JP2016170930A - Negative electrode active material layer and power storage device having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material layer that can enhance the initial capacity and initial efficiency of a power storage device.SOLUTION: A negative electrode active material layer includes first powder 2 which has at least one kind selected from Si, an Si compound, Sn and an Sn compound, and has an average particle diameter Dranging from 0.1 to 20 μm, and second powder 3 containing plate-like graphite particles which have a thickness ranging from 0.3 to 100nm and a length in the long axis direction ranging from 0.1 to 100 μm. The average particle diameter Dof the second powder 3 is smaller than the average particle diameter Dof the first powder 2. The second powder 3 can be interposed among the first powder because it has a smaller diameter than the first powder 2, and can form an efficient and excellent electrically conductive path. On the surface of the plate-like graphite particles are adsorbed aromatic vinyl copolymer made from vinyl aromatic monomers represented by the formula (1): -(CH-CHX)- (X represents phenyl, naphthyl, anthracenyl or pyrenyl).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a negative electrode active material layer and a power storage device including the negative electrode active material layer.

  A lithium ion secondary battery is a secondary battery having a high charge / discharge capacity and capable of high output. Currently, it is mainly used as a power source for portable electronic devices, and further expected as a power source for electric vehicles that are expected to be widely used in the future.

  In recent years, silicon-based materials such as silicon and silicon compounds, and tin-based materials such as tin and tin compounds, which have a charge / discharge capacity that greatly exceeds the theoretical capacity of carbon materials, have been studied as negative electrode active materials for lithium ion secondary batteries. . However, it is known that when a silicon-based material or a tin-based material is used as the negative electrode active material, the negative electrode active material expands and contracts with insertion and extraction of lithium (Li) in the charge / discharge cycle.

  As the negative electrode active material expands and contracts, a load is applied to the binder that holds the negative electrode active material on the current collector. As a result, the negative electrode active material is distorted due to a decrease in adhesion between the negative electrode active material and the current collector, a decrease in capacity due to an increase in electrode resistance due to the destruction of the conductive path in the electrode, and repeated expansion and contraction. Desorption of the active material from the electrode accompanying the refinement of the negative electrode active material that occurs is generated. Thus, the expansion and contraction of the negative electrode active material affect the cycle characteristics of the battery. Various studies have been conducted in order to suppress the deterioration of the cycle characteristics.

For example, the use of silicon oxide (SiO _x : x is about 0.5 ≦ x ≦ 1.5) as a negative electrode active material has been studied. It is known that SiO _x decomposes into Si and SiO ₂ when heat-treated. This is called disproportionation reaction, and is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. The Si phase obtained by separation is very fine. Further, the SiO ₂ phase covering the Si phase has a function of suppressing decomposition of the electrolytic solution. Therefore, the secondary battery using the negative electrode active material made of SiO _x decomposed into Si and SiO ₂ has excellent cycle characteristics.

As the silicon particles constituting the Si phase of SiO _x are finer, the secondary battery using the silicon particles as the negative electrode active material has improved cycle characteristics. Therefore, Japanese Patent No. 3865033 (Patent Document 1) describes a method of heating and sublimating metal silicon and SiO ₂ to form silicon oxide gas and cooling it to produce SiO _x . According to this method, the particle diameter of the silicon particles constituting the Si phase can be made 1-5 nm nanosize.

JP 2009-102219 (Patent Document 2) discloses that a silicon raw material is decomposed to an elemental state in a high-temperature plasma and rapidly cooled to a liquid nitrogen temperature to obtain silicon nanoparticles. A manufacturing method for fixing in a SiO ₂ —TiO ₂ matrix by a sol-gel method or the like is described.

  However, in the manufacturing method described in Patent Document 1, the matrix is limited to a sublimable material. Moreover, in the manufacturing method described in Patent Document 2, high energy is required for plasma discharge. Further, the silicon composites obtained by these production methods have a problem that the dispersibility of Si-phase silicon particles is low and they are likely to aggregate. When the Si particles are aggregated to increase the particle size, the secondary battery using the Si particles as the negative electrode active material has a low initial capacity and also deteriorates the cycle characteristics.

By the way, in recent years, nanosilicon materials that are expected to be used in various fields such as semiconductors, electrical / electronics, and the like have been developed. For example, in Physical Review B (1993), vol 48, 8172-8189 (Non-patent Document 1), hydrogen chloride (hereinafter referred to as HCl) and calcium disilicide (hereinafter referred to as CaSi ₂ ) are reacted. Thus, a method for synthesizing a layered polysilane is described, and it is described that the layered polysilane thus obtained can be used for a light emitting device or the like. Japanese Unexamined Patent Application Publication No. 2011-090806 (Patent Document 3) describes a lithium ion secondary battery using layered polysilane as a negative electrode active material.

  Japanese Patent Laid-Open No. 2004-349056 (Patent Document 4) describes a negative electrode material for a lithium ion secondary battery in which one end of a carbon fiber is bonded to the surface of an active material nucleus containing a silicon-based active material. Yes. The stress acting during charging / discharging can be relaxed by the carbon fiber, and the conductivity is improved by forming the conductive path, so that the cycle characteristics of the secondary battery are improved. However, the manufacturing method described in Patent Document 4 uses a technique of growing carbon fibers, which complicates the synthesis process.

  As described above, various negative electrode active materials have been studied, but a negative electrode active material layer that can further improve the initial capacity and the initial efficiency is demanded.

Japanese Patent No. 3865033 JP 2009-102219 A JP 2011-090806 A JP 2004-349056 A

Physical Review B (1993), vol48, 8172-8189

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a negative electrode active material layer capable of improving initial capacity and initial efficiency and a power storage device including the negative electrode active material layer.

The negative electrode active material layer of the present invention that solves the above problems includes a first powder having at least one selected from the group consisting of Si, Si compound, Sn, and Sn compound, a thickness of 0.3 nm to 100 nm, and a long axis direction. And a second powder having plate-like graphite particles having a length of 0.1 μm to 100 μm, wherein the average particle diameter D ₅₀ of the second powder is smaller than the average particle diameter D ₅₀ of the first powder. .

The average particle diameter D ₅₀ can be measured by particle size distribution measurement method. The average particle diameter D ₅₀ refers to the particle size cumulative value of the volume distribution in the particle size distribution measurement by laser diffraction method is equivalent to 50%. That is, the average particle diameter D ₅₀ refers to the median diameter measured by volume.

On the surface of the plate-like graphite particles, the following formula (1): — (CH ₂ —CHX) — (1)
(In Formula (1), X represents a phenyl group, a naphthyl group, an anthracenyl group, or a pyrenyl group, and these groups may have a substituent). It is preferable that the aromatic vinyl copolymer is adsorbed.

The average particle diameter _{D 50} of the first powder is preferably greater 20μm or less than 0.1 [mu] m.

The average particle diameter _{D 50} of the second powder is preferably 0.1μm or more 10μm or less.

  When the total of the first powder and the second powder is 100% by mass, the second powder is preferably contained in an amount of 5% by mass to 30% by mass, and more preferably 10% by mass to 20% by mass. preferable.

  It is preferable that a polyamideimide is included as a binder.

  A power storage device according to the present invention includes the negative electrode active material layer.

  The power storage device is preferably a lithium ion secondary battery.

The negative electrode active material layer of the present invention includes a first powder having at least one selected from the group consisting of Si, Si compound, Sn and Sn compound and a second powder having plate-like graphite particles, and an average of the second powder particle size _{D 50} is smaller than the average particle diameter _{D 50} of the first powder. Therefore, the second powder is favorably disposed in the gap between the first powders and the gap between the current collector and the first powder. The plate-like graphite particles constituting the second powder have a layer structure in which a plurality of graphene single layers are laminated, and are excellent in conductivity and strength. In addition, since the aspect ratio is large, it is excellent in flexibility and has the feature of following the surrounding structural change due to volume change. In the power storage device of the present invention in which the second powder having such plate-like graphite particles is well disposed in the negative electrode active material layer, the initial capacity and the initial efficiency are excellent.

It is sectional drawing which represents typically the preferable one aspect | mode of the negative electrode using the negative electrode active material layer of this invention. 3 is a graph showing the particle size distribution of the graphite-polymer composite of Production Example 1 and the graphite-polymer composite of Comparative Production Example 1. FIG. 2 is a photograph showing a scanning electron microscope (hereinafter referred to as SEM) observation result of a cross section of the negative electrode active material layer of Example 1. FIG.

  The best mode for carrying out the present invention will be described below. Unless otherwise specified, the numerical range “x to y” described in this specification includes the lower limit x and the upper limit y in the range. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from the numerical value range can be used as upper and lower numerical values.

Negative electrode active material layer of the present invention comprises a first powder and a second powder, the average particle diameter D ₅₀ of the second powder having a plate-like graphite particles, selected Si, Si compounds, from the group consisting of Sn and Sn compounds smaller than the average particle diameter D ₅₀ of the first powder with at least one kind.

<First powder>
The first powder has at least one selected from the group consisting of Si, Si compound, Sn, and Sn compound. The first powder can occlude and release charge carriers such as Li and functions as a negative electrode active material.

It is preferred first powder average particle size _{D 50} of at most greater 20μm than 0.1 [mu] m. More preferably the average particle diameter _{D 50} of the first powder is 0.3μm or more 15μm or less, and more preferably an average particle diameter _{D 50} of the first powder is 1μm or more 10μm or less.

When the average particle diameter D ₅₀ of the first powder is too small, the specific surface area of the first powder becomes larger, increases the contact area between the first powder and the electrolyte, it will proceed decomposition of the electrolyte solution, a power storage device The cycle characteristics may be deteriorated. Also there is a possibility that the secondary particle size of the first powder becomes excessively large due to aggregation and an average particle diameter D ₅₀ is too small.

When the average particle diameter D ₅₀ of the first powder is too large, difficult first powder is uniformly dispersed in the anode active material layer, the conductivity of the whole electrode becomes uneven, charging and discharging characteristics may be deteriorated.

  Si, Si compound, Sn, and Sn compound expand and contract during charge and discharge. If the crystallite size of Si, Si compound, Sn, and Sn compound is too large, the first powder may collapse due to stress concentration during expansion and contraction. Therefore, it is preferable that the crystallite size of Si, Si compound, Sn, and Sn compound in the first powder is, for example, 1 nm to 300 nm.

  The crystallite size is calculated by the Scherrer equation from the half width of the diffraction peak obtained by X-ray diffraction (XRD) measurement.

  As the Si, Si compound, Sn, and Sn compound, known materials used as negative electrode active materials can be employed.

Examples of the Si compound include SiO _x (0.3 ≦ x ≦ 1.6), SiB ₄ , SiB ₆ Mg ₂ Si, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaSi ₂ , and CrSi. ₂ , Cu ₅ Si, FeSi ₂ , MnSi ₂ , NbSi ₂ , TaSi ₂ , VSi ₂ , WSi ₂ , ZnSi ₂ , SiC, Si ₃ N ₄ Si ₂ N ₂ O, SnSiO ₃ , LiSiO and the like.

Examples of the Sn compound include SnO _w (0 <w ≦ 2), SnSiO ₃ , LiSnO, tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.).

Moreover, the silicon material obtained through a decalcification reaction from CaSi ₂ disclosed in International Publication No. 2014/080608 can also be used as the silicon-based material. As the silicon material, for example, a product obtained by treating CaSi ₂ with an acid such as hydrochloric acid or hydrogen fluoride is subjected to a heat treatment (also referred to as baking) at 300 ° C. to 1000 ° C., for example. can get.

  In addition, you may use the composite_body | complex (henceforth a Si / C composite_body | complex) which compounded Si and Si compound, and carbon as 1st powder.

(Si / C composite)
In the Si / C composite, the carbon layer covers at least the surface of Si or Si compound. Note that the carbon constituting the carbon layer may be amorphous carbon only, crystalline carbon only, or a mixture of amorphous carbon and crystalline carbon. Also good.

  The carbonization process is not particularly limited, but as the carbonization process, a carbon powder and a Si or Si compound powder are mixed (for example, mechanical milling), and a mixture obtained from a composite of a resin and a Si or Si compound. And a step of carbonizing the organic gas by bringing Si or a Si compound into contact with the organic gas in a non-oxidizing atmosphere and heating.

<Second powder>
The second powder of the present invention functions as a negative electrode active material and / or a conductive aid.

  The second powder has plate-like graphite particles having a thickness of 0.3 nm to 100 nm and a length in the major axis direction of 0.1 μm to 100 μm. The plate-like graphite particles have a layer structure in which a plurality of graphene single layers are laminated, and are excellent in conductivity and strength. In addition, since the aspect ratio is large, it is excellent in flexibility and has the feature of following the surrounding structural change due to volume change.

  The plate-like graphite particles are much smaller in thickness than the scale-like graphite that is natural graphite. The aspect ratio determined by the length / thickness in the major axis direction of the plate-like graphite particles is 10 to 1000, and more preferably 50 to 100.

  The thickness of the plate-like graphite particles is 0.3 nm to 100 nm, more preferably 1 nm to 100 nm, and further preferably 30 nm to 80 nm. The length of the plate-like graphite particles in the major axis direction is 0.1 μm to 100 μm, more preferably 0.2 μm to 50 μm, and still more preferably 0.3 μm to 10 μm. The length of the plate-like graphite particles in the minor axis direction is preferably 0.1 μm to 100 μm, more preferably 0.1 μm to 50 μm, and further preferably 0.1 μm to 8 μm.

The average particle diameter D ₅₀ of the second powder is smaller than the average particle diameter D ₅₀ of the first powder. Therefore, the plate-like graphite particles constituting the second powder are easily arranged between the first powders, and the plate-like graphite particles have good conductivity between the first powders and between the first powder and the current collector. A path can be formed.

Second powder having an average particle diameter D ₅₀ of, smaller than the average particle diameter D ₅₀ of the first powder, although not particularly limit the scope of the particle size, the average particle diameter D ₅₀ of the first powder greater than 0.1μm since it is preferable that 20μm or less, an average particle diameter _{D 50} of the second powder is preferably 0.1μm or more 10μm or less. The average particle diameter _{D 50} of the second powder is more preferably 0.1μm or more 8μm or less, the average particle diameter _{D 50} of the second powder, and further preferably 0.1μm or more 5μm or less.

  The plate-like graphite particles can be obtained, for example, by pulverizing known graphite having a graphite structure, specifically artificial graphite, scale-like graphite, massive graphite, earth-like graphite or the like so that the graphite structure is not destroyed. Commercially available graphene can be used as the plate-like graphite particles.

  Examples of the pulverization treatment include ultrasonic treatment, treatment with a ball mill, wet pulverization, explosion, mechanical pulverization, and wet high pressure pulverization. In the ultrasonic treatment, the oscillation frequency is preferably 15 kHz to 400 kHz, and the output is preferably 500 W or less. As the grinding treatment, ultrasonic treatment or wet grinding treatment is preferable. Moreover, as temperature at the time of a grinding | pulverization process, it can be set as -20 degreeC-100 degreeC, for example. Moreover, as a grinding | pulverization processing time, it can be set as 0.01 hour-50 hours, for example. In the case of wet high-pressure pulverization or the like, the pressure during the pulverization treatment is preferably 50 MPa to 400 MPa.

  It is preferable that functional groups such as a hydroxyl group, a carboxyl group, and an epoxy group are bonded to the surface of the plate-like graphite particles. When the functional group is bonded to the surface of the plate-like graphite particles, the affinity between the plate-like graphite particles and other organic substances such as a solvent and a polymer is increased.

  This is particularly useful for increasing the affinity between the later-described aromatic vinyl copolymer and the plate-like graphite particles. That is, the presence of these functional groups tends to increase the amount of adsorption of the aromatic vinyl copolymer to the plate-like graphite particles.

  Such functional groups are 0.01 atomic% or more and 50 atomic% or less, assuming that the total carbon atoms in the vicinity of the surface of the plate-like graphite particles, preferably in the region from the surface to a depth of 10 nm are 100 atomic%, More preferably, it is bonded to 20 atom% or less, particularly preferably 10 atom% or less. When the ratio of the carbon atom to which the functional group is bonded exceeds 50 atomic%, the hydrophilicity of the plate-like graphite particles tends to increase and the affinity with the organic substance tends to decrease. The functional groups near the surface of the plate-like graphite particles can be quantified by X-ray photoelectron spectroscopy (XPS).

Further, on the surface of the plate-like graphite particles, the following formula (1):
_{- (CH 2 -CHX) - (} 1)
(In Formula (1), X represents a phenyl group, a naphthyl group, an anthracenyl group, or a pyrenyl group, and these groups may have a substituent.)
It is preferable that the aromatic vinyl copolymer containing the vinyl aromatic monomer unit represented by these is adsorbed.

  Plate-like graphite particles adsorbed with an aromatic vinyl copolymer (hereinafter referred to as a graphite-polymer composite) exhibit suitable affinity with an organic solvent or a binder. Therefore, the dispersibility of the graphite-polymer composite in the liquid containing the binder is suitable.

  In the present specification, an aggregate of plate-like graphite particles and / or an aggregate of graphite-polymer composite is referred to as a second powder. That is, the plate-like graphite particles constituting the second powder may be formed by adsorbing an aromatic vinyl copolymer.

  The vinyl aromatic monomer unit is considered to be adsorbed on the six-membered ring structure on the surface of the plate-like graphite particles. And the vinyl aromatic monomer unit which has a vinyl aromatic monomer unit adsorb | sucks to plate-like graphite particle | grains because the said vinyl aromatic monomer unit adsorb | sucks to plate-like graphite particle | grains.

  The aromatic vinyl copolymer preferably contains a vinyl aromatic monomer unit and a monomer unit other than the vinyl aromatic monomer unit (hereinafter referred to as a second monomer unit). In the aromatic vinyl copolymer, the vinyl aromatic monomer unit is likely to be adsorbed on the plate-like graphite particles, and the second monomer unit is likely to be compatible with the solvent, the resin, and the functional group on the surface of the plate-like graphite particles. That is, the vinyl aromatic monomer unit of the aromatic vinyl copolymer has a function of adsorbing the aromatic vinyl copolymer to the plate-like graphite particles, and the second monomer unit is mainly composed of the graphite-polymer composite and the solvent or binder. Responsible for improving the affinity with other resins.

  An aromatic vinyl copolymer having a higher content of vinyl aromatic monomer units increases the amount of adsorption to the plate-like graphite particles. The content of the vinyl aromatic monomer unit is preferably 10% by mass to 98% by mass, more preferably 30% by mass to 98% by mass, and particularly preferably 50% by mass to 95% by mass with respect to the entire aromatic vinyl copolymer. preferable. If the content of the vinyl aromatic monomer unit is lower than 10% by mass, the adsorption amount of the aromatic vinyl copolymer to the plate-like graphite particles may be reduced. When the content of the vinyl aromatic monomer unit is higher than 98% by mass, the affinity between the graphite-polymer composite and the solvent or resin is lowered, and the dispersibility of the graphite-polymer composite in the solvent or resin is reduced. May decrease.

  Examples of the substituent of the formula (1) include an amino group, a carboxyl group, a carboxylic ester group, a hydroxyl group, an amide group, an imino group, a glycidyl group, an alkoxy group, a carbonyl group, an imide group, and a phosphate ester group. . In order to increase the dispersibility of the graphite-polymer composite in a solvent or resin, the substituent is preferably an alkoxy group, and the alkoxy group is preferably a methoxy group.

  Examples of the vinyl aromatic monomer unit include a styrene monomer unit, a vinyl naphthalene monomer unit, a vinyl anthracene monomer unit, a vinyl pyrene monomer unit, a vinyl anisole monomer unit, a vinyl benzoate monomer unit, and an acetyl styrene monomer unit. Of these, a styrene monomer unit, a vinyl naphthalene monomer unit, and a vinyl anisole monomer unit are preferable from the viewpoint of improving the dispersibility of the graphite-polymer composite in a solvent or a resin.

  The second monomer unit is at least selected from the group consisting of (meth) acrylic acid, (meth) acrylates, (meth) acrylamides, vinylimidazoles, vinylpyridines, maleic anhydride, vinylphosphoric acids, and maleimides. Monomer units derived from one monomer are preferred. In this specification, for example, “(meth) acrylic acid” means both “acrylic acid” and “methacrylic acid”.

  By adsorbing the aromatic vinyl copolymer containing the second monomer unit to the surface of the plate-like graphite particles, the affinity between the graphite-polymer composite and the solvent or resin is improved. In addition, the graphite-polymer composite can be satisfactorily dispersed in the resin.

  Examples of (meth) acrylates include alkyl (meth) acrylates and substituted alkyl (meth) acrylates. Examples of the substituted alkyl (meth) acrylate include hydroxyalkyl (meth) acrylate, aminoalkyl (meth) acrylate, and isocyanate alkyl (meth) acrylate.

  Examples of (meth) acrylamides include (meth) acrylamide, N-alkyl (meth) acrylamide, and N, N-dialkyl (meth) acrylamide.

  Examples of vinyl imidazoles include 1-vinyl imidazole.

  Examples of vinyl pyridines include 2-vinyl pyridine and 4-vinyl pyridine.

  Maleimides include maleimide, alkylmaleimide, arylmaleimide, and N-phenylmaleimide.

  From the viewpoint of improving the dispersibility of the graphite-polymer composite, the second monomer unit includes alkyl (meth) acrylate, hydroxyalkyl (meth) acrylate, aminoalkyl (meth) acrylate, and N, N-dialkyl (meth). Acrylamide, 2-vinylpyridine, 4-vinylpyridine and arylmaleimide are preferred, hydroxyalkyl (meth) acrylate, N, N-dialkyl (meth) acrylamide, 2-vinylpyridine and arylmaleimide are more preferred, and phenylmaleimide is particularly preferred. .

  From the viewpoint of improving the initial capacity and the initial efficiency of the power storage device of the present invention, the second monomer unit is particularly preferably acrylic acid, vinyl phosphoric acid, isocyanate alkyl (meth) acrylate, or phenylmaleimide.

  The organic group in the second monomer unit preferably has about 2 to 100 carbon atoms, more preferably about 2 to 50 carbon atoms. More preferably, it is about 2-20. If the carbon number is excessive, the organic group is bulky and the graphite-polymer composite is bulky, so that it may be difficult to mix a sufficient amount of the negative electrode active material into the negative electrode active material layer.

  Examples of the aromatic vinyl copolymer include, for example, a copolymer of styrene (hereinafter referred to as ST) and N, N-dimethylmethacrylamide (hereinafter referred to as DMMAA), 1-vinylnaphthalene ( Hereinafter referred to as VN) and DMMAA, 4-vinylanisole (hereinafter referred to as VA) and DMMAA, ST and N-phenylmaleimide (hereinafter referred to as PM). A copolymer of ST and 1-vinylimidazole (hereinafter referred to as VI), a copolymer of ST and 4-vinylpyridine (hereinafter referred to as 4VP), ST and N, N -Copolymer of dimethylaminoethyl methacrylate (hereinafter referred to as DMAEMA), copolymer of ST and methyl methacrylate (hereinafter referred to as MMA), ST and hydroxyethyl methacrylate (Hereinafter referred to as HEMA), ST and 2-vinylpyridine (hereinafter referred to as 2VP), ST and 2VP, ST and MMA. Polymer, copolymer of ST and polyethylene oxide (hereinafter referred to as PEO), copolymer of ST and acrylic acid, copolymer of ST and vinyl phosphoric acid, ST and 2-isocyanatoethyl methacrylate A copolymer is mentioned.

  In the aromatic vinyl copolymer, the copolymer may be a random copolymer or a block copolymer.

  The content of the second monomer unit relative to the vinyl aromatic monomer unit in the aromatic vinyl copolymer is preferably 0.1 mol to 10 mol with respect to 1 mol of the vinyl aromatic monomer unit. More preferably, it is 3 mol-3 mol, and more preferably 0.5 mol-2 mol. The vinyl aromatic monomer unit and the second monomer unit are particularly preferably present in a molar ratio of 1: 1. If the content ratio of the second monomer unit is too small, it may be difficult to increase the affinity between the graphite-polymer composite and the binder. Further, if the content ratio of the vinyl aromatic monomer unit is too small, it may be difficult to increase the adsorptivity of the aromatic vinyl copolymer to the plate-like graphite particles.

  The number average molecular weight of the aromatic vinyl copolymer is preferably 1,000 to 1,000,000, and more preferably 5,000 to 100,000. When the number average molecular weight of the aromatic vinyl copolymer is less than 1,000, the adsorptive capacity to the plate-like graphite particles tends to be lowered. On the other hand, when the number average molecular weight is larger than 1,000,000, the graphite-polymer The dispersibility of the composite in a solvent or resin tends to decrease, or the viscosity increases significantly, making it difficult to handle. The number average molecular weight of the aromatic vinyl copolymer was measured by gel permeation chromatography (column: Shodex GPC K-805L and Shodex GPC K-800RL (both manufactured by Showa Denko KK), eluent: chloroform). The value converted with standard polystyrene is used.

The content of the aromatic vinyl copolymer in the graphite-polymer composite is preferably 10 ⁻⁷ to 10 ⁻¹ parts by mass of the aromatic vinyl copolymer with respect to 100 parts by mass of the plate-like graphite particles. ^It is more preferably ⁻⁵ to 10 ⁻² parts by mass. If the amount of the aromatic vinyl copolymer relative to the plate-like graphite particles is less than 10 ⁻⁷ parts by mass, the amount of adsorption of the aromatic vinyl copolymer onto the plate-like graphite particles is insufficient, so that the graphite for the solvent or resin -Dispersibility of the polymer composite tends to decrease. On the other hand, when the amount of the aromatic vinyl copolymer relative to the plate-like graphite particles exceeds 10 ⁻¹ parts by mass, there may be a free aromatic vinyl copolymer that is not directly adsorbed on the plate-like graphite particles.

  The method for producing a graphite-polymer composite comprises mixing raw material graphite particles, an aromatic vinyl copolymer containing a vinyl aromatic monomer unit represented by the formula (1), a hydrogen peroxide, and a solvent. And a pulverizing step of subjecting the mixture obtained in the mixing step to a pulverizing treatment.

  Examples of the raw material graphite particles include known graphite having a graphite structure, such as artificial graphite, scale-like graphite, massive graphite, and earth-like graphite. Although there is no restriction | limiting in particular in the magnitude | size of raw material graphite particle | grains, it is preferable that the particle diameter of raw material graphite particle | grains is the range of 0.01 mm-5 mm, and it is more preferable that it is the range of 0.1 mm-1 mm. The particle diameter referred to here can be measured, for example, by a dry sieving method based on the JIS Z 8815 sieving test method general rules.

  As the aromatic vinyl copolymer, those described above can be used.

  Examples of the hydrogen peroxide include a complex of a compound having a carbonyl group and hydrogen peroxide, a compound in which hydrogen peroxide is coordinated to a compound such as a quaternary ammonium salt, potassium fluoride, rubidium carbonate, phosphoric acid, or uric acid. It is done. Examples of the compound having a carbonyl group include urea, carboxylic acid (benzoic acid, salicylic acid, etc.), ketone (acetone, methyl ethyl ketone, etc.), and carboxylic acid ester (methyl benzoate, ethyl salicylate, etc.). As the hydrogen peroxide, a complex of a compound having a carbonyl group and hydrogen peroxide is preferable.

  Such a hydrogen peroxide acts as an oxidizing agent and facilitates peeling between carbon layers without destroying the graphite structure of the raw graphite particles. That is, hydrogen peroxide enters between the carbon layers to oxidize the surface of the layer, and proceeds with cleavage. At the same time, the aromatic vinyl copolymer penetrates into the cleaved carbon layer and stabilizes the cleavage surface. Promoted. As a result, the aromatic vinyl copolymer is adsorbed on the surface of the plate-like graphite particles.

  Solvents are dimethylformamide (hereinafter referred to as DMF), chloroform, dichloromethane, chlorobenzene, dichlorobenzene, N-methylpyrrolidone (hereinafter referred to as NMP), hexane, toluene, dioxane, propanol, γ-picoline, acetonitrile, Dimethyl sulfoxide (hereinafter referred to as DMSO) and dimethylacetamide (hereinafter referred to as DMA) are preferable, and DMF, chloroform, dichloromethane, chlorobenzene, dichlorobenzene, NMP, hexane and toluene are more preferable.

  In the mixing step, the raw graphite particles, the aromatic vinyl copolymer, the hydrogen peroxide, and the solvent are mixed. The mixing amount of the raw material graphite particles is preferably 0.1 g / L to 500 g / L per liter of solvent, and more preferably 10 g / L to 200 g / L. If the mixing amount of the raw material graphite particles is less than 0.1 g / L per liter of solvent, the consumption of the solvent increases, which is economically disadvantageous. On the other hand, if it exceeds 500 g / L per liter of solvent, the viscosity of the liquid increases. May be difficult to handle.

  The mixing amount of the aromatic vinyl copolymer is preferably 0.1 part by mass to 1000 parts by mass, and more preferably 0.1 part by mass to 200 parts by mass with respect to 100 parts by mass of the raw graphite particles. When the mixing amount of the aromatic vinyl copolymer is less than 0.1 parts by mass with respect to 100 parts by mass of the raw material graphite particles, the dispersibility of the obtained plate-like graphite particles tends to be lowered. When the mixing amount of the copolymer exceeds 1000 parts by mass with respect to 100 parts by mass of the raw graphite particles, the aromatic vinyl copolymer does not dissolve in the solvent, and the viscosity of the liquid rises and handling becomes difficult. There is a fear.

  The mixing amount of the hydrogen peroxide is preferably 0.1 to 500 parts by mass, more preferably 1 to 100 parts by mass with respect to 100 parts by mass of the raw graphite particles. When the mixing amount of the hydrogen peroxide is less than 0.1 parts by mass with respect to 100 parts by mass of the raw graphite particles, the dispersibility of the obtained graphite-polymer composite tends to be lowered. When the amount exceeds 500 parts by mass with respect to part, the raw graphite particles are excessively oxidized, and the conductivity of the obtained graphite-polymer composite tends to be lowered.

  In the pulverizing step, the mixture obtained in the mixing step is pulverized to pulverize the raw graphite particles into plate-like graphite particles. This produces a graphite-polymer composite. Examples of the pulverization treatment include ultrasonic treatment, treatment with a ball mill, wet pulverization, explosion, mechanical pulverization, and wet high pressure pulverization. In the ultrasonic treatment, the oscillation frequency is preferably 15 kHz to 400 kHz, and the output is preferably 500 W or less. As the grinding treatment, ultrasonic treatment or wet grinding treatment is preferable. In the pulverization step, the raw graphite particles can be pulverized to obtain plate-like graphite particles without destroying the graphite structure of the raw graphite particles. Moreover, as temperature at the time of a grinding | pulverization process, it can be set as -20 degreeC-100 degreeC, for example. Moreover, as a grinding | pulverization processing time, it can be set as 0.01 hour-50 hours, for example. In the case of wet high-pressure pulverization or the like, the pressure during the pulverization treatment is preferably 50 MPa to 400 MPa.

  The plate-like graphite particles and the graphite-polymer composite may be oriented so that the plate surface of at least some of the plate-like graphite particles in the negative electrode active material layer is substantially parallel to the surface of the current collector. The orientation of the graphite particles may be broken by the presence of the first powder. By breaking the orientation, the gap between the carbon layers in the plate-like graphite particles faces in the direction intersecting the current collector surface. For this reason, the said clearance gap faces in the direction which cross | intersects with respect to the advancing direction of lithium ion, and the contact frequency of the lithium ion to the said clearance gap increases. For this reason, lithium ions can easily enter and exit the plate-like graphite particles, and the function of the plate-like graphite particles as the negative electrode active material can be fully utilized, so that the charge / discharge capacity of the power storage device can be improved.

  In addition to the first powder and the second powder, graphite can also be added to the negative electrode active material layer. Examples of graphite include natural graphite, granulated graphite, hard carbon, and soft carbon.

  The amount of graphite added is preferably 80% by mass or less, more preferably 50% by mass or less, and more preferably 20% by mass or less when the total amount of the first powder, the second powder, and graphite is 100% by mass. A range is more preferred. When there is too much content of graphite, the fall of a capacity | capacitance and the effect that the effect of a 2nd powder may fall may arise.

The average particle diameter D ₅₀ of the graphite is preferably smaller than the average particle diameter D ₅₀ of the first powder. The average particle diameter _{D 50} of the graphite is preferably 0.1μm or more 20μm or less, more preferably 0.1μm or 15μm or less, an average particle diameter _{D 50} of the graphite is in the 0.1μm or 10μm or less More preferably it is.

<Mixing ratio of first powder and second powder>
When the total of the first powder and the second powder is 100% by mass, the second powder is preferably contained in an amount of 5% by mass to 30% by mass, and the second powder is contained in an amount of 10% by mass to 20% by mass. More preferably.

<Negative electrode>
The negative electrode active material layer of the present invention is used for a negative electrode of a power storage device. The negative electrode has a current collector and a negative electrode active material layer disposed on the current collector surface. The current collector refers to a chemically inert electronic high conductor that keeps a current flowing through an electrode during discharging or charging of a power storage device. As a material that can be used for the current collector, for example, at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, and molybdenum In addition, a metal material such as stainless steel, and further a conductive resin can be used. The current collector may be covered with a known protective layer. Moreover, you may process the surface of an electrical power collector by a well-known method. The current collector can take a form such as a foil, a sheet, a film, a line, a bar, or a mesh. Therefore, metal foils, such as copper foil, nickel foil, aluminum foil, stainless steel foil, can be used suitably as a collector. When the current collector has a thin shape such as a foil, sheet, or film, the thickness of the current collector is preferably in the range of 1 μm to 100 μm.

  For example, in order to form a negative electrode active material layer of a negative electrode of a non-aqueous secondary battery using the negative electrode active material layer of the present invention, a negative electrode mixture containing a negative electrode active material is applied on a current collector, and the negative electrode mixture The binder contained in may be dried or cured. As the negative electrode mixture, a mixture of a negative electrode active material, a conductive additive, a binder, and, if necessary, other additives added with an appropriate amount of an organic solvent to form a slurry may be used. As a method for applying the slurry-like negative electrode mixture to the current collector, methods such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, a curtain coating method, and a gravure coating method can be employed.

  The above-mentioned graphite-polymer composite is suitably dispersed in the solvent because it does not aggregate in the solvent because the above-mentioned aromatic vinyl copolymer is adsorbed on the surface of the plate-like graphite particles. For this reason, the negative electrode mixture containing a graphite-polymer composite is easy to form a uniform negative electrode active material layer.

  The binder plays a role of directly or indirectly connecting the negative electrode active material, the conductive additive and other additives to the surface of the current collector. The binder is required to bind the above-described active material and the like to the current collector in as small an amount as possible, but the addition amount is 0.5% by mass of the total of the negative electrode active material, the conductive auxiliary agent, and the binder. -50 mass% is desirable. If the addition amount of the binder is less than 0.5% by mass, the moldability of the electrode may be lowered, and if it exceeds 50% by mass, the energy density of the electrode may be lowered.

  As the binder, polyvinylidene fluoride (Polyvinylidene DiFluoride: hereinafter referred to as PVDF), polytetrafluoroethylene (hereinafter referred to as PTFE), styrene-butadiene rubber (hereinafter referred to as SBR), polyimide (hereinafter referred to as SBR), and the like. PI), polyamideimide (hereinafter referred to as PAI), carboxymethylcellulose (hereinafter referred to as CMC), polyvinyl chloride (hereinafter referred to as PVC), methacrylic resin (hereinafter referred to as PMA). , Polyacrylonitrile (hereinafter referred to as PAN), modified polyphenylene oxide (hereinafter referred to as PPO), polyethylene oxide (hereinafter referred to as PEO), polyethylene (hereinafter referred to as PE), polypropylene (hereinafter referred to as PP). ), Polyacrylic acid (hereinafter referred to as It referred to as AA.) And the like.

  When a binder having high strength and high resistance such as PAI or PI is used as the binder, the effect of the present invention that the initial efficiency and the initial capacity are effectively improved becomes remarkable.

  The conductive assistant is added to increase the conductivity of the electrode. Examples of conductive assistants include carbon black, graphite, acetylene black (hereinafter referred to as “AB”), ketjen black (registered trademark) (hereinafter referred to as “KB”), vapor grown carbon fiber (Vapor Growth). Carbon Fiber: hereinafter referred to as VGCF) and various metal particles. These can be added alone or in combination of two or more. Although there are no particular limitations on the amount of conductive aid used, it can be, for example, about 20 parts by mass to 100 parts by mass with respect to 100 parts by mass of the active material. If the amount of the conductive auxiliary is less than 20 parts by mass, an efficient conductive path may not be formed, and if it exceeds 100 parts by mass, the electrode moldability may deteriorate and the energy density may be lowered. Note that when the silicon oxide combined with the carbon material is used as the active material, the amount of the conductive auxiliary agent added can be reduced or eliminated.

  There is no restriction | limiting in particular in an organic solvent, The mixture of a some solvent may be sufficient. N-methyl-2-pyrrolidone and N-methyl-2-pyrrolidone and ester solvents (ethyl acetate, n-butyl acetate, butyl cellosolve acetate, butyl carbitol acetate, etc.) or glyme solvents (diglyme, triglyme, tetraglyme, etc.) The mixed solvent is particularly preferred.

  Here, FIG. 1 is a cross-sectional view schematically showing a preferred embodiment of a negative electrode using the negative electrode active material layer of the present invention. As shown in FIG. 1, the negative electrode includes a current collector 1 and a negative electrode active material layer 5 formed on the surface of the current collector 1. The negative electrode active material layer 5 includes a first powder 2, a second powder 3, and a binder 4.

The second powder 3 had an average particle diameter _{D 50} is smaller than the average particle diameter _{D 50} of the first powder 2, a thickness of 0.3Nm～100nm, plate length in the long axis direction 0.1μm~500μm Graphite particles. Since the second powder 3 has an average particle diameter D ₅₀ smaller than the average particle diameter D ₅₀ of the first powder 2, the second powder 3 is a gap formed between the first powders 2, the current collector 1 and the first powder 1. It arrange | positions favorably in the clearance gap formed with the powder 2, etc.

  The plate-like graphite particles constituting the second powder 3 have a layer structure in which a plurality of graphene single layers are laminated, and are excellent in strength. Therefore, when the plate-like graphite particles are well dispersed and arranged in the negative electrode active material layer 5, the stress acting on the negative electrode active material layer 5 during charge / discharge is uniformly relieved. Further, since the plate-like graphite particles have high conductivity, the plate-like graphite particles are well dispersed and arranged in the negative electrode active material layer 5 so that a good conductive path is formed in the negative electrode active material layer 5. .

<Power storage device>
When the power storage device of the present invention is a lithium ion secondary battery, known positive electrodes, electrolytic solutions, and separators that are not particularly limited can be used. The positive electrode may be anything that can be used in a non-aqueous secondary battery. The positive electrode has a current collector and a positive electrode active material layer bound on the current collector. The positive electrode active material layer includes a positive electrode active material and a binder, and may further include a conductive additive and other additives. The positive electrode active material, the conductive additive, and the binder are not particularly limited as long as they can be used in the nonaqueous secondary battery.

As the positive electrode active material, a material capable of occluding and releasing charge carriers such as Li may be used. As the positive electrode active material, the layered compound Li _a Ni _b Co _c Mn _{d De} O _f (0.2 ≦ a ≦ 2, b + c + d + e = 1, 0 ≦ e <1, D is Li, Fe, Cr, Cu, At least one element selected from Zn, Ca, Mg, S, Si, Na, K, Al, Zr, Ti, P, Ga, Ge, V, Mo, Nb, W, La, 1.7 ≦ f ≦ 3 ), Li ₂ MnO ₃ . Further, as a positive electrode active material, a solid solution composed of a spinel such as LiMn ₂ O ₄ and Li ₂ Mn ₂ O ₄ and a mixture of a spinel and a layered compound, LiMPO ₄ , LiMVO _4, or Li ₂ MSiO ₄ (M in the formula) Are selected from at least one of Co, Ni, Mn, and Fe). Furthermore, as the positive electrode active material, tavorite compound (the M a transition metal) LiMPO ₄ F, such as LiFePO ₄ F represented by, Limbo ₃ such LiFeBO ₃ (M is a transition metal) include borate-based compound represented by be able to. Any metal oxide used as the positive electrode active material may have the above composition formula as a basic composition, and a metal element contained in the basic composition may be substituted with another metal element. Further, as the positive electrode active material, a positive electrode active material that does not contain lithium ions contributing to charge / discharge, for example, sulfur alone (S), a compound in which sulfur and carbon are combined, a metal sulfide such as TiS ₂ , V ₂ O, etc. ₅ , oxides such as MnO ₂ , polyaniline and anthraquinone, compounds containing these aromatics in the chemical structure, conjugated materials such as conjugated diacetic acid organic materials, and other known materials can also be used. Further, a compound having a stable radical such as nitroxide, nitronyl nitroxide, galvinoxyl, phenoxyl, etc. may be adopted as the positive electrode active material. When using a positive electrode active material that does not contain lithium, it is necessary to add ions to the positive electrode and / or the negative electrode in advance by a known method. Here, in order to add the ion, a metal or a compound containing the ion may be used.

  The current collector for the positive electrode may be any material generally used for the positive electrode of a lithium ion secondary battery, such as aluminum, nickel, and stainless steel, and is otherwise the same as the current collector for the negative electrode. The same conductive aids as those described for the negative electrode can also be used.

  The electrolytic solution includes a nonaqueous solvent and an electrolyte (also referred to as a supporting electrolyte or a supporting salt) dissolved in the nonaqueous solvent. In the power storage device of the present invention, the electrolytic solution is not particularly limited. For example, cyclic esters, chain esters, ethers and the like can be used as the non-aqueous solvent. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, ethyl methyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane. As the non-aqueous solvent, a compound in which a part or all of hydrogen in the chemical structure of the above specific solvent is substituted with fluorine may be employed.

  More preferably, the non-aqueous solvent is an aprotic organic solvent, such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC). It is preferable to use one or more selected from the above.

Examples of the electrolyte dissolved in these non-aqueous solvents include lithium salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiI, LiClO ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ .

Examples of the electrolytic solution include 0.5 mol / L to 1 of a lithium metal salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 in} a non-aqueous solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, and dimethyl carbonate. A solution dissolved at a concentration of about 7 mol / L can be used.

  A separator is used in the power storage device as necessary. The separator separates the positive electrode and the negative electrode, and allows passage of charge carriers while preventing a short circuit of current due to contact between both electrodes. If necessary, a separator capable of holding an electrolytic solution can also be selected.

  The type of the separator is not particularly limited. For example, polyethylene, polypropylene, polytetrafluoroethylene, polyimide, polyamide, polyaramid (aromatic polymer), polyester, polyacrylonitrile or other synthetic resin, cellulose, amylose or other polysaccharide, fibron, keratin, etc. Examples thereof include microporous materials, porous materials, non-woven fabrics, woven fabrics, and the like using one or more kinds of electrically insulating materials such as natural polymers such as lignin and suberin and ceramics. These preferably have a thin shape such as a sheet shape, a film shape, or a foil shape, and may have a single layer structure or a multilayer structure.

  The power storage device of the present invention can be manufactured as follows. A separator is interposed between the positive electrode and the negative electrode as necessary to form an electrode body. The shape of the electrode body is not particularly limited, and may be any of a stacked type in which a positive electrode, a separator, and a negative electrode are stacked, or a wound type in which a positive electrode, a separator, and a negative electrode are wound. For example, if the power storage device of the present invention is a battery, after connecting the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal connected to the outside using a current collecting lead or the like, What is necessary is just to seal an electrode body in a battery case with electrolyte solution. In addition, the power storage device of the present invention only needs to be charged and discharged within a voltage range suitable for the type of active material included in the electrode.

  The shape of the power storage device of the present invention is not particularly limited. For example, when the power storage device of the present invention is a lithium ion secondary battery, various shapes such as a cylindrical shape, a square shape, a coin shape, and a laminate shape can be employed.

  The power storage device of the present invention may be mounted on a vehicle, for example. The vehicle may be a vehicle that uses electric energy from a power storage device such as a lithium ion secondary battery for all or a part of its power source, and may be, for example, an electric vehicle or a hybrid vehicle. When a lithium ion secondary battery is mounted on a vehicle, a plurality of lithium ion secondary batteries may be connected in series to form an assembled battery.

  Hereinafter, embodiments of the present invention will be specifically described with reference to Examples and Comparative Examples.

  Hereinafter, the present invention will be described in more detail with reference to examples.

(First powder)
(Production Example A)
<Preparation of Si / C composite powder>
A mixed solution of 7 ml of an aqueous HF solution having a concentration of 46% by mass and 56 ml of an aqueous HCl solution having a concentration of 36% by mass was brought to 0 ° C. in an ice bath, and 3.3 g of CaSi ₂ was added thereto and stirred in an argon gas stream. . After confirming the completion of foaming, the mixed solution was warmed to room temperature, stirred for another 2 hours at room temperature, then added with 20 ml of distilled water and further stirred for 10 minutes. At this time, yellow powder floated.

  The obtained mixed solution was filtered, and the obtained residue was washed with 10 ml of distilled water and then with 10 ml of ethanol. The residue after washing was vacuum dried to obtain 2.5 g of layered polysilane.

1 g of this layered polysilane was weighed, and heat treatment was carried out in argon gas containing O ₂ in an amount of 1% by volume or less at 500 ° C. for 1 hour to obtain silicon aggregated particles.

  The obtained silicon agglomerated particles were put into a rotary kiln type reactor, and a carbonization process by thermal CVD was performed under a condition of 850 ° C. and a residence time of 5 minutes under a flow of propane gas. The furnace core tube of the reactor was disposed in the horizontal direction, and the rotational speed of the core tube was 1 rpm. A baffle plate is disposed on the inner peripheral wall of the core tube, and the contents accumulated on the baffle plate with the rotation of the core tube are configured to fall from the baffle plate at a predetermined height. The contents are stirred during the reaction. The Si / C composite powder obtained in this carbonization step is used as the first powder of Production Example A.

The average particle diameter D ₅₀ of the first powder of Production Example A was measured by a laser diffraction scattering particle size distribution measuring method. The average particle diameter D ₅₀ of the first powder of Production Example A was 5 [mu] m. Note that the average particle diameter D ₅₀ refers to the particle size cumulative value of the volume distribution in the particle size distribution measurement by laser diffraction method is equivalent to 50%. That is, the average particle diameter D ₅₀ means the median size measured by volume.

  The first powder of Production Example A corresponds to the first powder in the present invention.

(Second powder)
<Preparation of graphite-polymer composite>
(Graphite-polymer composite of Production Example 1)
One mole equivalent of N, N-dimethylmethacrylamide (hereinafter referred to as DMMAA), a catalyst amount of azobisisobutyronitrile (abbreviation, AIBN) and toluene with respect to 1 mole equivalent of styrene (hereinafter referred to as ST). Mixing as a solvent, a polymerization reaction was carried out at 60 ° C. for 6 hours in a nitrogen atmosphere. After allowing to cool, the mixture was purified by reprecipitation using chloroform and hexane to obtain an ST-DMMAA (50:50) copolymer. The number average molecular weight (Mn) of this ST-DMMAA (50:50) copolymer was 55,000.

Here, the number average molecular weight (Mn) was measured under the following conditions using gel permeation chromatography (“Shodex GPC101” manufactured by Showa Denko KK).
Column: Shodex GPC K-805L and Shodex GPC K-800RL (both manufactured by Showa Denko KK)
・ Eluent: Chloroform ・ Measurement temperature: 25 ° C.
Sample concentration: 0.1 mg / ml
・ Detection means: RI
In addition, the number average molecular weight (Mn) showed the value converted with standard polystyrene.

  12.5 g of graphite particles (“EXP-P” manufactured by Nippon Graphite Industries Co., Ltd., particle diameter: 100 μm to 600 μm), 1.25 g of urea-hydrogen peroxide inclusion complex, ST-DMMAA (50:50) copolymer 400 mg and 500 ml of DMF are mixed, and pulverized at 200 MPa using a starburst system, a laboratory machine, which is a wet high-pressure pulverizer, to obtain a dispersion containing plate-like graphite particles and ST-DMMAA copolymer. It was. This dispersion was pulverized again at 200 MPa, and the process of recovering the dispersion was repeated 40 times to obtain a dispersion after pulverization 40 times. The dispersion obtained after 40 times of pulverization was filtered under reduced pressure, and the filtrate was washed with about 500 ml of NMP, and then filtered under reduced pressure, and the filtrate was recovered in a paste state. The filtrate was vacuum-dried to obtain a graphite-polymer composite aggregate of Production Example 1 containing plate-like graphite particles and an aromatic vinyl copolymer. The obtained aggregate of the graphite-polymer composite of Production Example 1 was ultrasonically dispersed in NMP to obtain a paste having a solid content of 17% by mass. The aggregate of the graphite-polymer composite of Production Example 1 obtained in this step corresponds to the second powder in the present invention.

When the graphite-polymer composite of Production Example 1 constituting the second powder was observed with an SEM, it had a plate shape, a width of 0.8 μm to 5 μm, and a thickness of 20 nm to 80 nm. The particle size distribution of the graphite-polymer composite of Production Example 1 was measured. The particle size distribution was measured using a laser diffraction / scattering particle size distribution measuring method. FIG. 2 shows a particle size distribution diagram of the graphite-polymer composite of Production Example 1. As can be seen in FIG. 2, it was found that the graphite-polymer composite of Production Example 1 had a particle size of 4 μm or less. The Measurement of the average particle diameter D _50, graphite Preparation Example 1 - average particle diameter D ₅₀ of the polymer composite was 1 [mu] m.

(Graphite-polymer composite of Comparative Production Example 1)
A graphite-polymer composite of Comparative Production Example 1 was obtained in the same manner as the production method of the graphite-polymer composite of Production Example 1 except that the pulverization at 200 MPa was repeated 10 times.

When the SEM observation of the graphite-polymer composite of Comparative Production Example 1 was performed, the plate-like shape had a width of 2 μm to 50 μm and a thickness of 30 nm to 800 nm. FIG. 2 shows a particle size distribution diagram of the graphite-polymer composite of Comparative Production Example 1. As can be seen in FIG. 2, the graphite-polymer composite of Comparative Production Example 1 was found to have a particle size of 90 μm or less. The Measurement of the average particle diameter D _50, the graphite of Comparative Production Example 1 - average particle diameter D ₅₀ of the polymer composite was 10 [mu] m.

(Graphite-polymer composite of Production Example 2)
ST-2-Isocyanate ethyl methacrylate (50:50) using the same polymer production method and mixing ratio as the graphite-polymer composite of Production Example 1 except that 2-isocyanate ethyl methacrylate was used instead of DMMAA. A copolymer was obtained. The number average molecular weight (Mn) of this ST-2-isocyanatoethyl methacrylate (50:50) copolymer was 25,000.

  The graphite-polymer composite of Production Example 2 was prepared in the same manner as in the production method of the graphite-polymer composite of Production Example 1 except that this ST-2-isocyanatoethyl methacrylate copolymer was used as the aromatic vinyl copolymer. The body was made. The graphite-polymer composite of Production Example 2 was ultrasonically dispersed in NMP to prepare a paste having a solid content of 24% by mass.

Measurement of the average particle diameter D ₅₀ of the polymer composite, graphite Production Example 2 - - graphite Production Example 2 Average particle diameter D ₅₀ of the polymer composite was 1 [mu] m.

(Graphite-polymer composite of Production Example 3)
ST-PM (91) using the same polymer production method and mixing ratio as the graphite-polymer composite of Production Example 1 except that N-phenylmaleimide (hereinafter referred to as PM) was used instead of DMMAA. : 9) A copolymer was obtained. The number average molecular weight (Mn) of this ST-PM (91: 9) copolymer was 45,000.

  A graphite-polymer composite of Production Example 3 was produced in the same manner as the production method of the graphite-polymer composite of Production Example 1 except that this ST-PM copolymer was used as an aromatic vinyl copolymer. . The graphite-polymer composite of Production Example 3 was ultrasonically dispersed in NMP to prepare a paste having a solid content of 24% by mass.

Measurement of the average particle diameter D ₅₀ of the polymer composite, graphite of Preparation 3 - - graphite in Production Example 3 average particle diameter D ₅₀ of the polymer composite was 1 [mu] m.

(Graphite-polymer composite of Production Example 4)
An ST-vinyl phosphate (91: 9) copolymer was obtained using the same polymer production method and mixing ratio as the graphite-polymer composite of Production Example 1 except that vinyl phosphate was used instead of DMMAA. . The number average molecular weight (Mn) of this ST-vinyl phosphoric acid (91: 9) copolymer was 61,000.

  A graphite-polymer composite of Production Example 4 was produced in the same manner as in the production method of the graphite-polymer composite of Production Example 1 except that this ST-vinyl phosphate copolymer was used as an aromatic vinyl copolymer. did. The graphite-polymer composite of Production Example 4 was ultrasonically dispersed in NMP to prepare a paste having a solid content of 19% by mass.

Measurement of the average particle diameter D ₅₀ of the polymer composite, graphite of Preparation 4 - - graphite in Production Example 4 Average particle diameter D ₅₀ of the polymer composite was 2 [mu] m.

(Graphite-polymer composite of Production Example 5)
The ST-methyl acrylate (91: 9) copolymer was prepared using the same polymer production method and mixing ratio as the graphite-polymer composite of Production Example 1 except that methyl acrylate was used instead of DMMAA. Obtained. The number average molecular weight (Mn) of this ST-methyl acrylate (91: 9) copolymer was 40,000.

  This ST-methyl acrylate (91: 9) copolymer was dissolved in toluene, 1N aqueous potassium hydroxide solution was added, and the mixture was stirred at room temperature for 3 days. After completion of the reaction, the aqueous phase was separated, adjusted to pH 3 or lower with an aqueous nitric acid solution, and freeze-dried to obtain an ST-acrylic acid (91: 9) copolymer.

  A graphite-polymer composite of Production Example 5 is produced in the same manner as in the production method of the graphite-polymer composite of Production Example 1 except that this ST-acrylic acid copolymer is used as an aromatic vinyl copolymer. did. The graphite-polymer composite of Production Example 5 was ultrasonically dispersed in NMP to prepare a paste having a solid content of 24% by mass.

Measurement of the average particle diameter D ₅₀ of the polymer composite, graphite Production Example 5 - - graphite in Production Example 5 mean particle diameter D ₅₀ of the polymer composite was 1 [mu] m.

  The graphite-polymer composites of Production Examples 1 to 5 described above correspond to the second powder of the present invention.

<Surface analysis of graphite-polymer composite>
The graphite-polymer composite paste of Production Example 3 described above was applied onto an indium foil and dried to prepare a graphite-polymer composite coating film of Production Example 3. The graphite-polymer composite film of Production Example 3 was subjected to time-of-flight secondary ion mass spectrometry (TOF-SIMS, positive ion: m / z 0-250), and the graphite-polymer composite coating of Production Example 3 was applied. The molecules present on the surface of the membrane were analyzed. As a result, it was found that ST-PM (91: 9) copolymer was adsorbed on the surface of the coating film of the graphite-polymer composite of Production Example 3. From the ST-PM (91: 9) copolymer fragment pattern, among the ST-PM (91: 9) copolymer components, the copolymer component containing a large amount of vinyl aromatic monomer units is plate-like graphite particles. It was found to be easily adsorbed on the surface of

  Moreover, when X-ray photoelectron spectroscopy (XPS) measurement was performed on the obtained coating film of the graphite-polymer composite of Production Example 3, a hydroxyl group was found on the carbon atom in the vicinity of the coating film surface (a region having a depth of 10 nm from the surface). It was confirmed that they were bonded. Furthermore, the carbon amount and oxygen amount in the vicinity of the coating film surface were measured, and the atomic ratio between carbon and oxygen was determined. As a result, the oxygen atom was 1.13 with respect to 100 carbon atoms. The graphite particles as the raw material had about 2 oxygen atoms per 100 carbon atoms.

  That is, the atomic ratio of oxygen to carbon was smaller on the surface of the coating film of the graphite-polymer composite of Production Example 3 than on the surface of the raw graphite particles. From this, it was found that the aromatic vinyl copolymer was adsorbed and coated on the surface of the plate-like graphite particles.

Example 1
<Formation of negative electrode>
75 parts by mass of the first powder of Production Example A, 10 parts by mass of the graphite-polymer composite of Production Example 1 as solids, 5 parts by mass of AB powder, 10 parts by mass of polyamideimide, and NMP are mixed. Thus, a slurry-like negative electrode mixture was prepared. This slurry-like negative electrode mixture was applied to the surface of an electrolytic copper foil having a thickness of 20 μm as a current collector using a doctor blade to form a negative electrode mixture layer on the current collector.

Thereafter, the composite material comprising the current collector and the negative electrode mixture layer was dried at 80 ° C. for 20 minutes, and NMP was volatilized and removed from the negative electrode mixture layer to obtain a negative electrode active material layer. After drying, the current collector and the negative electrode active material layer were firmly and closely joined with a roll press. The density of the negative electrode active material layer was 1.1 mg / cm ³ . This was heated under vacuum at 200 ° C. for 2 hours to form a negative electrode having a negative electrode active material layer thickness of about 16 μm. This negative electrode active material layer was used as the negative electrode active material layer of Example 1.

<Production of lithium ion secondary battery>
A lithium ion secondary battery (half cell) was produced using the negative electrode produced by the above procedure as an evaluation electrode. The counter electrode was a metal lithium foil (thickness 500 μm).

The counter electrode was cut to φ13 mm, the evaluation electrode was cut to φ11 mm, and a separator (Hoechst Celanese glass filter and celgard 2400) was sandwiched between them to form an electrode body battery. This electrode body battery was accommodated in a battery case (CR2032 coin cell manufactured by Hosen Co., Ltd.). Also, in the battery case, a nonaqueous electrolyte solution in which LiPF ₆ was dissolved at a concentration of 1M was injected into a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a ratio of 1: 1 (volume ratio), and the battery case was sealed. The lithium ion secondary battery of Example 1 was obtained.

(Example 2)
75 parts by mass of the first powder of Production Example A, 5 parts by mass of the graphite-polymer composite of Production Example 1 as a solid content, 5 parts by mass of natural graphite powder having an average particle diameter D ₅₀ of 20 μm, and AB powder 5 parts by mass, 10 parts by mass of polyamideimide, and NMP were mixed, and a negative electrode active material layer was prepared by preparing a slurry-like negative electrode mixture. A negative electrode active material layer was obtained, and a lithium ion secondary battery of Example 2 was further obtained.

(Example 3)
70 parts by mass of the first powder of Production Example A, 15 parts by mass of the graphite-polymer composite of Production Example 1 as solids, 5 parts by mass of AB powder, 10 parts by mass of polyamideimide, and NMP are mixed. The negative electrode active material layer of Example 3 was obtained in the same manner as in Example 1 except that the negative electrode active material layer was prepared by preparing a slurry-like negative electrode mixture. The next battery was obtained.

Example 4
70 parts by mass of the first powder of Production Example A, 15 parts by mass of the graphite-polymer composite of Production Example 2 above, 5 parts by mass of AB powder, 10 parts by mass of polyamideimide, and NMP are mixed. The negative electrode active material layer of Example 4 was obtained in the same manner as in Example 1 except that the negative electrode active material layer was prepared by preparing a slurry-like negative electrode mixture. The next battery was obtained.

(Example 5)
70 parts by mass of the first powder of Production Example A, 15 parts by mass of the graphite-polymer composite of Production Example 3 above, 5 parts by mass of AB powder, 10 parts by mass of polyamideimide, and NMP are mixed. The negative electrode active material layer of Example 5 was obtained in the same manner as in Example 1 except that the negative electrode active material layer was prepared by preparing a slurry-like negative electrode mixture. The next battery was obtained.

(Example 6)
70 parts by mass of the first powder of Production Example A, 15 parts by mass of the graphite-polymer composite of Production Example 4 as solids, 5 parts by mass of AB powder, 10 parts by mass of polyamideimide, and NMP are mixed. A negative electrode active material layer of Example 6 was obtained in the same manner as in Example 1 except that a negative electrode active material layer was prepared by preparing a slurry-like negative electrode mixture. The next battery was obtained.

(Example 7)
70 parts by mass of the first powder of Production Example A, 15 parts by mass of the graphite-polymer composite of Production Example 5 as solids, 5 parts by mass of AB powder, 10 parts by mass of polyamideimide, and NMP are mixed. A negative electrode active material layer of Example 7 was obtained in the same manner as in Example 1 except that a negative electrode active material layer was prepared by preparing a slurry-like negative electrode mixture. The next battery was obtained.

(Comparative Example 1)
75 parts by mass of the first powder of Production Example A, 10 parts by mass of the graphite-polymer composite of Comparative Production Example 1 as a solid content, 5 parts by mass of AB powder, 10 parts by mass of polyamideimide, and NMP A negative electrode active material layer of Comparative Example 1 was obtained in the same manner as in Example 1 except that a negative electrode active material layer was prepared by mixing and preparing a slurry-like negative electrode mixture. A secondary battery was obtained.

(Comparative Example 2)
A slurry prepared by mixing 75 parts by mass of the first powder of Production Example A, 10 parts by mass of natural graphite powder having an average particle diameter D ₅₀ of 20 μm, 5 parts by mass of AB powder, 10 parts by mass of polyamideimide, and NMP. A negative electrode active material layer of Comparative Example 2 was obtained in the same manner as Example 1 except that a negative electrode active material layer was prepared and a negative electrode active material layer was prepared. Further, a lithium ion secondary battery of Comparative Example 2 was obtained. Obtained.

<SEM observation of negative electrode active material layer>
The cross section of the negative electrode active material layer of Example 1 was observed with SEM. The SEM observation result of the cross section of the negative electrode active material layer of Example 1 is shown in FIG.

As indicated by reference numerals in FIG. 3, the second powder 3 that is the graphite-polymer composite of Production Example 1 is good in the space formed by the first powders 2 of Production Example A that are Si / C composite powders. Had been placed in. Here, the average particle diameter _{D 50} of the Si / C composite powder of Production Example A was 5 [mu] m, the graphite of Preparation 1 - average particle diameter _{D 50} of the polymer complex is a 1 [mu] m. The average particle diameter D ₅₀ of the second powder 3 is smaller than the average particle diameter D ₅₀ of the first powder 2.

  The plate-like graphite particles constituting the graphite-polymer composite of Production Example 1 have a layer structure in which a plurality of graphene single layers are laminated, and are excellent in strength. Therefore, when the plate-like graphite particles are well dispersed and arranged in the negative electrode active material layer, the stress acting on the negative electrode active material layer during charge / discharge is uniformly relieved. Further, since the plate-like graphite particles have high conductivity, the plate-like graphite particles are well dispersed and arranged in the negative electrode active material layer, so that a good conductive path is formed in the negative electrode active material layer.

<Evaluation test 1>
The initial capacities of the lithium ion secondary batteries of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 were measured. Specifically, the lithium ion secondary batteries of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 were held in a thermostatic bath at 25 ° C. for 1 hour, and then charged and discharged. The charging end voltage was 1.0 V at the Li counter electrode, the discharging end voltage was 0.01 V at the Li counter electrode, charging and discharging were performed at a constant current of 0.1 mA, and the initial charge capacity and the initial discharge capacity were measured.

From the initial charge capacity and the initial discharge capacity, the initial efficiency (%) was calculated by the following formula.
Initial efficiency (%) = (initial charge capacity / initial discharge capacity) x 100

  The results are shown in Table 1.

Compared to the initial efficiency of the lithium ion secondary battery of Comparative Example 2, the initial efficiency of the lithium ion secondary battery of Comparative Example 1 was improved by 1.5%. This is considered to be the effect of adding a graphite-polymer composite composed of plate-like graphite particles instead of spherical graphite. On the other hand, the initial efficiencies of the lithium ion secondary batteries of Example 1 and Example 2 were further greatly increased to 1.4% and 1.0%, respectively, compared with the initial efficiencies of the lithium ion secondary batteries of Comparative Example 1. Improved. The Results By using the second powder having a mean particle diameter D ₅₀ is smaller than the average particle diameter D ₅₀ of the first powder, it was confirmed that the initial efficiency is significantly improved. Moreover, the initial efficiency of the lithium ion secondary battery of Example 1 that does not contain graphite and includes the second powder is higher than the initial efficiency of the lithium ion secondary battery of Example 2 that contains the second powder and graphite. It was. This is because the average particle diameter D ₅₀ of the graphite is larger than the average particle diameter D ₅₀ of the first powder and the second powder is presumed to result graphite has not been effectively placed between the active material.

<Evaluation Test 2>
The cycle test which repeats charging / discharging on the following conditions was performed using the lithium ion secondary battery of Examples 3-7, and the charging / discharging capacity | capacitance of each cycle was measured. The cycle test was conducted up to 30 cycles, with the end voltage of charge being 1.0 V at the Li counter electrode, the end voltage of discharge being 0.01 V at the Li counter electrode, and charging / discharging performed at a constant current of 0.1 mA as one cycle. The charge / discharge capacity in the first cycle and the number of each cycle was measured. Initial charge capacity, initial discharge capacity, and initial efficiency were determined in the same manner as in Evaluation Test 1. The capacity retention rate after 30 cycles was determined by the following formula.
Capacity maintenance rate after 30 cycles (%) = (charge capacity after 30 cycles / initial charge capacity) × 100

  Table 2 shows the initial discharge capacity, initial charge capacity, initial efficiency, and capacity retention rate after 30 cycles of the lithium ion secondary batteries of Examples 3 to 7.

  Compared with the lithium ion secondary battery of Example 3, all of the lithium ion secondary batteries of Examples 4 to 7 had high initial efficiency and capacity retention after 30 cycles. This result is presumed to be due to the type of polymer of each graphite-polymer composite used in the lithium ion secondary batteries of Examples 3 to 7.

  The polymer of the graphite-polymer composite used in the lithium ion secondary battery of Example 4 has an isocyanate group. The isocyanate group is very reactive and forms a urethane bond with a compound having a hydroxyl group. In the negative electrode active material layer of Example 4, PAI is used as the binder. The terminal of PAI is OH or an acid anhydride. Therefore, the isocyanate group of the graphite-polymer composite and the end of PAI generate a urethane bond, and the bond between the graphite-polymer composite and PAI is strengthened, whereby the initial efficiency of the lithium ion secondary battery of Example 4 is enhanced. And it is estimated that the capacity retention after 30 cycles was improved from the initial efficiency of the lithium ion secondary battery of Example 3 and the capacity retention after 30 cycles.

  The polymer of the graphite-polymer composite used in the lithium ion secondary battery of Example 5 has high chemical resistance because the amide portion has a ring-closed structure. Therefore, deterioration of the graphite-polymer composite due to the electrolytic solution or the like is suppressed, and the initial efficiency of the lithium ion secondary battery of Example 5 and the capacity retention after 30 cycles are the initial efficiency of the lithium ion secondary battery of Example 3 and It is estimated that the capacity retention rate was improved after 30 cycles.

  The polymer of the graphite-polymer composite used in the lithium ion secondary battery of Example 6 contains a phosphate group. It is presumed that the inclusion of this phosphate group facilitates the formation of a SEI (Solid Electrolyte Interface) film on the active material surface. If the SEI film is present, it is considered that continuous decomposition of the electrolytic solution can be suppressed and cycle characteristics can be improved. Therefore, it is estimated that the initial efficiency and the capacity retention rate after 30 cycles of the lithium ion secondary battery of Example 6 were improved from the initial efficiency and the capacity retention ratio after 30 cycles of Example 3.

  The polymer of the graphite-polymer composite used in the lithium ion secondary battery of Example 7 has a carboxyl group. Since the carboxyl group has a polar structure, it is presumed that the adhesion with other electrode members, current collectors, and the like becomes stronger. It can also be inferred that the conductive path in the negative electrode was effectively constructed by improving the adhesion. Therefore, it is estimated that the initial efficiency and the capacity retention rate after 30 cycles of the lithium ion secondary battery of Example 7 were improved from the initial efficiency and the capacity retention ratio after 30 cycles of Example 3.

  1: current collector, 2: first powder, 3: second powder, 4: binder, 5: negative electrode active material layer.

Claims (9)

A first powder having at least one selected from the group consisting of Si, Si compounds, Sn and Sn compounds;
A second powder having plate-like graphite particles having a thickness of 0.3 nm to 100 nm and a length in the major axis direction of 0.1 μm to 100 μm,
It said second powder having an average particle diameter D ₅₀ of the negative electrode active material layer being less than the average particle diameter D ₅₀ of the first powder. On the surface of the plate-like graphite particles, the following formula (1):
_{- (CH 2 -CHX) - (} 1)
(In Formula (1), X represents a phenyl group, a naphthyl group, an anthracenyl group, or a pyrenyl group, and these groups may have a substituent.)
The negative electrode active material layer according to claim 1, wherein an aromatic vinyl copolymer containing a vinyl aromatic monomer unit represented by: 3. The negative electrode active material layer according to claim 1, wherein an average particle diameter D ₅₀ of the first powder is greater than 0.1 μm and not greater than 20 μm. It said second powder having an average particle diameter D ₅₀ of the negative electrode active material layer according to any one of claims 1 to 3 is 0.1μm or more 10μm or less.   5. The negative electrode active according to claim 1, wherein the second powder is contained in an amount of 5% by mass to 30% by mass when the total of the first powder and the second powder is 100% by mass. Material layer.   The negative electrode active according to any one of claims 1 to 5, wherein the second powder is contained in an amount of 10% by mass to 20% by mass when the total of the first powder and the second powder is 100% by mass. Material layer.   The negative electrode active material layer according to claim 1, comprising polyamideimide as a binder.   A power storage device comprising the negative electrode active material layer according to claim 1.   The power storage device according to claim 8, wherein the power storage device is a lithium ion secondary battery.