JP2019067579A - Lithium ion secondary battery and negative electrode material for lithium ion secondary battery - Google Patents

Abstract

To provide a lithium ion secondary battery having long life and a negative electrode material used for the same.SOLUTION: A negative electrode material used for a negative electrode of a lithium ion secondary battery includes: composite particles having a structure where scale-like silicon particles are dispersed in a carbon material; and graphite-based particles. The carbon material is amorphous carbon or nano graphene, and the content of the composite particles with respect to the total amount of the composite particles and the graphite-based particles is 2 mass% or more and 50 mass% or less.SELECTED DRAWING: Figure 2A

Description

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

  From the viewpoint of environmental protection, energy saving, etc., a hybrid vehicle using an engine and a motor together as a power source has been developed and commercialized. Further, in the future, development of a fuel cell hybrid vehicle that uses a fuel cell instead of an engine is also popular.

  A secondary battery capable of repeatedly charging and discharging electricity as an energy source of this hybrid vehicle is an essential technology. Among them, a lithium ion secondary battery is a battery having a feature of high energy density, which has a high operating voltage and can easily obtain a high output, and in the future, it is becoming more and more important as a power source of electric vehicles. In the application to electric vehicles, it is regarded as important as a battery that the further high capacity and high output are required and the long life is long. Battery design to realize these items is important.

  Graphite-based carbon materials are widely used as negative electrode active materials of lithium ion secondary batteries. However, although the high capacity is desired as described above, in the carbon-based negative electrode, since the theoretical capacity of carbon is 372 mAh, it is impossible to desire more capacity.

  Therefore, application of various high theoretical capacity substances other than carbon, particularly metal particles, to the negative electrode has been studied.

  For example, Patent Document 1 discloses a technique using an electrode for a non-aqueous electrolyte secondary battery having graphite, amorphous carbon and silicon oxide as an electrode active material, for the purpose of increasing the discharge capacity.

  Furthermore, Patent Document 2 discloses a carbon material for a non-aqueous secondary battery negative electrode containing at least three types of carbon materials, which is a composite carbon particle containing silicon element, a natural graphite particle, and an artificial graphite particle. The negative electrode using the carbon material for non-aqueous secondary battery negative electrodes to contain is disclosed.

Unexamined-Japanese-Patent No. 2010-092834 JP, 2015-164127, A

  Silicon is an attractive material that has a theoretical capacity of 4197 mAh and can store 11.3 times more lithium than graphite.

  However, when the silicon particles are filled with lithium ions, the volume expands to about 3.1 times, so the silicon particles are mechanically broken while repeating the lithium ion filling and discharging. The resulting fine silicon particles are electrically isolated. In addition, a new electrochemical coating layer is formed on the fracture surface, which increases the irreversible capacity. There is room for improvement in that these phenomena at the negative electrode significantly reduce the life characteristics of the battery.

  An object of the present invention is to provide a lithium ion secondary battery having a long life and an anode material used therefor.

  The present invention is a negative electrode material for use in a negative electrode of a lithium ion secondary battery, comprising composite particles having a configuration in which scaly silicon particles are dispersed in a carbon material, and graphite-based particles, wherein the carbon material is amorphous It is carbon or nanographene, and the content of the composite particles is 2% by mass or more and 50% by mass or less based on the total amount of the composite particles and the graphite-based particles.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode material for lithium ion secondary batteries which has a long life can be provided.

It is a graph which shows the result of having measured distribution of pore volume about the composite particle of Example 1. FIG. It is a graph which shows distribution of the integral pore volume corresponding to FIG. 1A. FIG. 2 is a view schematically showing a composite particle of Example 1. FIG. 2B is an enlarged cross-sectional view of the nanographene coated scaly silicon particles of FIG. 2A. 3 is a scanning electron microscope image of the composite particles of Example 1. It is the scanning electron microscope image to which the smooth cross section of the composite particle of Example 1 was expanded. It is a schematic block diagram which shows the vapor phase growth apparatus of an Example. It is a schematic diagram which shows the cross-section of the negative electrode which concerns on one Embodiment of this invention. It is a schematic cross section which shows the internal structure of the lithium ion secondary battery which concerns on one Embodiment of this invention. 5 schematically shows a composite particle of Example 2. FIG. FIG. 7B is an enlarged cross-sectional view of the nanographene coated scaly silicon particles of FIG. 7A. 5 is a scanning electron microscope image of the composite particles of Example 2. 7 is a scanning electron microscope image in which the smooth cross section of the composite particle of Example 2 is enlarged.

  The negative electrode used for a lithium ion secondary battery is formed by apply | coating negative electrode compound material containing negative electrode material and a binder on negative electrode collectors, such as copper foil. A conductive material may be further added to the negative electrode mixture in order to reduce the electrical resistance.

  The negative electrode material of the present invention includes composite particles (hereinafter, also simply referred to as “composite particles”) having a configuration in which scaly silicon particles are dispersed in a carbon material, and graphite-based particles. In other words, graphite-based particles and composite particles of carbon and silicon are mixed and used.

  As the graphite-based particles, natural graphite, a composite carbonaceous material in which a film formed by natural chemical graphite by dry CVD (Chemical Vapor Deposition) method or wet spraying method is formed, resin raw material such as epoxy or phenol, or petroleum or coal The artificial graphite etc. which are manufactured by baking as a raw material the pitch type | system | group material obtained from these can be used. As the graphite-based particles, a material having a wide surface spacing d 002 of the carbon 002 plane is preferable because it is excellent in rapid charge / discharge and low temperature characteristics.

  Furthermore, it is desirable that the graphite-based particles be a graphite material in which the spacing d002 of the carbon 002 plane is 0.3345 nm or more and less than 0.338 nm.

  Moreover, in order to constitute an electrode, a highly conductive carbonaceous material such as graphitic, amorphous or activated carbon may be mixed.

  Moreover, you may use the material which has the characteristic of following (1)-(3) as graphite type particle | grains.

(1) the intensity ratio of the peak intensity in the range of 1300～1400Cm ^-1 as measured by Raman spectroscopy (ID) and the peak intensity in the range of 1580～1620Cm ^-1 as measured by Raman spectroscopy spectra (IG) in it R value (ID / IG) is 0.2 to 0.4 (2) half-value width Δ value of the peak in the range of 1300～1400Cm ^-1 as measured by Raman spectroscopy spectra is 40 cm ^-1 or more 100 cm ^{- 1} or less (3) The intensity ratio X value (I (110) / I (I (110)) of the peak intensity (I (110)) of the (110) plane to the peak intensity (I (004)) of the (004) plane) in X-ray diffraction 004)) 0.1 or more and 0.45 or less Further, the composite particles can prevent mechanical destruction associated with the filling and release of lithium ions by nanoizing the silicon particles.

  However, there is a problem that part of the silicon nanoparticles is electrically isolated due to the volume change caused by the filling and releasing of lithium ions, which causes the life characteristics to be largely deteriorated.

  Therefore, by using scaly silicon particles and reducing the direction of expansion and contraction, mechanical destruction is prevented.

  For example, the major axis of the scaly silicon particles may have an average value of 10 nm or more and 1 μm or less. If the thickness is less than 10 nm, agglomeration of silicon particles is severe, making it difficult to produce a negative electrode material having a desired structure. Moreover, when it exceeds 1 μm, the possibility of breakage is high due to mechanical expansion at the time of lithium storage.

  On the other hand, the thickness of the scaly silicon particles is preferably 5 nm or more and 100 nm or less on average. If it is less than 5 nm, the mechanical strength is weak, and there is a high possibility of micronization at the time of battery preparation. Moreover, when it exceeds 100 nm, there is a high possibility of breakage due to mechanical expansion during lithium storage.

However, since silicon particles with such a shape inherently have a large specific surface area of several tens of m ² / g or more, the amount of electrochemical film formation is proportional to that and the coulombic efficiency is low, and expansion and contraction are caused. It was not possible to achieve both the mechanical destruction control and the film suppression by the electrolytic solution.

  Therefore, by embedding the silicon particles in the carbon material, the exposed surface area is reduced, the reaction with the electrolytic solution is suppressed, and the deterioration of the active material is suppressed. For example, nanographene or amorphous carbon is used to disperse scaly silicon in such a carbon material.

From the viewpoint of suppressing deterioration of the active material, the specific surface area of the composite particles is desirably 0.1 m ² / g or more and 40 m ² / g or less.

  As the carbon material used for the composite particles, for example, it is desirable that the energy loss image in the STEM-ADF measurement of the cross section exhibits an energy loss distribution of 90 to 150 eV L-Si end and 280 to 400 eV C—K end.

  Moreover, expansion and contraction of silicon can be alleviated by the carbon material by using composite particles. For example, it is desirable that the composite particles have 80% or more of pores in the range of 100 nm or more and 800 nm or less in pore diameter within the range of 0.01 to 1 μm in pore diameter.

  The content of carbon in the composite particles is desirably 20% by mass to 70% by mass, and more desirably 30% by mass to 60% by mass. The content of carbon in the composite particles can be measured by high frequency combustion infrared absorption method.

  The binder (binder resin) is not particularly limited. For example, polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, polyamide imide, polyimide, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC) ), Polyalginic acid, polyacrylic acid, etc. can be used.

(About the mixing amount of composite particles)
The negative electrode material includes composite particles and graphite-based particles. Here, the negative electrode material does not contain a binder.

  The mixing ratio of the composite particles to the graphite-based particles is preferably 2:98 to 50:50 on a mass basis. In other words, the content of the composite particles relative to the total amount of the composite particles and the graphite-based particles is 2% by mass or more and 50% by mass or less.

  If the content of the composite particles is less than 2% by mass, the composite particles of carbon and silicon are initially charged but are not discharged thereafter, so that the capacity increase due to the addition of the composite particles of carbon and silicon is not observed. On the other hand, when the content of the composite particles exceeds 50% by mass, the binding property of the electrode is reduced, and the electrode is exfoliated at the electrode preparation stage.

  For this reason, from the viewpoint of high capacity and electrode production, the mixing amount (content) of the composite particles should be set in the range of 2% by mass to 50% by mass based on the total amount of the composite particles and the graphite-based particles. Is desirable.

(About particle size)
The composite particles can be pulverized and classified to produce particles having any average particle size and particle size distribution. The average particle diameter of the composite particles is desirably 5 μm or more and 30 μm or less. In order to process to an average particle size of less than 5 μm, the manufacturing cost becomes high. Moreover, when it exceeds 30 micrometers, it is too large and electrode formation by slurry application is difficult.

(Positive electrode)
The positive electrode is manufactured by applying a positive electrode mixture containing a positive electrode active material, an electron conductive material, and a binder to the surface of an aluminum foil as a current collector. In addition, a conductive material may be further added to the positive electrode mixture in order to reduce the electrical resistance.

The positive electrode active material is preferably a lithium composite oxide represented by the composition formula Li _α Mn _x M ¹ _y M ² _z O ₂ . Here, M ¹ is at least one of Co and Ni, and M ² is at least one selected from the group consisting of Co, Ni, Al, B, Fe, Mg, and Cr. Further, x + y + z = 1, 0 <α <1.2, 0.2 ≦ x ≦ 0.6, 0.2 ≦ y ≦ 0.4, and 0.05 ≦ z ≦ 0.4 are satisfied. In addition, among those in which the above composition formula holds, it is more preferable that M ¹ be Ni or Co and M ² be Co or Ni.

  In the composition, if Ni is increased, the capacity can be increased, and if Co is increased, the output at low temperature can be improved. In addition, if the amount of Mn is increased, the material cost can be suppressed. In addition, the additive element is effective in stabilizing the cycle characteristics.

Examples of other positive electrode active materials include general formulas LiM _a PO ₄ (M: Fe or Mn, 0.01 ≦ a ≦ 0.4) and LiMn _1-a M _a PO ₄ (M: bivalent other than Mn) The cation may be an orthorhombic phosphate compound having the symmetry of the space group Pmnb in which 0.01 ≦ a ≦ 0.4).

  The binder (binder) may be any one that brings the material constituting the positive electrode into close contact with the current collector for the positive electrode, and, for example, homopolymers or copolymers of vinylidene fluoride, tetrafluoroethylene, acrylonitrile, ethylene oxide, etc. Examples include coalescence, styrene-butadiene rubber and the like. The conductive material is, for example, a carbon material such as carbon black, graphite, carbon fiber, metal carbide or the like, and may be used alone or in combination.

(Separator)
As a separator which prevents a short circuit with a positive electrode and a negative electrode, the separator used for the well-known lithium ion battery can be used, The microporous film made from polyolefins, such as polyethylene and a polypropylene, a nonwoven fabric, etc. are mentioned. From the viewpoint of increasing the output of the battery, the thickness of the separator is preferably 20 μm or less, and more preferably 18 μm or less. By using a separator of such a thickness, the capacity per volume of the battery can be increased. However, if the separator is too thin, the handleability may be impaired, or the separation between the positive and negative electrodes may be insufficient to easily cause a short circuit. Therefore, the lower limit of the thickness is 10 μm.

(Electrolyte solution)
As the electrolytic solution, one in which a compound containing lithium ion is used as an electrolyte and this is dissolved in a non-aqueous solvent is used. Examples of the non-aqueous solvent include ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like from the viewpoint of low temperature characteristics and film formation on the negative electrode surface. Then, an additive such as vinylene carbonate (VC) may be added to this.

The lithium salt used in the electrolytic solution is not particularly limited, but as the inorganic lithium salt, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiI, LiCl, LiBr and the like can be used. Also, the organic lithium _{salt, LiB [OCOCF 3] 4,} LiB [OCOCF 2 CF 3] 4, LiPF 4 (CF 3) 2, LiN (SO 2 CF 3) 2, LiN (SO 2 CF 2 CF 3) 2 Etc. can be used. In particular, LiPF ₆ is a preferable material from the viewpoint of quality stability, and is widely used in consumer batteries. In addition, LiB [OCOCF ₃ ] ₄ is suitable in terms of dissociativeness and solubility, and is useful because it exhibits high conductivity at low concentrations.

  Hereinafter, examples of the present invention will be described. The following description shows specific examples of the content of the present invention, and the present invention is not limited to these descriptions, and various modifications by those skilled in the art may be made within the scope of the technical concept disclosed herein. Changes and modifications are possible.

  FIG. 1A is a graph showing the results of measurement of pore volume distribution for the composite particles of this example. The abscissa represents the pore diameter (pore diameter), and the ordinate represents the pore volume.

  As shown to this figure, the composite particle of a present Example has a peak in 0.2 micrometer of pore diameters vicinity. Moreover, there are also many voids having a pore diameter of 10 μm or more.

  FIG. 1B is a graph showing the distribution of integrated pore volume corresponding to FIG. 1A. The abscissa represents the pore diameter, and the ordinate represents the integrated pore volume.

  As shown in FIG. 1B, in the range of pore diameters of 0.01 to 1 μm, the pores having a pore diameter of 100 nm or more and 800 nm or less have pores of 80% or more based on the pore volume. These pores serve as a cushioning effect to ease expansion and contraction during cycling, making the surface coating less likely to break.

  FIG. 2A is a view schematically showing a composite particle of the present example.

  The composite particle shown in this figure has a structure in which nanographene coated scaly silicon particles 101 are embedded in nanographene 102. In other words, the nanographene coated scaly silicon particles 101 are dispersed in the nanographene 102 (carbon material) to form a particle shape as a whole.

  The major axes of the nanographene-coated scale-like silicon particles 101 are 10 nm or more and 1 μm or less on average. When the major diameter is less than 10 nm, the aggregation of silicon particles is severe, and it is difficult to produce composite particles having a desired structure. In addition, when the major axis exceeds 1 μm, there is a high possibility of breakage due to mechanical expansion at the time of lithium storage.

  The thickness of the nanographene-coated scale-like silicon particle 101 is preferably 5 nm or more and 200 nm or less before being coated with nanographene, and more preferably 5 nm or more and 100 nm or less. If the thickness is less than 5 nm, the mechanical strength is weak and there is a high possibility of micronization when producing the battery. In addition, when the thickness exceeds 200 nm, the possibility of breakage due to mechanical expansion at the time of lithium absorption is high.

  FIG. 2B is an enlarged cross-sectional view of the nanographene coated scaly silicon particles of FIG. 2A.

  The nanographene coated scaly silicon particles 101 have a structure in which the surface of the scaly silicon particles 103 is covered with the nanographene layer 104.

  The thickness of the nanographene layer 104 is preferably 2 nm or more and 10 nm or less. If the thickness is less than 2 nm, it is too thin and mechanical strength is insufficient. In addition, when the thickness is more than 10 nm, the nanographene layer is not arranged along the surface shape, and the original purpose of enhancing the electrical conductivity can not be achieved. For the preparation of the nanographene layer 104, a vapor deposition method using propylene gas was used. Details will be described later.

  The same vapor phase growth method using propylene gas was also used to produce a structure in which nanographene-coated scale-like silicon particles 101 were embedded in nanographene 102. Details will be described later.

  Here, graphene and nanographene are defined.

  Graphene refers to an assembly (molecule) of monolayers in which carbon atoms are densely packed in a two-dimensional honeycomb lattice. And, nanographene refers to graphene having a dimension of nanometer order. In the present specification, nanographene more specifically refers to a disk having a diameter of 100 nm or less, which is a typical dimension in a case where it is assumed that molecules constituting graphene are disk-like.

(Method of producing composite particles of carbon and silicon)
The composite particles of this example were produced by the following method.

  First, nanographene coated scaly silicon particles were obtained by covering the surface of scaly silicon particles with nanographene using a vapor phase growth method using propylene gas. Thereafter, in order to form nanographene-coated scale-like silicon particles embedded in nanographene, a similar vapor deposition method was used.

  Next, an apparatus for producing nanographene will be described.

  FIG. 4 is a schematic block diagram showing a chemical vapor deposition apparatus used for producing nanographene.

  The chemical vapor deposition apparatus shown in this figure is capable of producing a nanographene covering layer and a nanographene embedded layer by thermal vapor deposition. The film thickness (the amount of nanographene to be stacked) can be controlled by the growth time or the like.

  In the figure, the chemical vapor deposition apparatus includes a reaction furnace 401, piping for supplying propylene, argon, helium and the like to the reaction furnace 401, mass flow meters 403a, 403b and 403c attached to each piping, and plugs 404a, 404b and 404c. ,including. The sample boat 402 can be installed in the reaction furnace 401.

  First, an example of a method for producing a nanographene covering layer on the surface of scaly silicon particles will be described.

  The scaly silicon particles were placed in a sample boat 402 and placed near the center of the reaction furnace 401. The reaction furnace 401 is made of quartz and has a diameter of 5 cm and a length of 40 cm.

  Then, using the hydrogen line in the figure, hydrogen gas was flowed at a flow rate of 200 mL / min, the temperature of the reaction furnace 401 was raised from room temperature to 1000 ° C. at a rate of 10 ° C./min, and was further held at 1000 ° C. for 1 hour . By this heat treatment step, it is possible to reduce the natural oxide film formed on the surface of the scaly silicon particles.

  Thereafter, the hydrogen line (plug 404c) was closed, the plug 404b was opened, argon gas was flowed at a flow rate of 200 mL / min, the temperature was decreased at a rate of 10 ° C./min, and the temperature was decreased to 800 ° C.

  When reaching 800 ° C., the plug 404a was opened and propylene gas was introduced at a flow rate of 10 mL / min, and at the same time, the flow rate of argon gas was 190 mL / min to grow a nanographene covering layer for 1 hour.

  Thereafter, the plug 404a is closed to stop the supply of propylene gas. Argon gas was flowed at a flow rate of 200 mL / min, held for 15 minutes, and then naturally cooled.

  The nanographene covering layer produced by the above method has a structure in which nanographene is stacked in multiple layers, and has an electrical conductivity of 1000 S / m or more.

  Next, an example of a method of embedding nanographene-coated scale-like silicon particles in a nanographene embedded layer will be described.

  The nanographene coated scaly silicon particles were placed in a sample boat 402 and placed near the center of the reaction furnace 401.

  Then, argon gas was flowed at a flow rate of 200 mL / min, and the temperature was raised to 800 ° C. at a rate of 10 ° C./min. When reaching 800 ° C., propylene gas was introduced at a flow rate of 30 mL / min, and at the same time, the flow rate of argon gas was 170 mL / min, and a nanographene embedded layer was grown for 4 hours.

  After that, the propylene gas line was closed, argon gas was flowed at a flow rate of 200 mL / min, and after 15 minutes, natural cooling was performed.

  FIG. 3A is a scanning electron microscope image (SEM image) of the composite particle of this example.

  As shown in the figure, the composite particle 301 is an irregularly shaped particle having a particle size of about 100 μm. The fracture surface and the crack which arose by grinding in the case of manufacture can be observed.

  FIG. 3B is an enlarged image of the smooth cross section of the composite particle of this example.

  As shown in the figure, the composite particles have a configuration in which scaly silicon particles 302 are dispersed in a carbon material 303. It can be seen that the scaly silicon particles 302 are embedded in the carbon material 303 fairly densely. In addition, pores 304 also exist inside the composite particles.

  In this figure, it can be seen that the large scale scaly silicon particles 302 have a major axis of 600 nm or less and a thickness of 200 nm or less. The thickness of the scaly silicon particles 302 is mostly 5 nm or more and 100 nm or less. In this figure, although the nanographene covering layer is not clear, it is considered to be present as a thin film on the surface of the scaly silicon particles 302.

  In the present embodiment, the scale-like silicon particles have an average value of 0.2 μm in major axis and 0.01 μm in thickness. These dimensions are measured using SEM images.

(Electrode preparation method)
FIG. 5: is a schematic diagram which shows the cross-section of the negative electrode of an Example.

  In this figure, the negative electrode 105 mixes the graphite particle 106, the composite particle 107 containing carbon and silicon, and a binder (not shown) to form a current collector 108 (current collector foil). The composition applied to Graphite-based particles 106 and composite particles 107 are the main components of the negative electrode material.

  The negative electrode was produced by the following procedure.

  Water was used as a solvent so that the negative electrode material, and styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC) as a binder would have a mass ratio of 96: 2: 2. In other words, the mass ratio of the negative electrode material, SBR to CMC is 96: 2: 2. The ratio of the graphite-based particles 106 to the composite particles 107 was 90:10 on a mass basis.

The mixture of the negative electrode material and the binder as described above was mixed with water to prepare a negative electrode mixture slurry. Then, this negative electrode mixture slurry was applied to a 10 μm-thick current collector 108 and dried in vacuum. The negative electrode 105 obtained by drying was molded by a roll press to a density of 1.5 g / cm ³ .

  Next, the embodiment will be described more specifically.

(Preparation of lithium ion secondary battery)
The wound-type lithium ion secondary battery was manufactured as follows.

-Positive electrode First, layered LiMO ₂ (M represents one or more elements selected from the group consisting of Ni, Co and Mn) as a positive electrode active material, acetylene black as a conductive material, and a binder All were mixed so that polyvinylidene fluoride (PVdF) as was determined to be 93: 4: 3 in mass ratio. Then, N-methylpyrrolidone (NMP) was added as a solvent to this mixture, and the mixture was kneaded to prepare a positive electrode mixture slurry. Then, the positive electrode mixture slurry was applied to a 15 μm thick aluminum foil and dried in the air. The positive electrode obtained by drying was shape | molded by roll press, and the positive electrode was produced.

-Separator As a separator, a separator with a total thickness of 20 μm in which polypropylene, polyethylene and polypropylene were laminated in three layers was used.

Electrolyte As the electrolyte, 1.0 mol / L of an organic solvent in which ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) are mixed at a volume ratio of 1: 2: 2 is obtained. A solution in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved was used. 2% by mass of vinylene carbonate (VC) was added as an additive.

(Production of wound battery)
FIG. 6 is a schematic cross-sectional view showing the internal structure of the lithium ion secondary battery.

  The lithium ion secondary battery 200 shown in this figure includes a positive electrode 210, a separator 211, a negative electrode 212, a battery case 213 (battery can), a positive electrode current collection tab 214, a negative electrode current collection tab 215, an inner lid 216, an internal pressure release valve 217, A gasket 218, a PTC resistance element 219, a battery cover 220 and an axial center 221 are provided. In the battery cover 220, an inner cover 216, an internal pressure release valve 217, a gasket 218 and a PTC resistance element 219 are integrated. The positive electrode 210, the separator 211 and the negative electrode 212 are wound around the axial center 221. Here, PTC is an abbreviation for Positive Temperature Coefficient, and means positive temperature coefficient.

  The separator 211 is inserted between the positive electrode 210 and the negative electrode 212, and an electrode group wound around the axial center 221 is produced. The shaft center 221 may be any known one as long as it can support the positive electrode 210, the separator 211 and the negative electrode 212. In addition to the cylindrical shape shown in FIG. 6, the electrode group may be formed into various shapes, for example, one obtained by laminating strip electrodes, one obtained by winding the positive electrode 210 and the negative electrode 212 into an arbitrary shape such as a flat shape. it can. The shape of the battery case 213 may be a cylindrical shape, a flat oval shape, a flat oval shape, a square shape, or the like in accordance with the shape of the electrode group.

  The material of the battery case 213 is selected from materials having corrosion resistance to the non-aqueous electrolyte, such as aluminum, stainless steel, nickel plated steel, and the like. In addition, when the battery case 213 is electrically connected to the positive electrode 210 or the negative electrode 212, in the portion in contact with the non-aqueous electrolyte, deterioration of the material due to corrosion of the battery case 213 or alloying with lithium ions does not occur. Thus, the material of the battery case 213 is selected.

  The electrode group is housed in the battery case 213, the negative electrode current collection tab 215 is connected to the inner wall of the battery case 213, and the positive electrode current collection tab 214 is connected to the bottom surface of the battery cover 220. The electrolyte is injected into the battery container interior 213 before sealing the battery. As a method of injecting the electrolytic solution, there is a method of adding directly to the electrode group in a state in which the battery cover 220 is released, or a method of adding from an injection port provided on the battery cover 220.

  After that, the battery cover 220 is closely attached to the battery case 213, and the entire battery is sealed. If there is an electrolyte inlet, seal it as well. Well-known techniques such as welding, caulking and the like can be used to seal the battery.

  The following evaluation was performed using the above-mentioned battery.

(Carbon mass ratio measurement of composite particles of carbon and silicon)
Carbon mass ratio measurement of composite particles of carbon and silicon was performed by high frequency combustion infrared absorption method.

(Specific surface area measurement of composite particles of carbon and silicon)
The specific surface area of the composite particles of carbon and silicon was calculated from the measurement results of nitrogen gas adsorption measurement. As the measurement conditions, the adsorption gas is nitrogen (N ₂ ), and the measured relative pressure P / P ₀ is 0 to 1.

(Pore distribution measurement of composite particles of carbon and silicon)
The pore distribution was measured by mercury porosimetry in the range of a measured pore diameter of 0.01 to 10 μm.

(Measurement of void area rate)
The cross-sectional SEM image of the composite particle of carbon and silicon as shown in FIG. 3B was taken, and the image analysis was performed using the SEM image to calculate the ratio of pores occupied by the entire image (pore area ratio).

(Life evaluation method)
The battery was charged to 4.2 V at 100 mA / g constant current constant voltage charge. Thereafter, it was discharged to 3.0 V at a constant current discharge of 100 mA / g. This test was repeated 100 times.

  FIG. 7A is a view schematically showing a composite particle of the present example.

  The composite particles shown in this figure have a structure in which nanographene coated scaly silicon particles 701 are embedded in amorphous carbon 702. In other words, nanographene-coated scale-like silicon particles 701 are dispersed in amorphous carbon 702 (carbon material) to form a particle shape as a whole.

  The major diameter of the nanographene-coated scale-like silicon particle 701 is 10 nm or more and 1 μm or less. When the major diameter is less than 10 nm, the aggregation of silicon particles is severe, and it is difficult to produce an anode material having a desired structure. When the major axis exceeds 1 μm, the possibility of breakage is high due to mechanical expansion at the time of lithium storage.

  The thickness of the nanographene-coated scale-like silicon particle 701 is 5 nm or more and 100 nm or less. If the thickness is less than 5 nm, the mechanical strength is weak and there is a high possibility of micronization when producing the battery. If the thickness exceeds 100 nm, the possibility of breakage is high due to mechanical expansion at the time of lithium storage.

  FIG. 7B is an enlarged sectional view showing the silicon particle of FIG. 7A.

  As shown in FIG. 7B, the nanographene coated scaly silicon particles 701 have a structure in which the surface of scaly silicon particles 703 is covered with a nanographene layer 704 (graphite thin film). The nanographene layer 704 is preferably formed to be in direct contact with the surface of the scaly silicon particles 703.

  The thickness of the nanographene layer 704 is preferably 2 nm or more and 10 nm or less. If the thickness is less than 2 nm, it is too thin and mechanical strength is insufficient. In addition, when the thickness is more than 10 nm, the nanographene layer 704 is not arranged along the surface shape, and the original purpose of enhancing the electrical conductivity can not be achieved. The same method as in Example 1 was used to produce the nanographene layer 704.

  Composite particles containing amorphous carbon 702 (carbon material) of this example were produced by the following method.

  Coal tar was used as a carbon source, this carbon source and nanographene coated scaly silicon particles were mixed at a mass ratio of 3: 2, and high-temperature treatment was performed in a heat treatment furnace. In the high temperature treatment, the temperature was raised to 900 ° C. at a temperature rising rate of 10 ° C./hr while flowing nitrogen as an inert gas, and then held at 900 ° C. for 5 hours. Then it was naturally cooled.

  In addition to coal tar, petroleum pitch, naphthalene, anthracene, phenanthrolene, phenol resin, polyvinyl alcohol and the like can be used as the carbon raw material. As the inert gas, in addition to nitrogen, argon or the like can be used.

  The produced composite particles can be made to have any average particle size and particle size distribution by crushing and classification. The average particle size is desirably 5 μm or more and 30 μm or less. In order to process to an average particle size of less than 5 μm, the manufacturing cost becomes high. When the average particle size exceeds 30 μm, the size is too large, and it is difficult to form an electrode by slurry application.

  FIG. 8A is a scanning electron microscope image of the composite particle of this example.

  As shown in the figure, the composite particle 801 is an irregularly shaped particle having a particle size of about 50 μm. It is possible to observe a fractured surface produced by grinding during production.

  FIG. 8B is an enlarged image of the smooth cross section of the composite particle. This smooth cross section is also produced by the same method as that shown in FIG. 3B.

  As shown in FIG. 8B, the composite particles have a configuration in which scaly silicon particles 802 are dispersed in a carbon material 803. It can be seen that the scaly silicon particles 802 have fewer holes and a more compact structure than that shown in FIG. 3B.

  In FIG. 8B, it can be seen that even if the scaly silicon particles 802 are large, the major axis is 800 nm or less, and most scaly silicon particles 802 have a thickness of 200 nm or less. The thickness of the scaly silicon particles 302 is 5 nm or more and 100 nm or less.

  In the present embodiment, the scale-like silicon particles have an average value of 0.2 μm in major axis and 0.01 μm in thickness.

  The composite particles of carbon and silicon particles were evaluated in the same manner as in Example 1.

  A lithium ion secondary battery was produced in the same manner as in Example 1 except that the composite particles were changed to the composite particles shown in FIG. 7A, and the life was evaluated.

(Comparative example 1)
In this comparative example, composite particles having a structure in which nano graphene particles were coated at a ratio of 20% by mass with spherical silicon particles having an average particle diameter of 5 μm were produced. The method is basically the same as that of the first embodiment. Only the differences from the first embodiment will be described below.

  When the temperature reached 800 ° C., the plug 404a was opened and propylene gas was introduced at a flow rate of 10 mL / min, and at the same time the flow rate of argon gas was 190 mL / min, and the time for growing the nanographene covering layer was 2 hours.

  The prepared nanographene-coated spherical silicon particles were embedded in the nanographene embedded layer by the same method as in Example 1.

  Thereafter, the method of manufacturing the negative electrode, the method of manufacturing the battery using the negative electrode, and the like are also the same as in Example 1, and thus the description thereof is omitted here.

  In addition, about evaluation, it carried out similarly to Example 1, and performed.

(Comparative example 2)
In this comparative example, composite particles are obtained in the same manner as in Comparative Example 1 except that the spherical silicon particles having an average particle diameter of 5 μm in Comparative Example 1 are changed to spherical silicon particles having an average particle diameter of 100 nm (0.1 μm). Made and evaluated.

  In the present example, particles having a structure in which scaly silicon particles were coated with nanographene at a ratio of 20% by mass were produced. The method is basically the same as that of the first embodiment. Only the differences from the first embodiment will be described below. In the present example, the scaly silicon particles have an average value of 0.2 μm in major axis and 0.01 μm in thickness.

  Hydrogen gas was flowed at a flow rate of 200 mL / min, and the reactor 401 was heated from room temperature to 1000 ° C. at a rate of 10 ° C./min, and the holding time at 1000 ° C. was 30 minutes.

  The prepared nanographene-coated scale-like silicon particles were embedded in the nanographene embedded layer by the same method as in Example 1.

  Thereafter, the method of manufacturing the negative electrode, the method of manufacturing the battery using the negative electrode, and the like are also the same as in Example 1, and thus the description thereof is omitted here.

  In addition, about evaluation, it carried out similarly to Example 1, and performed.

  Table 1 shows the configurations and evaluation results of the examples and the comparative examples.

  In the case of Comparative Examples 1 and 2 in which a negative electrode material containing composite particles having a configuration in which spherical silicon particles are dispersed in a carbon material and graphite-based particles, the capacity retention ratio after 100 cycles is 40%, 45 % Was low. On the other hand, in Examples 1 to 3 using the negative electrode material including the composite particles having a configuration in which scaly silicon particles are dispersed in the carbon material and the graphite-based particles, the capacity retention ratio after 100 cycles is 75% or more. there were. From this result, it is possible to produce a lithium ion secondary battery having a long life by using a negative electrode material including composite particles having a configuration in which scaly silicon particles are dispersed in a carbon material, and graphite-based particles. I understand.

From the comparison of Examples 1 and 2 with Example 3, the void area ratio of the composite particles is 1% or more and 3% or less, the specific surface area of the composite particles is less than 45 m ² / g, and carbon contained in the composite material It was found that the capacity retention rate can be further improved by setting the amount to 30% by mass or more.

  101: nanographene coated scaly silicon particles, 102: nanographene embedded layer, 103: scaly silicon particles, 104: nanographene coated layer, 105: negative electrode, 106: graphitic particles, 107: composite particles, 108: current collector, 200: lithium ion secondary battery, 210: positive electrode, 211: separator, 212: negative electrode, 213: battery container, 214: positive electrode current collecting tab, 215: negative electrode current collecting tab, 216: inner lid, 217: internal pressure release valve, 218: Gasket, 219: PTC resistance element, 220: battery lid, 221: axial center, 301: composite particles, 302: scaly silicon particles, 303: carbon material, 304: holes.

Claims (8)

A negative electrode material used for a negative electrode of a lithium ion secondary battery,
A composite particle having a configuration in which scaly silicon particles are dispersed in a carbon material, and a graphite-based particle,
The carbon material is amorphous carbon or nanographene,
The negative electrode material for a lithium ion secondary battery, wherein a content of the composite particles is 2% by mass or more and 50% by mass or less based on a total amount of the composite particles and the graphite-based particles.   2. The lithium according to claim 1, wherein the composite particle has 80% or more of pores in the range of 100 nm or more and 800 nm or less of the pore diameter in the range of 0.01 to 1 μm of pore diameter based on pore volume. Negative electrode material for ion secondary battery.   The negative electrode material for a lithium ion secondary battery according to claim 1, wherein a pore area ratio of the composite particles is 1% or more and 3% or less. The specific surface area of the composite particles is less 0.1 m ² / g or more ^{40 m} 2 / g, a negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 3.   The carbon material constituting the composite particle exhibits an energy loss distribution of 90 to 150 eV L-Si end and 280 to 400 eV CK edge in an energy loss image in cross-section STEM-ADF measurement. The negative electrode material for lithium ion secondary batteries as described in any one of -4.   The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 5, wherein the scaly silicon particles are coated with nanographene.   The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the scaly silicon particles have an average value of a major axis of 10 nm to 1 μm and a thickness of 5 nm to 200 nm. material. A lithium ion secondary battery comprising a positive electrode and a negative electrode,
The said negative electrode is a lithium ion secondary battery containing the negative electrode material for lithium ion secondary batteries as described in any one of Claims 1-7.

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