JP6206045B2 - Negative electrode active material, negative electrode including negative electrode active material, and lithium ion secondary battery including negative electrode - Google Patents

Description

  The present invention relates to a negative electrode active material, a negative electrode including a negative electrode active material, and a lithium ion secondary battery including a negative electrode.

Lithium ion secondary batteries are widely applied as power sources for portable electronic devices because they are lighter and have a higher capacity than nickel cadmium batteries, nickel metal hydride batteries, and the like. It is also a promising candidate as a power source for use in hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries that serve as these power sources.
Currently, many researches have been conducted on negative electrode active materials for electrochemical devices such as lithium ion secondary batteries, such as silicon and silicon oxide, which have a larger charge / discharge capacity than carbon materials such as graphite. However, when such a material is used as the negative electrode active material, the negative electrode active material expands and contracts with charge and discharge, so that the binder expands due to repeated charge and discharge, and the active material particles and the current collector are separated. As a result, the cycle characteristics are significantly reduced as compared with the carbon material.
In order to solve the above-described problems, a technique for improving cycle characteristics by covering negative electrode active material particles with Si, Sn, Zn, or the like and suppressing expansion of the negative electrode active material particles has been proposed (Patent Document 1). ).

JP 2000-173593 A

  However, according to the method of Patent Document 1, the expansion of the negative electrode active material particles cannot be suppressed, and the cycle characteristics are not sufficiently improved.

The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a lithium ion secondary battery having high cycle characteristics.

The negative electrode active material according to the present invention is characterized in that the surface of the negative electrode active material particles is coated with a porous inorganic compound layer.

By using the negative electrode active material according to the present invention, the cycle characteristics of the lithium ion secondary battery are improved.

The thickness of the porous inorganic compound layer of the negative electrode active material according to the present invention is preferably 1 nm or more and 200 nm or less.

By using the negative electrode active material according to the present invention, the cycle characteristics of the lithium ion secondary battery are further improved.

The porosity of the porous inorganic compound layer of the negative electrode active material according to the present invention is preferably 10% or more and 50% or less.

By using the negative electrode active material according to the present invention, the cycle characteristics of the lithium ion secondary battery are further improved.

ADVANTAGE OF THE INVENTION According to this invention, the negative electrode active material for lithium ion secondary batteries which has sufficient cycling characteristics can be provided.

It is a schematic cross section of the lithium ion secondary battery of this embodiment. 2 is a STEM image of a porous inorganic compound layer obtained in Example 1. FIG. 2 is a schematic view of a negative electrode active material coated with a porous inorganic compound layer in Example 1. FIG.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings as the case may be. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings are not limited to the illustrated ratios.

(Lithium ion secondary battery)
FIG. 1 is a schematic cross-sectional view showing a lithium ion secondary battery according to this embodiment. As shown in FIG. 1, the lithium ion secondary battery 100 mainly includes a laminate 30, a case 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30. ing.

  The laminated body 30 is configured such that a pair of a positive electrode 10 and a negative electrode 20 are disposed to face each other with a separator 18 interposed therebetween. The positive electrode 10 is obtained by providing a positive electrode active material layer 14 on a plate-like (film-like) positive electrode current collector 12. The negative electrode 20 is obtained by providing a negative electrode active material layer 24 on a plate-like (film-like) negative electrode current collector 22. The main surface of the positive electrode active material layer 14 and the main surface of the negative electrode active material layer 24 are in contact with the main surface of the separator 18. Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

  Hereinafter, the positive electrode 10 and the negative electrode 20 are collectively referred to as electrodes 10 and 20, and the positive electrode current collector 12 and the negative electrode current collector 22 are collectively referred to as current collectors 12 and 22, and the positive electrode active material layer 14 and the negative electrode The active material layer 24 may be collectively referred to as the active material layers 14 and 24. First, the electrodes 10 and 20 will be specifically described.

(Positive electrode current collector)
The positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a binder, and a conductive auxiliary agent in an amount as necessary.

(Positive electrode active material)
Examples of the positive electrode active material include occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions (for example, PF ₆ ⁻ ) of the lithium ions. The electrode is not particularly limited as long as it can be reversibly advanced, and a known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and the general formula: LiNi _x Co _y Mn _z MaO ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr) Oxide, lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (where M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr, or VO) shown), and composite metal oxides of lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1) , etc.

(Positive electrode binder)
The binder binds the positive electrode active materials and the current collector 12 together with the positive electrode active materials. The binder is not particularly limited as long as the above-described bonding is possible, and examples thereof include fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). In addition to the above, for example, cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide resin, polyamideimide resin, or the like may be used as the binder. Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene. In this case, since the binder also exhibits the function of the conductive assistant particles, it is not necessary to add the conductive assistant. As the ion conductive conductive polymer, for example, those having ion conductivity such as lithium ion can be used. For example, polymer compounds (polyether-based polymer compounds such as polyethylene oxide and polypropylene oxide) , Polyphosphazene, etc.) and a lithium salt such as LiClO ₄ , LiBF ₄ , LiPF ₆ , or an alkali metal salt mainly composed of lithium, and the like. Examples of the polymerization initiator used for the combination include a photopolymerization initiator or a thermal polymerization initiator that is compatible with the above-described monomer.

The content of the binder in the positive electrode active material layer 14 is not particularly limited, but when added, it is preferably 0.5 to 5% by mass with respect to the mass of the positive electrode active material.

(Positive electrode conductive aid)
The conductive auxiliary agent is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive auxiliary agent can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal fine powders, and conductive oxides such as ITO.

Although content of the binder in the positive electrode active material layer 14 is not specifically limited, It is preferable that it is 1-10 mass% on the basis of the sum of the mass of a positive electrode active material, a conductive support agent, and a binder. By making content of a positive electrode active material and a binder into the said range, in the obtained positive electrode active material layer 14, the amount of binders is too small and the tendency which cannot form a strong positive electrode active material layer can be suppressed. Moreover, the amount of the binder that does not contribute to the electric capacity increases, and the tendency that it is difficult to obtain a sufficient volume energy density can be suppressed.

The content of the conductive additive in the positive electrode active material layer 14 is not particularly limited, but when added, it is preferably 0.5 to 5% by mass with respect to the mass of the positive electrode active material.

(Negative electrode current collector)
The negative electrode current collector 22 may be any conductive plate material, and for example, a metal thin plate (metal foil) of copper, nickel, stainless steel, or an alloy thereof can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material, a binder, and a conductive auxiliary agent in an amount as required.

(Negative electrode active material)
The negative electrode active material of this embodiment is composed of a negative electrode active material particle whose surface is covered with a porous inorganic compound layer.

The negative electrode active material of the present embodiment is a porous inorganic compound layer that exists on the surface of the negative electrode active material by changing the volume when lithium ions intercalate into the negative electrode active material and the negative electrode active material expands during charging. Since the voids compensate, the amount of expansion and contraction of the binder is reduced. For this reason, it is considered that the cutting of the conductive path of the negative electrode active material layer resulting from the expansion and contraction of the binder is suppressed, and the cycle characteristics are improved.

In addition, since the porous inorganic compound layer is not a dense film in which pores cannot be confirmed, it is considered to have the above function. Therefore, it can be recognized as porous in the range having the above functions. However, empirically, a case where the porosity is 10% or more can be defined as porous, and a case where the porosity is less than 10% can be defined as dense. Of course, it is not limited to this. In addition, such a porous inorganic compound layer can be confirmed by observing the cross section of the electrode containing the negative electrode active material with a transmission electron microscope such as STEM, thereby vacancy in the layer covering the surface of the negative electrode active material particles. Since the image is observed as a contrast, it can be confirmed by this, and the void ratio can be calculated by calculating the occupancy ratio of the holes by binarization processing by image analysis.

Examples of the negative electrode active material particles include graphite, non-graphitizable carbon, graphitizable carbon, and low-temperature calcined carbon capable of occluding and releasing lithium ions (intercalation / deintercalation or doping / dedoping). Mainly composed of carbon materials such as Al, Si, silicon oxide (in the specification, described as SiO), metals such as Sn, which can be combined with lithium, and oxides such as TiO ₂ , SnO ₂ and Fe ₂ O ₃ And particles containing a crystalline / amorphous compound, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like.

The porous inorganic compound layer is preferably a continuous film. With such a configuration, the influence on the binder due to the expansion and contraction of the negative electrode active material can be reduced. In addition, it is possible to suppress the dropping of the porous inorganic compound layer during charge and discharge, and the negative electrode active material layer and the pores of the separator are not blocked, so that ionic conductivity can be maintained, and cycle deterioration is reduced.

The continuous film is not an aggregate of particles, but is an integrated film in which the same material is formed continuously on the negative electrode active material surface. Therefore, the grain boundary normally observed in the aggregate of particles is not observed. Confirmation of the continuous film can also be easily confirmed by observing the cross section of the electrode containing the negative electrode active material with a transmission electron microscope such as STEM.

The coverage of the porous inorganic compound layer covering the negative electrode active material particles is not particularly limited as long as it has the above-described effect, but is preferably 50% or more.

The thickness of the porous inorganic compound layer is preferably 1 nm or more and 200 nm or less. Further, it is more preferably 5 nm or more and 100 nm or less.

When the thickness of the porous inorganic compound layer is within the above range, the void amount of the inorganic compound layer is suitable as an amount to compensate for the volume change of charge / discharge, and the amount of expansion / contraction of the binder can be reduced. The cutting of the conductive path of the negative electrode active material layer resulting from the expansion and contraction of is suppressed, and the cycle characteristics are improved. When the thickness of a porous inorganic compound layer exceeds the said range, the electroconductivity of a negative electrode negative electrode active material layer will reduce and cycling characteristics will fall.

Furthermore, the porosity of the porous inorganic compound layer is preferably 10% or more and 50% or less, and more preferably 20% or more and 40% or less.

When the porosity of the porous inorganic compound layer is within the above range, the amount of voids in the inorganic compound layer is appropriate as an amount to compensate for the volume change of charge / discharge, and the elongation of the binder can be reduced. Cutting of the conductive path of the negative electrode active material layer resulting from expansion and contraction is suppressed, and cycle characteristics are improved. When the porosity of the inorganic compound layer exceeds the above range, the strength of the inorganic compound layer decreases, and the inorganic compound falls off, thereby blocking the negative electrode active material layer and the pores of the separator, which may reduce the cycle characteristics. is there.

The porous inorganic compound layer is effective even when the entire surface of the negative electrode active material particles is not completely covered, but it is preferable that the entire surface is coated.

The porous inorganic compound layer covering the surface of the negative electrode active material particles is TiO ₂ , SnO ₂ , ZnO, NiO, ZrO ₂ , Nb ₂ O ₅ , VO ₂ , CrO ₂ , MoO ₂ , RuO ₂ , RuO ₂ , WO An oxide such as ₃ is conceivable. Alternatively, carbides and nitrides with these metals may be used. Moreover, what substituted other elements for the illustrated oxide, carbide, and nitride may be used. When the oxide is coated, it can be easily formed by a liquid phase method or the like.

As an example of the liquid phase method, specifically, the raw material for forming the porous inorganic compound layer is present in the solution as a complex, and the porous inorganic compound layer is deposited by depositing it on the surface of the negative electrode active material particles. Form. Next, specific procedures will be described in order.

First, an aqueous solution containing negative electrode active material particles and a metal fluoro complex is prepared.

Although the density | concentration of the metal fluoro complex in aqueous solution is restrict | limited to the solubility to the water of a metal fluoro complex, about 0.001-1M is preferable in general. Note that M = mol / L.

Further, this aqueous solution may contain a scavenger capable of extracting fluoride ions (F ⁻ ) from the metal fluoro complex. When a scavenger is added, the deposition rate of the metal oxide can be increased. The concentration when boric acid is used is limited to the solubility of boric acid in water, but is preferably about 0.01 to 0.6 M in the treatment solution.

Then, the aqueous solution containing the metal fluoro complex is impregnated with SiO powder. Specifically, the SiO powder may be put into an aqueous solution containing a metal fluoro complex and stirred as necessary. Further, instead of mixing the metal fluoro complex-containing aqueous solution and the scavenger from the beginning, a carbon material may be dispersed in the scavenger aqueous solution, and the metal fluoro complex aqueous solution may be dropped therein. When a scavenger is not used, a carbon material may be dispersed in water, and a metal fluoro complex aqueous solution may be dropped therein.

The surface of the negative electrode active material particles is preferably hydrophobized. Since the surface of the negative electrode active material particles is hydrophobized, wettability is reduced, and the deposited region is limited. Therefore, the deposited inorganic compound layer easily becomes a porous layer including voids. Since the inorganic compound layer continuously deposited following the deposition process is based on the porous layer including the voids, it is similarly a porous layer including voids. As a result of the above total precipitation process, the deposited inorganic compound layer is considered to be porous.

The hydrophobizing method is a method of coating with a hydrophobic substance such as carbon or the like, or a surface of the negative electrode active material particle, a hydrophobizing agent such as a surfactant, silicone oil, alkylhalogenosilane, alkylalkoxysilane, alkyl The method of making the gas of silylating agents, such as a disilazane, contact is mentioned.

Examples of the metal fluoro complex include a tin fluoro complex, a silicon fluoro complex, a titanium fluoro complex, a zirconium fluoro complex, an indium fluoro complex, a magnesium fluoro complex, a zinc fluoro complex, and an aluminum fluoro complex. Among these, a tin fluoro complex, Titanium fluoro complexes are preferred.

Specifically, examples of the metal fluoro complex include fluorinated zirconic acid (H ₂ ZrF ₆ ), fluorinated silicic acid (H ₂ SiF ₆ ), fluorinated titanic acid (H ₂ TiF ₆ ), or salts thereof, fluorinated Tin fluoride (SnF ₂ , SnF ₄ ), indium fluoride (InF ₃ ), copper fluoride (CuF ₂ ), magnesium fluoride (MgF ₂ ), zinc fluoride (ZnF ₂ ), aluminum fluoride (AlF ₃ ), etc. At least one selected from the group consisting of: Of these, at least one selected from the group consisting of titanic fluoride, salts thereof, and tin fluoride is preferable.

Examples of the salt of the metal fluoro complex include potassium salt, calcium salt, ammonium salt and the like. For example, K ₂ ZrF ₆ , K ₂ SiF ₆ , K ₂ TiF ₆ , CaZrF ₆ , CaSiF ₆ , CaTiF ₆ , (NH ₄ ) ₂ ZrF ₆ , (NH ₄ ) ₂ SiF ₆ , (NH ₄ ) ₂ TiF _{6 and the} like.

In addition, for example, such a metal fluoro complex can be obtained by converting a metal compound that is not a fluoro complex into a hydrofluoric acid (HF) aqueous solution, an ammonium hydrogen fluoride (NH ₄ F.HF) aqueous solution, an ammonium fluoride (NH ₄ F) aqueous solution, or the like. It can also be obtained by dissolving. For example, when iron oxyhydroxide (FeOOH) and cobalt hydroxide (Co (OH) ₂ ) are dissolved in an NH ₄ F / HF aqueous solution, a metal fluoro complex such as FeF ₆ ³⁻ or CoF ₆ ⁴⁻ is dissolved in the aqueous solution. Therefore, it can be used in the present invention.

Examples of the scavenger include boric acid (H ₃ BO ₃ ), aluminum (Al), ferrous chloride (FeCl ₂ ), ferric chloride (FeCl ₃ ), sodium hydroxide (NaOH), ammonia (NH ₃ ), Titanium (Ti), iron (Fe), nickel (Ni), magnesium (Mg), copper (Cu), zinc (Zn), silicon (Si), silicon dioxide (SiO ₂ ), calcium oxide (CaO), bismuth oxide (Bi ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), magnesium oxide (MgO) and the like can be mentioned, among which boric acid is preferable. In addition, supplements are not necessarily used.

In aqueous solutions, for example
^{_{MF x (x-2n) +}} nH 2 O⇔MO n + xF - + 2nH + (1)
Equilibrium reaction is established, and if there is H ₃ BO ₃ or Al as a scavenger,
H ₃ BO ₃ + 4H ⁺ + 4F ⁻ = HBF ₄ + 3H ₂ O (2)
Al + 6H ⁺ + 6F ⁻ = H ₃ AlF ₆ + 3 / 2H ₂ (3)
Thus, the balance of equation (1) is shifted to the right.

Specifically, boric acid becomes HBF ₄ to react with fluoride ions to equation (2). Fluoride equilibrium the hydride ion is consumed (1) to promote the generation MO _n is a metal oxide proceeds to the right. Further, Al also reacts with fluoride ions as shown in the formula (3) to become H ₃ AlF ₆ . As a result, in Eq. (1), the equilibrium proceeds in the direction in which MO _n which is a metal oxide is generated. When the reaction rate of the formula (1) is sufficiently fast depending on the type of the metal fluoro complex, or when the metal oxide to be generated itself functions as a scavenger, the scavenger may not be used.

The porous inorganic compound layer may contain F and / or B. For example, the F concentration with respect to the whole negative electrode active material (the negative electrode active material particles + the porous inorganic compound layer + F + B) may be 50 to 5000 mass ppm, and the B concentration may be 10 to 1000 mass ppm.

(Negative electrode binder and negative electrode conductive additive)
In addition to the material used for the positive electrode 10 described above, an acrylic resin such as PAA can also be used for the binder and the conductive assistant. In addition, the content of the binder and the conductive auxiliary agent is also adjusted as appropriate when the volume change of the negative electrode active material and the adhesion to the foil must be taken into account, and the same content as the content in the positive electrode 10 described above. Should be adopted. When adding, it is preferable that the addition amount of a binder is 2-20 mass% with respect to the mass of a negative electrode active material. It is preferable that the addition amount of a conductive support agent is 0.5-5 mass% with respect to the mass of a negative electrode active material.

With the components described above, the electrodes 10 and 20 can be produced by a commonly used method. For example, a paint containing an active material (a positive electrode active material or a negative electrode active material), a binder (a positive electrode binder or a negative electrode binder), a solvent, and a conductive additive (a positive electrode conductive additive or a negative electrode conductive aid) is placed on the current collector. It can manufacture by apply | coating and removing the solvent in the coating material apply | coated on the electrical power collector.

  As the solvent, for example, N-methyl-2-pyrrolidone, N, N-dimethylformamide, water and the like can be used.

There is no restriction | limiting in particular as an application | coating method, The method employ | adopted when producing an electrode normally can be used. Examples thereof include a slit die coating method and a doctor blade method.

The method for removing the solvent in the paint applied on the current collectors 12 and 22 is not particularly limited, and the current collectors 12 and 22 applied with the paint are dried, for example, in an atmosphere of 80 ° C. to 150 ° C. Just do it.

  Then, the electrodes on which the active material layers 14 and 24 are formed in this way may then be pressed by a roll press device or the like as necessary. The linear pressure of the roll press can be, for example, 10 to 50 kgf / cm.

Next, other components of the lithium ion secondary battery 100 will be described.

(Separator)
The separator is not particularly limited as long as it is stable with respect to the electrolytic solution and has excellent liquid retention, but generally includes a porous sheet of polyolefin such as polyethylene and polypropylene, or a nonwoven fabric.

(Electrolytes)
The electrolyte is contained in the positive electrode active material layer 14, the negative electrode active material layer 24, and the separator 18. The electrolyte is not particularly limited, and for example, in the present embodiment, an electrolytic solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, so that the withstand voltage during charging is limited to a low level. As the electrolytic solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. It does not specifically limit as lithium salt, The lithium salt used as an electrolyte of a lithium ion secondary battery can be used. For example, as the lithium salt, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiFSI, and LiBOB, organic acid anion salts such as LiCF ₃ SO ₃ , (CF ₃ SO ₂ ) ₂ NLi, and the like are used. be able to.

Moreover, as the organic solvent, for example, aprotic high dielectric constant solvents such as ethylene carbonate and propylene carbonate, aprotic low viscosity such as acetic acid esters and propionic acid esters such as dimethyl carbonate and ethyl methyl carbonate, etc. A solvent is mentioned. It is desirable to use these aprotic high dielectric constant solvents and aprotic low viscosity solvents in combination at an appropriate mixing ratio. Furthermore, ionic liquids using imidazolium, ammonium, and pyridinium type cations can be used. The counter anion is not particularly limited, and examples thereof include BF ₄ ⁻ , PF ₆ ⁻ , (CF ₃ SO ₂ ) ₂ N ^{− and the} like. The ionic liquid can be used by mixing with the organic solvent described above.
The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 2.0 M from the viewpoint of electrical conductivity. The conductivity of the electrolyte at 25 ° C. is preferably 0.01 S / m or more, and is adjusted by the type of electrolyte salt or its concentration.

When the electrolyte is a solid electrolyte or gel electrolyte, poly (vinylidene fluoride) or the like can be contained as a polymer material.
Furthermore, you may add various additives in the electrolyte solution of this embodiment as needed. Examples of additives include vinylene carbonate and methyl vinylene carbonate for the purpose of improving cycle life, biphenyl and alkyl biphenyl for the purpose of preventing overcharge, various carbonate compounds for the purpose of deoxidation and dehydration, Carboxylic anhydride, various nitrogen-containing and sulfur-containing compounds can be mentioned.

(Case)
The case 50 seals the laminated body 30 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can suppress leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, as the case 50, as shown in FIG. 1, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point, such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). Etc. are preferred.

(Lead)
The leads 60 and 62 are made of a conductive material such as aluminum.
Then, the leads 60 and 62 are welded to the positive electrode current collector 12 and the negative electrode current collector 22 by a known method, respectively, and a separator is provided between the positive electrode active material layer 14 of the positive electrode 10 and the negative electrode active material layer 24 of the negative electrode 20. 18 may be inserted into the case 50 together with the electrolytic solution with the 18 interposed therebetween, and the entrance of the case 50 may be sealed.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment. For example, the lithium ion secondary battery is not limited to the shape shown in FIG. It may be a type or the like.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

[Example 1]
(Preparation of positive electrode)
96% by weight of Li _1.05 Ni _0.77 Co _0.20 Al _0.025 O ₂ (manufactured by JFE Mineral Co., Ltd.) as the positive electrode active material, 2% by weight of carbon black as the conductive assistant, and 0% of graphite A paste-like positive electrode slurry was prepared by mixing and dispersing 0.5% by weight, 1.5% by weight of PVDF as a binder, and a solvent of N-methyl-2-pyrrolidone. Then, using a comma roll coater, the positive electrode active material layer was uniformly applied to both surfaces of an aluminum foil having a thickness of 20 μm. Next, the N-methyl-2-pyrrolidone solvent in the positive electrode active material was dried in an air atmosphere at 110 ° C. in a drying furnace. The coating amount of the positive electrode active material was 22.0 mg / cm ² . In addition, the thickness of the coating film of the positive electrode active material layer apply | coated to both surfaces of the said aluminum foil was adjusted to the substantially same film thickness. The positive electrode on which the positive electrode active material was formed was pressure-bonded to both surfaces of the positive electrode current collector by a roll press to produce a positive electrode so that the density of the positive electrode active material layer was 3.6 g / cm ³ . . The positive electrode sheet was obtained by the above.

(Preparation of negative electrode active material)
The negative electrode active material was coated with the porous inorganic compound layer by the following procedure.

(NH ₄ ) ₂ TiF ₆ (Morita Chemical Industry Co., Ltd.) was dissolved in water at 40 ° C. for 5 minutes to give a 0.3 M aqueous solution. After dispersing 40 g of SiO particle powder hydrophobized using hexamethyldisilazane in this aqueous solution, H ₃ BO ₃ (manufactured by Kanto Chemical Co., Inc.) is 1.5 times the concentration of the (NH ₄ ) ₂ TiF ₆ aqueous solution. And stirred at a rotational speed of 500 rpm. After 3 hours, the solution was filtered, washed with water, and dried at 60 ° C. to recover SiO particles coated with Ti oxide.

(Preparation of negative electrode)
As a negative electrode active material, 87.9% by weight of SiO, 2.1% by weight of acetylene black, 10% by weight of polyamide-imide resin, and a solvent of N-methyl-2-pyrrolidone are mixed and dispersed to form a negative electrode active material layer A slurry for formation was prepared. This slurry was applied to one surface of a 10 μm thick copper foil so that the amount of the negative electrode active material applied was 3.3 mg / cm ² and dried at 100 ° C. to form a negative electrode active material layer. The negative electrode on which the negative electrode active material is formed is pressure-bonded to both sides of the negative electrode current collector by a roll press, and the negative electrode is produced so that the density of the negative electrode active material layer is 1.5 g / cm ^3. did. Thus, a negative electrode sheet was obtained. The obtained electrode cross section was observed with STEM. The result is shown in FIG. It was confirmed that a porous inorganic compound layer was formed on the surface of the negative electrode active material particles.

<Measurement method of thickness of porous inorganic compound layer>
The thickness of the porous inorganic compound layer on the negative electrode active material surface was measured according to the following procedure. First, the cross section of the negative electrode active material particles is observed with a STEM, and the area of the porous inorganic compound layer portion is binarized and calculated by image analysis from the obtained cross-sectional STEM image. The thickness of the porous inorganic compound layer was obtained by dividing the cross-sectional area of the porous inorganic compound layer by the outer periphery of the negative electrode active material particles. The above operation was performed for each of the arbitrary 10 particles. For example, referring to FIG. 2 obtained in Example 1, the particle surrounded by a circle located at the center of the photograph of the STEM image obtained as shown in FIG. The focused particles are schematically shown in FIG. As shown in FIG. 3, the area of the portion of the porous inorganic compound layer (A) was calculated by binarization processing by image analysis, and was divided by the outer periphery of the negative electrode active material particles (B). The value obtained as described above was defined as the thickness (C) of the porous inorganic compound layer.

<Measurement method of porosity of porous inorganic compound layer>
The porosity of the porous inorganic compound layer on the negative electrode active material surface was measured according to the following procedure. The porous inorganic compound layer on the negative electrode active material surface was photographed using STEM (manufactured by JEOL). In the cross-sectional view of the porous inorganic compound layer on the surface of one arbitrarily selected negative electrode active material particle, a measurement region was arbitrarily selected, and the area of voids in the region was measured. For example, porosity = {(area of void) / (area of square of 100 nm × 100 nm)} × 100 was arbitrarily calculated for 10 places in the porous inorganic compound layer on the same particle surface. The above operation was performed for any 100 particles. What averaged the porosity obtained by said method was made into the porosity of a porous inorganic compound layer.

(Production of evaluation lithium-ion secondary battery)
Using the positive electrode and the negative electrode prepared above, a separator made of a polyethylene microporous film is sandwiched between them and placed in an aluminum laminate pack. In this aluminum laminate pack, a 1M LiPF ₆ solution (solvent: EC) is used as an electrolytic solution. / DEC = 3/7 (volume ratio)) was injected, and then vacuum-sealed to produce a lithium ion secondary battery for evaluation.

[Examples 2 to 10]
(NH ₄ ) ₂ TiF ₆ (Morita Chemical Industry Co., Ltd.) was dissolved in water at 40 ° C. for 5 minutes to obtain an aqueous solution having a concentration shown in Table 1. After dispersing 40 g of SiO powder hydrophobized with hexamethyldisilazane in this aqueous solution, H ₃ BO ₃ (manufactured by Kanto Chemical Co., Inc.) was added to a concentration 1.5 times that of the (NH ₄ ) ₂ TiF ₆ aqueous solution. And stirred at a rotational speed of 500 rpm. After 3 hours, the solution was filtered and washed with water, and then dried at 60 ° C., and the SiO particles covered with the Ti oxide were collected to obtain a negative electrode active material. A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1 except that the negative electrode active material produced by the above method was used, and the cycle characteristics were evaluated.

[Examples 11 to 17]
(NH ₄ ) ₂ TiF ₆ (Morita Chemical Industry Co., Ltd.) was dissolved in water at 40 ° C. for 5 minutes to give a 0.3 M aqueous solution. After dispersing 40 g of SiO powder hydrophobized with hexamethyldisilazane in this aqueous solution, H ₃ BO ₃ (manufactured by Kanto Chemical Co., Inc.) was added to a concentration 1.5 times that of the (NH ₄ ) ₂ TiF ₆ aqueous solution. Then, the mixture was stirred at the rotational speed shown in Table 1. After 3 hours, the solution was filtered and washed with water, and then dried at 60 ° C., and the SiO particles covered with the Ti oxide were collected to obtain a negative electrode active material. A lithium ion secondary battery for evaluation was produced in the same manner as in Example 1 except that the negative electrode active material produced by the above method was used, and the cycle characteristics were evaluated.

[Example 18]
A lithium ion secondary battery for evaluation was prepared in the same manner as in Example 1 except that (NH ₄ ) ₂ TiF ₆ (Morita Chemical Co., Ltd.) was changed to K ₂ ZrF ₆ (Pure Chemical Co., Ltd.) The cycle characteristics were evaluated.

[Comparative Example 1]
In Comparative Example 1, a lithium ion secondary battery for evaluation was produced in the same manner as in Example 1 except that the negative electrode active material coating treatment was not performed, and the cycle characteristics were evaluated.

[Comparative Example 2]
(NH ₄ ) ₂ TiF ₆ (manufactured by Morita Chemical Co., Ltd.) and H ₃ BO ₃ (manufactured by Kanto Chemical Co., Ltd.) were dissolved in water to prepare aqueous solutions having respective concentrations of 0.01M and 0.05M. 10 g of SiO powder was added to this aqueous solution, and the mixture was stirred and dispersed at room temperature at a rotation speed of 100 rpm. After 24 hours, the solution was filtered, washed with water, and then dried to recover SiO particles coated with Ti oxide. Moreover, the porosity of the inorganic compound layer of the obtained negative electrode active material was 0% by STEM observation, and it confirmed that it was dense.

[Comparative Example 3]
In Comparative Example 3, the negative electrode active material was coated with the porous inorganic compound layer by the ball mill treatment shown below.

SiO particle powder hydrophobized with hexamethyldisilazane and TiO ₂ particles were blended at a weight ratio of 80:20. Next, using a planetary ball mill apparatus, a ball mill treatment for repeating mechanical press-contact of the blend was performed for 24 hours. The ball mill container and the balls were made of stainless steel, and powder adjustment and ball mill treatment were performed in an argon atmosphere. The SiO particles coated with TiO ₂ were collected by the above procedure. Using this, a lithium ion secondary battery for evaluation was produced, and the cycle characteristics were evaluated. In addition, as a result of STEM observation, the obtained negative electrode active material has no inorganic compound as a layer, and the grain boundaries of TiO ₂ particles can be confirmed, and the particles are scattered on the surface of the SiO particles. It was confirmed.

<Method for evaluating cycle characteristics>
About the lithium ion secondary battery for evaluation produced by the Example and the comparative example, the cycle characteristic was measured using the secondary battery charging / discharging test apparatus (made by Hokuto Denko Co., Ltd.). The charge / discharge cycle of constant current and constant voltage charge to 0.5V at 0.5C and constant current discharge to 2.5V at 1C was repeated 500 cycles, the capacity retention rate after 500 cycles was measured, and the cycle characteristics were evaluated.

In Table 1, it shows about the inorganic compound of Examples 1-19 and Comparative Examples 1-3, film quality, precursor density | concentration, stirring speed, the thickness and porosity of a porous inorganic compound, and a cycle maintenance factor.

  When the cross section of the porous inorganic compound layer in Examples 1-18 was observed by STEM, it was confirmed that it was a porous continuous film in all Examples.

  Examples 1 to 18 showed a high cycle retention rate. In the batteries of Comparative Examples 1 to 3, when the porous inorganic compound layer was not coated, when the inorganic compound layer was dense and when it was not a continuum, the cycle retention rate was significantly reduced.

  By using the lithium ion secondary battery of the present invention, a lithium ion secondary battery having improved cycle characteristics can be provided.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode collector, 24 ... Negative electrode active material layer, 30 ... Laminate, 50 ... Case, 52 ... Metal foil, 54 ... Polymer film, 60, 62 ... Lead, 100 ... Lithium ion secondary battery.

Claims (4)

A negative electrode active material, wherein the surface of the negative electrode active material particles is coated with a porous inorganic compound layer, and the porosity of the porous inorganic compound layer is 10% or more and 50% or less .   The negative electrode active material according to claim 1, wherein the porous inorganic compound layer has a thickness of 1 nm to 200 nm. The negative electrode containing the negative electrode active material as described in any one of Claims 1 thru | or 2 . A lithium ion secondary battery comprising the negative electrode according to claim 3 , a positive electrode, and an electrolyte.