JP2021157867A - Negative electrode material for lithium ion secondary battery - Google Patents

Abstract

To provide a negative electrode material for lithium ion secondary batteries, with which a lithium ion secondary battery can maintain a high discharge capacity ratio even by charging/discharging.SOLUTION: A negative electrode material for lithium ion secondary batteries includes: a silicon particle; a diamond oxide catalyst fine particle close to a silicon particle; a first conductive material coated on a silicon particle and having a porous structure; and a second conductive material of fibrous carbon that extends from a diamond oxide catalyst fine particle. A silicon particle is held in the form of a cluster of grapes on a second conductive material.SELECTED DRAWING: Figure 2

Description

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

Lithium-ion secondary batteries have a high energy density and are attracting attention as batteries for electric vehicles and power storage. Electric vehicles include zero-emission electric vehicles that do not have an engine, hybrid electric vehicles that have both an engine and a rechargeable battery, and plug-in electric vehicles that are charged from a grid power source. In particular, zero-mission electric vehicles and plug-in electric vehicles are required to have high energy density lithium-ion secondary batteries.

In order to increase the capacity of lithium-ion secondary batteries, it is necessary to develop high-capacity electrode active materials. In particular, looking at the negative electrode, a silicon negative electrode having a theoretical capacity eight times or more that of the current graphite negative electrode is expected as a next-generation high-capacity negative electrode to replace graphite.

For example, Patent Document 1 describes a nanocarbon material composite in which Si fine particles are encapsulated in the voids of marimocarbon in which fibrous nanocarbon materials are radially formed with diamond oxide catalyst fine particles as nuclei, and a method for producing the same. It is disclosed.

In Patent Document 2, the median value in the number-based distribution of the aspect ratios of the particles (A) containing an element other than the carbon element that can store and release lithium ions and the primary particles that can store and release lithium ions is 1. A three-dimensional entangled network structure is formed by one or more carbon fibers (C) containing graphite particles (B) having a value of .4 to 3.0 and carbon fibers (C). A negative electrode material for a lithium ion secondary battery is disclosed in which the particles (A) are fused to the body and the structure is fused to at least a part of the surface of the graphite particles (B).

In Patent Document 3, a negative electrode material for a lithium ion secondary battery including a composite material composed of silicon particles, a graphitic material, and a carbonaceous material, which has been subjected to a treatment for imparting compressive force and shearing force, and is a carbonaceous material. A negative electrode material for a lithium ion secondary battery is disclosed, which comprises a composite material A having a structure in which the silicon particles having a coating film A made of A on at least a part of the surface and the graphitic material are in close contact with each other. ..

Patent Document 4 discloses a negative electrode having silicon particles and graphene, the periphery of the silicon particles having a region covered with the graphene, and the graphene having voids through which lithium ions pass. ing.

Japanese Unexamined Patent Publication No. 2014-177389 International Publication No. 2016/017583 Japanese Unexamined Patent Publication No. 2008-235247 Japanese Unexamined Patent Publication No. 2015-181136

In such silicon particles in the silicon negative electrode, the volume change due to charging and discharging is large. Therefore, in order to increase the capacity and rate of the silicon negative electrode, a conductive network capable of following the volume change of the silicon particles during charging and discharging is required. Such a conductive network is formed by adding a conductive agent together with the silicon particles.

The conductive agent is introduced by being mixed with the silicon particles and the binder at the time of producing the negative electrode, and the conductive agent is attached to the surface of the silicon particles. Since there is no change in the volume of the silicon particles before charging, the electrons are distributed throughout the negative electrode via the conductive agent.

However, in the charging process, as the charging progresses, the volume of the silicon particles gradually changes, and when the charging capacity exceeds 3000 mAh / g, the volume of the silicon particles expands to three times or more that before charging. If the conductive network is disconnected in this process, electrons may not be supplied to some silicon particles, and then charging may not proceed.

On the other hand, in the discharge process, electrons are deprived of the silicon particles and lithium ions are emitted from the silicon particles. As the discharge capacity increases, the size of the silicon particles gradually decreases, and when the discharge is completed, the silicon particles return to the size before charging. At this time, if the conductive material is peeled off from the surface of the silicon particles, the conductive network between the conductive agent and the silicon particles is cut, and the silicon particles escape from the conductive network, the electrons are conductive from the silicon particles during the discharge. It will not be transmitted to the network, and then it will not be able to be charged again.

In this way, it is important for the conductive network to follow the volume change of the silicon particles in the charge / discharge process of the negative electrode, and if this does not work sufficiently, the negative electrode cannot be charged / discharged in the middle of the charge / discharge cycle. , The initial capacitance of the negative electrode is reduced, and the rate characteristics and the life of the charge / discharge cycle are also reduced.

Further, since the surface of the silicon particles is covered with an ultrathin oxide layer, the electron resistance of the surface is high. If the electronic resistance on the surface of the silicon particles is high, it becomes difficult to charge and discharge a large current, and the input / output density of the lithium ion secondary battery decreases. That is, it is also necessary to modify the surface of the silicon particle surface to impart electron conductivity to the layer having high electronic resistance.

Therefore, it is an object of the present invention to provide a negative electrode material for a lithium ion secondary battery in which the lithium ion secondary battery maintains a high discharge capacity ratio even when charged and discharged.

As a result of diligent research conducted by the present inventors in order to solve the above-mentioned problems, a layered first layer covering a lithium ion secondary battery, silicon particles as a negative electrode material for the lithium ion secondary battery, and silicon particles. A lithium ion secondary battery is mounted by mounting a negative electrode manufactured using a negative electrode material containing a conductive material and a fibrous second conductive material extending from diamond oxide catalyst fine particles in the vicinity of silicon particles. The present invention has been completed by finding that the above-mentioned problems can be solved by preventing the silicon particles from coming out of the conductive network even when charged and discharged.

That is, the present invention comprises silicon particles, diamond oxide catalyst fine particles in close proximity to the silicon particles, a first conductive material having a porous structure coated on the silicon particles, and a fibrous form extending from the diamond oxide catalyst fine particles. It comprises a second conductive material which is carbon, and is characterized by a negative electrode material for a lithium ion secondary battery in which silicon particles are held in a vine tuft shape in the second conductive material.

According to the present invention, there is provided a negative electrode material for a lithium ion secondary battery in which the lithium ion secondary battery maintains a high discharge capacity ratio even when charged and discharged. Issues, configurations and effects other than those described above will be clarified by the following description of the embodiments.

It is a figure which shows the cross section of one Embodiment of the lithium ion secondary battery which concerns on this invention. It is a figure which shows typically how the negative electrode material which concerns on this invention changes from the state before charging (discharging state, the figure on the left side) to the state of charging (the figure on the right side).

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the dimensions and shapes of each part are exaggerated for clarification, and the actual dimensions and shapes are not accurately depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of the parts shown in these drawings. It is needless to say that the embodiment described later is an example, and other embodiments may be possible by combining each embodiment, combining with a known or well-known technique, or replacing the embodiment.

The "~" described in the present specification is used to mean that the numerical values described before and after it are included as the lower limit value and the upper limit value. In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value described in another stepwise. The upper or lower limit of the numerical range described herein may be replaced with the values shown in the examples.

Hereinafter, the present invention will be described in detail based on the embodiments.

(Structure of Lithium Ion Secondary Battery of One Embodiment of the Present Invention)
FIG. 1 schematically shows the internal structure of an embodiment of a lithium ion secondary battery according to the present invention. The lithium ion secondary battery 101 is an electrochemical device that stores or makes available electrical energy by storing and releasing lithium ions into electrodes in a non-aqueous electrolyte. This is called by another name of a lithium ion battery, a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery, and any of the batteries is the subject of the present embodiment.

The lithium ion secondary battery 101 according to the present embodiment has a configuration in which an electrode group including a positive electrode 107, a negative electrode 108, and a separator 109 is housed in a battery container 102 in a sealed state. As the electrode group, various configurations can be adopted, such as a configuration in which strip-shaped electrodes are laminated, a configuration in which strip-shaped electrodes are wound and formed into a cylindrical or flat shape. The battery container 102 can be selected from any shape such as a cylindrical shape, a flat oval shape, and a square shape, depending on the shape of the electrode group. After accommodating the electrode group from the opening provided in the upper part of the battery container 102, the opening is closed by the lid 103 and sealed.

The outer edge of the lid 103 is joined to the opening of the battery container 102 by welding, caulking, adhesion, or the like over the entire circumference, and the battery container 102 is sealed in a sealed state. The lid 103 has a liquid injection port for injecting the electrolytic solution L into the battery container 102 after sealing the opening of the battery container 102. The liquid injection port is sealed by a liquid injection plug 106 after the electrolytic solution L is injected into the battery container 102. It is also possible to provide a safety mechanism to the liquid injection plug 106. As the safety mechanism, a pressure valve for releasing the pressure inside the battery container 102 may be provided.

A positive electrode external terminal 104 and a negative electrode external terminal 105 are fixed to the lid 103 via an insulating seal member 112, and a short circuit between both terminals (positive electrode external terminal 104 and negative electrode external terminal 105) is prevented by the insulating seal member 112. .. The positive electrode external terminal 104 is connected to the positive electrode 107 via the positive electrode lead wire 110, and the negative electrode external terminal 105 is connected to the negative electrode 108 via the negative electrode lead wire 111. The material of the lead wire insulating seal member 112 can be selected from fluororesin, thermosetting resin, glass hermetic seal, etc., and any insulating material that does not react with the electrolytic solution L and has excellent airtightness is used. can do.

The electrode group is formed by laminating a positive electrode 107 and a negative electrode 108 via a separator 109. The separator 109 is arranged between the positive electrode 107 and the negative electrode 108 to prevent short circuits between them, and is also inserted between the electrode group and the battery container 102 so that the positive electrode 107 and the negative electrode 108 are short-circuited through the battery container 102. I try not to. The electrolytic solution L is held on the surface of the separator 109 and each electrode (positive electrode 107 and negative electrode 108) and inside the pores.

As the material of the battery container 102, any material can be selected as long as it is not corroded by the electrolytic solution L or dissolved in the electrolytic solution L. For example, stainless steel, steel, nickel-plated steel plate, plastic, etc. Aluminum (for example, a laminated sheet of aluminum) can be used.

As the separator 109, for example, a polyolefin-based polymer sheet made of polyethylene, polypropylene, or the like, or a multi-layered sheet in which a polyolefin-based polymer and a fluorine-based polymer sheet typified by tetrafluoride polyethylene are welded is used. Can be done. Further, a mixture of ceramics and binder may be formed in a thin layer on the surface of the separator 109 so that the separator 109 does not shrink when the battery temperature rises. Further, as the separator 109, a cellulose separator made of cellulose fibers may be used. The separator 109 preferably has a pore diameter of 0.01 μm to 10 μm and a pore ratio of 20% to 90%, for example, in order to allow lithium ions to permeate during charging and discharging of the lithium ion secondary battery 101.

(Manufacturing of positive electrode)
The positive electrode 107 is composed of a positive electrode active material, a conductive agent, a binder, and a positive electrode current collector.

Examples of the positive electrode active material are LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ . In addition, LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , Li ₄ Mn ₅ O ₁₂ , LiMn _2-x M _x O ₂ (where M = Co, Ni, Fe, Cr, Zn, Ta, x = 0. 01-0.2), Li ₂ Mn ₃ MO ₈ (where M = Fe, Co, Ni, Cu, Zn), Li _1-x A _x Mn ₂ O ₄ (where A = Mg, B, Al, Fe, Co, Ni, Cr, Zn, Ca, x = 0.01 to 0.1), LiNi _1-x MxO ₂ (where M = Co, Fe, Ga, x = 0.01 to 0.2) , LiFeO ₂ , Fe ₂ (SO ₄ ) ₃ , LiCo _1-x M _x O ₂ (where M = Ni, Fe, Mn, x = 0.01-0.2), LiNi _1-x M _x O ₂ (However, M = Mn, Fe, Co, Al, Ga, Ca, Mg, x = 0.01 to 0.2), Fe (MoO ₄ ) ₃ , FeF ₃ , LiFePO ₄ , LiMnPO ₄ , Li ₂ MnO ₃ -LimnO ₂ system solid solution and the like can be listed. In the present embodiment, it is preferable to select _{LiNi 1/3} Mn _1/3 Co _1/3 O _{2 as the positive electrode active material.} However, this embodiment is not limited to these materials because the positive electrode material is not restricted in any way.

The average particle size (D ₅₀ ) of the positive electrode active material is specified to be equal to or less than the thickness of the mixture layer. If the positive electrode active material powder contains coarse particles having a size equal to or larger than the thickness of the mixture layer, the coarse particles are removed in advance by sieving classification, wind flow classification, or the like to prepare particles having a size equal to or less than the thickness of the mixture layer. The average particle size can be measured using a particle size distribution meter using laser scattering.

Further, when the positive electrode active material is an oxide, the oxide generally has low conductivity (that is, high electrical resistance), so carbon powder is added as a conductive agent to improve the conductivity between the oxide particles.

As the conductive agent, for example, known materials such as graphite, amorphous carbon, graphitized carbon, carbon black, activated carbon, conductive fibers, and carbon nanotubes can be used. Conductive fibers include vapor-grown carbon, fibers produced by carbonizing pitch (a by-product of petroleum, coal, coal tar, etc.) at high temperatures, and carbon fibers produced from acrylic fibers (Polyacrylonitrile). be. Further, the material of the conductive agent is a material that does not oxidize and dissolve at the charge / discharge potential of the positive electrode (usually 2.5 V to 2.8 V), and is a metal material having a lower electric resistance than the positive electrode active material, for example, titanium. A fiber made of a corrosion-resistant metal such as gold, a carbide such as SiC or WC, or a nitride such as _{Si 3} N _{4 or BN may be used.} The conductive agent can be produced by an existing production method such as a melting method or a chemical vapor deposition method. The conductive agent may cover the surface of the positive electrode active material and be integrated. The content of the conductive agent may be usually 1% to 10%, preferably 5% to 10%, based on the weight of the positive electrode active material. If the amount of the conductive agent is too small, the electrical conductivity will be insufficient and the positive electrode will have high resistance. On the contrary, if the amount of the conductive agent is too large, the content of the positive electrode active material will decrease and the battery capacity will decrease.

As the binder, for example, polyvinylidene fluoride (hereinafter, also referred to as PVDF) can be used. As the binder, one previously dissolved in 1-methyl-2-pyrrolidone (hereinafter, also referred to as NMP) can be used. The binder content is usually 1% to 10%, preferably 5% to 10%, based on the weight of the positive electrode active material. If the amount of binder is too small, the structure of the positive electrode is weakened and the cycle life is shortened. On the contrary, if the amount is too large, the resistance of the positive electrode is increased and the battery capacity is reduced.

The positive electrode current collector includes an aluminum foil having a thickness of usually 1 μm to 100 μm, an aluminum perforated foil having a hole diameter of usually 0.11 mm to 10 mm and a thickness of usually 10 μm to 100 μm, and expanded metal. A foamed metal plate or the like is used, and stainless steel, titanium, or the like can be applied in addition to aluminum as the material. In the present invention, any material can be used for the current collector without being limited by the material, shape, manufacturing method, etc., as long as the battery does not change such as dissolution and oxidation during use.

When forming a mixture layer on the surface of the positive electrode current collector, first, the mixing ratio (weight percentage display) of the positive electrode active material, the conductive agent, and the binder is determined, for example, the positive electrode active material is usually 80% by weight to 95% by weight. %, The conductive agent is usually 1% by weight to 10% by weight, and the binder is usually 1% by weight to 10% by weight, and these are blended. The mixing ratio of the conductive agent is preferably 3% by weight or more in order to sufficiently exert conductivity and enable charging / discharging of a large current. As a result, the resistance of the entire positive electrode 107 is reduced, and the ohmic resistance loss when a large current flows can be reduced. Further, when improving the energy density of the lithium ion secondary battery 101, it is desirable that the mixing ratio of the positive electrode active material is in a high range of 85% by weight to 95% by weight.

Next, while stirring and mixing the positive electrode active material, the NMP solution containing the binder, and the conductive agent, NMP as an organic solvent is added to prepare a slurry having smooth fluidity (also referred to as a positive electrode slurry) to prepare a positive electrode collection. It is applied to an electric body, and the organic solvent is evaporated and dried. The method of applying the positive electrode slurry to the positive electrode current collector is not particularly limited, and known methods such as a doctor blade method, a dipping method, and a spray method can be adopted. Further, after applying the positive electrode slurry to the current collector, the organic solvent is evaporated and dried, and the positive electrode mixture layer formed on the surface of the positive electrode current collector is pressure-molded by a roll press to obtain the positive electrode 107. Can be produced. It is also possible to stack a plurality of positive electrode mixture layers on the positive electrode current collector by performing the process from application of the positive electrode slurry to drying a plurality of times.

The thickness of the positive electrode mixture layer is preferably equal to or larger than the average particle size of the positive electrode active material. This is because if the thickness of the positive electrode mixture layer is less than the average particle size of the positive electrode active material, the electron conductivity between adjacent particles decreases. _{The average particle size (D 50} ) of the positive electrode active material of the present embodiment is measured by a laser scattering method and is usually in the range of 1 μm to 4 μm. When this positive electrode active material is used, the thickness of the positive electrode mixture layer is usually 10 μm or more. When the thickness of the positive electrode mixture layer is within the above range, a higher effect of the present embodiment can be obtained. The upper limit of the thickness of the positive electrode mixture layer is usually 30 μm or less. If it is thicker than that, the charging level of the positive electrode active material near the surface of the mixture and the surface of the current collector will vary unless the amount of the conductive agent is increased, and uneven charging / discharging may occur. Increasing the amount of the conductive agent increases the volume of the positive electrode and reduces the energy density of the battery.

(Manufacturing of negative electrode)
The negative electrode 108 is composed of a negative electrode active material, a binder, and a negative electrode current collector. The negative electrode active material is the negative electrode material of the present invention, and includes silicon particles, diamond oxide catalyst fine particles, a first conductive material, and a second conductive material.

In the negative electrode material of the present invention, the silicon particles are crystalline or amorphous metal particles. A trace amount of silicon oxide may be present on the surface of the particle, a trace amount of oxygen or carbon may be contained inside the particle, and silicon may be the main component of the particle as a whole. Further, the shape of the silicon particles can have any shape such as a spherical shape, a lump shape, or a plate shape. Since the lithium ion secondary battery becomes smaller as the density of the negative electrode mixture layer increases, it is desirable that the silicon particles are spherical or agglomerated. Here, the agglomerate is an agglomerate of secondary particles or more in which silicon particles of primary particles are agglomerated, and the aspect ratio of the longest side to the shortest side passing through the center of the agglomerate is 1 to 3. It is assumed that it is an atypical agglomerate that is close to a spherical shape (aspect ratio is 1).

Due to the volume change of the silicon particles during charging and discharging of the lithium ion battery 101, a large stress is generated in the silicon particles, and the silicon particles may collapse. This phenomenon is called pulverization of silicon particles, and as this phenomenon progresses, the conductivity of the negative electrode deteriorates, and the number of chargeable and dischargeable silicon particles decreases. In order to avoid this, the stress can be reduced and the pulverization can be suppressed by reducing the particle size of the silicon particles in advance. However, if the particles are too fine, the filling property in the negative electrode is lowered, and the capacitance density per volume of the lithium ion secondary battery is lowered. Further, when the particles become finer, the surface area thereof increases, so that the amount of side reactions other than charging and discharging increases, and the initial capacity of the lithium ion secondary battery decreases. Therefore, in order to suppress pulverization during the initial charge / discharge cycle and increase the capacitance density of the negative electrode, the average particle size of the primary particles of the silicon particles is usually 5 nm to 500 nm. In order to further improve the rate characteristics and cycle life, the average particle size of the primary particles of the silicon particles is preferably 100 nm or less, more preferably 70 nm or less. Further, in order to further increase the capacitance density of the negative electrode, the average particle size of the primary particles of the silicon particles is preferably 10 nm or more, more preferably 20 nm or more. The average particle size of the primary particles of the silicon particles can be, for example, the average value of the equivalent circle diameters of 10 to 50 particles randomly selected in a transmission electron micrograph (TEM photograph).

The average particle size of the secondary particles of the silicon particles is usually 5 _{μm to 10 μm for the secondary particle size D 50,} which is 50% of the cumulative volume in the volume-based particle size distribution when measured with a laser scattering particle size distribution measuring device. _{The secondary particle size D 90 at} which the cumulative volume in the volume-based particle size distribution is 90% is usually 15 μm to 25 μm.

Next, in FIG. 2, the structure of the negative electrode material of the present invention will be described with a focus on one silicon particle 201. On the left side of FIG. 2, silicon particles 201 satisfying the above conditions are shown. A layered conductive material 202 is formed on the surface of the silicon particles 202. This is the first conductive material 202. The layered form means that the first conductive material 202 is a carbonaceous material having a disordered layer structure or a graphite material having no disorder in the layered structure, and has one layer, two layers, three layers, or four layers on the surface of the silicon particles 202. It means a form in which the graphene layer is covered with more than one layer. The graphene layer may have a microscopic layer structure, and the entire surface of the silicon particles does not have to have a continuous graphene structure. The first conductive material 202 coats the silicon particles 201 in a layered manner, and can maintain the electron conductivity by following the volume change of the silicon particles 201. Further, the first conductive material 202 has a porous structure, which enables insertion and desorption of lithium ions.

Further, the silicon particles 201 coated with the first conductive material 202 are a plurality of fibrous second conductive materials 203 extending radially from the diamond oxide catalyst fine particles 204 in the vicinity of the silicon particles 201 (FIG. In 2, it is included by 2). Since the second conductive material 203 is connected to the first conductive material 202 coated with different silicon particles 201, even if the silicon particles 201 change in volume and their positions shift, the silicon particles Allows the transfer of electrons between 201.

The first conductive material 202 is preferably an agglomerate of fine particles or primary particles, and the layer formed by the first conductive material 202 has a porous structure having pores. The size of the pores is a size that allows solvated lithium ions to be taken in and out, and is usually 1/100 to 1 (equal particle size) of the particle size of the silicon particles 201. Although it depends on the size of the silicon particles 201, it is usually 10 nm to 1000 nm. The size of the pores was determined by, for example, 10 to 50 pores randomly selected by measuring the pore distribution with a porosimeter by injecting mercury and binarizing an SEM photograph of the electrode surface or cross section. It can be the average value of the circle equivalent diameter of.

The amount of pores in the layer is not limited, but is usually the pore volume obtained by measuring the pore distribution with a porosimeter by mercury intrusion, and the pore area calculated by binarized image processing of the SEM photograph of the electrode cross section. Can be the ratio of.

Since there are pores in the layer formed by the first conductive material 202, lithium ions can permeate through the pores, and the silicon particles can occlude and release lithium ions. In the first conductive material 202, the layer made of the first conductive material 202 may be densified by heat treatment, provided that pores remain in the layer.

Examples of the material that can be used for the first conductive material 202 include carbon black, furnace black, graphite, and amorphous carbon. The average particle size of the primary particles of the first conductive material 202 may be usually 1/10 or less of the average particle size of the primary particles of the silicon particles 201. When the average particle size of the first conductive material 202 is within the above range, the first conductive material 202 can cover the surface of the silicon particles 201.

The content of the first conductive material 202 is, for example, the weight ratio of the silicon particles 201 to the first conductive material 202 (silicon particles 201: first) when the first conductive material 202 is carbon black. The conductive material 202) is usually in the range 97: 3 to 70:30, preferably 95: 5 to 75:25, in other words, the weight ratio of the first conductive material 202 to the weight of the silicon particles 201. It is desirable that (the first conductive material 202 / silicon particles 201) is usually in the range of 0.031 to 0.429, preferably 0.053 to 0.333. When the weight ratio of the first conductive material 202 to the weight of the silicon particles 201 is smaller than 0.031, the amount of the first conductive material 202 tends to be insufficient, and the silicon particles 201 have high electronic resistance. The surface remains. On the other hand, when the weight ratio of the first conductive material 202 to the weight of the silicon particles 201 exceeds 0.429, the ratio of the silicon particles 201 tends to decrease, and the capacitance density of the negative electrode tends to decrease.

The coating thickness (layer thickness) of the first conductive material 202 is usually 100 nm or more. The coverage of the first conductive material 202 with respect to the surface area of the silicon particles 201 is usually 70% to 90%. With such a coverage, the surface exposed on the surface of the silicon particles 201, that is, the surface of the silicon particles 201 not covered with the first conductive material 202 can be set to 10% to 30%. Lithium ions pass through the first conductive material 202, are occluded and released into the silicon particles 201, and the capacity of the negative electrode is increased.

The first conductive material 202 may contain, in addition to carbon, a material that does not occlude, release, or dissolve lithium ions when the negative electrode is charged or discharged. The first conductive material 202 is selected from the group consisting of, but not limited to, for example, titanium, iron, nickel, alumina, silica, lithium titanate, barium titanate, alkali metal oxides, alkaline earth metal oxides and the like. Can include one or more species.

Examples of the method of coating the surface of the silicon particles 201 with the first conductive material 202 include a mechanical coating method using a ball mill or mechanical fusion, and a vapor phase coating using a gas pyrolysis reaction. In the ball mill method, the powder of the silicon particles 201 and the carbon powder which is the raw material of the first conductive material 202 are mixed, and the impact of high-speed rotation causes the surface of the silicon particles 201 to become the first conductive material 202. It can be coated with carbon. Further, according to the vapor phase coating method, the powder of the silicon particles 201 and the hydrocarbon (acetylene, propylene, etc.) are heated to a high temperature of, for example, 800 ° C. or higher to decompose the hydrocarbons, and the surface of the silicon particles 201 is first surfaced. It is possible to coat carbon which becomes the conductive material 202 of.

For example, the silicon particles 201 coated with the first conductive material 202 are made by mixing carbon black (CB; Carbon Black) as the first conductive material 202 and the silicon particles 201, and using a dry ball mill. It can be produced by using and mixing. The composition of the silicon particles 201 coated with the first conductive material 202 can be determined from the mixing ratio of the CB and the silicon particles 201.

The porous structure of the layer formed from the first conductive material 202 can be controlled by the amount of the first conductive material 202 added to the surface area of the silicon particles 201. The surface area can be estimated from the average surface area per silicon particle 201 by measuring the particle size of the silicon particle 201, the specific surface area of the powder of the silicon particle 201, and the like. The amount of the first conductive material 202 added can be appropriately determined according to the average particle size of the silicon particles 201, that is, the specific surface area. It is desirable that the first conductive material covers almost the entire surface area of the silicon particles 201. By changing the coating amount and the layer thickness of the first conductive material 202, the average coating amount per silicon particle 201 and the average pore size can be controlled.

The presence of the first conductive material 202 on the surface of the silicon particles 201 can be confirmed from the peaks of silicon and carbon in the photoelectron spectroscopy measurement. As a simple method, the layer thickness can be measured by scraping the surface of silicon particles by ion sputtering while performing photoelectron spectroscopy measurement. As a more rigorous method, the thickness of the first conductive material 202 can be measured from the TEM photograph. The porous structure can be observed with a scanning electron microscope (SEM).

The coverage (θ) of the first conductive material 202 is the total specific surface area (S) of the silicon particles 201, and the thickness (t), weight (w), and density (w) of the first conductive material 202. It can be calculated from ρ). The density of the first conductive material 202 can be calculated by adhering the first conductive material 202 on a flat plate and obtaining the weight and thickness, but if it is a known material, the literature value is used. You may use it. The formula for calculating the coverage (θ) of the first conductive material 202 is shown below.
θ = w / (t ・ ρ ・ S) Expression 1

Subsequently, a method for producing the second conductive material 203 including the silicon particles 201 coated with the first conductive material 202 will be described. Here, "inclusion" means that a plurality of fibrous second conductive materials 203 enclose the silicon particles 201 coated with the first conductive material 202, for example, FIG. As shown in 2, a plurality of fibrous second conductive materials 203 are arranged in contact with silicon particles 201 coated with the first conductive material 202.

First, an outline of a method for producing the second conductive material 203 will be described. First, the silicon particles 201 coated with the first conductive material 202 and the diamond oxide catalyst fine particles 204 are mixed. Then, when a hydrocarbon is supplied to this mixture and the hydrocarbon is thermally decomposed, a plurality of second conductive materials 203 (two in FIG. 2) radiate from the diamond oxide catalyst fine particles 204 as a starting point. It grows up. Eventually, the second conductive material 203 grows while incorporating the plurality of silicon particles 201 in a vine tuft shape, and becomes a marimo-like morphology (spherical or deformed spherical shape). The "vine tuft" is a form in which at least one primary particle or secondary particle of silicon particles 201 is included (contacted) in at least two or more second conductive materials 203 and dispersed. Meaning, "marimo-like" means a form in which the second conductive material 203 is radially grown from the oxide diamond catalyst fine particles 204.

Here, the diamond oxide catalyst fine particles 204, which is the starting point of growth of the second conductive material 203, are catalyst fine particles containing diamond oxide and a metal catalyst supported on the diamond oxide. As the metal catalyst supported on diamond oxide, Ni, Co, Pd and the like can be used. The weight ratio of diamond oxide to the metal catalyst (diamond oxide: metal catalyst) is usually 70:30 to 90:10, preferably 75:25 to 85:15.

To support a metal catalyst on diamond oxide, the diamond oxide is immersed in an aqueous solution of a metal salt containing a metal serving as a metal catalyst, left for a predetermined time, for example, 12 hours, and then excess water is evaporated, dried, and then dried. For example, it is fired in air at 400 ° C. to 500 ° C. to decompose and oxidize the metal salt, and convert the metal salt into a metal oxide. Next, the metal oxide consisting of Ni, Co, Pd, etc. supported on the oxidized diamond oxidized by firing in air is reduced to a metal in a reducing atmosphere such as hydrogen to form a metal catalyst. Oxidized diamond catalyst fine particles 204 can be obtained.

Subsequently, the details of the method for producing the second conductive material 203 will be described. First, the diamond oxide catalyst fine particles 204 and the silicon particles 201 (hereinafter, also referred to as carbon-coated silicon particles) whose surface is coated with the first conductive material 202 are mixed so that they are uniformly dispersed. The weight ratio of the diamond oxide catalyst fine particles 204 to the carbon-coated silicon particles (diamond oxide catalyst fine particles 204: carbon-coated silicon particles) is usually 1: 1 to 1:99, preferably 1: 4 to 1: 9. Mix in.

In the present invention, the diamond oxide catalyst fine particles 204 remain in the negative electrode material (negative electrode). The diamond oxide catalyst fine particles 204 and Ni, Co or Pd as a metal catalyst contained therein and their alloys do not dissolve in the potential range in which the negative electrode operates in charge and discharge (usually 0 V to 2 V based on the Li reference electrode). However, since lithium ions are not occluded and released, it is desirable to use as little as possible. By reducing the amount of the diamond oxide catalyst fine particles 204 and the metal catalyst contained therein, the weight of the negative electrode can be reduced and the capacitance density of the negative electrode can be increased.

For mixing, a method of stirring and mixing two kinds of powders in a dry manner can be adopted. If the carbon-coated silicon particles are agglomerated, it is desirable to mix them wet. Examples of the solvent used for wet mixing include alcohols, hydrocarbons, ketones, aromatic compounds, 1-methyl-2-pyrrolidone, water and the like. For handling, a solvent that does not easily volatilize at room temperature is desirable. Ethanol and propanol (primary or secondary) can be selected for alcohols, hexane can be selected for hydrocarbons, and toluene can be selected for aromatic compounds. Moreover, you may use these 2 or more kinds of mixed solvents. For example, a mixed solvent of propanol and hexane can be used as the solvent. Such a solvent is mixed with diamond oxide catalyst fine particles 204 and carbon-coated silicon particles, and the mixture is stirred by rotating a stirrer or a propeller. The weight ratio of the solvent, the diamond oxide catalyst fine particles 204 and the carbon-coated silicon particles is not limited, but can usually be in the range of 1:19 to 19: 1. In order to increase the silicon content in the negative electrode material synthesized in the present invention, it is desirable to set the ratio in the range of 1:19 to 2: 8. Since the diamond oxide catalyst fine particles 204 and the carbon-coated silicon particles may be suspended in the solvent, the mass ratio of the powder to the solvent can be appropriately adjusted in the range of 1% to 50%. After mixing, the suspension is dried and the solvent is vaporized to prepare a powder in which the diamond oxide catalyst fine particles 204 and the carbon-coated silicon particles are uniformly dispersed.

By bringing the diamond oxide catalyst fine particles 204 and the carbon-coated silicon particles close to each other in this way, when the fibrous second conductive material 203 grows from the diamond oxide catalyst fine particles 204, a plurality of carbon-coated silicon particles are formed. It can be included in the fibrous second conductive material 203. Here, "proximity" means that the diamond oxide catalyst fine particles 204 are arranged in a range close to the carbon-coated silicon particles. For example, as shown in FIG. 2, the second conductive material 203 and the silicon particles 201 Including that they are placed in contact with each other. The contact state may be at least one carbon-coated silicon particle that is actually in contact. The diamond oxide catalyst fine particles 204 may be separated from the carbon-coated silicon particles as long as the fibrous second conductive material 203 is in contact with the carbon-coated silicon particles. This is because a plurality of fibrous second conductive materials 203 grow from the diamond oxide catalyst fine particles 204, so that at least one of the closest carbon-coated silicon particles is sufficient, and the other carbon-coated silicon particles are present. Since it is included in the growing second conductive material 203, it may be separated from the diamond oxide catalyst fine particles 204 without being in contact with the fine particles 204.

The fibrous carbon (carbon nanofibers), which is the second fibrous conductive material 203 that grows radially from the surface of the diamond oxide catalyst fine particles 204, uses a fluid vapor phase synthesizer and is a raw material gas (reaction gas). ), It can be produced by growing carbon nanofibers on the oxide diamond catalyst fine particles 204 by a thermal decomposition reaction. If the average particle size of the primary particles of the diamond oxide catalyst fine particles 204 is too large, it will be difficult to float in the reaction vessel of the fluidized gas phase synthesizer, so the average particle size is usually 500 nm or less, preferably 100 nm to 400 nm. The average particle size of the primary particles of the diamond oxide catalyst fine particles can be, for example, the average value of the equivalent circle diameters of 10 to 50 particles randomly selected in the TEM photograph. For example, when methane is used as the raw material gas and Ni, Co or Pd is used as the metal catalyst to be supported on diamond oxide, the growth temperature is usually 400 ° C. to 600 ° C. Further, when the carbon nanofibers are grown by suspending the diamond oxide catalyst fine particles 204 at a predetermined temperature and stirring them, the particle size of the marimo-like negative electrode material produced increases in proportion to the stirring time, that is, the reaction time. Therefore, when the catalyst content is high, the reaction time may be short, and when the catalyst content is low, the reaction time may be long.

The second conductive material 203 grows fibrously and radially from the diamond oxide catalyst fine particles 204 close to the coated carbon silicon particles, and as a result, the second conductive material 203 is oxidized together with the coated carbon silicon particles. A plurality of other carbon-coated silicon particles in the vicinity of the diamond catalyst fine particles 204 are involved and entangled to grow further. Therefore, the plurality of carbon-coated silicon particles are held in a vine tuft shape by the plurality of fibrous second conductive materials 203, and the negative electrode active material which is the negative electrode material of the present invention is synthesized.

After the second conductive material 203 has grown for a predetermined time, the supply of the raw material gas to the reaction vessel is stopped, the heating by the electric furnace is stopped, and the temperature is lowered to about room temperature. The negative electrode active material, which is the negative electrode material of the present invention, is recovered from the reaction vessel.

As shown in FIG. 2, the obtained second conductive material 203 grows radially from the diamond oxide catalyst fine particles 204, comes into contact with the first conductive material 202, and connects the plurality of silicon particles 201. Since the second conductive material 203 spreads and grows like a marimo, it functions as a conductive network of the entire negative electrode material. Silicon particles 201 coated with the first conductive material 202 adhere to the second conductive material 203 in a vine-like shape and are connected to the second conductive material 203. Since the second conductive material 203 is fibrous carbon, it is also referred to as FC (Fiber-like Carbon).

If the length of the second conductive material 203 is usually ½ or more of the average particle size of the primary particles of the silicon particles 201, the silicon particles 201 can be connected to each other. The upper limit of the length of the second conductive material 203 is not limited, but is usually 1 to 3 times the average particle size of the primary particles of the silicon particles 201. Further, since the second conductive material 203 does not contribute to charging / discharging, it is desirable that the volume is small. The average diameter of the second conductive material 203 may be usually 1/10 or less of the average particle size of the primary particles of the silicon particles 201, for example, 100 nm or less, preferably 50 nm or less, and more preferably 10 nm. It is as follows. When the average diameter of the second conductive material 203 is in the above range, the volume ratio of the second conductive material 203 can be reduced. The average diameter and length of the second conductive material 203 correspond to, for example, a circle having a diameter of 10 to 50 randomly selected in the TEM photograph if the average diameter is used. It can be the average value of the diameter, and if it is the length, it can be the average value of the lengths of 10 to 50 second conductive materials 203 randomly selected in the SEM photograph, for example. The length and diameter of the second conductive material 203 can be controlled by appropriately changing parameters such as catalyst content, raw material gas composition, flow rate, reaction tank temperature, and reaction time.

The content of the second conductive material 203 is the ratio of the total weight of the silicon particles 201 and the first conductive material 202 to the weight of the second conductive material 203 (silicon particles 201 and the first conductive material 202). : The second conductive material 203) is usually in the range of 85: 15-98: 2, in other words, the weight ratio of the second conductive material 203 to the total weight of the silicon particles 201 and the first conductive material 202 ( The second conductive material 203 / (silicon particles 201 + first conductive material 202)) is usually adjusted to be in the range of 0.020 to 0.176. When the content of the second conductive material 203 is within the above range, the first conductive material 202 and the second conductive material 203 have an optimum composition, and the volume changes during charging and discharging. However, the conductive network becomes strong.

The composition of the second conductive material 203 can be determined by the following formula by measuring the amount of silicon contained in the negative electrode active material after production. The gravimetric method (JIS G 1212 Annex 1, silicon dioxide gravimetric method (1)) can be applied to the analysis of silicon. The composition of the first conductive material 202 is the mixed composition before the dry mixing.

Second Conductive Material Composition (% by Weight) = 100-Silicon Composition (% by Weight) -First Conductive Material Composition (% by Weight) Equation 2

FIG. 2 shows how the negative electrode material changes from the state before charging (discharged state, left side figure) to the charged state (right side figure). The weight ratio of the second conductive material 203 to the total weight of the silicon particles 201 and the first conductive material 202 (second conductive material 203 / (silicon particles 201 + first conductive material 202)) is in the above range. Then, the conductive network is maintained even if the state changes from the discharged state to the charged state, and the state can be returned to the discharged state as it is.

The negative electrode material (also referred to as a silicon particle composite) of the present invention is composed of primary particles and secondary particles, and is contained in the negative electrode. The primary particle is composed of 1 silicon particle 201 and the first conductive material 202, and is the smallest unit obtained by mechanically crushing the silicon particle composite. The secondary particles are agglomerate particles in which a plurality of primary particles of silicon particles 201 are electrically connected by a first conductive material 202 and aggregated in the form of particles. Inside the negative electrode, the secondary particles are electrically connected by the second conductive material 203, so that all the silicon particles can be charged and discharged. The plurality of carbon-coated silicon particles electrically connected by the second conductive material 203 become agglomerates and become powdery, so that they can be distinguished from other agglomerates. Its aggregates, as measured by particle size distribution measurement by laser _{scattering, D 50} is usually 1 m to 100 m, further _{D 50} is 5Myuemu～20myuemu, smooth at the time of application of an anode slurry to the current collector It is more preferable because a negative electrode surface is formed.

The secondary particles of the silicon particle composite of the present invention are composed of the primary particles of a plurality of silicon particles 201 and the first conductive material 202, and the average particle size (D ₅₀ ) thereof is equal to or less than the thickness of the negative electrode mixture layer. Is specified to be. If the silicon powder contains coarse particles having a size equal to or larger than the thickness of the mixture layer, the coarse particles are removed in advance by sieving classification, wind flow classification, or the like to prepare particles having a size equal to or less than the thickness of the mixture layer. The average particle size can be measured using a particle size distribution meter using laser scattering.

Graphite can be further mixed with the negative electrode material of the present invention. Since graphite itself has conductivity, when it comes into contact with the first conductive material 202 or the second conductive material 203, it is possible to impart conductivity to the entire negative electrode material. Further, since graphite has a theoretical capacity of 372 Ah / kg, lithium ions can be occluded and released.

If the amount of graphite added is controlled as follows, a high-capacity negative electrode material can be obtained. The total amount of carbon contained in the two types of materials, the first conductive material 202 and the second conductive material 203, or the three types of the first conductive material 202, the second conductive material 203, and graphite. When the total amount of carbon contained in the material is defined as the total carbon amount, the ratio of the total carbon amount to the weight of silicon (total carbon amount / silicon) is usually 0.5 to 1.5.

The negative electrode 108 is manufactured using the negative electrode material of the present invention. First, a negative electrode slurry is prepared by mixing a negative electrode active material, which is the negative electrode material of the present invention, a binder, and a solvent.

Examples of the binder include, but are not limited to, polyamideimide, PVDF, a copolymer of vinylidene fluoride and hexafluoropropylene (PVDF-HFP), a binder of styrene butadiene rubber and carboxymethyl cellulose, and the like. As the binder, fluororubber, ethylene / propylene rubber, polyacrylic acid, polymethylmethacrylate, chitosan, alginic acid, polyimide, polyamide and the like can also be used. As the binder, any binder that does not decompose on the surface of the negative electrode in the lithium ion secondary battery and does not dissolve in the electrolytic solution can be used in the present invention. Polyamideimide is desirable as the binder.

As the binder, it is desirable to select a material that covers at least a part of the surface of the silicon particles 201 and allows lithium ions to pass through. At the time of charging, if the lithium ions in the electrolytic solution pass through the binder and reach the surface of the silicon particles 201, the lithium ions can be occluded. On the contrary, at the time of discharge, lithium ions are released from the surface of the silicon particles 201 to the electrolytic solution via the binder. By covering the surface of the silicon particles 201 with the binder, the reduction reaction of the electrolytic solution is suppressed, so that the initial charge / discharge efficiency (Coulomb efficiency) can be increased and the cycle life can be improved. Since the binder acts as a lithium ion conductor and the first conductive material 202 acts as an electron conductor, even if the volume of the silicon particles 201 changes due to charging and discharging, the binder and the like follow the volume change, and ions and electrons. It is considered that the conductivity of the is less likely to deteriorate.

The binder may be formed on at least a part of the surface of the silicon particles 201 as a composite layer integrated with the first conductive material 202. This is because the composite layer functions to transfer both electrons and ions.

The solvent used to prepare the negative electrode slurry may be any solvent that dissolves the binder. For example, when polyamide-imide is used as the binding, 1-methyl-2-pyrrolidone can be used. The solvent is selected according to the type of binder. A known kneader or disperser can be used for the dispersion treatment of the negative electrode material.

A known technique can be applied to the preparation of the negative electrode slurry, and a solvent capable of dispersing the negative electrode active material and the binder can be used.

Next, the negative electrode slurry is applied to the negative electrode current collector, the solvent is evaporated and dried, and the negative electrode mixture layer, which is a mixed layer composed of the negative electrode material and the binder, is formed on the negative electrode current collector. The method of applying the negative electrode slurry to the negative electrode current collector is not particularly limited, and known methods such as a doctor blade method, a dipping method, and a spray method can be adopted. Further, it is also possible to laminate a plurality of negative electrode mixture layers on the negative electrode current collector by performing the process from application of the slurry to drying a plurality of times.

The negative electrode current collector that can be used in the present invention includes a copper foil having a thickness of usually 10 μm to 100 μm, a hole having a hole diameter of usually 0.1 mm to 10 mm, and a copper perforation having a thickness of usually 10 μm to 100 μm. Foil, expanded metal, foamed metal plate, etc. are used, and in addition to copper, stainless steel, titanium, nickel, etc. can also be applied. In the present invention, any negative electrode current collector can be used without being limited by the material, shape, manufacturing method, and the like.

It is desirable that the thickness of the negative electrode mixture layer is equal to or larger than the average particle size measured by the laser scattering method of the negative electrode active material which is the negative electrode material of the present invention. This is because if the thickness of the negative electrode mixture layer is less than the average particle size, the electron conductivity between adjacent particles is lowered. The average particle size of the secondary particles of the silicon particle composite, which is the negative electrode material of the present invention, is usually _{5 μm to 20 μm for D50 when measured with a laser scattering particle size distribution measuring device.} Therefore, when the thickness of the negative electrode mixture layer is made _{larger than D 50} , the higher effect of the present invention can be obtained. Further, it is desirable that the upper limit of the thickness of the negative electrode mixture layer is 50 μm or less. This is because if the thickness is higher than that, the charging level of the negative electrode active material near the surface of the mixture and the surface of the negative electrode current collector will vary unless the amount of the conductive agent is increased, and uneven charging / discharging may occur. Further, when the amount of the conductive agent is increased, the volume of the negative electrode becomes bulky and the energy density of the battery decreases.

A negative electrode slurry is applied, the solvent is vaporized at, for example, 80 ° C. to 120 ° C., the binder is cured by heat treatment at, for example, 200 ° C. to 400 ° C., and pressure molding is performed by a roll press to form a negative electrode on the negative electrode current collector. The negative electrode 108 on which the mixture layer is formed can be produced. (Also referred to as a negative density) density of the negative electrode mixture layer is usually in the range of ^{0.5g / cm 3 ~1.3g / cm 3} . Since the negative electrode density is within the above range, a space is secured in the negative electrode mixture layer, and even when the silicon particles 201 expand as shown in FIG. 2, the space remains in the negative electrode mixture layer and the electrolyte channel becomes Be retained. Therefore, even in the charged state, lithium ions can be diffused inside the negative electrode, and lithium ions can be occlusioned. Further, even in the case of discharging on the contrary, since the electrolytic solution exists in the vicinity of the silicon particles 201, lithium ions can be discharged to the electrolytic solution.

The negative electrode containing the negative electrode material of the present invention has a high capacity and excellent rate characteristics. Therefore, by using the negative electrode material of the present invention, a lithium ion secondary battery having a high capacity and excellent rate characteristics is mounted. It becomes possible to provide a power storage device.

(Manufacturing of electrolyte)
In this embodiment, the electrolytic solution contains a solvent and an electrolyte.

Solvents that can be used in the electrolytic solution are propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, 1,2-dimethoxyethane, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-Dioxolane, formamide, dimethylformamide, methyl propionate, ethyl propionate, phosphate triester, trimethoxymethane, dioxolane, diethyl ether, sulfolane, 3-methyl-2-oxazolidinone, tetrahydrofuran, 1,2-di There is one or more non-aqueous solvents selected from the group consisting of ethoxyethane, chloroethylene carbonate, chloropropylene carbonate and the like. Other solvents may be used as long as they do not decompose on the positive electrode or the negative electrode built in the battery of the present invention.

Further, the electrolyte, the chemical formula _{LiPF 6, LiBF 4, LiClO 4} , LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, or multi such imide lithium salts represented by lithium trifluoromethane sulfonimide There are different types of lithium salts. A non-aqueous electrolyte solution prepared by dissolving these salts in the above-mentioned solvent can be used as the battery electrolyte solution. Other electrolytes may be used as long as they do not decompose on the positive electrode or the negative electrode built in the battery of the present invention.

_{As a typical example of the electrolytic solution that can be used in the present embodiment, lithium hexafluorophosphate (LiPF 6} ) or lithium borofluoride is used as an electrolyte in a solvent obtained by mixing ethylene carbonate with dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or the like. There is a solution in which (LiBF _{4) is dissolved.}

In the present embodiment, the type of solvent and electrolyte, and the mixing ratio of the solvent are not limited. In addition to the above-mentioned electrolytic solution, other electrolytic solutions can also be used.

For example, a solid polymer electrolyte (polymer electrolyte) can be used instead of the non-aqueous electrolyte solution. In this case, an ionic conductive polymer such as ethylene oxide, acrylonitrile, polyvinylidene fluoride, methyl methacrylate, or polyethylene oxide of hexafluoropropylene can be used as the electrolyte. When these solid polymer electrolytes are used, there is an advantage that the separator 109 can be omitted.

It is also possible to use a mixture of polyvinylidene fluoride and a non-aqueous electrolyte solution (gel electrolyte).

In addition, ionic liquids can be used. For example, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF ₄ ), a mixed complex of lithium salt LiN (SO ₂ CF ₃ ) ₂ (LiTFSI), triglime and tetraglime, cyclic quaternary ammonium cation. (N-methyl-N-propylpyrrolidinium is exemplified.) And imide-based anion (bis (fluorosulfonyl) imide is exemplified.) Select a combination that does not decompose at the positive and negative sides, and present the present. It can be used in the lithium ion secondary battery of the present invention.

(Assembly of lithium ion secondary battery)
The structure of the electrode group may be the square structure shown in FIG. 1, or may be various shapes such as a cylindrical structure, a flat shape, or a strip-shaped structure. can. The shape of the battery container may be selected from a cylindrical shape, a flat oval shape, a square shape, and the like according to the shape of the electrode group.

The positive electrode 107, the separator 109, and the negative electrode 108 are laminated to form an electrode group. The separator 109 functions as a medium for moving lithium ions between the positive electrode and the negative electrode while preventing a short circuit between the positive electrode 107 and the negative electrode 108.

A separator 109 is also inserted between the electrode arranged at the end of the electrode group and the battery container 102 so that the positive electrode 107 and the negative electrode 108 are not short-circuited through the battery container 102.

The electrode group is electrically connected to an external terminal via a lead wire at the upper part. Specifically, the positive electrode 107 is connected to the positive electrode external terminal 104 of the lid 103 via the positive electrode lead wire 110. The negative electrode 108 is connected to the negative electrode external terminal 105 of the lid 103 via the negative electrode lead wire 111. The shapes and materials of the lead wires 110 and 111 are arbitrary as long as they have a structure capable of reducing ohm loss when an electric current is passed and do not react with the electrolytic solution, depending on the structure of the battery container 102. You can choose. For example, the lead wires 110 and 111 can take any shape such as a wire shape, a plate shape, and a foil shape.

After that, the lid 103 is brought into close contact with the battery container 102, and the entire battery is sealed. For example, the lid 103 can be attached to the battery container 102 by caulking. Known techniques such as welding and adhesion may be applied to the method of sealing the battery.

As a method of injecting the electrolytic solution, there are a method of removing the lid 103 from the battery container 102 and adding it directly to the electrode group, or a method of adding it from a liquid injection plug 106 installed on the lid 103.

The liquid injection plug 106 is used when adding an electrolyte to the positive electrode 107 or the negative electrode 108, but the liquid injection plug 106 is unnecessary when the electrode holds the electrolyte in advance.

Since the lithium ion secondary battery of the present invention contains the negative electrode material, it has a high capacity and excellent rate characteristics.

Next, the present embodiment will be described in more detail based on Examples and Comparative Examples, but the present embodiment is not limited to these Examples.

In the examples, except for the matters described below, the above-described (configuration of the lithium ion secondary battery according to the embodiment of the present invention), (manufacturing of the positive electrode), (manufacturing of the negative electrode), (manufacturing of the electrolytic solution). ) And (Assembly of a lithium ion secondary battery), a lithium ion secondary battery having the structure shown in FIG. 1 was manufactured.

(Manufacturing of positive electrode 107)
As the positive electrode active material, LiNi _1/3 Mn _1/3 Co _1/3 O ₂ was used.

PVDF was used as the binder of the positive electrode slurry, and 1-methyl-2-pyrrolidone was used as the solvent. A known kneader and disperser were used for the dispersion treatment of the positive electrode slurry.

(Manufacturing of negative electrode 108)
As the negative electrode active material, a negative electrode material having the composition shown in Table 1 below was used.

The carbon-coated silicon was produced as follows. First, silicon powder and carbon black (CB) are mixed in a composition of 70:30 to 90:10 by weight, and then stirred using a planetary ball mill to surface the silicon particles with carbon black (first conductivity). Material) was attached to produce carbon-coated silicon. The mixing ratio of silicon particles and carbon black (CB) is shown in the column of weight composition of the negative electrode material in Table 1.

Next, the production of the second conductive material was carried out as follows. First, the carbon-coated silicon and the oxide diamond catalyst fine particles were collected at a weight ratio of 1: 9. This was mixed by a wet 1, 2 or 3 dispersion method. In wet 1, 2-propanol was used, in wet 2, hexane was used, and in wet 3, a mixed solvent of 2-propanol and hexane having a volume ratio of 1: 9 was used.

After the dispersion treatment, the solvent was dried at room temperature, and the solvent was completely removed under vacuum.

Subsequently, the mixture of the carbon-coated silicon and the oxide diamond catalyst fine particles produced above was placed in the reaction vessel. The reaction vessel was heated to 600 ° C. by an electric furnace, and then propane was used as the raw material gas and Ar was used as the dilution gas. The flow rate of propane was 10% and the flow rate was 100 sccm (sccm was cm ³ / min). Propane as a raw material gas was supplied to the reaction vessel so as to be. The reaction time was 60 minutes, and after 60 minutes had passed, the energization of the electric furnace was stopped and the reaction vessel was naturally cooled to produce a negative electrode active material. The weight ratio of the diamond oxide catalyst fine particles to the carbon-coated silicon particles was 1: 9.

Further, a negative electrode slurry was prepared by using polyamide-imide as a binder and 1-methyl-2-pyrrolidone as a solvent. A known kneader and disperser were used for the dispersion treatment of the negative electrode slurry. The negative electrode slurry was applied once on a rolled copper foil having a thickness of 10 μm by the doctor blade method, dried at 100 ° C., and then heat-treated at 300 ° C. in vacuum to cure the binder.

(Manufacturing of electrolytic solution L)
The electrolytic solution L was prepared by dissolving 1 molar concentration (1M = 1 mol / dm ³ ) of LiPF ₆ in a mixed solvent of ethylene carbonate (denoted as EC) and ethyl methyl carbonate (denoted as EMC). The mixing ratio of EC and EMC was 1: 2 by volume. The electrolyte concentration was set to 1 molar concentration as a reference value. Further, 1% by weight of vinylene carbonate was added to the electrolytic solution.

(Assembly of lithium ion secondary battery 101)
As the separator 109, a polyethylene single layer having a thickness of 25 μm and a porosity of 45% was used. The lid 103 was attached to the battery container 102 by caulking. The rated capacity (calculated value) of the manufactured lithium ion secondary battery 101 was set to 3 Ah. When the electrode coating amount was changed, the area and the number of electrodes were changed, and the electrode size was adjusted so that the rated capacity could be obtained.

(Examples 1 to 7)
Table 1 shows the specifications of the negative electrode material of the examples, the composition of the negative electrode mixture containing the binder, and the negative electrode density. The second conductive material of the present invention is labeled as fibrous carbon (FC; Fiber-like carbon).

The first conductive material is carbon black (CB), and the weight composition of CB is the composition of silicon particles and CB before mixing. The composition of the second conductive material was determined from the weight change of the negative electrode active material obtained after the raw material gas was thermally decomposed by the CVD apparatus (see Equation 2).

Examples 1 to 4 are embodiments of the negative electrode active material when the composition of CB / Si is changed in the range of 0.333 to 0.053 and the weight ratio of FC / (Si + CB) is kept constant. The particle size distribution of the carbon-coated silicon particle powder of Example 3 was measured by a laser scattering particle size distribution measuring device using a dispersion liquid obtained by mixing and dispersing the powder, water, and a small amount of a surfactant. As a result, D ₅₀ was 7.8 _{μm and D 90} was 20 μm. When the carbon-coated silicon particles and the diamond oxide catalyst fine particles were mixed, 2-propanol was used as a solvent and the particles were mixed by a wet method (wet 1). Using this, a second conductive material (FB) was grown by CVD.

Example 5 is an example in which a negative electrode active material prepared in Example 3 mixed with natural graphite powder having an average particle size of 20 μm is used as the negative electrode.

In Example 6, hexane was used as a solvent, and carbon-coated silicon particles and diamond oxide catalyst fine particles were mixed by a wet method (wet 2). Using this, a second conductive material (FB) was grown by CVD.

In Example 7, carbon-coated silicon particles and diamond oxide catalyst fine particles were mixed by a wet method using a mixed solvent of 2-propanol and hexane having a volume ratio of 1: 9 as a solvent (wet 3). Using this, a second conductive material (FB) was grown by CVD.

In Example 8, carbon-coated silicon particles and a diamond oxide catalyst were mixed by a dry method (dry method). Using this, a second conductive material (FB) was grown by CVD.

Table 2 shows the evaluation results of the lithium ion secondary battery of the example. In the evaluation of the lithium ion secondary battery, the batteries of each example were subjected to the initial aging treatment. First, charging was started from the open circuit state. At 0.2 CA current, the voltage was maintained when 4.2 V was reached, and charging was continued until the current became 1/10. After that, a rest period of 30 minutes was provided, and discharge was started at 0.2 CA. When the battery voltage reached 2.8 V, the discharge was stopped and the battery was paused for 30 minutes. Similarly, charging and discharging were repeated 5 times to complete the initial aging process of the battery.

Next, charging was set as the condition at the time of initialization, and discharge tests of 0.5 CA and 1 CA were carried out. Table 2 shows the discharge capacity.

In each of the examples, the initial charge / discharge efficiency exceeded 85%, and the initial capacity was in agreement with the design value. In addition, the discharge capacity ratio of 0.5CA and 1CA to 0.2CA was also high. With the configuration of the present invention, it was possible to obtain a negative electrode having a high capacity and excellent rate characteristics.

(Comparative Examples 1 to 3)
In the comparative example, a lithium ion secondary battery was manufactured in the same manner as in the examples except that the negative electrode active material described below was used.

In Comparative Example 1, only the powder of silicon particles coated with the first conductive material was used as it was as the negative electrode active material, and the composition of the second conductive material was set to zero.

In Comparative Example 2, as the negative electrode active material, a powder of silicon particles was directly mixed with diamond oxide catalyst fine particles to grow a second conductive material, and the composition of the first conductive material was zero. And said.

In Comparative Example 3, as the negative electrode active material, a powder obtained by simply mixing a powder of silicon particles, a first conductive material, and a second conductive material in a dry manner was used.

Table 3 shows the specifications of the negative electrode material of the comparative example, the composition of the negative electrode mixture containing the binder, and the negative electrode density.

Table 4 shows the evaluation results of the lithium ion secondary battery of the comparative example. The evaluation method was the same as in the examples. In Comparative Example 1, the discharge capacity of 0.2CA was equivalent to that of Examples 1 and 2, but the rate characteristics of 0.5CA and 1CA were lowered. In addition, the initial charge / discharge efficiency of Comparative Example 1 was low. In Comparative Example 2, the initial charge / discharge efficiency was low, and the discharge capacity was also greatly reduced. In Comparative Example 3, although the initial charge / discharge efficiency and the discharge capacity of 0.2CA were high, the rate characteristics of 0.5CA and 1CA were lowered.

Although the embodiment of the present embodiment has been described in detail above, the specific configuration is not limited to this embodiment, and even if there is a design change or the like within a range that does not deviate from the gist of the present embodiment. They are included in this embodiment.

101 Lithium ion secondary battery 102 Battery container 103 Lid 104 Positive electrode external terminal 105 Negative electrode external terminal 106 Injection plug 107 Positive electrode 108 Negative electrode 109 Separator 110 Positive electrode lead wire 111 Negative electrode lead wire 112 Insulation seal member L Electrolyte 201 Silicon particles 202 First Conductive material 203 Second conductive material 204 Diamond oxide catalyst fine particles

Claims (6)

Silicon particles, diamond oxide catalyst fine particles close to the silicon particles, a first conductive material having a porous structure coated with silicon particles, and a second fibrous carbon extending from the diamond oxide catalyst fine particles. Including conductive materials
A negative electrode material for a lithium ion secondary battery in which silicon particles are held in a tuft of vines in a second conductive material. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the first conductive material is carbon black, and the weight ratio of the silicon particles to the first conductive material is 95: 5 to 75:25. The negative electrode material for a lithium ion secondary battery according to claim 1, wherein the coating thickness of the first conductive material is 100 nm or more, and the coating ratio with respect to the surface area of silicon particles is 70% to 90%. The negative electrode material for a lithium ion secondary battery according to claim 1, further comprising graphite. A lithium ion secondary battery containing the negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4 as a negative electrode active material. The step of preparing the silicon particles coated with the first conductive material by coating the surface of the silicon particles with the first conductive material, and
The step of mixing the silicon particles coated with the first conductive material and the diamond oxide catalyst fine particles,
The negative electrode material for a lithium ion secondary battery according to any one of claims 1 to 4, which comprises a step of growing a second conductive material from the diamond oxide catalyst fine particles by a thermal decomposition reaction of hydrocarbons. Method.

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