from the results of analysis of the outer surface of the negative electrode active material 10 by energy dispersive X-ray spectroscopy (SEM-EDX) in combination with a scanning electron microscope may be, for example, 60 mass% or more. The mass ratio of carbon determined by SEM-EDX analysis of the outer surface of the negative electrode active material 10 can be used to determine the proportion of carbon on the surface of the negative electrode active material 10, i.e., the degree of coverage by the coating layer 2. When the mass ratio of carbon on the outer surface of the negative electrode active material 10 is 60 mass% or more, side reactions between the silicon phase 12 contained in the composite particles 1 and the electrolyte are further suppressed, thereby further improving the initial efficiency of the battery. To further improve the initial efficiency of the battery, the mass ratio of carbon on the outer surface of the negative electrode active material 10 may be 70 mass% or more.

[0018] The SEM-EDX analysis of the outer surface of the negative electrode active material 10 can be performed by performing elemental mapping analysis of the outer surface of the negative electrode active material 10 using EDX from a backscattered electron image of a cross section of the negative electrode active material 10. The area of ​​the target element is calculated using image analysis software. The observation magnification is, for example, 2000 to 20,000 times. When measuring from the state of a battery, 10 negative electrode active materials 10 with a maximum particle diameter of 5 μm or more are randomly selected from a cross-sectional image of a backscattered electron image of a negative electrode mixture layer containing the negative electrode active material, and elemental mapping analysis is performed on the outer surface of each negative electrode active material 10 using EDX. The measured values ​​of the area of ​​the target element contained in the 10 particles are averaged to calculate the content of the target element.

[0019] The ratio (MB / MA) of the mass fraction of silicon in the negative electrode active material 10 (MB) calculated from the analysis results of the outer surface of the negative electrode active material 10 by SEM-EDX to the mass fraction of silicon in the negative electrode active material 10 (MA) calculated from the analysis results of the negative electrode active material 10 by inductively coupled plasma optical emission spectroscopy (ICP analysis) may be, for example, 0.75 or less, or may be 0.70 or less. The ratio (MB / MA) is an index indicating the mass fraction of silicon located on the surface of the negative electrode active material 10 relative to the mass fraction of silicon in the entire negative electrode active material 10, i.e., the extent to which the surface of the composite particle 1 is covered by the coating layer 2. When the ratio (MB / MA) is 0.75 or less, side reactions between the silicon phase 12 contained in the composite particle 1 and the electrolyte are further suppressed, thereby further improving the initial efficiency of the battery.

[0020] In ICP analysis of the negative electrode active material 10, a sample of the negative electrode active material 10 is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is filtered off. The resulting filtrate is then analyzed by ICP to measure the spectral intensity of each element. Subsequently, a calibration curve is created using commercially available standard solutions of the elements, and the content of each element contained in the negative electrode active material 10 is calculated. From this calculation result, the mass proportion of the elements constituting the negative electrode active material 10 can be determined.

[0021] The mass percentage of silicon in the negative electrode active material 10 may be, for example, 50% by mass or more. The mass percentage of silicon here is the mass percentage (MA) calculated from the analysis results of the negative electrode active material 10 by the above-mentioned ICP analysis. A silicon mass percentage of 50% by mass or more can achieve even higher capacity. Furthermore, according to the configuration of the negative electrode active material 10 according to embodiment 1, even if the silicon mass percentage is 50% by mass or more, it is possible to sufficiently reduce electrode swelling due to an increase in the volume of the silicon phase 12 during charging.

[0022] The average pore diameter of the negative electrode active material 10 measured by nitrogen gas adsorption may be, for example, 1 nm or more and 10 nm or less. Pores with an average pore diameter of 1 nm or more and 10 nm or less are suitable for providing a space size that alleviates the expansion of the silicon phase 12. Therefore, the negative electrode active material 10 having an average pore diameter in the above range can more effectively reduce the swelling of the electrode.

[0023] The ratio (C / A) of the true density (C) of the pulverized negative electrode active material 10 determined by the liquid-phase displacement method using 1-butanol to the true density (A) may be, for example, 1.05 or more. Here, the true density (C) of the pulverized negative electrode active material 10 will be described. In order to effectively mitigate the expansion of the silicon phase 12, it is desirable that the closed pores contained in the negative electrode active material 10 are not too large. When the negative electrode active material 10 is pulverized, it is estimated that large pores contained in the negative electrode active material 10 often cause the negative electrode active material 10 to crack. In other words, it is believed that pores that are too large among the pores present before pulverization of the negative electrode active material 10 are lost by pulverization. Therefore, the ratio (C / A) of the true density (C) of the negative electrode active material 10 after pulverization to the true density (A) of the negative electrode active material 10 before pulverization serves as an indicator of the presence of pores of a size suitable as spaces for mitigating the expansion of the silicon phase 12. Therefore, when the ratio (C / A) is 1.05 or more, the negative electrode active material 10 can more effectively reduce swelling of the electrode.

[0024] The pulverized material of the negative electrode active material 10 prepared when determining the true density (C) has a median diameter (D 50 That is, when measuring the true density (C), the negative electrode active material 10 has a median diameter (D 50 The median diameter (D) of the pulverized product is in the range of 1 μm or more and 2 μm or less. 50 ) means the particle size (volume average particle size) at which the volume integrated value is 50% in the particle size distribution measured by a laser diffraction scattering method.

[0025] In order to increase the energy density per volume, the ratio (C / A) may be, for example, 1.50 or less.

[0026] The average pore diameter of the pulverized material measured by nitrogen gas adsorption may be, for example, 1 nm or more and 10 nm or less. Negative electrode active material 10 having an average pore diameter of 1 nm or more and 10 nm or less has spaces of a size more suitable for mitigating the expansion of silicon phase 12. Therefore, negative electrode active material 10 having an average pore diameter in the above range can more effectively reduce electrode swelling.

[0027] The tap bulk density of the negative electrode active material 10 may be, for example, 0.5 mg / mL or more and 0.9 mg / mL or less, which allows the negative electrode active material 10 to more effectively reduce swelling of the electrode.

[0028] Each component of the negative electrode active material 10 according to the first embodiment will be specifically described below.

[0029] (Composite Particle) As described above, the composite particle 1 includes the carbon phase 1 and the silicon phase 12 dispersed in the carbon phase 1.

[0030] The average particle size of the composite particle 1 may be 1 μm or more and 25 μm or less, or 4 μm or more and 15 μm or less. This configuration facilitates alleviating stress caused by volumetric changes in the composite particle 1 during charging and discharging, making it easier to obtain good charge-discharge cycle characteristics. Furthermore, by ensuring that the surface area of ​​the composite particle 1 is of an appropriate size, capacity reduction due to side reactions with the battery electrolyte is suppressed.

[0031] The average particle size of the composite particles 1 refers to the particle size (volume-average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. For example, an "MT-3000II" manufactured by Microtrac Bell Co., Ltd. can be used as the measuring device.

[0032] The composite particles 1 can be removed from the battery by, for example, the following method, which is just an example.

[0033] First, a fully discharged battery is disassembled to remove the negative electrode. The negative electrode is washed with, for example, anhydrous ethyl methyl carbonate or dimethyl carbonate to remove non-aqueous electrolyte components. The negative electrode mixture layer is peeled off from the negative electrode core current collector (e.g., copper foil), and the mixture layer is crushed in a mortar to obtain a sample powder. Next, the sample powder is dried in a dry atmosphere for 1 hour and, for example, immersed in gently boiled 6 M hydrochloric acid for about 10 minutes to remove elements derived from materials other than the composite particles. Next, the sample powder is washed with ion-exchanged water, filtered, and dried for 1 hour at 200 °C, for example. Note that a fully discharged state refers to a state in which the depth of discharge (DOD) is 90% or more (the state of charge (SOC) is 10% or less).

[0034] The carbon phase 11 is a matrix made of a carbonaceous material. Because the carbon phase 11 is conductive, contact between the composite particle 1 and its surroundings is easily maintained even if voids are formed around the composite particle 1. This improves battery characteristics such as the initial charge / discharge efficiency and charge / discharge cycle characteristics of the battery.

[0035] The carbon phase 11 has a Raman spectrum of 1600 cm in the Raman spectrum observed by laser Raman spectroscopy using a laser beam having a wavelength of 532 nm under the condition (2) above. -1 The intensity of the G band observed near 1360 cm -1 The carbon phase 11 is formed so that the intensity ratio of the D band observed near the carbon phase 11 satisfies 1.1 or more and 1.5 or less. Therefore, the carbon phase 11 may be composed of, for example, amorphous carbon (i.e., non-crystalline carbon), and activated carbon, hard carbon, etc. can be used. Amorphous carbon generally refers to a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.34 nm.

[0036] The silicon phase 12 is a phase of simple silicon, which repeatedly absorbs and releases Li ions as the battery is charged and discharged. The capacity is generated by a Faraday reaction involving the silicon phase 12. The silicon phase 12 has a large capacity. Furthermore, the silicon phase 12 also expands and contracts to a large extent as the battery is charged and discharged. However, in the negative electrode active material of the present disclosure, the silicon phase 12 is dispersed within the carbon phase 11, and therefore the stress caused by the expansion and contraction of the silicon phase 12 is alleviated by the carbon phase 11.

[0037] The silicon phase 12 may be particulate. The silicon phase 12 is particulate, for example, at least before the first charge. The average particle size of the silicon phase 12 may be 1 nm or more and 1000 nm or less. The average particle size of the silicon phase 12 may be 500 nm or less, 200 nm or less, or 50 nm or less. After the first charge, the average particle size of the silicon phase 12 may be 400 nm or less, or 100 nm or less. By dispersing the fine silicon phase 12 within the carbon phase 11 as described above, the volume change of the composite particle 1 during charge and discharge is reduced, and the structural stability of the negative electrode active material can be improved.

[0038] The average particle size of the silicon phases 12 can be measured using an SEM image obtained by SEM observation of a cross section of the negative electrode active material 10 in which the cross sections of the silicon phases 12 are exposed. Specifically, the average particle size of the silicon phases 12 is determined by averaging the maximum diameters of 100 silicon phases 12 arbitrarily selected from the cross-sectional SEM image of the negative electrode active material 10.

[0039] The silicon phase 12 may contain crystalline silicon. The silicon phase 12 is composed of, for example, a plurality of crystallites. The crystallite size of the silicon phase 12 may be 30 nm or less, 20 nm or less, or 15 nm or less. As a result, the volume change due to expansion and contraction of the silicon phase 12 during charge and discharge can be reduced, and the effect of improving the charge and discharge cycle characteristics becomes more significant. The crystallite size of the silicon phase 12 is calculated by the Scherrer equation from the half-width of the diffraction peak derived from the Si(111) plane in the X-ray diffraction pattern obtained by X-ray diffraction measurement using Cu-Kα radiation.

[0040] The lower limit of the crystallite size of the silicon phase 12 is not particularly limited, but is, for example, 1 nm. An example of a suitable crystallite size of the silicon phase 12 is 1 nm or more and 15 nm or less, and may be 5 nm or more and 11 nm or less. When the crystallite size of the silicon phase 12 is 1 nm or more, for example, the surface area of ​​the silicon phase 12 can be kept small, making it difficult for deterioration of the silicon phase 12, which is accompanied by the generation of irreversible capacity, to occur. When the crystallite size is 15 nm or less, the expansion and contraction of the silicon phase 12 can be easily made uniform, and stress generated in the negative electrode active material 10 is effectively alleviated.

[0041] From the viewpoint of increasing capacity and improving charge / discharge cycle characteristics, the mass proportion of the silicon 12 phase in the composite particle 1 may be 30 mass% or more and 80 mass% or less, or 40 mass% or more and 70 mass% or less.

[0042] The mass proportion of the silicon phase 12 in the composite particle 1 can be determined by quantifying the amount of silicon that constitutes the silicon phase 12 in the composite particle 1 by ICP analysis.

[0043] The porosity of the composite particle 1 before the initial charge / discharge may be 50% or less. By keeping the porosity of the composite particle 1 at 25% or less, the number of areas that can become the starting point for cracks and breaks when charge / discharge is repeated is reduced. This suppresses deterioration of the negative electrode active material and further improves the charge / discharge cycle characteristics of the battery. The porosity of the composite particle 1 may be 33% or less, or may be 15% or less. The lower limit of the porosity is not particularly limited, but an example is 1%.

[0044] The porosity of the composite particle 1 means the proportion of voids in the cross section of the composite particle 1. The porosity of the composite particle 1 can be measured using an SEM image of a cross section of the negative electrode active material in which the cross section of the composite particle 1 is exposed. The porosity of the composite particle 1 is calculated by extracting void regions by binarizing the SEM image using image analysis software (e.g., ImageJ), and dividing the total area of ​​the voids by the total area of ​​the particle cross section.

[0045] The porosity of the composite particle 1 can be controlled to some extent by adjusting, for example, the firing temperature, firing atmosphere, and compressive force applied to the particles during firing in the manufacturing process of the composite particle 1.

[0046] The composite particle 1 may have a Vickers hardness of 300 HV or more. Since the conductive layer does not affect the Vickers hardness, the Vickers hardness of the composite particle 1 is substantially the same as that of the mother particle in the composite particle 1. When the composite particle 1 has a high Vickers hardness, the volume change of the silicon phase 12 during charge and discharge can be easily suppressed, and deterioration of the particle structure can be reduced. As a result, the effect of improving the charge and discharge cycle characteristics becomes more significant. The Vickers hardness of the composite particle 1 is more preferably 350 HV or more, and may be 400 HV or more, or 500 HV or more.

[0047] The Vickers hardness of the composite particle 1 can be measured using a Vickers hardness tester. Specifically, the composite particle 1, from which the coating layer 2 has been removed from the negative electrode active material 10, is embedded in a thermosetting resin and polished with No. 400 abrasive paper to expose the cross section of the composite particle 1. The cross section is then polished to a mirror finish using No. 2000 abrasive paper and buff polishing. The Vickers hardness is measured under conditions of a load of 1 kg and a holding time of 15 seconds. The upper limit of the Vickers hardness of the composite particle 1 is not particularly limited, but is, for example, 1500 HV.

[0048] The content of each element in the composite particle 1 and the negative electrode active material 10 can be measured by ICP analysis or SEM-EDX. For example, the content of each element on the outer surface of the composite particle 1 and the negative electrode active material 10 is measured by SEM-EDX, and bulk analysis of the composite particle 1 and the negative electrode active material 10 (i.e., the element content as particles) is measured by ICP analysis.

[0049] <ICP> A sample of Composite Particle 1 is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is removed by filtration. The resulting filtrate is then analyzed by ICP to measure the spectral intensity of each element. Next, a calibration curve is created using commercially available standard solutions of the elements, and the content of each element contained in Composite Particle 1 is calculated.

[0050] <SEM-EDX> Elemental mapping analysis is performed by EDX from a backscattered electron image of a cross section of the negative electrode active material in which the cross section of the composite particle 1 is exposed. The area containing the target element is calculated using image analysis software. The observation magnification is, for example, 2000 to 20,000 times. When measuring from the state of the battery, 10 composite particles 1 with a maximum particle diameter of 5 μm or more are randomly selected from the cross section image of the backscattered electron image of the negative electrode mixture layer containing the negative electrode active material, and elemental mapping analysis is performed on each of them by EDX. The measured values ​​of the area containing a predetermined element contained in the 10 particles are averaged to calculate the content of the target element.

[0051] Desirable measurement conditions for cross-sectional SEM-EDX analysis are shown below: Processing equipment: SM-09010 (Cross Section Polisher) manufactured by JEOL Processing conditions: Acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 Pa to 2 x 10 -3 Pa Measuring device: Hitachi High-Tech, SU8600 Acceleration voltage during analysis: 12 kV Working distance: 4 mm

[0052] (Coating Layer) As described above, the coating layer 2 contains carbon. The coating layer 2 coats at least a portion of the composite particle 1. The coating layer 2 may coat the entire surface of the composite particle 1. The coating layer 2 is, for example, a thin film layer containing a carbon material. Such a coating layer 2 can improve the conductivity of the negative electrode active material 10 while suppressing a side reaction between the silicon phase 12 in the composite particle 1 and the electrolyte solution.

[0053] Examples of the carbon material used for the coating layer 2 include graphite such as natural graphite, artificial graphite, and graphitized mesophase carbon, amorphous carbon, soft carbon, and hard carbon. The carbon material may be amorphous carbon. This makes it easy to form a thin coating layer 2 that covers the surface of the base particles. Examples of amorphous carbon include carbon black, burned pitch, coke, and activated carbon.

[0054] Examples of raw materials for the carbon material used to form the coating layer 2 include low-molecular-weight organic compounds and polymeric organic materials. Examples of low-molecular-weight organic compounds include methane, ethane, propane, isopropane, butane, isobutane, ethylene, propylene, acetylene, butene, vinyl chloride, vinyl fluoride, difluoroethylene, chloroethane, fluoroethane, difluoroethane, chloromethane, fluoromethane, difluoromethane, trifluoromethane, methylamine, formaldehyde, benzene, toluene, xylene, styrene, naphthalene, phenanthrene, anthracene, pyrene, and phenol. Examples of polymeric organic materials include polystyrene, polyethylene, polypropylene, poly(meth)acrylic acid, poly(meth)acrylic acid ester, phenolic resin, novolac resin, polyvinyl alcohol, polylactic acid, resorcinol-formaldehyde resin, polyvinylpyrrolidone, nylon, coal tar, pitch, creosote oil, starch, cellulose, and processed sugars.

[0055] When the composite particles 1 described above are coated, the raw material of the carbon material may be introduced as a gas in the heat treatment step, or may be added and mixed with the composite particles 1 in advance and then subjected to the heat treatment step.

[0056] The thickness of the coating layer 2 is desirably thin enough not to affect the average particle size of the composite particles 1, i.e., to not significantly increase the average particle size of the composite particles 1. The thickness of the coating layer 2 may be 1 nm or more and 200 nm or less, or 5 nm or more and 100 nm or less, taking into consideration the suppression of side reactions, ensuring conductivity, and the diffusibility of ions such as Li ions. The thickness of the coating layer 2 can be measured by SEM observation or transmission electron microscope (TEM) observation of a cross section of the negative electrode active material 10 in which the cross sections of the composite particles 1 and the coating layer 2 are exposed.

[0057] (Method for Producing Negative Electrode Active Material) The negative electrode active material 10 according to the first embodiment can be produced, for example, by the following production method.

[0058] 2 is a flowchart showing an example of a method for producing a negative electrode active material according to embodiment 1. As shown in FIG. 2, the method according to embodiment 1 includes the steps of: (A) attaching at least one substance selected from the group consisting of a thermally decomposable substance, a volatile substance, and a soluble substance to first precursor particles formed of a composite material containing silicon and carbon and having pores to produce second precursor particles (S1); (B) forming a coating layer that covers at least a portion of the surface of the second precursor particles (S2); and (C) removing the substance attached to the surface of the second precursor particles (S3). Note that steps (B) and (C) may be performed within the same process (e.g., consecutive heat treatment processes), or step (C) may be performed after step (B).

[0059] (Step (A)) The first precursor particles are obtained, for example, by pulverizing a mixture of raw silicon and a carbon source while stirring it in a ball mill or the like to form fine particles, then heat-treating the mixture in an inert atmosphere and pulverizing the sintered product obtained by the heat treatment. Examples of the carbon source that can be used include amorphous carbon such as activated carbon, as described above as the material for the carbon phase 11, sugars such as carboxymethyl cellulose (CMC), and water-soluble resins such as polyvinylpyrrolidone. The pores formed in the first precursor particles can be controlled, for example, by adjusting the carbon source, the heat treatment temperature, or the compressive force applied to the particles during firing.

[0060] The first precursor particles may be commercially available products such as silicon-carbon composite powders having the desired pores.

[0061] The second precursor particles are produced by impregnating the first precursor particles with at least one substance selected from the group consisting of a thermally decomposable substance, a volatile substance, and a soluble substance. For example, the substance to be impregnated into the first precursor particles is mixed with a solvent to produce a mixed solution, and the mixed solution is applied to the first precursor particles and dried to produce the second precursor particles. This fills the pores of the first precursor particles with the substance to be impregnated.

[0062] Examples of thermally decomposable substances include higher alcohols, oils and fats, plasticizers, surfactants, organic acids, etc. Examples of higher alcohols, oils and fats, plasticizers, surfactants, and organic acids are as follows:

[0063] Higher alcohols: octyl alcohol, nonyl alcohol, decyl alcohol, undecyl alcohol, dodecyl alcohol, myristyl alcohol, cetyl alcohol, palmityl alcohol, behenyl alcohol, stearyl alcohol

[0064] Fats and oils: soybean oil, coconut oil, linseed oil, cottonseed oil, rapeseed oil, tung oil, castor oil, olive oil, oleic acid, lauric acid, myristic acid, palmitic acid, behenic acid, stearic acid

[0065] Plasticizers: Dimethyl phthalate, diethyl phthalate, dibutyl phthalate, bis(2-ethylhexyl) phthalate, dioctyl phthalate, diisononyl phthalate, diisodecyl phthalate, butyl benzyl phthalate, dioctyl adipate, bis(2-ethylhexyl) adipate, diisononyl adipate, bis(2-ethylhexyl) azelaate, dibutyl sebacate, bis(2-ethylhexyl) sebacate, tributyl phosphate, tri(2-ethylhexyl) phosphate, triphenyl phosphate, tricresyl phosphate

[0066] Surfactants: polyalkylene oxides and derivatives thereof such as polyoxyethylene, polyoxypropylene, polyoxyethylene alkyl ethers, polyoxypropylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxypropylene alkylphenyl ethers, polyoxyethylene lanolin derivatives, polyoxyethylene glycerin fatty acid esters, polyoxyethylene sorbitan acid esters, polyoxypropylene glycerin fatty acid esters, polyoxypropylene sorbitan acid esters, ethylene glycol fatty acid esters, propylene glycol fatty acid esters, polyglycerin, glycerin fatty acid esters, and polyglycerin fatty acid esters.

[0067] Organic acids: aliphatic monocarboxylic acids such as octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, linoleic acid, and oleic acid; aromatic monocarboxylic acids such as benzoic acid, salicylic acid, and toluic acid; polycarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, fumaric acid, maleic acid, phthalic acid, and terephthalic acid; and hydroxycarboxylic acids such as lactic acid, tartaric acid, citric acid, and malic acid.

[0068] Examples of volatile substances include lower alcohols and lower carboxylic acids. Examples of lower alcohols include ethylene glycol, glycerin, butanol, hexanol, glucose, and derivatives thereof. Examples of lower carboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, heptanoic acid, and derivatives thereof.

[0069] Examples of soluble substances include water-soluble inorganic salts (lithium hydroxide, lithium acetate, lithium chloride, sodium chloride, calcium chloride, ammonium chloride, sodium carbonate, sodium bicarbonate, sodium sulfate, potassium sulfate, magnesium sulfate, zinc sulfate, sodium phosphate, potassium phosphate, lithium benzoate, sodium benzoate, potassium benzoate, lithium acetate, sodium acetate, and potassium acetate).

[0070] For reasons of availability, cost, and thermal decomposition, for example, plasticizers, fats and oils, and organic acids are used, and for example, bis(2-ethylhexyl) adipate, stearic acid, etc. are preferably used.

[0071] (Step (B)) Next, a coating layer is formed to cover at least a part of the surface of the second precursor particles. To form the coating layer, for example, the carbon material described above as the material for the coating layer 2 can be used.

[0072] Examples of such methods include a CVD method using a hydrocarbon gas such as acetylene or methane as a raw material, and a method in which coal pitch, petroleum pitch, phenolic resin, or the like is mixed with second precursor particles and heated to carbonize them. Carbon black or the like may also be attached to the surfaces of the second precursor particles. For example, a coating layer may be formed on the surfaces of the second precursor particles by heating a mixture of the second precursor particles and a carbon material in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere) at 700°C or higher and 950°C or lower.

[0073] (Step (C)) The substance attached to the surface of the second precursor particle is removed. The method for removing the attached substance can be appropriately selected depending on the properties of the attached substance used. For example, when a thermally decomposable substance and / or a volatile substance is used, the attached substance can be removed by heat treatment at a temperature required to remove these substances. When a soluble substance is used, the attached substance can be removed using a liquid that can dissolve the substance.

[0074] The thermally decomposable substance may have a mass loss rate of 80% or more at 400°C, based on the mass at 25°C. The volatile substance may have a mass loss rate of 80% or more at 400°C, based on the mass at 25°C. Such thermally decomposable substances and volatile substances are suitable as substances to be attached because they can be easily removed by heat treatment.

[0075] According to the above manufacturing method, when forming the coating layer, it is possible to prevent the material of the coating layer from penetrating into the pores contained in the first precursor particles and blocking the pores. Therefore, when composite particles having desired pores are used as the first precursor particles, the coating layer can be formed while maintaining the pores. Therefore, it is possible to manufacture a negative electrode active material having closed pores that satisfies the above-mentioned condition (1), such as the negative electrode active material 10 of the first embodiment.

[0076] (Embodiment 2) An electrode according to embodiment 2 includes the negative electrode active material according to embodiment 1. The negative electrode active material according to embodiment 1 can reduce swelling of the electrode due to charging, as described in embodiment 1. Therefore, the electrode according to embodiment 2 can reduce swelling due to charging.

[0077] Fig. 3 is a cross-sectional view showing a schematic configuration of an example of an electrode according to embodiment 2. The electrode according to embodiment 2 is a negative electrode. As shown in Fig. 3, a negative electrode 20 according to embodiment 2 includes, for example, a negative electrode mixture layer 21 and a negative electrode current collector 22. The negative electrode mixture layer 21 is disposed on the negative electrode current collector 22 and contains the negative electrode active material according to embodiment 1.

[0078] The anode mixture layer 21 may further contain another anode active material, such as a carbon material, in addition to the anode active material according to embodiment 1. By including a carbon material as the anode active material, a conductive path in the anode active material can be ensured even when the anode active material according to embodiment 1 expands and contracts during charge and discharge, thereby further suppressing capacity degradation associated with charge and discharge cycles. Furthermore, by including a carbon material, it becomes easier to control the porosity of the anode mixture layer 21, thereby enabling control of the porosity in consideration of the permeability of the electrolyte. The carbon material functioning as the anode active material is, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon.

[0079] The negative electrode mixture layer 21 further includes, for example, a binder. Examples of the binder include fluorine-containing resins such as PVDF, polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and SBR. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like.

[0080] In order to reduce the resistance of the negative electrode 20, the negative electrode mixture layer 21 may further contain a conductive additive. Examples of the conductive additive include a carbon material and a conductive polymer compound. Examples of the carbon material include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of the conductive polymer compound include polyaniline, polypyrrole, and polythiophene.

[0081] The negative electrode current collector 22 may be a sheet or film made of a metal material such as stainless steel, nickel, copper, or an alloy thereof. The sheet or film may be porous or non-porous. Examples of the sheet or film include metal foil and metal mesh. A carbon material such as carbon may be applied to the surface of the negative electrode current collector 22 as a conductive auxiliary material.

[0082] (Embodiment 3) A battery according to embodiment 3 includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains the negative electrode active material according to embodiment 1. With this configuration, the battery according to embodiment 3 can reduce swelling of the negative electrode due to charging.

[0083] 4 is a longitudinal cross-sectional view schematically illustrating an example of a battery according to embodiment 3. The battery 100 is a cylindrical battery including a cylindrical battery case, a wound electrode group 54, and an electrolyte (not shown). The electrode group 54 is housed in the battery case and is in contact with the electrolyte.

[0084] The battery case is composed of a case body 55, which is a cylindrical metal container with a bottom, and a sealing body 56 that seals the opening of the case body 55. A gasket 67 is disposed between the case body 55 and the sealing body 56. The gasket 67 ensures the airtightness of the battery case. Within the case body 55, insulating plates 57 and 58 are disposed on both ends of the electrode group 54 in the direction of the winding axis of the electrode group 54.

[0085] Case body 55 has, for example, a step 61. Step 61 can be formed by partially pressing the side wall of case body 55 from the outside. Step 61 may be formed in an annular shape on the side wall of case body 55 along the circumferential direction of an imaginary circle defined by case body 55. In this case, sealing body 56 is supported by, for example, the surface of step 61 on the opening side.

[0086] Sealing body 56 includes a filter 62, a lower valve body 63, an insulating member 64, an upper valve body 65, and a cap 66. These components are stacked in this order in sealing body 56. Sealing body 56 is attached to the opening of case body 55 so that cap 66 is located on the outside of case body 55 and filter 62 is located on the inside of case body 55.

[0087] Each of the above-mentioned members constituting the sealing body 56 has, for example, a disk or ring shape. The above-mentioned members, except for the insulating member 64, are electrically connected to each other.

[0088] The electrode group 54 has a positive electrode 51, a separator 52, and a negative electrode 53. The positive electrode 51, the separator 52, and the negative electrode 53 are all strip-shaped. The width direction of the strip-shaped positive electrode 51 and the negative electrode 53 is, for example, parallel to the winding axis of the electrode group 54. The separator 52 is disposed between the positive electrode 51 and the negative electrode 53. The positive electrode 51 and the negative electrode 53 are spirally wound with the separator 52 interposed between these electrodes.

[0089] When observing the cross section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 54, the positive electrodes 51 and negative electrodes 53 are stacked alternately in the radial direction of an imaginary circle defined by the case body 55, with a separator 52 interposed between these electrodes.

[0090] The positive electrode 51 is electrically connected to a cap 66, which also serves as a positive electrode terminal, via a positive electrode lead 59. One end of the positive electrode lead 59 is connected, for example, to near the center of the positive electrode 51 in the longitudinal direction of the positive electrode 51. The positive electrode lead 59 passes through a through-hole formed in the insulating plate 57 and extends from the positive electrode 51 to the filter 62. The other end of the positive electrode lead 59 is welded, for example, to the surface of the filter 62 on the electrode group 54 side.

[0091] The negative electrode 53 is electrically connected to the case body 55, which also serves as a negative electrode terminal, via a negative electrode lead 60. One end of the negative electrode lead 60 is connected to, for example, an end of the negative electrode 53 in the longitudinal direction of the negative electrode 53. The other end of the negative electrode lead 60 is welded to, for example, the inner bottom surface of the case body 55.

[0092] Each component of the battery 100 will be specifically described below.

[0093] The positive electrode 51 includes a material having the property of absorbing and releasing metal ions (e.g., lithium ions). The positive electrode 51 includes, for example, a positive electrode active material. The positive electrode 51 includes, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0094] The positive electrode current collector can be, for example, a sheet or film made of a metal material such as aluminum, stainless steel, titanium, or an alloy thereof. Aluminum and its alloys are suitable as materials for the positive electrode current collector because they are inexpensive and easy to form into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, or the like may be used as the sheet or film. A carbon material such as carbon may be applied to the surface of the positive electrode current collector as a conductive auxiliary material.

[0095] The positive electrode mixture layer includes a positive electrode active material. The positive electrode active material can be a material capable of absorbing and releasing metal ions (e.g., lithium ions). Examples of the positive electrode active material include lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, and transition metal oxynitrides. In particular, using a lithium-containing transition metal oxide or a lithium-containing transition metal phosphate as the positive electrode active material can reduce battery manufacturing costs and increase average discharge voltage. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0096] The positive electrode mixture layer may further contain a binder. As the binder, the materials described as binders usable for the negative electrode mixture layer in the second embodiment can also be used for the positive electrode mixture layer.

[0097] The positive electrode mixture layer may further contain a conductive additive. As the conductive additive, the materials described in the second embodiment as conductive additives that can be used in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0098] The negative electrode 53 contains the negative electrode active material according to embodiment 1. The negative electrode 53 is, for example, the negative electrode 20 according to embodiment 2.

[0099] The electrolyte solution used as the electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte solution may be, for example, 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0100] The non-aqueous solvent may be a cyclic carbonate, a chain carbonate, a cyclic ether, a chain ether, a nitrile, an amide, etc. One selected from these solvents may be used, or two or more may be used in combination.

[0101] Examples of lithium salts that can be used include lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium perchlorate (LiClO), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One selected from these electrolyte salts may be used, or two or more may be used in combination.

[0102] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator 52 has high ion permeability and adequate mechanical strength and insulating properties. The separator 52 can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator 52 can be made of a polymer, for example. The polymer can be a polyolefin such as polypropylene or polyethylene.

[0103] In the battery according to the third embodiment, the electrolyte may be impregnated into a polymer provided as a separator, for example. That is, the battery according to the third embodiment may have a structure in which the electrolyte and the polymer are used in combination.

[0104] The battery according to the third embodiment may further include a solid electrolyte as the electrolyte. That is, the battery according to the present disclosure may have a hybrid structure in which an electrolytic solution and a solid electrolyte are used in combination. Examples of solid electrolyte materials include halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, and organic polymer solid electrolytes. In the present disclosure, the term "halide solid electrolyte" refers to a solid electrolyte containing a halogen element as the main anion component. The term "sulfide solid electrolyte" refers to a solid electrolyte containing sulfur as the main anion component. The term "oxide solid electrolyte" refers to a solid electrolyte containing oxygen as the main anion component. The term "main anion component" refers to the anion with the largest mass among all anions constituting the solid electrolyte.

[0105] As an example of the structure of the battery according to the third embodiment, the configuration example shown in FIG. 4 is described, i.e., a cylindrical nonaqueous electrolyte secondary battery in which a wound electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and an electrolyte solution are housed in an outer casing. However, the battery according to the present disclosure is not limited to this configuration example. The battery according to the third embodiment may have any shape, such as a prismatic shape, a coin shape, a button shape, or a laminate shape. Furthermore, instead of the wound electrode group in the battery according to the third embodiment, an electrode group of another shape, such as an electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween, may be used.

[0106] (Other Embodiments) (Additional Notes) The above description of the embodiments discloses the following techniques.

[0107] (Technology 1) A negative electrode active material comprising composite particles including a carbon phase and a silicon phase dispersed within the carbon phase, and a coating layer containing carbon covering at least a portion of the surface of the composite particles, wherein the negative electrode active material simultaneously satisfies the following (1) and (2): (1) The ratio (B / A) of the true density (B) determined by a gas-phase substitution method using helium to the true density (A) determined by a liquid-phase substitution method using 1-butanol is 1.04 or more; (2) In a Raman spectrum of the carbon phase observed by laser Raman spectroscopy using a laser beam with a wavelength of 532 nm, -1 The intensity of the G band observed near 1360 cm -1 The intensity ratio of the D band observed in the vicinity is 1.1 or more and 1.5 or less.

[0108] This configuration makes it possible to improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0109] (Technology 2) The negative electrode active material according to Technology 1, wherein the mass ratio of carbon to the total of carbon, oxygen, and silicon calculated from the analysis results of the outer surface of the negative electrode active material by energy dispersive X-ray spectroscopy combined with a scanning electron microscope is 60 mass% or more.

[0110] This configuration can more reliably improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0111] (Technology 3) The anode active material according to Technology 1 or 2, wherein a ratio (MB / MA) of the mass fraction of silicon in the anode active material (MB) calculated from the analysis results of the outer surface of the anode active material by energy dispersive X-ray spectroscopy combined with a scanning electron microscope to the mass fraction of silicon in the anode active material (MA) calculated from the analysis results of the anode active material by inductively coupled plasma atomic emission spectroscopy is 0.75 or less.

[0112] This configuration can more reliably improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0113] (Technology 4) The negative electrode active material according to any one of Technologies 1 to 3, wherein the ratio (B / A) is 1.50 or less.

[0114] This configuration can more reliably improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0115] (Technology 5) The negative electrode active material according to any one of Technologies 1 to 4, wherein a mass ratio of silicon in the negative electrode active material is 50 mass% or more.

[0116] This configuration can more reliably improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0117] (Technology 6) The negative electrode active material according to any one of Technologies 1 to 5, wherein the negative electrode active material has an average pore diameter of 1 nm or more and 10 nm or less as measured by a nitrogen gas adsorption method.

[0118] This configuration can more reliably improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0119] (Technology 7) The ratio (C / A) of the true density (C) of the pulverized product of the negative electrode active material determined by a liquid phase substitution method using 1-butanol to the true density (A) is 1.05 or more, and the pulverized product has a median diameter (D) of 1 μm or more and 2 μm or less. 50 7. The negative electrode active material according to any one of claims 1 to 6,

[0120] This configuration can more reliably improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0121] (Technology 8) The negative electrode active material according to Technology 7, wherein the ratio (C / A) is 1.50 or less.

[0122] This configuration makes it possible to increase the energy density per volume.

[0123] (Technology 9) The negative electrode active material according to Technology 7 or 8, wherein the average pore diameter of the pulverized material measured by nitrogen gas adsorption method is 1 nm or more and 10 nm or less.

[0124] This configuration can more reliably improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0125] (Technology 10) The negative electrode active material according to any one of Techniques 1 to 9, wherein the tap bulk density of the negative electrode active material is 0.5 mg / mL or more and 0.9 mg / mL or less.

[0126] This configuration can more reliably improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0127] (Technology 11) A battery comprising: a negative electrode containing the negative electrode active material according to any one of technologies 1 to 10; a positive electrode; and an electrolyte.

[0128] This configuration makes it possible to improve the initial efficiency of the battery and reduce swelling of the electrodes at the same time.

[0129] (Technology 12) A method for producing a negative electrode active material, comprising: (A) attaching at least one substance selected from the group consisting of a thermally decomposable substance, a volatile substance, and a soluble substance to first precursor particles formed of a composite material containing silicon and carbon and having pores, to produce second precursor particles; (B) forming a coating layer that covers at least a portion of the surface of the second precursor particles; and (C) removing the substance attached to the surface of the second precursor particles.

[0130] This configuration makes it possible to produce a negative electrode active material that can improve the initial efficiency of the battery while reducing swelling of the electrode.

[0131] (Technology 13) The method for producing a negative electrode active material according to Technology 12, wherein the thermally decomposable substance and the volatile substance each have a mass reduction rate of 80% or more at 400°C based on the mass at 25°C.

[0132] This configuration makes it possible to produce a negative electrode active material that can improve the initial efficiency of the battery while reducing swelling of the electrode.

[0133] The present disclosure will be described in more detail below using examples. The following examples are merely examples of embodiments, and are not intended to limit the scope of the present disclosure.

[0134] <Preparation of Negative Electrode Active Material> [Example 1] Step (A): 10 g of silicon-carbon composite powder (GSS manufactured by Giga Solar Materials Co.) as a silicon-carbon composite precursor (first precursor particles) and 3.0 g of bis(2-ethylhexyl) adipate (manufactured by Tokyo Chemical Industry Co., Ltd.) as an impregnating substance were stirred in 30 g of ethanol at 25°C for 10 minutes. The resulting black suspension was reduced in pressure to 0.02 MPa, held for 1 minute, and then released to atmospheric pressure. This operation was repeated twice. The resulting mixture was air-dried on a petri dish at 25°C for 12 hours and then dried at 60°C for 5 hours to obtain a black powder as second precursor particles.

[0135] Steps (B) and (C): The obtained black powder was mixed with 1.0 g of coal pitch (MCP250, manufactured by JFE Chemical Corporation), and the mixture was heated to 800° C. in an inert atmosphere and heat-treated at 800° C. for 3 hours. The heating rate up to 800° C. was 10° C. / min.

[0136] In this way, the negative electrode active material of Example 1 was obtained.

[0137] Example 2 A negative electrode active material was prepared in the same manner as in Example 1, except that the amount of coal pitch in step (B) was changed to 2.0 g.

[0138] [Example 3] In step (A), stearic acid (Tokyo Chemical Industry Co., Ltd.) was used instead of bis(2-ethylhexyl) adipate. Furthermore, in step (B), 0.9 g of resorcinol (Fujifilm Wako Pure Chemical Industries, Ltd.) and 0.1 g of hexamethylenetetramine (Fujifilm Wako Pure Chemical Industries, Ltd.) were used instead of coal pitch. Except for these points, a negative electrode active material was produced in the same manner as in Example 1.

[0139] Example 4 In step (A), unlike Example 1, silicon-carbon composite precursors (first precursor particles) were synthesized by the following method.

[0140] 5 g of activated carbon (manufactured by Wako Pure Chemical Industries, Ltd.), 30 mg of carboxymethyl cellulose (CMC), and 1.5 g of silicon nanoparticles (manufactured by Sigma-Aldrich) with an average particle size of 10 nm were added to 30 g of water and mixed, followed by stirring for 10 minutes. The resulting black suspension was reduced in pressure to 0.02 MPa, held for 1 minute, and then released to atmospheric pressure. This operation was repeated twice. The resulting mixture was dried at 60°C to obtain a black powder. The black powder was heated to 800°C in an inert atmosphere and heat-treated at 800°C for 3 hours. The heating rate to 800°C was 10°C / min. This was used as a silicon-carbon composite precursor (first precursor particle).

[0141] A negative electrode active material was produced in the same manner as in Example 1, except that the silicon-carbon composite precursor (first precursor particles) was different.

[0142] Comparative Example 1 Silicon-carbon composite powder (GSS manufactured by Giga Solar Materials Co.) was heated to 800°C in an inert atmosphere and then heat-treated at 800°C for 3 hours. The heating rate up to 800°C was 10°C / min. This resulted in the negative electrode active material of Comparative Example 1.

[0143] [Comparative Example 2] 10 g of silicon-carbon composite powder (GSS manufactured by Giga Solar Materials Co.) and 1.0 g of coal pitch (MCP250 manufactured by JFE Chemical Corporation) were mixed, heated to 800°C in an inert atmosphere, and heat-treated at 800°C for 3 hours. At this time, the rate of temperature increase up to 800°C was 10°C / min. As a result, a negative electrode active material of Comparative Example 2 was obtained.

[0144] Comparative Example 3 10 g of silicon-carbon composite powder (GSS manufactured by Giga Solar Materials Co.), 0.9 g of resorcinol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and 0.1 g of hexamethylenetetramine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed, and the mixture was heated to 800°C in an inert atmosphere and heat-treated at 800°C for 3 hours. The heating rate up to 800°C was 10°C / min. This produced a negative electrode active material of Comparative Example 3.

[0145] Comparative Example 4 A silicon-carbon composite precursor was prepared in the same manner as in Example 4. 1.0 g of coal pitch (MCP250, manufactured by JFE Chemical Corporation) was mixed with the prepared silicon-carbon composite precursor, and the mixture was heated to 800°C in an inert atmosphere and heat-treated at 800°C for 3 hours. The heating rate up to 800°C was 10°C / min. This produced a negative electrode active material of Comparative Example 4.

[0146] Comparative Example 5 A silicon-carbon composite precursor was prepared in the same manner as in Example 4. 2.0 g of coal pitch (MCP250, manufactured by JFE Chemical Corporation) was mixed with the prepared silicon-carbon composite precursor, and the mixture was heated to 1200°C in an inert atmosphere and heat-treated at 1200°C for 3 hours. The heating rate up to 1200°C was 10°C / min. This produced a negative electrode active material of Comparative Example 5.

[0147] Comparative Example 6 25 g of carboxymethyl cellulose (CMC), 5.0 g of silicon nanoparticles (manufactured by Sigma-Aldrich) with an average particle size of 10 nm, and 1.0 g of stearic acid were mixed, heated to 800°C in an inert atmosphere, and heat-treated at 800°C for 3 hours. At this time, the heating rate to 800°C was 10°C / min. This was used as a silicon-carbon composite precursor. 1.0 g of coal pitch (MCP250, manufactured by JFE Chemical Corporation) was mixed with the prepared silicon-carbon composite precursor, heated to 800°C in an inert atmosphere, and heat-treated at 800°C for 3 hours. At this time, the heating rate to 800°C was 10°C / min. This gave the negative electrode active material of Comparative Example 6.

[0148] <Analysis of Negative Electrode Active Material> [Measurement of True Density] For the negative electrode active materials of each Example and Comparative Example, the true density (A) was measured by a liquid phase substitution method using 1-butanol, and the true density (B) was measured by a gas phase substitution method using helium. The results are shown in Table 1.

[0149] The specific measurement method was as follows.

[0150] (True density by liquid-phase displacement method using 1-butanol) Measurement was performed according to the method specified in JIS R 7212. The mass (m1) of a 15 mL PTFE (polytetrafluoroethylene) pycnometer was measured. Next, a sample was placed at the bottom and its mass (m2) was measured. 1-butanol was gently added to the pycnometer. Next, the pycnometer was gently shaken to confirm that no large bubbles were generated. After that, it was placed in a vacuum desiccator under reduced pressure and left to stand for 20 minutes or more. After the generation of bubbles had stopped, the pycnometer was removed. It was further filled with 1-butanol, stoppered, and left to stand in a thermostatic bath at 25°C for 10 minutes or more. After that, the liquid level of the 1-butanol was aligned with the marked line, and the mass (m4) was measured. Next, the same pycnometer was filled with only 1-butanol, and the marked line was aligned in the same manner as above, and the mass (m3) was measured. True density was calculated using the following formula.

[0151] True density = (m2 - m1) / ((m2 - m1) - (m4 - m3)) x (density of 1-butanol (25°C))

[0152] [Measurement by Raman Spectroscopy] The carbon phase in the negative electrode active material of each Example and Comparative Example was measured by laser Raman spectroscopy using a laser beam with a wavelength of 532 nm. -1 The intensity of the G band observed near 1360 cm -1 The intensity ratio of the D band observed near the nucleus was calculated. The results are shown in Table 1.

[0153] The specific measurement method was as follows.

[0154] The particles of the negative electrode active material were placed on an observation table, and the focus was adjusted to an arbitrarily selected outer surface of the particle, and measurements were made under the following conditions.

[0155] <Measurement conditions> Measurement device: DXR3xi manufactured by Thermo Fisher Scientific Objective lens magnification: 20x Laser light wavelength: 532 nm Laser light output: 5 mW Exposure time: 0.020 seconds Number of accumulations: 100

[0156] [Analysis of Carbon Mass Ratio by SEM-EDX] The outer surface of the negative electrode active material of each Example and Comparative Example was analyzed by SEM-EDX. The mass ratio of carbon to the total of carbon, oxygen, and silicon was calculated from the analysis results. The results are shown in Table 1.

[0157] The specific analysis method was as follows.

[0158] Under the following measurement conditions, a field of view was selected in which no particles of negative electrode active material overlapped with other negative electrode active materials, and SEM-EDX analysis of the outer surface of the negative electrode active material was performed. The mass proportions were calculated from the intensities attributed to carbon, oxygen, and silicon, respectively.

[0159] <Measurement conditions> Measurement device: Hitachi High-Tech, SU8600 Acceleration voltage during analysis: 12 kV Working distance: 4 mm

[0160] [Analysis of Silicon Mass Fraction by ICP Analysis and SEM-EDX] ICP analysis was performed on the negative electrode active materials of each Example and Comparative Example to determine the silicon mass fraction (MA) in the negative electrode active material. Furthermore, the silicon mass fraction (MB) in the negative electrode active material was determined from the results of SEM-EDX analysis of the outer surface of the negative electrode active material. The ratio (MB / MA) calculated from the obtained results is shown in Table 1.

[0161] The specific analysis method was as follows.

[0162] The sample was placed in a platinum crucible and heated to 800°C for 3 hours in an electric furnace under an oxygen atmosphere for ashing. Nitric acid and hydrofluoric acid were added to the ashed sample, which was then dissolved by heating at 60°C and filtered. The resulting sample solution was measured under the following measurement conditions, and the mass percentage of silicon in the negative electrode active material was determined according to a calibration curve prepared based on a silicon standard solution for ICP analysis (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0163] <ICP analysis> Measurement device: iCAP PRO, manufactured by Thermo Fisher Scientific Measurement wavelength: 251 nm Measurement direction: axial

[0164] The method for analyzing the mass proportion of silicon on the outer surface of the negative electrode active material by SEM-EDX was the same as the method for analyzing the outer surface of the negative electrode active material of each example and comparative example by SEM-EDX, which was described above in [Analysis of mass proportion of carbon by SEM-EDX].

[0165] [Average Pore Diameter by Nitrogen Gas Adsorption Method] The average pore diameter of each of the negative electrode active materials of the Examples and Comparative Examples was measured by nitrogen gas adsorption method. The results are shown in Table 2.

[0166] The specific measurement method was as follows.

[0167] The negative electrode active material was heated at 300° C. for 3 hours in a nitrogen stream, and then the nitrogen adsorption / desorption isotherm was measured using the following apparatus and conditions.

[0168] Measurement device: BELSORP-mini (manufactured by Microtrac BEL) Measurement temperature: 77K

[0169] The total pore volume V (mL / g) was calculated from the amount of nitrogen adsorbed at a relative pressure p / p = 0.990 in the adsorption isotherm obtained by the above method.

[0170] The nitrogen adsorption isotherm obtained by the above method was analyzed by the multipoint method using the BET adsorption isotherm, and the specific surface area S (m 2 The average pore diameter was calculated based on the following formula:

[0171] Average pore diameter (nm) = (total pore volume (mL / g) / specific surface area (m 2 / g) × 4000

[0172] [True Density of Crushed Negative Electrode Active Material] The negative electrode active materials of the Examples and Comparative Examples were crushed in a vibration mill using the following apparatus and conditions.

[0173] Grinding device: Mixer Mill MM400 (manufactured by Verder Scientific) Grinding container: 5 mL zirconia container Grinding media: 1 mm diameter zirconia beads, 1.5 mL Sample amount: 1.0 g Grinding speed: 30 Hz

[0174] The median diameter (D 50 ) were as shown in Table 2. The true density (C) of the obtained pulverized product was measured in the same manner as for the true density (A). The ratio (C / A) of the true density (C) to the true density (A) is shown in Table 2.

[0175] [Average pore diameter of pulverized negative electrode active material measured by nitrogen gas adsorption method] A pulverized negative electrode active material was obtained in the same manner as when measuring the true density (C). The average pore diameter of the pulverized negative electrode active material was measured in the same manner as the average pore diameter measurement performed on the composite particles of the negative electrode active material. The results are shown in Table 2.

[0176] [Tap Bulk Density of Negative Electrode Active Material] The tap bulk density of the composite particles of the negative electrode active material of each Example and Comparative Example was measured. The results are shown in Table 2.

[0177] The specific measurement method was as follows.

[0178] The negative electrode active material was dried in a vacuum dryer at 60°C for 8 hours, and then passed through a sieve with 106 μm openings and filled into a cylindrical glass container with a diameter of 2.4 cm. The glass container filled with the active material was repeatedly tapped 50 times, with each tap being defined as a free fall from a height of 5 cm. The density was measured repeatedly until the rate of change in density calculated from the volume and mass of the composite particles was 2% or less before and after every 50 taps, and this was taken as the tapped bulk density.

[0179] <Battery Evaluation> (Production of Negative Electrode) The negative electrode active material of each Example and Comparative Example, PAA, CMC, SBR, and carbon nanotubes (CNT) were mixed in a mass ratio of negative electrode active material: PAA: CMC: SBR: CNT = 100: 5: 5: 5: 0.1, and water was added as a dispersion medium to prepare a negative electrode slurry. Next, the negative electrode slurry was applied to one side of a copper foil serving as a negative electrode current collector, and the coating was dried and then rolled. The negative electrode was cut into a size of 2 cm x 2 cm to produce a negative electrode in which a negative electrode mixture layer was formed on one side of the negative electrode current collector. At this time, a negative electrode current collector exposed portion was provided on a part of the negative electrode, and a nickel lead was attached. The initial thickness T0 of the produced negative electrode mixture layer was measured.

[0180] (Preparation of Counter Electrode) A lithium metal foil was cut into a size of 2.5 cm x 2.5 cm, and a nickel lead was attached to it to form a counter electrode.

[0181] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF) at a concentration of 1 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC=20:5:75.

[0182] (Preparation of non-aqueous electrolyte secondary battery (evaluation cell)) An electrode assembly was prepared by arranging a counter electrode and a negative electrode facing each other with a polyolefin separator interposed therebetween. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and after injecting a non-aqueous electrolyte, the opening of the exterior body was sealed to obtain a battery. This was used as an evaluation cell.

[0183] (Measurement of Initial Efficiency of Negative Electrode) The initial efficiency of the negative electrodes of each of the Examples and Comparative Examples was determined.

[0184] (1) Charging The evaluation cell was charged at 0.1 C in a temperature environment of 25°C until the cell voltage reached 5 mV, and then rested for 20 minutes. Next, it was charged at 0.01 C until the cell voltage reached 5 mV, and then rested for 20 minutes. It was further charged at 0.001 C until the cell voltage reached 5 mV, and then rested for 20 minutes.

[0185] (2) Discharge: The battery was discharged at 0.1 C until the cell voltage reached 1 V, and then rested for 20 minutes. Next, the battery was discharged at 0.01 C until the cell voltage reached 1 V, and then rested for 20 minutes. The battery was further charged at 0.001 C until the cell voltage reached 1 V, and then rested for 20 minutes.

[0186] (3) Initial Efficiency The charge capacity Cmc (mAh / g) per unit mass of the negative electrode active material and the discharge capacity Cmd (mAh / g) per unit mass of the negative electrode active material were calculated using the charge / discharge capacity results, the mass (g) of the negative electrode active material, and the density (g / mL) of the negative electrode active material in the negative electrode mixture layer when the evaluation cell was produced. For the evaluation cells of each Example and Comparative Example, the initial efficiency of the negative electrode was calculated using the following formula. The initial efficiency of the negative electrode is the ratio of the discharge capacity to the initial charge capacity of the negative electrode active material.

[0187] Initial efficiency E (%) = [Cmd (mAh / g) / Cmc (mAh / g)] × 100

[0188] (Measurement of Expansion Coefficient of Negative Electrode During Charge) The expansion coefficient of the negative electrode of each Example and Comparative Example was determined by the following method. After charging, the evaluation cell was disassembled to remove the negative electrode, and the thickness (T) of the negative electrode in the charged state (fully charged state) and the thickness (T0) of the negative electrode in the discharged state (fully discharged state) were determined. The thickness increase rate (T×100 / T0−100) of the negative electrode in the charged state relative to the thickness of the negative electrode in the discharged state was calculated, and this was defined as the expansion coefficient (%) of the negative electrode during charge.

[0189] The results are shown in Table 3. Table 3 also shows the expansion rate during charging per volume capacity.

[0190]

[0191]

[0192]

[0193] (Discussion) The negative electrode active materials of Examples 1 to 4, in which the ratio (B / A) of true density (B) to true density (A) was 1.04 or more and the ratio (ID / IG) of the D band intensity to the G band intensity in the Raman spectrum was 1.1 or more and 1.5 or less, were able to achieve both an improvement in the initial efficiency of the battery and a reduction in electrode swelling. In contrast, the negative electrode active materials of Comparative Examples 1 to 6, which did not satisfy either or both of these conditions, were unable to simultaneously achieve both an improvement in the initial efficiency of the battery and a reduction in electrode swelling.

[0194] The technology of the present disclosure is useful for batteries such as lithium ion secondary batteries.

Claims

1. A negative electrode active material comprising composite particles containing a carbon phase and a silicon phase dispersed within the carbon phase, and a carbon-containing coating layer covering at least a portion of the surface of the composite particles, the negative electrode active material simultaneously satisfying the following (1) and (2): (1) The ratio (B / A) of the true density (B) determined by a gas-phase substitution method using helium to the true density (A) determined by a liquid-phase substitution method using 1-butanol is 1.04 or more; (2) In a Raman spectrum of the carbon phase observed by laser Raman spectroscopy using a laser beam with a wavelength of 532 nm, -1 The G band intensity observed near 1360 cm -1 The intensity ratio of the D band observed in the vicinity is 1.1 or more and 1.5 or less.

2. The negative electrode active material according to claim 1, wherein the mass ratio of carbon to the total of carbon, oxygen, and silicon calculated from the results of analysis of the outer surface of the negative electrode active material by energy dispersive X-ray spectroscopy combined with a scanning electron microscope is 60 mass% or more.

3. The negative electrode active material according to claim 1, wherein the ratio (MB / MA) of the mass fraction of silicon in the negative electrode active material (MB) calculated from the results of analysis of the outer surface of the negative electrode active material by energy dispersive X-ray spectroscopy in combination with a scanning electron microscope to the mass fraction of silicon in the negative electrode active material (MA) calculated from the results of analysis of the negative electrode active material by inductively coupled plasma atomic emission spectroscopy is 0.75 or less.

4. The negative electrode active material according to claim 1, wherein the ratio (B / A) is 1.50 or less.

5. The negative electrode active material according to claim 1, wherein the mass ratio of silicon in the negative electrode active material is 50 mass % or more.

6. The negative electrode active material according to claim 1, wherein the average pore diameter of the negative electrode active material measured by nitrogen gas adsorption is 1 nm or more and 10 nm or less.

7. The ratio (C / A) of the true density (C) of the pulverized negative electrode active material determined by a liquid phase displacement method using 1-butanol to the true density (A) is 1.05 or more, and the pulverized material has a median diameter (D) of 1 μm or more and 2 μm or less. 50 The negative electrode active material according to claim 1 , 8. The negative electrode active material according to claim 7, wherein the ratio (C / A) is 1.50 or less.

9. The negative electrode active material according to claim 7, wherein the average pore diameter of the pulverized material measured by nitrogen gas adsorption is 1 nm or more and 10 nm or less.

10. The negative electrode active material according to claim 1, wherein the tap bulk density of the negative electrode active material is 0.5 mg / mL or more and 0.9 mg / mL or less.

11. A battery comprising: a negative electrode containing the negative electrode active material according to any one of claims 1 to 10; a positive electrode; and an electrolyte.

12. A method for producing a negative electrode active material, comprising: (A) attaching at least one substance selected from the group consisting of a thermally decomposable substance, a volatile substance, and a soluble substance to first precursor particles formed of a composite material containing silicon and carbon and having pores, to produce second precursor particles; (B) forming a coating layer that covers at least a portion of the surface of the second precursor particles; and (C) removing the substance attached to the surface of the second precursor particles.

13. The method for producing a negative electrode active material according to claim 12, wherein the thermally decomposable substance and the volatile substance each have a mass reduction rate of 80% or more at 400°C, based on the mass at 25°C.

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