JP6547309B2 - Negative electrode material for lithium ion secondary battery, paste for lithium ion secondary battery negative electrode, negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a negative electrode material for a lithium ion secondary battery, a paste for a lithium ion secondary battery negative electrode using the negative electrode material, a negative electrode for a lithium ion secondary battery, and a lithium ion secondary battery.

  2. Description of the Related Art In recent years, with the spread of portable electronic devices such as smartphones and portable music players, the demand for small-sized, lightweight, high energy density secondary batteries is increasing. Among them, lithium ion secondary batteries are rapidly being promoted to hybrid electric vehicles (HEVs) and electric vehicles (EVs) in addition to the above-mentioned portable electronic devices, and the industrial application range continues to expand. There is.

  In a lithium ion secondary battery electrode, the electrode is composed of a mixture and a current collector. The composite material is composed of an active material, and optionally a conductive aid and a binder.

  With regard to the high capacity and high energy density of lithium ion batteries, as for the negative electrode active material, in addition to the conventionally widely used graphite materials, metals such as silicon, germanium, tin and the like which form an alloy with lithium ions, and those It is considered to use the oxide of In particular, with respect to a negative electrode active material containing silicon, silicon and silicon oxide are actively studied because the theoretical capacity per unit mass is large and significant improvement in energy density is expected.

  However, since the negative electrode active material containing silicon has a large volume expansion and contraction associated with lithium ion absorption and release, and the charge and discharge are repeated, the electron conductivity of the negative electrode mixture deteriorates, so the capacity of the negative electrode gradually increases. descend. As a result, a secondary battery using such an electrode has a problem that cycle characteristics deteriorate. On the other hand, considering the balance with the current capacity of the positive electrode, the charge and discharge capacity of the negative electrode material is currently excessive for the actual market demand level, so the capacity may be slightly reduced depending on the application of the battery Also, materials having higher cycle characteristics have been required.

  For such a subject, for example, Patent Document 1 proposes using a silicon powder whose surface is covered with a carbon layer by a chemical vapor deposition method as an active material. Further, Patent Document 2 proposes a method in which silicon powder is mechanically alloyed with graphite powder and then carbon is coated by a chemical vapor deposition method. Further, Patent Document 3 and Patent Document 4 disclose a method of coating and granulating a silicon particle surface with a Si-C-O-based composite.

JP 2000-215887 A JP 2002-216751 A JP 2004-335335 A JP 2005-310759 A

  In the negative electrode material described in Patent Document 1, although the electron conductivity is improved, the collapse of the active material inside due to expansion and contraction can not be prevented, and the cycle characteristics are poor. Further, since the negative electrode material described in Patent Document 2 is a fusion of solid and solid of silicon and graphite, a uniform carbon film is not formed, and both the conductivity and film strength are insufficient, and the cycle characteristics are insufficient. . Furthermore, the coating layer of the negative electrode material described in Patent Document 3 and Patent Document 4 also does not have sufficient strength and may collapse without being able to withstand the expansion and contraction of silicon contained in the granulated body, and the cycle characteristics still It was inadequate.

  The present invention has been made to solve the above problems, and has a high capacity compared to a conventional graphite negative electrode material, and a lithium ion secondary battery having higher cycle characteristics than a conventional silicon-based negative electrode material An object of the present invention is to provide a negative electrode material for an electrode.

  The present inventors arrived at the present invention as a result of examining the above problems. That is, the gist of the present invention is lithium ion comprising composite particles containing 5% by volume to 30% by volume of active material particles comprising silicon particles or silicon particles coated with carbon in a matrix containing silicon oxycarbide (SiOC) ceramics It is an anode material for secondary batteries.

  The negative electrode material for a lithium ion secondary battery electrode according to the present invention comprises silicon in a matrix containing high strength silicon oxycarbide (SiOC) ceramics having lithium ion detachment / insertion ability and lithium ion conductivity / electron conductivity. Since the particles are supported, the durability of the composite particles to the expansion and contraction of silicon particles is high. In addition, by making the content of the silicon particles as low as 5 to 30% by volume in the composite particles, the silicon particles are included in the matrix with little exposure, so expansion and contraction of the silicon particles exposed from the matrix It is possible to prevent the collapse of the electrode.

Transmission electron micrograph of negative electrode material of the present invention produced in Example 1

  Hereinafter, the present invention will be described in detail. In addition, in this specification, "-" shall mean the range which includes the value of the both ends.

  The negative electrode material for a lithium ion secondary battery electrode of the present invention (hereinafter, sometimes simply referred to as “the negative electrode material of the present invention”) contains active material particles composed of silicon particles or silicon particles coated with carbon.

  The method for obtaining silicon particles is not particularly limited, and chemical reduction method, plasma jet method, laser ablation method, flame method, direct current arc plasma method, high frequency thermal plasma method, laser pyrolysis method and the like can be used. The silicon particles produced by these techniques are dangerously handled because they are rapidly oxidized in the atmosphere. Therefore, gradual oxidation treatment is performed to enhance safety. Here, since the gradual oxidation treatment burns when the ultrafine particles generated in the evaporation chamber are taken out to the atmosphere as it is, oxygen is gradually supplied into the chamber to form a thin oxide film on the particle surface for stabilization. Processing to In addition, silicon particles having an average particle diameter of 100 nm can also be purchased from Sigma-Aldrich Japan Co., Ltd. or the like.

  Since silicon particles are very easily oxidized, the surface is often oxidized to form a silicon oxide layer under normal handling, but in the present invention, silicon particles containing such silicon oxide are also included in the silicon particles. It shall be. However, from the viewpoint of improving the capacity of the negative electrode material, it is preferable to use silicon particles which are not oxidized as much as possible. Such silicon particles can be obtained by further reducing the silicon particles produced by the above method in a reducing atmosphere.

  As active material particles used for the negative electrode material of the present invention, silicon particles coated with carbon can also be used. The carbon coating can prevent the loss of the reversible capacity due to the natural oxidation of the silicon particle surface. In particular, when the silicon particles are micronized in order to improve the capacity retention rate, the surface area is increased, so the influence of the oxidation of the silicon particles becomes more remarkable.

  It is desirable that the active material particles composed of such carbon-coated silicon particles contain substantially no silicon oxide inside the coating layer. When the amount of silicon oxide inside the covering layer is large, when lithium ions are absorbed, the initial efficiency may be reduced as in the case of the influence of the surface silicon oxide.

  Here, the fact that substantially no silicon oxide is contained in the inside of the covering layer may be sufficient if the influence on the reversible capacity in the battery characteristics is substantially small, but in particular, X-ray photoelectron analysis of the negative electrode material ( In ESCA), the peak area of silicon oxide near 104 eV is smaller than 25% with respect to the sum of peak areas of silicon and silicon-carbon near 100 eV. In X-ray photoelectron analysis (ESCA), the peak area of silicon oxide near 104 eV to the sum of peak areas of silicon and silicon-carbon near 100 eV is more preferably 20% or less, still more preferably 10% or less, and silicon oxide Most preferably, no product peaks are detected.

  The thickness of the covering layer made of carbon is preferably 1 nm or more and 20 nm or less, and more preferably 2 nm or more and 10 nm or less. When the thickness of the covering layer is less than 1 nm, it becomes difficult to effectively cover the surface of the silicon particles. If the thickness of the covering layer is larger than 20 nm, the diffusion of lithium ions at the time of charge and discharge may be inhibited. In addition, as the proportion of silicon particles decreases, the capacity of the composite negative electrode material tends to decrease. The thickness of the coating layer covering the silicon particle surface can be measured using a transmission electron microscope (TEM).

  Here, although it is preferable that all is a carbon as a coating layer which consists of carbon, as long as the function as an active material particle is not impaired, another element may be contained.

  The method of carbon-coating the surface of silicon particles is not particularly limited, and vacuum deposition, ion plating, sputtering, thermal CVD, plasma CVD, photo CVD, etc. can be used. As a method of obtaining silicon particles which do not contain silicon oxide in the inside of the covering layer, the silicon particles whose surface has been naturally oxidized may be reduced by hydrogen reduction and then the covering layer may be formed by CVD or the like. The coating layer may be formed without exposing silicon particles produced in an inert atmosphere to an oxidizing atmosphere.

  In addition, it is preferable that the silicon particles coated with carbon have a low content of silicon carbide. Silicon carbide is charge / discharge inactive and has an adverse effect on reversible capacity. The low content of silicon carbide is sufficient if the battery characteristics have substantially small influence on the reversible capacity, but in particular, in the peak area of silicon around 99.6 eV in X-ray photoelectron analysis (ESCA) In contrast, the peak area of silicon carbide near 100.9 eV is preferably smaller than 100%, more preferably 70% or less, and still more preferably 30% or less.

  It is preferable that the average particle diameter of the active material particle which the negative electrode material of this invention contains is 10 nm or more and 160 nm or less. When the average particle size is less than 10 nm, the dispersibility of silicon particles in the composite negative electrode material is deteriorated, that is, local aggregation progresses, so the absolute amount of local volume change increases and cycle characteristics tend to be deteriorated. . Therefore, the average particle size of the active material particles is more preferably 20 nm or more, and still more preferably 30 nm or more. On the other hand, when the average particle size of the active material particles is larger than 160 nm, pulverization of silicon particles during charge and discharge cycles and cracking due to an increase in the absolute amount of local volume change are likely to occur. Properties tend to deteriorate. Therefore, the average particle size of the active material particles is more preferably 120 nm or less, still more preferably 100 nm or less, still more preferably 80 nm or less, and most preferably 75 nm or less . In the present invention, the "average particle diameter" means a number average particle diameter.

The particle size of the active material particles comprising silicon particles and silicon particles coated with carbon can be determined by image analysis type particle size distribution measurement data of data of electron micrographs such as a scanning electron microscope (SEM) and a transmission electron microscope (TEM). And the projected area equivalent circle diameter of the primary particles (active material particles). Specifically, primary particles included in a plurality of particle photographs taken by SEM are recognized using image analysis type particle size distribution measurement software, and individual particle diameters are evaluated as the projected area circle equivalent diameter of the primary particles. The number average particle diameter, particle size distribution, etc. can be calculated from the particle diameter data obtained from the primary particle projected area total of 10 μm ² . Examples of the image analysis type particle size distribution measurement software include "Mac-VIEW" manufactured by Mountech Co., Ltd., "A Image Kun" manufactured by Asahi Kasei Engineering Corporation, and the like.

  The negative electrode material of the present invention comprises composite particles containing active material particles comprising silicon particles or silicon particles coated with carbon described above in a dense matrix containing silicon oxycarbide (SiOC) ceramics. Since the active material particles are included in the dense SiOC matrix, expansion and contraction of the volume of the composite particles are alleviated, so deformation and collapse of the electrode can be suppressed, and the capacity retention rate can be improved. In addition, the particle size of the negative electrode material can be made larger than that of bare silicon particles or carbon coated silicon particles, and the mixing ratio of the binder resin can be reduced, and it is mixed with the binder resin or solvent to make a paste It is possible to improve the dispersibility at the time of coating and the coating property at the time of coating on the current collector.

Here, including SiOC ceramics indicates that 20% by volume to 100% by volume of SiOC is included as a constituent material of the matrix. The content of SiOC ceramics in the matrix is preferably 30% by volume to 100% by volume, more preferably 50% by volume to 100% by volume, and still more preferably 75% by volume to 100% by volume. As components other than SiOC in the matrix, silicon carbide (SiC), silicon carbonitride (SiCN), silicon carboboronitride (SiNBC), Ti oxycarbide (TiOC), Al oxycarbide (AlOC), hard carbon And soft carbon, graphite, carbon materials such as graphene, etc. can be preferably used. The SiOC ceramic in the composite particle is a ²⁹ Si solid state NMR using tetramethylsilane (TMS) as a reference substance, and the bonding state around the Si atom is Si-O ₂ C ₂ (-34 ppm) and / or Si-O ₃ C It can be detected as containing (-72 ppm).

  The compactness of the matrix is assessed by the porosity. The porosity is a numerical value that indicates the percentage of voids in the composite particles. The porosity can be calculated from the ratio of the void area to the total area by observing the cross section of the composite particle with an electron microscope. In the composite particle of the present invention, the internal porosity is preferably 10% or less. More preferably, it is 5% or less, more preferably 2% or less. If the porosity is 10% or less, the voids are unlikely to be the source of matrix breakage during expansion and contraction of the silicon particles, and the collapse of the composite particles can be further suppressed.

  The negative electrode material of the present invention is obtained by mixing active material particles with a matrix precursor and calcining. The method of mixing the active material particles and the matrix precursor is not particularly limited, but the active material particles are further added to a solution or a suspension obtained by mixing the matrix precursor with a solvent, and stirring with a magnetic stirrer, It is desirable to mix uniformly by ultrasonic dispersion, a revolution / revolution mixer, a bead mill, a ball mill, a planetary mixer, Creamix, Harmotech, etc.

  As a precursor of SiOC in the matrix, polysiloxane and polycarbosilane are preferable, and these may be mixed and used in any ratio. In addition, polysiloxane as a matrix precursor may be added as a polymer, or may be synthesized by reacting in a system a silane compound represented by an alkoxysilane compound part. Also, as precursors of components other than SiOC in the matrix, polycarbosilane for silicon carbide (SiC), polycarbosilazane for silicon carbonitride (SiCN), and silicon carboboronitride (SiNBC) For polyorganoborosilazane and Ti oxycarbide (TiOC), polytitanoxane, for Al oxycarbide (AlOC), polyaluminoxane, for hard carbon, a phenol resin, and for soft carbon, pitch and glucose can be preferably used.

  The solvent is not particularly limited, but is preferably one capable of dissolving the matrix precursor or one capable of dispersing without aggregation, and water, glycol derivatives such as N-methyl pyrrolidone, diacetone alcohol, propylene glycol monomethyl ether acetate, toluene And aromatic hydrocarbons represented by benzene, various alcohols, various ketones, various ethers, esters, and the like. The solvent may be used as a mixture of two or more in order to improve the solubility of the solute or the dispersibility of the dispersion medium.

  In addition, solvent removal may be performed before firing. Although the method and equipment for removing the solvent are not particularly limited, it is desirable to carry out under reduced pressure or under an inert atmosphere. Here, the inert atmosphere refers to a nitrogen atmosphere, an argon atmosphere, and the like. Examples of equipment usable for desolvation include various dryers such as a hot air dryer and a vacuum dryer, and an oil bath.

  The firing may be performed in the air, but is more preferably performed in an inert atmosphere or a reducing atmosphere. As an inert atmosphere, a nitrogen atmosphere, an argon atmosphere or the like can be used. Further, as a reducing atmosphere, a mixture of hydrogen and the like to a gas used in an inert atmosphere can be used. The calcination temperature is preferably 500 ° C to 1400 ° C. In addition, the firing may be performed once or twice or more. In the case where the firing is performed a plurality of times, it is more preferable to perform at a temperature higher than the first firing in any of the second and subsequent firings. The equipment used for firing is not particularly limited, but firing equipment such as an electric furnace, a muffle furnace, a box furnace, a roller hearth kiln, and a rotary kiln can be used.

  In addition, after the removal of the solvent, during the calcination, pulverization may be performed at a desired stage after the calcination. If firing is performed after grinding, sintering of the matrix proceeds, and micro-scratches that have entered the matrix by grinding can be reduced or repaired. The grinding method is not particularly limited. Furthermore, the fired composite particles can be classified to adjust the desired particle size.

  The composite particles constituting the negative electrode material of the present invention contain 5 to 30% by volume of active material particles composed of silicon particles or silicon particles coated with carbon in the particles. The content of the active material particles (percentage of the volume of the active material particles to the volume of the composite particles) is preferably 5 to 20% by volume, more preferably 5 to 15% by volume, and 5 to 10% by volume It is further preferred that When the content of the silicon active material is less than 5% by volume, the improvement effect of the negative electrode capacity may be reduced. In addition, when the content of the silicon active material is greater than 30% by volume, the portion of the composite particle not covered with the matrix but not the active material particle is increased, and the strength of the composite particle is decreased. Expansion and contraction may cause collapse of the composite particles, and furthermore, collapse of the electrode, which may deteriorate the capacity retention rate.

The content (volume%) of the active material particles in the composite particles can be approximated by the cross-sectional area percentage of the active material particle cross section with respect to the total cross-sectional area of the composite particle cross section when the observation area is sufficiently large. In the present invention, when the cross section of the composite particle is observed for 4000 μm ² or more, the total of the cross sections of the active material particle cross section present in the composite particle cross section to the total (S _c ) of the cross sections of the observed composite particle The percentage of S _a ), that is, the value of (S _a / S _c ) × 100 (%) is regarded as the content (volume%) of the active material particles in the composite particles. Specifically, the composite particle is observed by an electron microscope of the cross section of the cross section of the sample taken out with an ion milling device, and the area of the cross section of the active material particle is extracted from the observation image by image processing software etc. The content of the active material particles can be determined by calculating the percentage of the area of the extraction portion to the area. When the matrix is made of SiOC, silicon particles and silicon particles coated with carbon are observed as white contrast on the screen of the reflection electron image mode of the SEM.

  The number of active material particles contained in each composite particle is not particularly limited, but it is preferable to include a plurality of active material particles.

It is preferable that the average particle diameter (number average particle diameter) of the composite particle which comprises the negative electrode material of this invention is 0.5-50 micrometers. The average particle diameter of the composite particles is more preferably 0.5 to 30 μm, and still more preferably 0.5 to 20 μm. When the average particle diameter of the composite particles is smaller than 0.5 μm, the area of the grinding surface on which the active material particles are exposed increases, so the strength of the composite particles tends to decrease and the capacity retention ratio tends to deteriorate. In addition, when the average particle diameter of the composite particles is larger than 50 μm, the coating uniformity tends to be lowered due to streaks, curling and the like when applying the negative electrode paste containing the composite particles to the current collector. In addition, the average particle diameter of the composite particles is identified by using data of an electron micrograph such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM) using image analysis type particle size distribution measurement software, and primary particles ( The projected area of the composite particle can be evaluated as the equivalent circle diameter. Specifically, primary particles included in a plurality of particle photographs taken by SEM are recognized using image analysis type particle size distribution measurement software, and individual particle diameters are evaluated as the projected area circle equivalent diameter of the primary particles. The number average particle diameter, particle size distribution, etc. can be calculated from the particle diameter data obtained from the primary particle projected area total of 0.25 mm ² . Examples of the image analysis type particle size distribution measurement software include "Mac-VIEW" manufactured by Mountech Co., Ltd., "A Image Kun" manufactured by Asahi Kasei Engineering Corporation, and the like.

  The paste for a lithium ion secondary battery negative electrode (hereinafter sometimes simply referred to as “paste for negative electrode”) of the present invention contains the above-mentioned negative electrode material, a binder resin, and a solvent. The paste for the negative electrode of the present invention can be obtained by mixing the binder resin with a solvent and adjusting to an appropriate viscosity, and then adding the negative electrode material of the present invention, a conductive auxiliary agent etc. if necessary, and thoroughly kneading.

  The binder resin is not particularly limited, and thermoplastic resins such as polytetrafluoroethylene, polyvinylidene fluoride (PVdF), polyethylene, polypropylene and the like, rubbers such as styrene butadiene rubber (SBR), nitrile butadiene rubber, fluororubber and the like are used. Or mixtures of one or more of polymers having polysaccharides, polysaccharides such as carboxymethylcellulose, polyimide precursor and / or polyimide resin, polyamideimide resin, polyamide resin, polyacrylic acid, sodium polyacrylate, acrylic resin, polyacrylonitrile, etc. It can be used as

  The solvent is not particularly limited, and water, N-methylpyrrolidone, diacetone alcohol, glycol derivatives represented by propylene glycol monomethyl ether acetate, various alcohols, various ketones, various ethers, esters, etc. are used. be able to. The solvent may be used as a mixture of two or more for the purpose of improving the solubility of the binder in the solvent and the coating property of the negative electrode paste.

  The conductive aid is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance, and carbon blacks such as furnace black, ketjen black, acetylene black, etc., graphite such as natural graphite, artificial graphite, and graphene And conductive materials such as carbon fibers and metal fibers, and conductive powders such as metal powders such as copper, nickel, aluminum and silver.

  The conditions and method for producing the paste for the negative electrode of the present invention are not particularly limited, but in kneading, a revolution mixer is used, or media dispersion such as a bead mill or ball mill is performed, or three rolls are used uniformly. It is preferable to disperse. Moreover, the order which mixes the above-mentioned negative electrode material, binder resin, a solvent, etc. is not specifically limited.

  The negative electrode for a lithium ion secondary battery of the present invention contains the above-described negative electrode material, and can be typically obtained by applying the negative electrode paste of the present invention to a current collector and drying it.

  As the current collector, metal foils such as aluminum foil, nickel foil, titanium foil, copper foil, stainless steel foil and the like can be used, and copper foil and aluminum foil are most widely used.

  In order to apply the paste for lithium ion secondary battery negative electrode of the present invention to a metal foil, methods such as blade coating, spin coating, roll coating, slit die coating, spray coating, dip coating, screen printing and the like can be used. The application is carried out on one side or both sides. When applying on both sides, first apply one side and dry the solvent at a temperature of 50 to 400 ° C for 1 minute to 20 hours in air, in vacuum, or in an inert gas atmosphere such as nitrogen or argon Although it is common to apply and dry on the reverse side, it is also possible to apply on both sides simultaneously by a method such as roll coating or slit die coating. The coating thickness of the negative electrode paste is preferably 1 to 500 μm.

  For example, in the case of using a polyimide precursor as a binder resin, after coating, heat treatment may be performed at 100 to 500 ° C. for 1 minute to 24 hours to convert the polyimide precursor to polyimide, whereby a reliable negative electrode can be obtained. . The heat treatment conditions are preferably at 200 to 450 ° C. for 30 minutes to 20 hours. The heat treatment is preferably performed in an inert gas such as nitrogen gas or in vacuum in order to suppress the mixing of water.

  The obtained electrode can also be pressed by, for example, a roll press to control thickness and volume density.

  EXAMPLES Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited by these examples.

Example 1
[Preparation of Polysiloxane Solution (PS-1)]
73.31 g of diphenyldimethoxysilane (0.3 mol), 22.23 g of vinyltrimethoxysilane (0.15 mol), 12.32 g of 3- (3,4-epoxycyclohexyl) propyltrimethoxysilane (0. 2). 05 mol), 69.41 g of phenyltrimethoxysilane (0.35 mol), 35.15 g of 3-acryloxypropyltrimethoxysilane (Acryl-TMS, 0.15 mol) and 44.96 g of ethyl acetoacetate, Into a three-necked flask. While stirring the solution at room temperature, an aqueous phosphoric acid solution in which 1.0 g of phosphoric acid was dissolved in 54.0 g of water was added over 30 minutes. Then, the flask was immersed in an oil bath at 40 ° C. and stirred for 30 minutes, then the oil bath was set to 80 ° C. and heated for 30 minutes, and the oil bath was further heated to 120 ° C. The reaction was terminated 3 hours after the start of heating. Methanol generated during the reaction and unconsumed water were removed by distillation. Ethyl acetoacetate was added such that a solution of the obtained polysiloxane in ethyl acetoacetate had a polymer concentration of 45.4% by mass, to obtain a polysiloxane solution (PS-1).

[Preparation of Active Material Particles]
The silicon oxide on the surface of the silicon particles synthesized by the high frequency thermal plasma method and subjected to gradual oxidation treatment was reduced under the conditions of a hydrogen atmosphere of 40 volume% hydrogen and a reduction temperature of 700 ° C. to obtain silicon particles without an oxide film. . Subsequently, the surface of silicon particles was coated with pyrolytic carbon at a processing temperature of 1000 ° C. using methane: nitrogen = 1: 1 as a source gas. The active material particles obtained in this manner are observed by SEM, and from the obtained image, the active material contained in the SEM image is obtained using the image analysis type particle size distribution measurement software (Mac-VIEW, manufactured by Mountech Co., Ltd.) Each particle diameter was measured as a projected area circle equivalent diameter of particles, and a number average particle diameter was calculated from data of particle diameter obtained from a total of 10 μm ² as an active material particle projected area. In addition, the thickness of the coating layer formed on the surface of the silicon particles was measured by a transmission electron microscope. As a result, active material particles having an average particle diameter of 70 nm were obtained, in which the surface of silicon particles having an average particle diameter of 64 nm was coated with a coating layer consisting of carbon of 3 nm. [Preparation of Composite Particles]
0.9 parts by weight of the active material particles prepared as described above, 23.7 parts by weight of the polysiloxane solution (PS-1), and 75.3 parts by weight of propylene glycol monomethyl ether acetate are mixed and stirred to obtain a suspension The Thereafter, the solvent was removed by vacuum drying at 200 ° C. for 2 hours.

The dried product was placed in a lump in an alumina container with a lid, and firing was performed in a nitrogen atmosphere at 600 ° C. for 2 hours in a temperature controlled box furnace capable of controlling the atmosphere. Then, after sufficiently cooling and putting in an automatic mortar, it was fired again under the condition of 1000 ° C. for 1 hour under argon atmosphere and sieved. The composite particles (negative electrode material) obtained in this manner are observed by SEM, and the obtained image is included in the SEM image using image analysis type particle size distribution measurement software (Mac-VIEW, manufactured by Mountech Co., Ltd.) The individual particle diameters were measured as the projected area circle equivalent diameter of the composite particles to be used, and the number average particle diameter was calculated from the data of particle diameters obtained from a total of 0.25 mm ² as the composite particle projected area. It was 10 um.

[Fabrication of negative electrode]
90 parts by weight of the obtained negative electrode material, 121 parts by weight of an aqueous solution of sodium carboxymethylcellulose (Dacel, CMC 2200) having a solid concentration of 1% by mass, and an SBR aqueous dispersion having a solid concentration of 48% by mass (JSR, TRD-2001 A slurry paste was obtained by adding and stirring 8 parts by weight, 5 parts by weight of acetylene black as a conductive additive, and an appropriate amount of water. The obtained paste was applied onto an electrolytic copper foil using a doctor blade, and dried at 80 ° C. for 30 minutes to form an electrode. Furthermore, the coated part of this electrode was punched into a circle having a diameter of 16 mm, and vacuum drying was performed at 80 ° C. for 12 hours to produce a negative electrode.

[Measurement of content of active material particles in composite particles]
The produced negative electrode was treated with an ion milling apparatus (IM 4000, manufactured by Hitachi High-Technologies Corporation) to cut out a cross section. This sample is cross-sectionally observed using an electron microscope (S-5500, manufactured by Hitachi High-Technologies Corporation), and the observed image is an active material in composite particles using image processing software ImageJ (manufactured by the National Institutes of Health). A black and white binarization process was performed on the particle part and the other part. A total of 4000 μm ² is observed and image-processed as the composite particle cross sectional area, and the ratio of the total (white portion) of the cross section of the active material particle to the total of the cross section of the composite particle (sum of black and white portions) The content (volume%) of the active material particles was calculated.

[Measurement of porosity of composite particles]
The produced negative electrode was treated with an ion milling apparatus (IM 4000, manufactured by Hitachi High-Technologies Corporation), and a cross section was taken out. This sample is cross-sectionally observed using an electron microscope (S-5500, manufactured by Hitachi High-Technologies Corporation), and an image with a magnification of 5000 is obtained from the composite particles using the image processing software ImageJ (manufactured by the National Institutes of Health). A black and white binarization process was performed on the void portion and the other portion. Ten cross-sectional observation images were processed, and when the porosity was calculated from the average value of the cross-sectional area of the void portion to the average value of the cross-sectional area of the composite particles, it was 0.8%.

[Preparation of coin-type lithium secondary battery and evaluation of electrode characteristics]
Metal lithium was used as a counter electrode with the negative electrode produced as mentioned above. As an electrolytic solution, an electrolytic solution in which 1 M LiPF ₆ was added to a mixed solvent of ethylene carbonate: dimethyl carbonate = 7: 3 (volume ratio) and 10% by mass of fluoroethylene carbonate with respect to the solvent was used. Moreover, the separator manufactured the coin-type lithium secondary battery (half cell) using Celgard # 2400 (made by Celguard) cut out to diameter 17 mm.

  Charging was performed at a constant current corresponding to 0.1 C to the negative electrode until the voltage of the half cell reached 20 mV, and after reaching 20 mV, charging was performed by reducing the current at a constant voltage. And charge was ended when current fell below 0.05C. The discharge was performed at a constant current corresponding to 0.1 C to the negative electrode until the voltage of the half cell reached 1.5 V, and the discharge was ended when the voltage reached 1.5 V. The charge and discharge under the above conditions were repeated two cycles.

The half cell was disassembled using a coin cell disassembling machine, the negative electrode was recovered, and reassembly into a coin-type lithium secondary battery (full cell) was performed. A positive electrode composed of 100 parts by weight of lithium cobaltate, 5 parts by weight of acetylene black, and 3 parts by weight of polyvinylidene fluoride was used as a counter electrode together with the recovered negative electrode. As an electrolytic solution, an electrolytic solution in which 1 M LiPF ₆ was added to a mixed solvent of ethylene carbonate: dimethyl carbonate = 7: 3 (volume ratio) and 10% by mass of fluoroethylene carbonate with respect to the solvent was used. Moreover, the separator manufactured the coin-type lithium secondary battery (full cell) using Celgard # 2400 (made by Celguard) cut out to diameter 17 mm.

  Charging was performed at a constant current equivalent to 0.1 C with respect to the negative electrode until the voltage of the full cell reached 4.18 V, and after reaching 4.18 V, charging was performed by reducing the current at a constant voltage. Then, charging was terminated when the current dropped below 0.05 C, and the capacity up to here was used as the charging capacity. Discharge was performed at a constant current equivalent to 0.1 C with respect to the negative electrode until the voltage of the full cell reached 2.5 V, and when the voltage reached 2.5 V, the discharge was terminated, and the capacity up to here was used as the discharge capacity.

  The charge / discharge measurement was performed for 100 cycles, and the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle (first time) in the full cell was calculated as a capacity retention ratio (%).

Example 2
In preparation of composite particles, 1.4 parts by weight of active material particles, 22.6 parts by weight of polysiloxane solution (PS-1) and 76.0 parts by weight of propylene glycol monomethyl ether acetate Each electrode characteristic was evaluated in the same manner as in Example 1 except that it was changed to a part.

[Example 3]
In preparation of composite particles, 1.9 parts by weight of carbon-coated silicon powder, 21.5 parts by weight of polysiloxane solution (PS-1), and 76.6 parts of propylene glycol monomethyl ether acetate Each electrode characteristic was evaluated in the same manner as in Example 1 except that it was changed to parts by weight.

Example 4
Each electrode was prepared in the same manner as in Example 1 except that the negative electrode material was prepared using active material particles in which the surface of silicon particles having an average particle diameter of 144 nm was coated with a 3 nm carbon coating layer as in Example 1. The characteristics were evaluated.

[Example 5]
Each electrode was prepared in the same manner as in Example 1 except that the negative electrode material was prepared using active material particles in which the surface of silicon particles having an average particle diameter of 94 nm was coated with a 3 nm carbon coating layer as in Example 1. The characteristics were evaluated.

[Example 6]
Each electrode was prepared in the same manner as in Example 1 except that the negative electrode material was prepared using active material particles in which the surface of silicon particles having an average particle diameter of 34 nm was coated with a 3 nm carbon coating layer as in Example 1. The characteristics were evaluated.

[Example 7]
Each electrode was prepared in the same manner as in Example 1 except that the negative electrode material was prepared using active material particles in which the surface of silicon particles having an average particle diameter of 27 nm was coated with a 3 nm carbon coating layer as in Example 1. The characteristics were evaluated.

[Example 8]
Each electrode was prepared in the same manner as in Example 1 except that the negative electrode material was prepared using active material particles in which the surface of silicon particles having an average particle diameter of 9 nm was coated with a 3 nm carbon coating layer as in Example 1. The characteristics were evaluated.

[Example 9]
Each electrode characteristic was evaluated like Example 1 except having produced an anode material using active material particles which consist of silicon particles whose average particle diameter is 64 nm.

Comparative Example 1
In preparation of the negative electrode material, 4.0 parts by weight of active material particles, 16.7 parts by weight of polysiloxane solution (PS-1), and 79.4 parts by weight of propylene glycol monomethyl ether acetate Each electrode characteristic was evaluated in the same manner as in Example 1 except that it was changed to a part. The negative electrode material thus obtained had incomplete coverage with the matrix material, a large amount of exposed carbon-coated silicon powder, and the porosity could not be measured.

Comparative Example 2
As a raw material of the negative electrode material, 0.9 parts by weight of active material particles as in Example 1 and 99.1 parts by weight of a glucose solution having a solid content concentration of 42.9% were mixed and stirred. The glucose aqueous solution was prepared by dissolving 16.7 parts by weight of D (+)-glucose (Wako Pure Chemical Industries, Ltd.) in 22.2 parts by weight of pure water. Thereafter, the solvent was removed by vacuum drying at 200 ° C. for 2 hours.

  The dried product was put into a lump and put into an alumina container with a lid, and firing was carried out in an argon atmosphere in a box furnace with temperature control capable of atmosphere control at 800 ° C. for 2 hours. The mixture was sufficiently cooled, placed in an automatic mortar and sieved to obtain composite particles having an average particle diameter of 10 um. The content of silicon in the negative electrode material was 10%. The electrode characteristics were evaluated in the same manner as in Example 1 except for the preparation of the negative electrode material.

  The structures of the negative electrode materials prepared in Examples and Comparative Examples and the evaluation results of the battery characteristics of lithium ion secondary batteries using the respective negative electrode materials are shown in Table 1.

Claims (7)

Silicon oxycarbide (SiOC) active material particles composed of silicon particles coated with carbon-containing in a matrix comprising a ceramic containing 5 vol% to 30 vol%, the lithium ion secondary porosity is composed of a composite particle is less than 10% Negative electrode material for next battery.   The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the composite particles contain a plurality of the active material particles.   The negative electrode material for a lithium ion secondary battery according to claim 1, wherein an average particle diameter of the active material particles is 10 nm to 120 nm. The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3 , wherein the composite particles have an average particle size of 0.5 μm to 50 μm. The paste for lithium ion secondary battery negative electrodes containing the negative electrode material for lithium ion secondary batteries in any one of Claims 1-4 , binder resin, and a solvent. The negative electrode for lithium ion secondary batteries containing the negative electrode material for lithium ion secondary batteries in any one of Claims 1-4 . A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 6 .