JP2011076985A - Negative electrode active material for lithium ion secondary battery, slurry composition for lithium ion secondary battery negative electrode, negative electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

A negative electrode active material for a lithium ion secondary battery with improved charge / discharge cycle characteristics, a slurry composition for a lithium ion secondary battery using the negative electrode active material for a lithium ion secondary battery, and a lithium ion secondary battery A negative electrode and a lithium ion secondary battery are provided.
A negative electrode active material for a lithium ion secondary battery including alloy particles having a metal concentration gradient. The alloy particles can be doped and dedoped with lithium ions, and include a first metal having a volume change rate of 100 to 1000% and a second metal having a volume change rate of 0 to 100%. The metal concentration becomes maximum in the surface layer portion of the alloy particles.
[Selection figure] None

Description

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

  In recent years, mobile terminals such as notebook personal computers, mobile phones, and PDAs have become widespread, and lithium ion secondary batteries are often used as power sources for these mobile terminals. As a required characteristic of a power source for mobile terminals, there is an increase in capacity, but at present, the capacity of a battery using a graphite-based negative electrode active material is in a saturated state, and it is difficult to significantly increase the capacity. Then, in order to achieve a higher capacity, use of silicon (Si), tin (Sn), or an alloy thereof as a negative electrode active material has been studied (Patent Document 1).

  However, when charging / discharging is performed in such a lithium ion secondary battery, the negative electrode active material expands and contracts when lithium is absorbed and released, and cracks are generated in the negative electrode layer. If charging / discharging is further repeated in this state, the negative electrode active material cannot withstand rapid expansion and contraction, cracks in the negative electrode layer propagate, and the negative electrode layer peels off and slides down. For this reason, electrical conductivity is lost, charging and discharging cannot be performed, and cycle characteristics are lowered.

  Therefore, it is necessary to improve such a problem and improve cycle characteristics. Therefore, in order to improve the cycle characteristics, Patent Document 2 reports a technique for producing alloy hollow particles. However, such alloy hollow particles cannot always obtain sufficient cycle characteristics. .

Japanese Patent Laid-Open No. 10-162823 JP 2007-123100 A

  The present invention has been made in view of the above problems, and has a negative electrode active material for a lithium ion secondary battery with improved charge / discharge cycle characteristics, and a lithium ion secondary using the negative electrode active material for a lithium ion secondary battery. It aims at providing the slurry composition for secondary batteries, the negative electrode for lithium ion secondary batteries, and a lithium ion secondary battery.

  As a result of intensive studies to solve the above problems, the present inventor has used lithium ion secondary battery negative electrode active materials made of alloy particles having a metal concentration gradient, so that lithium ion secondary batteries having good charge / discharge cycle characteristics can be obtained. It was found that a secondary battery could be provided.

That is, this invention which solves the said subject contains the following matters as a summary.
(1) A negative electrode active material for a lithium ion secondary battery comprising alloy particles having a metal concentration gradient.
(2) The alloy particles can be doped and dedoped with lithium ions, the first metal having a volume change rate defined by the following formula (I) of 100 to 1000%, and the first metal having a volume change rate of 0 to 100%. The negative electrode active material for a lithium ion secondary battery according to (1), which contains two metals.
(Volume change rate) = (Volume of metal doped with lithium ions) / (Volume of metal not doped with lithium ions) (I)
(3) The negative electrode active material for a lithium ion secondary battery according to (2), wherein the concentration of the first metal is maximized in a surface layer portion of the alloy particles.
(4) The negative electrode active material for a lithium ion secondary battery according to any one of (1) to (3), wherein the alloy particles have a core-shell structure having a core portion and a shell portion.
(5) The negative electrode active material for a lithium ion secondary battery according to (4), wherein the shell portion has the metal concentration gradient.
(6) The negative electrode active material for a lithium ion secondary battery according to (2), wherein the first metal is Si or Sn, and the second metal is Ni.
(7) The negative electrode active material for a lithium ion secondary battery according to (4), wherein the core part is polymer particles.
(8) A slurry composition for a negative electrode of a lithium ion secondary battery, comprising the negative electrode active material for a lithium ion secondary battery according to any one of (1) to (7).
(9) A negative electrode for a lithium ion secondary battery obtained by applying the slurry composition for a negative electrode of the lithium ion secondary battery according to (8) above to a current collector and drying it.
(10) A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to (9).

  Since the alloy particles used in the present invention have a metal concentration gradient, the stress difference due to the volume change of the alloy particles when doping and dedoping lithium ions can be reduced. Therefore, the lithium ion secondary battery using the negative electrode active material for lithium ion secondary battery made of the alloy particles can prevent the negative electrode from peeling and sliding due to expansion and contraction of the negative electrode active material during charge and discharge. High capacity and good charge / discharge cycle characteristics can be maintained.

<Alloy particles>
The negative electrode active material for a lithium ion secondary battery according to the present invention comprises alloy particles having a metal concentration gradient. Since the metal constituting the alloy particles has a concentration gradient, the stress difference due to the volume change of the alloy particles when doping and dedoping lithium ions can be reduced.

  The alloy particles can be doped and dedoped with lithium ions, and preferably contain a first metal and a second metal. The first metal preferably has a volume change rate defined by the following formula (I) of 100 to 1000%, and more preferably 300 to 700%. When the volume change rate of the first metal is in this range, stress due to charging / discharging can be relaxed, so that the charge / discharge cycle characteristics are excellent and high capacity can be achieved.

The volume change rate of the second metal is preferably 0 to 100%, more preferably 0 to 50%. When the volume change rate of the second metal is within this range, the stress on the inside of the particles can be relaxed, so that particles having excellent charge / discharge cycle characteristics can be obtained.
(Volume change rate) = (Volume of metal doped with lithium ions) / (Volume of metal not doped with lithium ions) (I)

  The volume change of the alloy particles can be measured by microscopic observation of the alloy particles in each of the charged state and the discharged state.

  More preferably, the alloy particles include a first metal and a second metal having a volume change rate in the above range, and the concentration of the first metal is maximized in the surface layer portion of the alloy particles. When the alloy particles include the first metal and the second metal having a volume change rate in the above range, and the concentration of the first metal is maximized in the surface layer portion of the alloy particles, the lithium ions are doped and dedoped. The stress difference due to the volume change of the alloy particles can be relaxed.

  The first metal can be doped and dedoped with lithium ions, and the metal having a volume change rate defined by the formula (I) of 100 to 1000% is preferably silicon (Si) or tin (Sn). The second metal can be doped and dedoped with lithium ions, and the metal having a volume change rate defined by the formula (I) of 0 to 100% includes silver (Ag), aluminum (Al), copper (Cu), Gallium (Ga), nickel (Ni), and zinc (Zn) are preferable, and Ni is more preferable. The alloy particles used in the present invention may contain other metals in addition to the first metal and the second metal. Further, the metal concentration gradient of the alloy particles may be continuous or discontinuous in the radial direction of the alloy particles as long as the concentration of the first metal is maximum in the surface layer portion of the alloy particles. That is, it is preferable that the concentration of the first metal is minimized in the innermost shell of the alloy particles and the concentration of the first metal is maximized in the outermost shell (surface layer portion). The concentration gradient can be confirmed by dissolving a small amount of the alloy particle surface with an acid and then performing elemental analysis (for example, X-ray fluorescence analysis (XRF)) of the treatment liquid or the particle itself.

  The alloy particles used in the present invention preferably have a core-shell structure having a core part and a shell part, and more preferably the shell part has a concentration gradient.

<Shell part>
It is preferable that the shell part of the alloy particle used for this invention consists of the metal layer a1 containing a 1st metal and a 2nd metal.

<Core part>
The core part of the alloy particles used in the present invention is composed of polymer particles, metal particles, oxides, carbon particles, etc. from the viewpoint of suppressing volume changes of the alloy particles when doping and dedoping lithium ions. It is preferable that the polymer particle and the carbon particle are used, and it is particularly preferable that the polymer particle is used.

<Polymer particles>
The polymer particles may be dispersed in water or may be in the form of beads. Usually, the polymer particles are dispersed in water. Those dispersed in water are usually water-dispersed polymers, and examples thereof include diene-based water-dispersed polymers, acrylic-based water-dispersed polymers, fluorine-based water-dispersed polymers, urethane-based polymers, and silicone-based polymers. Among these, from the viewpoint of being electrochemically stable, a diene water dispersion polymer and an acrylic water dispersion polymer are preferable.

  The diene-based water-dispersed polymer is a homopolymer or copolymer having a repeating unit formed by polymerizing conjugated dienes such as butadiene, isoprene, 1,3-pentadiene, and a mixture thereof in an amount of 50 mol% or more. is there.

  Specific examples of diene-based water-dispersed polymers include polybutadiene, polyisoprene, styrene / butadiene random copolymer, styrene / isoprene random copolymer, acrylonitrile / butadiene copolymer, styrene / acrylonitrile / butadiene copolymer, styrene / Examples thereof include butadiene block copolymers, styrene / butadiene / styrene block copolymers, styrene / isoprene block copolymers, and styrene / isoprene / styrene block copolymers.

  The acrylic water-dispersed polymer is a homopolymer or copolymer having 50 mol% or more of repeating units formed by polymerizing a derivative of acrylic acid, methacrylic acid or crotonic acid and a mixture thereof.

  Acrylic acid derivatives include acrylic acid, methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-amyl acrylate, isoamyl acrylate, n-hexyl acrylate , 2-ethylhexyl acrylate, hydroxypropyl acrylate, lauryl acrylate, acrylonitrile, and polyethylene glycol diacrylate. Methacrylic acid derivatives include methacrylic acid, methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate. , 2-ethylhexyl methacrylate, hydroxypropyl methacrylate, lauryl methacrylate, methacrylonitrile, glycidyl methacrylate, and tetraethylene glycol dimethacrylate. Examples of the crotonic acid derivatives include crotonic acid, methyl crotonic acid, ethyl crotonic acid, propyl crotonic acid, butyl crotonic acid, isobutyl crotonic acid, n-amyl crotonic acid, isoamyl crotonic acid, n-hexyl crotonic acid, and 2-ethylhexyl crotonic acid. And hydroxypropyl crotonic acid.

  Specific examples of the acrylic polymer include 2-ethylhexyl acrylate / methacrylonitrile / ethylene glycol dimethacrylate copolymer, 2-ethylhexyl acrylate / methacrylonitrile / tetraethylene glycol dimethacrylate copolymer, acrylic acid 2 -Ethylhexyl / methacrylonitrile / methoxypolyethylene glycol / ethylene glycol dimethacrylate copolymer, butyl acrylate / acrylonitrile / diethylene glycol dimethacrylate copolymer, 2-ethylhexyl methacrylate / ethyl acrylate / acrylonitrile / polyethylene glycol diacrylate copolymer Examples include coalescence.

  The fluorine-based water-dispersed polymer is a homopolymer or copolymer having a repeating unit obtained by polymerizing a fluorine-containing monomer.

  The urethane polymer is a homopolymer or copolymer having a urethane bond formed by condensation of an isocyanate group and an alcohol group.

  The silicone-based polymer is a homopolymer or copolymer having a main skeleton with a siloxane bond.

  In the present invention, from the viewpoint of adhesion with the metal layer a1, the polymer particles preferably have an anionic group or a cationic group. When the polymer particles have an anionic group or a cationic group, the adhesion of the polymer particles to the metal layer a1 is further improved.

  An anionic group is a group in which the substituent has an anionic chemical functionality, and examples of the anionic chemical functional group include carboxylate, sulfate, sulfonate, phosphate, phosphonate, or a mixture thereof.

  In order to contain an anionic group in the polymer particles, an anionic group-containing ethylenically unsaturated monomer may be copolymerized, and the anionic group-containing ethylenically unsaturated monomer is not particularly limited. Ethylenically unsaturated monocarboxylic acid monomers such as acrylic acid and methacrylic acid; ethylenically unsaturated polyvalent carboxylic acid monomers such as itaconic acid, maleic acid, fumaric acid and butenetricarboxylic acid; monobutyl fumarate Ethylenically unsaturated polyvalent carboxylic acid partial ester monomers such as monobutyl maleate and mono-2-hydroxypropyl maleate; multivalent carboxylic acid anhydrides such as maleic anhydride and citraconic anhydride; Unsaturated carboxylic acid monomer; styrene sulfonic acid, allyloxybenzene sulfonic acid, methallyloxybenzene sulfonic acid, vinyl Rusuruhon acid, allyl sulfonic acid, methallyl sulfonic acid, 4-sul phonic monomer having a sulfonic acid group, such as A Sid butyl methacrylate; and the like. These monomers can be used alone or in combination of two or more.

A cationic group is a group in which the substituent has cationic chemical functionality, and the substituent has the formula R ₁ R ₂ R ₃ R ₄ N ⁺ (A ⁻ ), where R ₁ is Is as follows.

R ₁ is of the formula —CH ₂ —CHOH—CH ₂ — or —CH ₂ —CH ₂ —, wherein R ₂ , R ₃ , R ₄ each independently represents 1-20 carbon atoms. is selected from alkyl or arylalkyl group having, a ^- is a halide ion, sulfate ion, phosphate ion, or a tetrafluoroborate ion.

  In order to contain a cationic group in the polymer particles, a cationic group-containing ethylenically unsaturated monomer may be copolymerized. As the cationic group-containing ethylenically unsaturated monomer, dimethylaminoethyl ( (Meth) acrylate, diethylaminoethyl (meth) acrylate, dipropylaminoethyl (meth) acrylate, diisopropylaminoethyl (meth) acrylate, dibutylaminoethyl (meth) acrylate, diisobutylaminoethyl (meth) acrylate, di-t-butylaminoethyl (Meth) acrylate, dimethylaminopropyl (meth) acrylamide, diethylaminopropyl (meth) acrylamide, dipropylaminopropyl (meth) acrylamide, diisopropylaminopropyl (meth) acrylamide, dibutyl (Meth) acrylic acid ester or (meth) acrylic acid amides having a dialkylamino group such as minopropyl (meth) acrylamide, diisobutylaminopropyl (meth) acrylamide, di-t-butylaminopropyl (meth) acrylamide, dimethylaminostyrene, Styrenes having a dialkylamino group such as dimethylaminomethylstyrene, vinylpyridines such as 4-vinylpyridine and 2-vinylpyridine, N-vinyl heterocyclic compounds such as N-vinylimidazole, aminoethyl vinyl ether, dimethylaminoethyl Acid neutralized or quaternary ammonium salts of monomers having amino groups such as vinyl ethers such as vinyl ether; Diary such as dimethyl diallyl ammonium chloride and diethyl diallyl ammonium chloride Such type quaternary ammonium salts.

  The content of the anionic group or the cationic group in the polymer particle is the content of the anionic group-containing ethylenically unsaturated monomer or the cationic group-containing ethylenically unsaturated monomer, and is 1 to 10% by mass. It is preferable that it is 2 to 5% by mass. When the cationic group or anionic group content in the polymer particles is within the above range, polymer particles excellent from the viewpoints of adhesion and production can be obtained.

  The solid content concentration of the polymer water-dispersed polymer is usually 15 to 70% by mass, preferably 20 to 65% by mass, and more preferably 30 to 60% by mass. When the solid content concentration is within this range, workability in producing alloy particles is good.

  The glass transition temperature of the polymer particles used in the present invention is preferably −70 ° C. or higher and 80 ° C. or lower, more preferably −50 ° C. or higher and 60 ° C. or lower, and further preferably −40 ° C. or higher and 50 ° C. or lower. When the glass transition temperature of the polymer particles is within the above range, strain due to expansion and contraction associated with charge / discharge can be absorbed in a low temperature environment and a high temperature environment, and charge / discharge cycle characteristics can be maintained. Examples of the method for controlling the glass transition temperature of the polymer particles include methods such as changing the monomer ratio in the polymer, controlling the molecular weight, controlling the regularity, and utilizing the interaction of contained side chains.

  The glass transition temperature here refers to the glass transition temperature of the polymer determined by a differential scanning calorimeter.

  The elastic modulus of the polymer particles used in the present invention is preferably 10 MPa or more and 1000 MPa or less, more preferably 15 MPa or more and 800 MPa or less, and further preferably 20 MPa or more and 600 MPa or less. When the modulus of elasticity of the polymer particles is in the above range, crushing during electrode pressing can be prevented, so that a desired density can be obtained, and strain due to expansion and contraction associated with charge / discharge can be absorbed. Examples of the method for controlling the glass transition temperature of the polymer particles include methods such as changing the monomer ratio in the polymer, controlling the molecular weight, controlling the regularity, and utilizing the interaction of contained side chains.

  Here, the elastic modulus indicates the tensile elastic modulus of the polymer obtained by forming the polymer particles into a film shape and obtaining by a tensile tester based on JIS K6251; 2004.

  The volume average particle size of the polymer particles is usually from 0.01 μm to 100 μm, and more preferably from 0.1 μm to 50 μm. When the average particle diameter of the polymer particles is within the above range, the dispersibility in the slurry is excellent. Examples of the measurement of the particle size distribution include a Coulter counter, a microtrack, and a transmission electron microscope.

<Carbon particles>
Examples of the carbon particles include carbonaceous materials and graphite materials. A carbonaceous material generally indicates a carbon material having a low graphitization (low crystallinity) obtained by heat-treating (carbonizing) a carbon precursor at 2000 ° C. or lower. A graphitic material having high crystallinity close to that of the graphite obtained by heat treatment as described above will be shown.

  Examples of the carbonaceous material include graphitizable carbon that easily changes the carbon structure depending on the heat treatment temperature, and non-graphitizable carbon having a structure close to an amorphous structure typified by glassy carbon. Examples of graphitizable carbon include carbon materials made from tar pitch obtained from petroleum and coal, such as coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, pyrolytic vapor-grown carbon fibers, etc. Is mentioned. MCMB is carbon fine particles obtained by separating and extracting mesophase spherules produced in the process of heating the pitches at around 400 ° C., and mesophase pitch-based carbon fiber is mesophase pitch obtained by growing and coalescing the mesophase spherules. Is a carbon fiber made from a raw material.

  Examples of the non-graphitizable carbon include phenol resin fired bodies, polyacrylonitrile-based carbon fibers, pseudo-isotropic carbon, and furfuryl alcohol resin fired bodies (PFA).

  Examples of the graphite material include natural graphite and artificial graphite. As artificial graphite, artificial graphite heat treated at 2800 ° C or higher, graphitized MCMB heat treated at 2000 ° C or higher, graphitized mesophase pitch carbon fiber heat-treated at 2000 ° C or higher mesophase pitch carbon fiber, etc. are used. Is done.

<Metal particles>
As the metal particles, lithium metal, a single metal that forms a lithium alloy, and an alloy are used.

  Examples of single metals and alloys that form lithium alloys include silver (Ag), aluminum (Al), barium (Ba), bismuth (Bi), copper (Cu), gallium (Ga), germanium (Ge), and indium (In ), Nickel (Ni), phosphorus (P), lead (Pb), antimony (Sb), silicon (Si), tin (Sn), strontium (Sr), zinc (Zn) and other compounds containing metals. It is done. Among them, Si, Sn or Pb simple metals, alloys containing these atoms, or compounds of these metals are used.

<Oxides etc.>
As the oxide, the oxide of the above metal particles is used. In addition, sulfides can also be used, and sulfides of the above metal particles are used.

  Examples of the oxide include an oxide such as tin oxide, manganese oxide, titanium oxide, niobium oxide, and vanadium oxide, and a lithium-containing metal composite oxide material containing a metal element selected from the group consisting of Si, Sn, Pb, and Ti atoms. Used.

As the lithium-containing metal composite oxide, a lithium titanium composite oxide represented by Li _x Ti _y M _z O ₄ (0.7 ≦ x ≦ 1.5, 1.5 ≦ y ≦ 2.3, 0 ≦ z ≦ 1.6, M includes Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb), among which Li _4/3 Ti _5/3 O ₄ , Li ₁ Ti ₂ O ₄ and Li _4/5 Ti _11/5 O ₄ are used.

  The alloy particles used in the present invention preferably have a core-shell structure in which the core portion is made of polymer particles and the shell portion is made of the metal layer a1, and the metal layer a1 has a metal concentration gradient. The metal layer a1 is preferably formed by metal plating. Examples of the metal plating include electroless plating, electrolytic plating, vacuum plating, and hot dipping, but electrolytic plating is preferable because it can be manufactured at low cost. The method of forming a metal concentration gradient in the metal layer a1 includes pH control of the plating bath, change of the plating solution composition, partial etching treatment of the metal layer a1, and the like, from the viewpoint of easy adjustment of the metal concentration gradient. It is preferable to form a metal concentration gradient in the metal layer a1 by controlling the pH of the plating bath.

  The shell portion in the present invention further includes a metal layer a2 formed by electroless plating in addition to the metal layer a1 including the first metal and the second metal, and the metal layer a1 is formed on the metal layer a2. Preferably it is formed. That is, the alloy particles in the present invention preferably have the metal layer a2 and the metal layer a1 in this order from the core part side.

  In the present invention, since the shell portion has such a configuration, the metal layer a1 having a large grain boundary and excellent elongation relaxes the stress difference due to the volume change during the charge / discharge cycle, and not only suppresses cracking but also during slurry kneading. It is also possible to suppress cracking. Thereby, the uniformity of a slurry can be improved and the dispersion | variation in battery capacity can also be suppressed. Furthermore, for the purpose of relaxing and protecting the expansion and contraction in the charge / discharge cycle, another metal layer can be provided on the metal layer a1. As the other metal layer, either electrolytic plating or electroless plating can be used, but electrolytic plating is preferable from the viewpoint of adhesion.

  In the case of forming the metal layer a2, any metal that can be formed by electrolytic plating can be appropriately selected from known materials such as aluminum (Al), chromium (Cr), manganese (Mn), and iron (Fe). , Cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), palladium (Pd), silver (Ag), tin (Sn), platinum (Pt), gold (Au) Etc. Among these, Ni, Cu, Sn, and alloys thereof are preferable because they are excellent in cost and mechanical properties.

  These metals and alloys may contain a third component for the purpose of facilitating the formation of plating and increasing the mechanical strength. Examples of the third component include phosphorus (P), boron (B), silicon (Si ), Manganese (Mn), chromium (Cr), iron (Fe), and the like.

<Electroless plating>
The electroless plating is not particularly limited, but it is preferable to use a known autocatalytic electroless plating. Examples of electroless plating include electroless copper plating using a reducing agent such as ammonium hypophosphite or hypophosphorous acid, ammonium borohydride, hydrazine, formalin, etc., and electroless plating using sodium hypophosphite as a reducing agent. Nickel-phosphorous plating, electroless nickel-boron plating using dimethylamine borane as a reducing agent, electroless palladium plating, electroless palladium-phosphorous plating using sodium hypophosphite as a reducing agent, electroless gold plating, electroless silver Electroless plating such as electroless nickel-cobalt-phosphorous plating using plating or sodium hypophosphite as a reducing agent can be used. Electroless copper plating and electroless nickel-phosphorous plating are more preferable because they are advantageous for forming ability on polymer particles and forming an electrolytic plating layer in the next step.

  When plating is performed by an electroless plating method, an activated catalyst is applied on the polymer particles, and then contacted with an electroless plating solution to perform electroless plating. Examples of the catalyst include silver, palladium, zinc, cobalt and the like, and are generally activated and used. The method for depositing the plating catalyst and activating the catalyst is not particularly limited. For example, after contacting an alkaline aqueous solution such as a potassium permanganate aqueous solution or a sodium permanganate aqueous solution with the polymer particles as necessary, hydroxylamine sulfate Neutralization and reduction treatment with an acidic aqueous solution such as a mixed solution of sulfuric acid and sulfuric acid, and then adding a metal compound such as silver, palladium, zinc, cobalt, or a salt or complex thereof to water or an organic solvent such as alcohol or chloroform. Examples include a method of reducing a metal after dipping in a solution (which may contain an acid, an alkali, a complexing agent, a reducing agent, etc., if necessary) dissolved at a concentration of 001 to 10% by weight. .

  The electroless plating solution used for electroless plating is not particularly limited, but a known autocatalytic electroless plating solution is preferably used. Examples of the electroless plating solution include an electroless copper plating solution containing ammonium hypophosphite or hypophosphorous acid, ammonium borohydride, hydrazine, formalin, or the like as a reducing agent, and sodium hypophosphite as a reducing agent. Electroless nickel-phosphorous plating solution, electroless nickel-boron plating solution using dimethylamine borane as reducing agent, electroless palladium plating solution, electroless palladium-phosphorous plating solution using sodium hypophosphite as reducing agent, electroless An electroless plating solution such as a gold plating solution, an electroless silver plating solution, or an electroless nickel-cobalt-phosphorous plating solution using sodium hypophosphite as a reducing agent can be used.

<Electrolytic plating>
Electrolytic plating is a method in which an article is energized in an electrolytic solution as a cathode to deposit a plated metal on the surface. The anode for electrolytic plating in the present invention is usually a plating of a metal element or a metalloid element capable of forming an alloy with lithium. Examples of metal elements or metalloid elements that can be plated and can form an alloy with lithium include silver (Ag), aluminum (Al), barium (Ba), bismuth (Bi), copper (Cu), and gallium (Ga). , Germanium (Ge), indium (In), nickel (Ni), phosphorus (P), lead (Pb), antimony (Sb), silicon (Si), tin (Sn), strontium (Sr), zinc (Zn) Or alloys thereof. Among them, it is preferable to deposit the metal layer a1 using an alloy (Si—Ni alloy or Sn—Ni alloy) in which the first metal is Si or Sn and the second metal is Ni. The metal forming the alloy is preferably two or more metals that do not form an intermetallic compound with each other.

  By using these metal elements or metalloid elements and controlling the pH of the plating bath when depositing the metal layer a1, the metal layer a1 can be provided with a metal concentration gradient, so that good charge / discharge cycle characteristics are achieved. In addition, a lithium ion secondary battery having a large capacity can be obtained. The metal layer a1 may further contain a third component for the purpose of facilitating the formation of plating, for the purpose of increasing mechanical strength, and for the purpose of relaxing the volume expansion during the insertion of lithium. As the third component, Zn, Sn, Fe, Ni, Ag, Co, Si, etc. are mentioned.

  The method for forming the metal concentration gradient is preferably pH control of the plating bath. For example, when the metal layer a1 is formed of a Sn-Ni alloy, Ni is preferentially precipitated by setting the pH to about 8, and then the pH is changed. To precipitate Sn. By controlling the pH in this way, the Sn concentration can be maximized in the surface layer portion of the alloy particles.

  The metal concentration gradient may be continuous or discontinuous, but is preferably continuous in order to alleviate the volume change during charge and discharge, and more preferably has a metal concentration gradient continuously in the radial direction.

  By using electrolytic plating, it is possible to perform plating in a short time and to produce particles at a low cost. Moreover, since the metal layer a1 having a metal concentration gradient produced by metal plating has high mechanical strength and excellent ductility, it can be made difficult to crack due to expansion / contraction associated with charge / discharge, and as a result, has excellent charge / discharge characteristics. Alloy particles can be obtained.

  For the cathode of electrolytic plating in the present invention, metal can be used, and plating can be deposited on the particles by contacting polymer particles subjected to electroless plating. The metal for the cathode may have any shape, but those having a mesh shape or a wool shape are preferable because particles are convected.

  The thickness of the metal layer a1 in the present invention is preferably 2 nm to 3 μm, more preferably 5 nm to 1 μm. Most preferably, it is 50 nm or more and 500 nm or less. The thickness of the metal layer a2 in the present invention is preferably 1 nm to 10 μm, more preferably 1 nm to 5 μm.

  When the thickness of the metal layer a1 is within this range, a lithium ion secondary battery capable of achieving both charge / discharge cycle characteristics and high capacity can be obtained. When the thickness of the metal layer a2 is in this range, the capacity per volume of the particles increases, and the workability when a slurry is prepared using the particles is excellent. The thickness of the metal layer a2 is adjusted by changing the composition of the polymer particles or the concentration of the metal catalyst solution used in the electroless plating process, changing the amount of catalyst present on the polymer particles, or changing the plating time. The thickness of the metal layer a1 is adjusted by the plating time, the amount of electricity, and the like. The thickness of the metal layer a1 and the metal layer a2 can be measured by cross-sectional observation with a transmission electron microscope.

  The slurry composition for lithium ion secondary batteries of this invention comprises the said negative electrode active material for lithium ion secondary batteries. It is preferable that the slurry composition for lithium ion secondary batteries of this invention contains a binder and a dispersion medium other than the said negative electrode active material for lithium ion secondary batteries.

<Binder>
The binder is a solution or dispersion in which binder (polymer) particles having binding properties are dissolved or dispersed in water or an organic solvent (hereinafter, these may be collectively referred to as “binder dispersion”). ). When the binder dispersion is water-based, it is usually a polymer water-dispersed polymer, for example, a diene polymer water-dispersed polymer, an acrylic polymer water-dispersed polymer, a fluorine-based polymer water-dispersed polymer, or a silicon-based polymer water. A dispersion polymer etc. are mentioned. A diene polymer water-dispersed polymer or an acrylic polymer water-dispersed polymer is preferred because it has excellent binding properties to the electrode active material and the strength and flexibility of the resulting electrode.

  When the binder dispersion is non-aqueous (using an organic solvent as a dispersion medium), usually, polyethylene, polypropylene, polyisobutylene, polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, polytetrafluoroethylene, poly Vinyl-based polymers such as vinyl acetate, polyvinyl alcohol, polyvinyl isobutyl ether, polyacrylonitrile, polymethacrylonitrile, polymethyl methacrylate, polymethyl acrylate, polyethyl methacrylate, allyl acetate, and polystyrene; dienes such as polybutadiene and polyisoprene Polymers such as polyoxymethylene, polyoxyethylene, polycyclic thioether, polydimethylsiloxane, etc. ether polymers containing heteroatoms in the main chain; polylactone, polycyclic anhydride, polyethylene N-terephthalate, polycarbonate, and other condensed ester polymers; Nylon 6, nylon 66, poly-m-phenylene isophthalamide, poly-p-phenylene terephthalamide, polypyromellitimide, and other condensed amide polymers -What was melt | dissolved in methylpyrrolidone (NMP) is mentioned.

  The diene polymer aqueous dispersion polymer is an aqueous dispersion of a polymer containing monomer units obtained by polymerizing conjugated dienes such as butadiene and isoprene. The proportion of the monomer unit obtained by polymerizing the conjugated diene in the diene polymer is usually 40% by weight or more, preferably 50% by weight or more, more preferably 60% by weight or more. Examples of the polymer include homopolymers of conjugated dienes such as polybutadiene and polyisoprene; and copolymers of monomers that are copolymerizable with conjugated dienes. Examples of the copolymerizable monomer include α, β-unsaturated nitrile compounds such as acrylonitrile and methacrylonitrile; unsaturated carboxylic acids such as acrylic acid and methacrylic acid; styrene, chlorostyrene, vinyltoluene, and t-butyl. Styrene monomers such as styrene, vinyl benzoic acid, methyl vinyl benzoate, vinyl naphthalene, chloromethyl styrene, hydroxymethyl styrene, α-methyl styrene, divinylbenzene; olefins such as ethylene, propylene; butadiene, isoprene, etc. Diene monomers; halogen atom-containing monomers such as vinyl chloride and vinylidene chloride; vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate and vinyl benzoate; vinyl esters such as methyl vinyl ether, ethyl vinyl ether and butyl vinyl ether Ethers; methyl vinyl ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, such as isopropenyl vinyl ketone; N- vinylpyrrolidone, vinylpyridine, and a heterocyclic containing vinyl compounds such as vinyl imidazole.

  The acrylic polymer aqueous dispersion polymer is an aqueous dispersion of a polymer containing monomer units obtained by polymerizing an acrylic ester and / or a methacrylic ester. The proportion of monomer units obtained by polymerizing acrylic acid ester and / or methacrylic acid ester is usually 40% by weight or more, preferably 50% by weight or more, more preferably 60% by weight or more. Examples of the polymer include a homopolymer of acrylic acid ester and / or methacrylic acid ester, and a copolymer with a monomer copolymerizable therewith. Examples of the copolymerizable monomer include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, and fumaric acid; two or more carbons such as ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, and trimethylolpropane triacrylate. -Carboxylic acid ester having a carbon double bond; styrene, chlorostyrene, vinyl toluene, t-butyl styrene, vinyl benzoic acid, vinyl benzoate methyl, vinyl naphthalene, chloromethyl styrene, hydroxymethyl styrene, α-methyl styrene, Styrenic monomers such as divinylbenzene; Amide monomers such as acrylamide, N-methylolacrylamide, and acrylamide-2-methylpropanesulfonic acid; α, β-non such as acrylonitrile and methacrylonitrile Japanese nitrile compounds; olefins such as ethylene and propylene; diene monomers such as butadiene and isoprene; monomers containing halogen atoms such as vinyl chloride and vinylidene chloride; vinyl acetate, vinyl propionate, vinyl butyrate, vinyl benzoate Vinyl esters such as methyl vinyl ether, ethyl vinyl ether, butyl vinyl ether, etc .; Vinyl ketones such as methyl vinyl ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, isopropenyl vinyl ketone; N-vinyl pyrrolidone, vinyl Heterocycle-containing vinyl compounds such as pyridine and vinylimidazole can be mentioned.

  The amount of these binders used is preferably from 0.1 to 10 parts by weight, more preferably from 0.5 to 8 parts by weight, in terms of solid content with respect to 100 parts by weight of the electrode active material (negative electrode active material). When the usage-amount of a binder is this range, the intensity | strength and softness | flexibility of the electrode which are obtained will become favorable.

<Active material>
The slurry composition for a negative electrode of a lithium ion secondary battery of the present invention may be used in combination with the negative electrode active material of the present invention and a general negative electrode active material for a lithium ion secondary battery.

  Examples of general negative electrode active materials for lithium ion secondary batteries include carbonaceous materials such as amorphous carbon, graphite, natural graphite, mesocarbon microbeads (MCMB), pitch-based carbon fibers, and conductive polymers such as polyacene. Etc. Preferred are graphite, natural graphite, and mesocarbon microbeads, and more preferred is graphite. Since the alloy particles can be deposited on the active material by using a general negative electrode active material for a lithium ion secondary battery, the time stability of the slurry can be improved. The addition amount of these general negative electrode active materials for lithium ion secondary batteries is preferably 0% by mass to 75% by mass, and more preferably 25% by mass to 50% by mass with respect to the entire active material. Within such a range, both high capacity and charge / discharge cycle characteristics can be achieved.

<Conductive agent>
The slurry composition for a lithium ion secondary battery negative electrode of the present invention may contain a conductive agent. As the conductive agent, conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor-grown carbon fiber, and carbon nanotube can be used. By using a conductive agent, electrical contact between electrode active materials can be improved, and when used in a lithium ion secondary battery, discharge rate characteristics can be improved. The usage-amount of a electrically conductive agent is 0-20 mass parts normally with respect to 100 mass parts of whole negative electrode active materials, Preferably it is 1-10 mass parts. Here, the whole negative electrode active material means the sum total of the negative electrode active material of this invention and the general negative electrode active material for lithium ion secondary batteries used as needed.

<Thickener>
The slurry composition for a lithium ion secondary battery negative electrode of the present invention may contain a thickener. Examples of thickeners include cellulosic polymers such as carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, and ammonium salts and alkali metal salts thereof; (modified) poly (meth) acrylic acid and ammonium salts and alkali metal salts thereof; ) Polyvinyl alcohols such as polyvinyl alcohol, copolymers of acrylic acid or acrylate and vinyl alcohol, maleic anhydride or copolymers of maleic acid or fumaric acid and vinyl alcohol; polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, modified Examples include polyacrylic acid, oxidized starch, phosphate starch, casein, and various modified starches. As for the compounding quantity of a thickener, 0.5-1.5 mass parts is preferable with respect to 100 mass parts of the whole negative electrode active material. When the blending amount of the thickener is within this range, the coating property and the adhesion with the current collector are good. In the present invention, “(modified) poly” means “unmodified poly” or “modified poly”, and “(meth) acryl” means “acryl” or “methacryl”.

<Dispersion medium>
As the dispersion medium used in the slurry composition for a negative electrode of the lithium ion secondary battery of the present invention, either water or an organic solvent can be used.

  Examples of the organic solvent include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methylpyrrolidone, cyclohexane, xylene, and cyclohexanone. These solvents may be used alone or in admixture of two or more and appropriately selected from the viewpoint of drying speed and environment.

<Manufacture of slurry composition for negative electrode of lithium ion secondary battery>
Although the mixing method of the slurry composition for lithium ion secondary battery negative electrodes is not specifically limited, For example, the method using mixing apparatuses, such as a stirring type, a shaking type, and a rotation type, is mentioned. Further, a method using a dispersion kneader such as a homogenizer, a ball mill, a sand mill, a roll mill, and a planetary kneader can be used. The solid content concentration of the slurry is 30% or more and 80% or less, and preferably 40% or more and 60% or less from the viewpoint of slurry stability and coating property.

<Anode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery according to the present invention is formed by coating the lithium ion secondary battery negative electrode slurry composition on a current collector and drying it to form a negative electrode active material layer.

<Current collector>
The current collector used in the present invention is not particularly limited as long as it has conductivity and is electrochemically durable, but a metal material is preferable because of its heat resistance, for example, iron, copper, aluminum Nickel, stainless steel, titanium, tantalum, gold, platinum and the like. As the current collector used for the negative electrode for a lithium ion secondary battery, copper is particularly preferable. The shape of the current collector is not particularly limited, but a sheet shape having a thickness of about 0.001 to 0.5 mm is preferable.

  In order to increase the adhesive strength with the negative electrode active material layer, the current collector is preferably used after roughening in advance. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, an abrasive cloth paper with a fixed abrasive particle, a grindstone, an emery buff, a wire brush provided with a steel wire or the like is used. Further, an intermediate layer may be formed on the current collector in order to increase the adhesive strength and conductivity with the negative electrode active material layer.

<Method for producing negative electrode for lithium ion secondary battery>
The method for producing a negative electrode for a lithium ion secondary battery of the present invention is not particularly limited. For example, the slurry composition for a negative electrode of a lithium ion secondary battery is applied to at least one side, preferably both sides, of a current collector, and dried by heating. Thus, a negative electrode active material layer is formed. The method for applying the slurry composition for a lithium ion secondary battery negative electrode to the current collector is not particularly limited. Examples of the method include a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. Examples of the drying method include drying with warm air, hot air, low-humidity air, vacuum drying, and drying by irradiation with (far) infrared rays or electron beams. The drying time is usually 5 to 30 minutes, and the drying temperature is usually 40 to 180 ° C.

  Next, it is preferable to lower the porosity of the negative electrode active material layer by pressure treatment using a mold press or a roll press. The preferable range of the porosity is 5 to 15%, more preferably 7 to 13%. If the porosity is too high, charging efficiency and discharging efficiency are deteriorated. When the porosity is too low, there is a problem in that it is difficult to obtain a high volume capacity, or the negative electrode active material layer is easily peeled off from the current collector, and a defect is likely to occur. Further, when a curable binder is used, it is preferably cured.

  The thickness of the negative electrode active material layer of the negative electrode for a lithium ion secondary battery of the present invention is usually 5 μm or more and 300 μm or less, preferably 30 μm or more and 250 μm or less.

  The lithium ion secondary battery of this invention is equipped with a positive electrode, a negative electrode, and electrolyte solution, and a negative electrode is the said negative electrode for lithium ion secondary batteries.

<Positive electrode>
The positive electrode used in the present invention is composed of a positive electrode active material layer containing at least a positive electrode active material, a conductive agent, and a binder, which is bound on a current collector.

As the positive electrode active material is not particularly _{limited, LiCoO 2, LiNiO 2, LiMnO} 2, LiMn 2 O 4, LiFePO 4, lithium-containing composite metal oxides such as _{LiFeVO 4; TiS 2, TiS 3} , amorphous MoS ₃ Transition metal sulfides such as Cu ₂ V ₂ O ₃ , amorphous V ₂ O—P ₂ O ₅ , MoO ₃ , V ₂ O ₅ , V ₆ O _13, etc. Further, a conductive polymer such as polyacetylene or poly-p-phenylene can be used. An iron-based oxide having poor electrical conductivity may be used as an active material covered with a carbon material by allowing a carbon source material to be present during reduction firing. These compounds may be partially element-substituted. Among these, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , and LiFePO ₄ are preferable, and LiCoO ₂ and LiNiO ₂ are more preferable because a high capacity battery can be manufactured.

  As a binder, the thing similar to what was illustrated by the term of the slurry composition for lithium ion secondary battery negative electrodes of this invention can be used. The amount of the binder in the positive electrode active material layer is preferably 0.1 to 5 parts by mass, more preferably 0.2 to 4 parts by mass, and particularly preferably 0.5 to 3 parts by mass with respect to 100 parts by mass of the positive electrode active material. Part. When the amount of the binder is within the above range, it is possible to prevent the active material from dropping from the electrode without inhibiting the battery reaction.

  As the conductive agent, conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor-grown carbon fiber, and carbon nanotube can be used. Examples thereof include carbon powders such as graphite, and fibers and foils of various metals. As the reinforcing material, various inorganic and organic spherical, plate-like, rod-like or fibrous fillers can be used. By using the conductive agent, the electrical contact between the positive electrode active materials can be improved, and when used in a lithium ion secondary battery, the discharge rate characteristics can be improved. The usage-amount of a electrically conductive agent is 0-20 mass parts normally with respect to 100 mass parts of positive electrode active materials, Preferably it is 1-10 mass parts.

<Electrolyte>
The electrolytic solution used in the present invention is not particularly limited. For example, a solution obtained by dissolving a lithium salt as a supporting electrolyte in a non-aqueous solvent can be used. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and other lithium salts. In particular, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li that are easily soluble in a solvent and exhibit a high degree of dissociation are preferably used. These can be used individually or in mixture of 2 or more types. The amount of the supporting electrolyte is usually 1% by mass or more, preferably 5% by mass or more, and usually 30% by mass or less, preferably 20% by mass or less, with respect to the electrolytic solution. If the amount of the supporting electrolyte is too small or too large, the ionic conductivity decreases, and the charging characteristics and discharging characteristics of the battery decrease.

  The solvent used in the electrolytic solution is not particularly limited as long as it can dissolve the supporting electrolyte. Usually, dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene. Alkyl carbonates such as carbonate (BC) and methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfolane and dimethyl sulfoxide Sulfur-containing compounds are used. In particular, dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate are preferable because high ion conductivity is easily obtained and the use temperature range is wide. These can be used individually or in mixture of 2 or more types. Moreover, it is also possible to use the electrolyte solution by containing an additive. As the additive, carbonate compounds such as vinylene carbonate (VC) are preferable.

Examples of the electrolytic solution other than the above include a gel polymer electrolyte obtained by impregnating a polymer electrolyte such as polyethylene oxide and polyacrylonitrile with an electrolytic solution, and an inorganic solid electrolyte such as LiI and Li ₃ N.

<Method for producing lithium ion secondary battery>
The manufacturing method of the lithium ion secondary battery of the present invention is not particularly limited. For example, the negative electrode and the positive electrode are overlapped via a separator, and this is wound or folded according to the shape of the battery and placed in the battery container, and the electrolytic solution is injected into the battery container and sealed. Further, if necessary, an expanded metal, an overcurrent prevention element such as a fuse or a PTC element, a lead plate and the like can be inserted to prevent an increase in pressure inside the battery and overcharge / discharge. The shape of the battery may be any of a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, a flat shape, and the like.

<Separator>
As the separator, known ones such as a microporous film or non-woven fabric made of polyolefin such as polyethylene or polypropylene or an aromatic polyamide resin; a porous resin coat containing inorganic ceramic powder;

  Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited thereto. In addition, unless otherwise indicated, the part and% in a present Example are a mass reference | standard.

  In Examples and Comparative Examples, various physical properties are evaluated as follows.

<Metal concentration gradient of particles>
50 mg of the prepared particles are immersed in 1 mol / L hydrochloric acid for a certain time, and the particles are washed with deionized water. Subsequently, elemental analysis of the particles is performed using an ICP emission analyzer. This operation is repeated, and the difference between the weight% of the outermost metal element and the weight% of the inner metal element is determined according to the following criteria. A larger value indicates that a metal concentration gradient is formed in the metal layer of the particle.
A: 10 wt% or more B: 7 wt% or more and less than 10 wt% C: 5 wt% or more and less than 7 wt% D: 1 wt% or more and less than 5 wt% E: less than 1 wt%

<Initial charging characteristics>
Using the produced coin-type battery, the capacity when charged to 1.5 V at a constant current of 0.1 C at 20 ° C. is determined according to the following criteria. A larger value indicates a higher-capacity active material, that is, an active material with excellent initial charge characteristics.
A: 700 mAh / g or more B: 500 mAh / g or more and less than 700 mAh / g C: 300 mAh / g or more and less than 500 mAh / g D: 100 mAh / g or more and less than 300 mAh / g C: less than 100 mAh / g

<Charge / discharge cycle characteristics>
Using the produced coin-type battery, a charge / discharge cycle is performed in which the battery is charged to 1.5 V at a constant current of 0.1 C at 20 ° C. and discharged to 0.2 V at a constant current of 0.1 C. The charge / discharge cycle is performed up to 50 cycles, and the ratio of the discharge capacity at the 50th cycle to the initial discharge capacity is defined as the capacity maintenance rate, and the determination is made according to the following criteria. It shows that the capacity | capacitance loss by repeated charging / discharging is so small that this value is large, ie, charging / discharging cycling characteristics are excellent.
A: 75% or more B: 60% or more and less than 75% C: 50% or more and less than 60% D: 40% or more and less than 50% C: less than 40%

Example 1
(Production of polymer particles)
In an autoclave equipped with a stirrer, 300 parts of ion-exchanged water, 33 parts of styrene, 65 parts of 2-ethylhexyl acrylate, 2.0 parts of dimethylaminoethyl methacrylate, 0.05 part of t-dodecyl mercaptan as a molecular weight regulator, polymerization initiator After adding 0.3 parts of potassium persulfate and stirring sufficiently, it heated and polymerized at 70 degreeC and the dispersion liquid containing a polymer particle was obtained. The polymerization conversion rate determined from the solid content concentration was approximately 99%. The volume average particle size of the obtained polymer particles is 0.1 μm, the cationic groups are nitrogen cations, the content of the cationic groups in the polymer particles is 2%, the glass transition temperature is −20 ° C., and the elasticity at room temperature of 25 ° C. The rate was 20 MPa.

(Preparation of alloy particles)
One part of polymer particles obtained by drying a dispersion containing polymer particles was added to a catalyst solution composed of 0.01 part of palladium chloride, 0.005 part of aqua regia and 10 parts of ultrapure water at 5 ° C. for 24 hours. It was immersed for a period of time to adsorb palladium as a catalyst. Then, electroless plating composed of 15 parts of Ni (PH ₂ O ₂ ) ₂ .6H ₂ O, 12 parts of H ₃ BO ₃ , 2.5 parts of CH ₃ COONa, and 1.3 parts of (NH ₃ ) ₂ SO ₄ By immersing in a bath at 30 ° C. for 5 minutes, polymer particles having a metal layer a2 formed by electroless plating were obtained. Subsequently, SnCl ₂ .2H ₂ O 0.175 mol L ⁻¹ , NiCl ₂ .6H ₂ O 0.075 mol L ⁻¹ , H ₂ NCH ₂ COOH 0.125 mol L ⁻¹ , K ₄ P ₂ O ₇ 0.5 mol L ⁻¹ The constituted electrolytic plating bath was adjusted to pH 8.12 using NH ₄ OH, and the polymer particles on which the metal layer a2 was formed were electroplated to form the metal layer a1 to obtain alloy particles. Ni mesh and Sn plate were used for the electrode of electroplating. The electroplating was performed for 1 hour, NH ₄ OH was dropped into the plating bath during the electroplating, and the pH of the plating bath at the end of the electroplating was 9.3. The obtained alloy particles were confirmed to have a core-shell structure. In the obtained alloy particles, the metal layer a1 had a thickness of about 2 nm, and the metal layer a2 had a thickness of about 10 nm.

  From the elemental analysis of the metal layer a1 of the alloy particles, the electrolytic plating bath at pH 8.12 is Sn 73%, Ni 27%, and the electrolytic plating portion at pH 9.3 is Sn 83%, Ni 17%. It was confirmed that an alloy was formed. That is, it was confirmed that the metal layer a1 has a metal concentration gradient continuously in the radial direction.

  Here, as the first metal capable of doping and dedoping lithium ions, the volume change rate of Sn is 676%, and as the second metal, the volume change rate of Ni is 0%.

(Production of slurry composition for electrode)
As carboxymethylcellulose, carboxymethylcellulose (“Serogen BSH-12” manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) having a solution viscosity of 8000 mPa · s was used to prepare a 1.5% aqueous solution.

  In a planetary mixer with a disper, 50 parts of the above alloy particles and 50 parts of artificial graphite (graphite) having an average particle diameter of 24.5 μm are put as a negative electrode active material, and 66.7 parts of the above 1.5% aqueous solution is added thereto. After adjusting the solid content concentration to 53.5% with ion-exchanged water, the mixture was mixed at 25 ° C. for 60 minutes. Next, after adjusting so that solid content concentration might be 44% with ion-exchange water, it mixed for 15 minutes at 25 degreeC. Next, 3 parts of a styrene-acrylonitrile copolymer (glass transition temperature—10 ° C.) was added to this solution and further mixed for 10 minutes. This was defoamed under reduced pressure to obtain a glossy and fluid slurry composition for electrodes.

(Manufacture of batteries)
Using a comma coater, the electrode slurry composition was applied to one side of a 18 μm thick copper foil so that the film thickness after drying was about 100 μm, dried at 60 ° C. for 20 minutes, and then at 150 ° C. for 2 hours. A negative electrode active material layer was formed by heat treatment. Subsequently, it rolled with the roll press and the electrode plate for negative electrodes of thickness 50 micrometers was obtained. When the coating thickness of the obtained negative electrode active material layer was measured, the film thickness was almost uniform.

  The negative electrode plate is cut out into a disk shape having a diameter of 15 mm and used as a negative electrode. A separator made of a disk-shaped polypropylene porous film having a diameter of 18 mm and a thickness of 25 μm is used as a negative electrode on the negative electrode active material layer surface side. Metal lithium and expanded metal were laminated in order, and this was stored in a stainless steel coin-type outer container (diameter 20 mm, height 1.8 mm, stainless steel thickness 0.25 mm) provided with polypropylene packing. The electrolyte is poured into the container so that no air remains, and the outer container is fixed with a 0.2 mm thick stainless steel cap through a polypropylene packing, and the battery can is sealed, and the diameter is A lithium ion coin type battery having a thickness of 20 mm and a thickness of about 2 mm was produced.

In addition, as an electrolytic solution, LiPF ₆ was added at 1 mol / liter in a mixed solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at EC: DEC = 1: 2 (volume ratio at 20 ° C.). A solution dissolved at a concentration was used.

  Table 1 shows the evaluation results of the performance of this battery.

(Comparative Example 1)
The polymer particles on which the metal layer a2 obtained by electroless plating obtained in Example 1 was formed were mixed with 105 parts of K ₂ SnO ₃ .3H ₂ O, 200 parts of Na ₂ Sn (OH) ₆ , 15 parts of KOH, and CH ₃ COOK. A lithium ion coin type battery was obtained and evaluated in the same manner as in Example 1 except that the pH was set to 9.12 in an electrolytic plating bath composed of 7 parts and electrolytic plating was performed for 1 hour. The results are shown in Table 1.

(Comparative Example 2)
A lithium ion coin type battery was obtained and evaluated in the same manner as in Example 1 except that silica powder was used as the negative electrode active material. The results are shown in Table 1.

  According to the present invention, as shown in Example 1, by using a negative electrode active material containing alloy particles having a metal concentration gradient, a lithium ion secondary battery having excellent initial charge characteristics and charge / discharge cycle characteristics is obtained. be able to.

  On the other hand, in Comparative Examples 1 and 2 using a negative electrode active material containing alloy particles having no metal concentration gradient, initial charge characteristics and charge / discharge cycle characteristics are inferior. This is considered to be due to the fact that the charge / discharge cycle characteristics are remarkably deteriorated because the particles are repeatedly expanded and contracted along with the insertion / extraction of lithium ions, and the negative electrode active material is miniaturized.

Claims (10)

   A negative electrode active material for a lithium ion secondary battery, comprising alloy particles having a metal concentration gradient. The alloy particles can be doped and dedoped with lithium ions, the first metal having a volume change rate defined by the following formula (I) of 100 to 1000%, and the first metal having a volume change rate of 0 to 100%. The negative electrode active material for a lithium ion secondary battery according to claim 1, comprising two metals.
(Volume change rate) = (Volume of metal doped with lithium ions) / (Volume of metal not doped with lithium ions) (I)   The negative electrode active material for a lithium ion secondary battery according to claim 2, wherein the concentration of the first metal is maximized in a surface layer portion of the alloy particles.   The negative electrode active material for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the alloy particles have a core-shell structure having a core portion and a shell portion.   The negative electrode active material for a lithium ion secondary battery according to claim 4, wherein the shell portion has the metal concentration gradient.   The negative electrode active material for a lithium ion secondary battery according to claim 2, wherein the first metal is Si or Sn, and the second metal is Ni.   The negative electrode active material for a lithium ion secondary battery according to claim 4, wherein the core part is a polymer particle.   The slurry composition for lithium ion secondary battery negative electrodes containing the negative electrode active material for lithium ion secondary batteries in any one of Claims 1-7.   The negative electrode for lithium ion secondary batteries formed by apply | coating the slurry composition for lithium ion secondary battery negative electrodes of Claim 8 to a collector, and drying.   A lithium ion secondary battery using the negative electrode for a lithium ion secondary battery according to claim 9.