JP2020017375A - Negative electrode material for power storage device - Google Patents

Abstract

To provide a negative electrode material with which a power storage device high in storage capacity and suppressed in deterioration of storage capacity due to repeated charge/discharge can be obtained.SOLUTION: A negative electrode material for a power storage device comprises a plurality of particles. Each of the particles includes: a mother particle whose material is an Si-based alloy; and a coating layer whose material is a carbon-based material, the coating layer covering the mother particle. The Si-based alloy contains Si: 50 at.% or more and 95 at% or less, Cr: 5 at% or more and 20 at% or less, Ti: 5 at% or more and 20 at% or less, and an element A: 10 at% or less. The element A is one or two or more selected from the group consisting of V, Fe, Ni, Mo, Nb, Co, Al and Sn.SELECTED DRAWING: None

Description

  The present invention relates to a material suitable for a negative electrode of a power storage device that involves movement of lithium ions during charge and discharge, such as a lithium ion secondary battery, an all solid lithium ion secondary battery, and a hybrid capacitor.

  In recent years, portable telephones, portable music players, portable terminals, and the like have rapidly spread. These portable devices have a lithium ion secondary battery. Electric vehicles and hybrid vehicles also have lithium ion secondary batteries. Further, lithium ion secondary batteries and hybrid capacitors are used as stationary power storage devices for home use.

  In a lithium ion secondary battery, a negative electrode occludes lithium ions during discharging. When charging a lithium ion secondary battery, lithium ions are released from the negative electrode. The negative electrode has a current collector and an active material fixed to the surface of the current collector.

  Carbon-based materials such as natural graphite, artificial graphite, and coke are used as an active material in the negative electrode. The theoretical capacity of this carbon-based material for lithium ions is only 372 mAh / g. An active material having a large capacity is desired.

Attention has been paid to Si as an active material in a negative electrode. Si reacts with lithium ions. The reaction forms a compound. Typical compounds _{are Li} 22 Si _5. By this reaction, a large amount of lithium ions is occluded in the negative electrode. Si can increase the storage capacity of the negative electrode.

  When the active material layer containing Si absorbs lithium ions, the active material layer expands due to the formation of the above-described compound. The expansion rate of the active material is about 400%. When lithium ions are released from the active material layer, the active material layer contracts. The active material falls off the current collector by repeating the expansion and contraction. This drop reduces the storage capacity. The repetition of expansion and contraction may inhibit the conductivity between the active materials. The life of a conventional lithium ion secondary battery in which the negative electrode contains Si is not long.

  The electric conductivity of Si alone is lower than those of carbon-based materials and metal-based materials. If Si is used in combination with a conductive material such as a carbon-based material for the negative electrode, the electrical conductivity can be improved. Various proposals have been made for improving the charge / discharge characteristics of a combination of Si and a conductive material.

  JP-A-2017-216172 discloses a negative electrode material for a lithium ion secondary battery including a Si phase having a crystallite size of 20 nm or less and a silicide phase having a crystallite size of 30 nm or less. In this negative electrode material, a graphite material is dispersed in a matrix of Si.

  Japanese Patent Application Laid-Open No. 2013-168328 discloses a composite material in which particles including a solid phase mainly containing Si and a Si intermetallic compound phase containing Cr or Ti are composited with a carbon material. This composite material is for a negative electrode of a lithium ion secondary battery. The composite further contains a conductivity enhancer.

JP 2017-216172 A JP 2013-168328 A

In the negative electrode material disclosed in JP-A-2017-216172, many by-products such as Li ₂ O are formed on the surface of the Si material. This by-product is generated by a decomposition reaction of the electrolytic solution. This by-product is insulating. Due to the formation of many by-products, the conductivity of the negative electrode is hindered, and the cycle characteristics of the secondary battery deteriorate. In addition, the capacity of the secondary battery is reduced due to the consumption of lithium ions in the electrolytic solution by-products.

  In the negative electrode material disclosed in JP 2013-168328 A, a Si—C bond is formed. Si bonded to C cannot store lithium ions. The formation of the Si—C bond causes a reduction in the capacity of the secondary battery. Further, since the Si-C compound is insulating, the conductivity of the negative electrode material is impaired by the Si-C, and the cycle characteristics of the secondary battery are impaired.

  A similar problem occurs in various power storage devices other than the lithium ion secondary battery.

  An object of the present invention is to provide a negative electrode material capable of obtaining a power storage device having a large power storage capacity and suppressing a reduction in the power storage capacity due to repeated charging and discharging.

The negative electrode material for an electric storage device according to the present invention includes a large number of particles. Each particle has a base particle whose material is a Si-based alloy and a coating layer that covers the base particle and whose material is a carbon-based material. This Si-based alloy
Si: 50 at. % At least 95 at. % Or less Cr: 5 at. % At least 20 at. % Or less Ti: 5 at. % At least 20 at. % Or less and element A: 10 at. % Or less. The balance is unavoidable impurities. The element A is at least one element selected from the group consisting of V, Fe, Ni, Mo, Nb, Co, Al, and Sn. This Si-based alloy has a Si phase and a silicide phase. In this Si-based alloy, the ratio (II / I) of the diffraction peak intensity II of the (111) plane, which is the silicide domain peak, to the diffraction peak intensity I of the (111) plane, which is the Si main peak, is 1.0 or more. is there. The ratio Pp of the particles present as primary particles and having a particle diameter of 10 μm or less to the total particles in the base powder, which is an aggregate of a large number of base particles, is 50% by volume or more. The content Pc of the carbon-based material in the negative electrode material is 0.010% by mass or more and 5.0% by mass or less. The ratio (D2 / D1) of the average particle diameter D2 of the coating layer to the average particle diameter D1 of the Si-based alloy is 0.8 or less.

  Preferably, the Si phase is amorphous or low crystalline.

  Preferably, the crystallite size of the Si phase is 30 nm or less. Preferably, the crystallite size of the silicide phase is 40 nm or less.

  Preferred carbon-based materials are acetylene black, Ketjen black, carbon nanofibers and carbon nanotubes.

  Preferably, the average particle size of the negative electrode material is 0.1 μm or more and 25 μm or less.

Preferably, the BET specific surface area of the negative electrode material is from 2.0 m ² / g to 40.0 m ² / g.

  The power storage device using the negative electrode material according to the present invention has a large power storage capacity because the material of the base particles is a Si-based alloy. In this negative electrode material, the coating layer suppresses the decomposition reaction of the electrolytic solution, suppresses the formation of an excessive resistance film, and suppresses the exhaustion of the electrolytic solution. In this negative electrode material, stress due to expansion and contraction of charge and discharge is reduced. In this power storage device, a reduction in the power storage capacity due to repeated charging and discharging is suppressed.

FIG. 1 is a micrograph showing one particle of a negative electrode material according to an embodiment of the present invention. FIG. 2 is a photomicrograph showing the base particles of the particles of FIG. FIG. 3 is a chart showing the results of electron beam diffraction of the Si-based alloy of the base particles of FIG. FIG. 4 is a micrograph showing a mother powder, which is an aggregate of the mother particles of FIG. FIG. 5 is a micrograph showing a state before the mother powder of FIG. 4 is crushed.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to the drawings as appropriate.

  This embodiment is a negative electrode material of a lithium ion secondary battery. This lithium ion secondary battery is an example of a power storage device that involves movement of lithium ions during charging and discharging. The negative electrode of the lithium ion secondary battery has a current collector, an active material, a conductive material, and a binder. The active material, the conductive material, and the binder are attached to the current collector in a mixed state. The conductive material assists the conductivity of the active material. The binder fixes the active material particles to other active material particles. The binder further fixes the active material particles to the current collector. As the active material, the negative electrode material according to the present invention can be used.

  The negative electrode material is composed of many particles. The collection of many particles is a powder. FIG. 1 shows one particle. Although not shown, the particles have base particles and a coating layer. The coating layer covers the base particles. The coating layer may cover the entire surface of the mother particle. The coating layer may partially cover the surface of the base particle.

FIG. 2 shows the base particles. The material of the base particles is a Si-based alloy. This Si-based alloy
Si: 50 at. % At least 95 at. % Or less Cr: 5 at. % At least 20 at. % Or less Ti: 5 at. % At least 20 at. % Or less and element A: 10 at. % Or less.

Preferably, the composition of the Si-based alloy is
Si: 50 at. % At least 95 at. % Or less Cr: 5 at. % At least 20 at. % Or less Ti: 5 at. % At least 20 at. % Or less Element A: 10 at. % Or less and the balance: unavoidable impurities.

  In the negative electrode material, Si forms a Si phase. The main component of the Si phase is Si. The Si phase may include another element that is dissolved in the Si matrix.

  Si bonds with lithium ions. Si also releases lithium ions. In other words, the negative electrode material containing Si stores and releases lithium ions. The lithium ion storage charges the lithium ion secondary battery. The discharge of the lithium ions discharges the lithium ion secondary battery.

  Si has a theoretical capacity of about 10 times or more as compared with a carbon-based material generally used as a negative electrode material. The storage capacity of a lithium ion secondary battery in which the negative electrode material contains Si is large. From the viewpoint of the storage capacity, the content of Si in the Si-based alloy is 50 at. % Or more, preferably 60 at. % Is more preferable, and 70 at. % Is particularly preferable. In view of the fact that a Si-based alloy can contain a sufficient amount of another element, the Si content is 95 at. %, Preferably 90 at. % Or less, more preferably 85 at. % Is particularly preferred.

  Preferably, the Si phase is amorphous or low crystalline. In this Si phase, there are many paths for moving lithium ions. Low crystallinity means that the crystallite size of Si is 30 nm.

As described above, the Si-based alloy contains Cr and Ti. In this Si-based alloy, an intermetallic compound such as Si ₂ (Cr, Ti) precipitates. This intermetallic compound forms a silicide phase. The Si phase and the silicide phase are finely mixed. When Si absorbs lithium ions, the Si phase expands. The silicide phase relaxes the stress during the expansion. When Si releases lithium ions, the Si phase contracts. The silicide phase relaxes the stress at the time of this contraction. Relaxation of the stress suppresses the negative electrode material from falling off the negative electrode. The relaxation of the stress also suppresses the electrical isolation of the Si phase. Therefore, a lithium ion secondary battery having a negative electrode containing a silicide phase has excellent cycle characteristics.

  The Cr content in the Si-based alloy is 5 at. % At least 20 at. % Or less is preferable. When the content is 5 at. % Or more has excellent cycle characteristics. From this viewpoint, the content is 6 at. % Or more is more preferable, and 7 at. % Is particularly preferable. When this content is 20 at. % Or less, the lithium ion secondary battery has excellent storage capacity. From this viewpoint, the content is 18 at. % Or less, more preferably 15 at. % Is particularly preferred.

  The content of Ti in the Si-based alloy is 5 at. % At least 20 at. % Or less is preferable. When the content is 5 at. % Or more has excellent cycle characteristics. From this viewpoint, the content is 7 at. % Or more, more preferably 9 at. % Is particularly preferable. When this content is 20 at. % Or less, the lithium ion secondary battery has excellent storage capacity. From this viewpoint, the content is 18 at. % Or less, more preferably 16 at. % Is particularly preferred.

  The total content of Cr and Ti in the Si-based alloy is 10 at. % At least 40 at. % Or less is preferable. When the content is 10 at. % Or more has excellent cycle characteristics. From this viewpoint, this content is 14 at. % Or more, more preferably 16 at. % Is particularly preferable. When this content is 40 at. % Or less, the lithium ion secondary battery has excellent storage capacity. From this viewpoint, this content is 35 at. % Or less, more preferably 30 at. % Is particularly preferred.

  As described above, the Si-based alloy can include the element A. The element A is at least one element selected from the group consisting of V, Fe, Ni, Mo, Nb, Co, Al, and Sn. Element A is not an essential component.

  V, Fe, Ni, Mo, Nb and Co form a solid solution in silicide. V, Fe, Ni, Mo, Nb and Co expand the crystal lattice of silicide. This silicide is excellent in diffusivity of lithium ions moving inside itself. The lithium ion secondary battery having this silicide is excellent in cycle characteristics and reaction efficiency. Further, V, Fe, Ni, Mo, Nb and Co have a catalytic action. Therefore, these elements can enhance battery characteristics.

  Al and Sn can be dissolved in Si. Al and Sn increase the conductivity of the Si phase. The negative electrode material having this Si phase has a small electric resistance. The lithium ion secondary battery having this Si phase is excellent in charge and discharge efficiency. Al and Sn can each be dispersed in the negative electrode material as a single substance. Simple Al and Sn are flexible and excellent in toughness. Simple Al and Sn can relieve stress during expansion and contraction of the Si phase.

  From these viewpoints, the content of the element A in the Si-based alloy is 1 at. % Or more, preferably 2 at. % Is more preferable, and 3 at. % Is particularly preferable. From the viewpoint of the storage capacity, this content is 10 at. % Or less, and 8 at. % Or less, more preferably 7 at. % Is particularly preferred.

  FIG. 3 shows the diffraction peak intensity I of the (111) plane, which is the Si main peak, and the diffraction peak intensity II of the (111) plane, which is the silicide domain peak. The ratio (II / I) of the diffraction peak intensity II to the diffraction peak intensity I is preferably 1.0 or more. In a Si alloy having a ratio (II / I) of 1.0 or more, the Si phase and the silicide phase are finely dispersed and mixed. In the negative electrode material containing this Si-based alloy, cracking, electrical isolation and falling off of the particles due to stress during charging and discharging hardly occur. A lithium ion secondary battery having this negative electrode material has excellent cycle characteristics. In this respect, the ratio (II / I) is more preferably equal to or greater than 2.0, and particularly preferably equal to or greater than 5.0. The ratio (II / I) is preferably 10.0 or less. In X-ray diffraction, a CuKα ray having a wavelength of 1.54059 angstroms is used as an X-ray source. The measurement is performed in a range where 2θ is 20 degrees or more and 80 degrees or less. Thereby, a diffraction spectrum is obtained. The intensity value of the (111) plane, which is the Si main peak, and the intensity value of the (111) plane, which is the silicide domain peak, are selected from the diffraction spectra obtained when 2θ is in the range of 20 ° to 80 °. (II / I) is calculated.

  The crystallite size of the Si phase is preferably 30 nm or less. In the negative electrode material having a crystallite size of 30 nm or less, cracking of particles, electrical isolation, and falling off from the current collector due to stress during charging and discharging are suppressed. In this respect, the crystallite size is more preferably equal to or less than 20 nm, and particularly preferably equal to or less than 10 nm. In the present invention, when a phase is amorphous, the crystallite size of this phase is considered to be zero.

  The crystallite size of the silicide phase is preferably 40 nm or less. In the negative electrode material having a crystallite size of 40 nm or less, lithium ions can easily move in the compound phase. In the negative electrode material having a crystallite size of 40 nm or less, the stress of expansion and contraction of the Si phase generated during charge and discharge is reduced. From these viewpoints, the crystallite size is more preferably equal to or less than 20 nm, and particularly preferably equal to or less than 15 nm.

Crystallite size can be confirmed by X-ray diffraction. In X-ray diffraction, a CuKα ray having a wavelength of 1.54059 angstroms is used as an X-ray source. The measurement is performed in a range where 2θ is not less than 20 degrees and not more than 80 degrees. In the obtained diffraction spectrum, a broader diffraction peak is observed as the crystallite size is smaller. The crystallite size can be determined from the half width of the peak obtained by powder X-ray diffraction analysis using the following Scherrer equation.
D = (K × λ) / (β × cosθ)
In this equation, D represents the size of the crystallite (angstrom), K represents the Scherrer's constant, λ represents the wavelength of the X-ray tube, and β represents the spread of the diffraction line due to the size of the crystallite. , Θ represent diffraction angles.

  The control of the crystallite size of the Si phase and the silicide phase can be performed by adjusting the components of the raw materials. The control of the crystallite size can also be performed by controlling the cooling rate at the time of solidification after dissolving the raw material powder. Further, the crystallite size can also be controlled by pulverization of particles described below and pulverization of secondary particles.

  The surface of the particles shown in FIG. 1 is smoother than the surface of the particles shown in FIG. There are few irregularities on this surface. This surface state is achieved by the coating layer. The material of the coating layer is a carbon-based material.

  The Si-based alloy is active in the reaction with the electrolyte. When the negative electrode material does not have a coating layer, the decomposition reaction of the electrolytic solution on the Si-based surface layer is remarkable, and many resistance films are formed. This resistance film increases the electric resistance of the negative electrode. This resistance film impairs the storage capacity of the lithium ion secondary battery. Further, due to expansion and contraction of the base particles during charge and discharge, the resistive film is peeled off and deposited in the negative electrode. This volume impairs the cycle characteristics of the lithium ion secondary battery. The coating layer suppresses a reaction between the Si-based alloy and the electrolyte. In a lithium ion secondary battery having a coating layer, formation of a resistive film is suppressed, and depletion of the electrolyte is also suppressed.

  Examples of carbon-based materials suitable for the coating layer include acetylene black, Ketjen black, other carbon blacks, carbon nanofibers, carbon nanotubes, and amorphous carbon. Acetylene black, Ketjen black, carbon nanofibers and carbon nanotubes are preferred. The coating layer may contain two or more carbon-based materials.

  The content Pc of the carbon-based material in the negative electrode material is preferably from 0.010% by mass to 5.0% by mass. In the negative electrode material having a content Pc of 0.010% by mass or more, the reaction of the Si-based alloy with the electrolytic solution is sufficiently suppressed. In this respect, the content Pc is more preferably equal to or greater than 0.05% by mass, and particularly preferably equal to or greater than 0.10% by mass. A negative electrode material having a content Pc of 5.0% by mass or less has excellent storage capacity. In this respect, the content Pc is more preferably equal to or less than 4.7% by mass, and particularly preferably equal to or less than 4.5% by mass.

  Preferably, the carbon-based material is not substantially contained in the base particles. Therefore, the bond between Si and C does not substantially occur inside the base particles.

  The coating layer is formed by attaching a plurality of fine particles whose material is a carbon-based material to the surface of base particles whose material is a Si-based alloy. The ratio (D2 / D1) of the average particle diameter D2 of the coating layer to the average particle diameter D1 of the Si alloy is preferably 0.8 or less. In other words, it is preferable that the difference between the particle size D1 and the particle size D2 is large. In the negative electrode material having the ratio (D2 / D1) of 0.8 or less, the carbon-based material enters the fine irregularities of the base particles shown in FIG. Therefore, the coating layer can be formed densely and uniformly on the surface layer of the Si-based alloy. With this coating layer, the decomposition reaction of the electrolyte solution on the surface layer of the Si-based alloy is sufficiently suppressed. In this respect, the ratio (D2 / D1) is more preferably equal to or less than 0.10, and particularly preferably equal to or less than 0.05. The ratio (D2 / D1) is preferably equal to or greater than 0.00010. The average particle diameter D1 means the average value of the diameter of the base particles (primary particles) whose material is a Si-based alloy. The average particle diameter D2 means the average value of the diameter of the fine particles (primary particles) whose material is a carbon-based material.

  Examples of the method for producing the mother powder include a water atomizing method, a single-roll quenching method, a twin-roll quenching method, a gas atomizing method, a disk atomizing method, and a centrifugal atomizing method. Preferred production methods are a single roll cooling method, a gas atomizing method and a disk atomizing method. Hereinafter, an example of these manufacturing methods will be described in detail. Manufacturing conditions are not limited to those described below.

  In the single-roll cooling method, a raw material is charged into a quartz tube having a pore at the bottom. This raw material is heated and melted by a high-frequency induction furnace in an argon gas atmosphere. The raw material flowing out of the pores is dropped on the surface of the copper roll and cooled, and a ribbon is obtained. This ribbon is put into the pot together with the ball. Examples of the material of the ball include zirconia, SUS304 and SUJ2. Examples of the pot material include zirconia, SUS304 and SUJ2. The pot is filled with nitrogen gas or argon gas, and the pot is sealed. This ribbon is pulverized by milling to obtain a mother powder. Examples of the milling include a ball mill, a bead mill, a planetary ball mill, an attritor, and a vibrating ball mill.

  In the gas atomization method, a raw material is charged into a quartz crucible having a pore at the bottom. This raw material is heated and melted by a high-frequency induction furnace in an argon gas atmosphere. In an argon gas atmosphere, argon gas is injected into the raw material flowing out of the pores. The raw material is quenched and solidified to obtain a powder. This powder is subjected to mechanical processing such as mechanical milling to obtain a mother powder. Examples of the milling method include a ball mill method, a bead mill method, a planetary ball mill method, an attritor method, and a vibrating ball mill method.

  In the disk atomizing method, a raw material is charged into a quartz crucible having a pore at the bottom. This raw material is heated and melted by a high-frequency induction furnace in an argon gas atmosphere. In an argon gas atmosphere, the raw material flowing out of the pores is dropped on a disk rotating at a high speed. The rotation speed is from 40,000 rpm to 60,000 rpm. The raw material is quenched by the disk and solidified to obtain a powder. This powder is subjected to mechanical processing such as mechanical milling to obtain a mother powder. Examples of the milling method include a ball mill method, a bead mill method, a planetary ball mill method, an attritor method, and a vibrating ball mill method.

  The mother powder obtained by these methods is shown in FIG. In FIG. 5, the base particles are aggregated.

  This mother powder is crushed by machining. Examples of the crushing method include a ball mill, a bead mill, a planetary ball mill, an attritor and a vibrating ball mill, a dry jet mill, a wet jet mill method, and the like. The crushed mother powder is shown in FIG. Due to the disintegration, many base particles are present as primary particles (particles that are not aggregated).

  This mother powder is coated. By the coating treatment, the surfaces of the base particles are coated with the carbon-based material, and the particles shown in FIG. 1 are obtained. Specific examples of the coating treatment include a ball mill, a bead mill, a planetary ball mill, an attritor and a vibrating ball mill, mechanochemical, CVD and PVD. The coating may be made by carbonization of the base particles. Even by the coating process, the carbon-based material does not enter the inside of the base particles.

  The ratio Pp of the particles present as primary particles and having a particle size of 10 μm or less to the total particles in the mother powder shown in FIG. 4 is preferably 50% by volume or more. In the negative electrode material obtained from the base powder having the ratio Pp of 50% by volume or more, the difference in the reaction rate between the surface and the inside of the Si phase with lithium ions is small. In this negative electrode material, stress concentration when occluding lithium ions hardly occurs. A lithium ion secondary battery having this negative electrode material has excellent cycle characteristics. In this respect, the proportion Pp is more preferably equal to or greater than 60% by volume, and particularly preferably equal to or greater than 70% by volume. The larger the ratio Pp, the better. The particle size of the primary particles is determined by dispersing the particles in a solvent such as water (a dispersing agent can be used if it is difficult to disperse), feeding the particles into a laser diffraction / scattering type particle size distribution analyzer, irradiating the particles with laser light, It is determined from the scattering pattern obtained therefrom.

  As described above, the negative electrode material is composed of many particles (see FIG. 1). The collection of many particles is a powder. The average particle size of this powder is preferably 0.1 μm or more and 25 μm or less. In a negative electrode material having an average particle diameter of 0.1 μm or more, formation of a resistive film can be suppressed. In this respect, the average particle size is more preferably equal to or greater than 0.2 μm, and particularly preferably equal to or greater than 0.3 μm. A negative electrode material having an average particle size of 25 μm or less sufficiently reacts with lithium ions. Therefore, the capacity of the lithium ion secondary battery having this negative electrode material is large. In this respect, the average particle size is more preferably equal to or less than 20 μm, and particularly preferably equal to or less than 10 μm.

  The average particle diameter is a particle diameter at a point where the cumulative curve becomes 50% when the cumulative curve is obtained with the total volume of the powder being 100%. The average particle size is measured by a laser diffraction / scattering type particle size distribution measuring device.

The BET specific surface area of the powder as the negative electrode material is preferably from 2.0 m ² / g to 40.0 m ² / g. In a powder having a BET specific surface area of 2.0 m ² / g or more, the Si-based alloy can react with lithium ions in a wide area. Therefore, the negative electrode using this powder has a large storage capacity. Further, in a powder having a specific surface area of 2.0 m ² / g or more, a difference in stress between the inside of the particle and the surface of the particle during charging and discharging is small. Therefore, in the negative electrode using this powder, the pulverization of the particles is suppressed, and the storage capacity is maintained. From these viewpoints, the specific surface area is more preferably more than ^{2.5m 2 / g, 3.0m 2 /} g or more is particularly preferable. In the powder having a specific surface area of 40.0 m ² / g or less, the decomposition reaction of the electrolytic solution on the surface of the particles is suppressed. Therefore, in the negative electrode using this powder, reduction of lithium ions is suppressed, and formation of a solid electrolyte is suppressed. In this negative electrode, the storage capacity is maintained. From these viewpoints, the specific surface area BET specific surface area is more preferably 30.0 m ² / g or less, 20.0 m ² / g or less is particularly preferred. The BET specific surface area is measured according to the standard of “JIS Z 8830: 2013”.

  Hereinafter, the effects of the present invention will be clarified by examples, but the present invention should not be construed as being limited based on the description of the examples.

  The effect of the negative electrode material according to the present invention was confirmed using a bipolar coin cell. First, raw materials having the compositions shown in Table 1-3 were prepared. Powder was produced from each raw material by gas atomization and mechanical milling. Each powder, a conductive material (acetylene black), a binder (polyimide, polyvinylidene fluoride, etc.) and a dispersion (N-methylpyrrolidone) were mixed to obtain a slurry. This slurry was applied on a copper foil as a current collector. This slurry was dried under reduced pressure with a vacuum dryer. The drying temperature was 200 ° C. or higher when polyimide was the binder, and was 160 ° C. or higher when polyvinylidene fluoride was the binder. The solvent was evaporated by this drying to obtain an active material layer. The active material layer and the copper foil were pressed with a roll. The active material layer and the copper foil were punched into a shape suitable for a coin cell to obtain a negative electrode.

A mixed solvent of ethylene carbonate and dimethyl carbonate was prepared as an electrolyte. The mass ratio between the two was 3: 7. Further, lithium hexafluorophosphate (LiPF ₆ ) was prepared as a supporting electrolyte. The amount of the supporting electrolyte is 1 mol per liter of the electrolytic solution. This supporting electrolyte was dissolved in the electrolytic solution.

  A separator and a positive electrode having a shape suitable for a coin cell were prepared. This positive electrode was stamped from a lithium foil. The separator was immersed in the electrolyte under reduced pressure, and allowed to stand for 5 hours to sufficiently penetrate the separator with the electrolyte.

  A negative electrode, a separator and a positive electrode were assembled in a coin-type cell. The coin cell was filled with the electrolyte. Note that the electrolyte needs to be handled in an inert atmosphere where the dew point is controlled. Therefore, the cell was assembled in a glove box in an inert atmosphere.

In the coin-shaped cell, charging was performed under the conditions of a temperature of 25 ° C. and a current density of 0.50 mA / cm ² until the potential difference between the positive electrode and the negative electrode became 0 V. Thereafter, discharging was performed until the potential difference became 1.5 V. This charge and discharge were repeated for 50 cycles. The initial discharge capacity X and the discharge capacity Y after 50 cycles of charging and discharging were measured. Further, the ratio (retention rate) of the discharge capacity Y to the discharge capacity X was calculated. The results are shown in Table 1-5 below. The initial discharge capacity is preferably 500 mAh / g or more. The maintenance ratio is preferably 80% or more.

  In Table 1-5 below, No. No. 1-40 is a composition of a negative electrode material according to an example of the present invention. 41-90 is the composition of the negative electrode material according to the comparative example.

  As shown in Table 1-5, the negative electrode materials of the respective examples are excellent in the balance between the initial discharge capacity and the retention. From the evaluation results, the superiority of the present invention is clear.

  The negative electrode described above can be applied not only to a lithium ion secondary battery, but also to various power storage devices such as an all-solid lithium ion secondary battery and a hybrid capacitor.

Claims (7)

A negative electrode material for an electricity storage device comprising a large number of particles,
Each particle has a base particle whose material is a Si-based alloy, and a coating layer that covers the base particle and whose material is a carbon-based material,
The Si-based alloy is
Si: 50 at. % At least 95 at. % Or less Cr: 5 at. % At least 20 at. % Or less Ti: 5 at. % At least 20 at. % Or less and element A: 10 at. % Or less, with the balance being unavoidable impurities,
Said element A is one or more selected from the group consisting of V, Fe, Ni, Mo, Nb, Co, Al and Sn;
The Si-based alloy has a Si phase and a silicide phase,
In the above Si-based alloy, the ratio (II / I) of the diffraction peak intensity II of the (111) plane, which is the silicide domain peak, to the diffraction peak intensity I of the (111) plane, which is the Si main peak, is 1.0 or more. Yes,
A ratio Pp of particles present as primary particles and having a particle diameter of 10 μm or less to all particles in a mother powder which is an aggregate of a large number of mother particles is 50% by volume or more;
A content Pc of the carbon-based material in the negative electrode material is 0.010% by mass or more and 5.0% by mass or less;
A negative electrode material in which the ratio (D2 / D1) of the average particle diameter D2 of the coating layer to the average particle diameter D1 of the Si alloy is 0.8 or less.   The negative electrode material according to claim 1, wherein the Si phase is amorphous or low crystalline.   3. The negative electrode material according to claim 1, wherein the crystallite size of the Si phase is 30 nm or less.   The negative electrode material according to any one of claims 1 to 3, wherein the silicide phase has a crystallite size of 40 nm or less.   The negative electrode material according to any one of claims 1 to 4, wherein the carbon-based material is acetylene black, Ketjen black, carbon nanofiber, or carbon nanotube.   The negative electrode material according to any one of claims 1 to 5, wherein the average particle size is 0.1 µm or more and 25 µm or less. Negative electrode material according to any one of claims 1 to 6 having a BET specific surface area is less than 2.0 m ² / g or more 40.0m ² / g.