JPWO2020067528A1 - Negative electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

The present invention includes a negative electrode active material layer containing a negative electrode active material and a negative electrode binder, and an insulating layer provided on the surface of the negative electrode active material layer and containing insulating fine particles and an insulating layer binder. A negative electrode for a secondary battery, wherein the coating amount of the insulating fine particles on the surface of the negative electrode active material layer is 20 to 70%. According to the present invention, it is possible to provide a negative electrode for a lithium ion secondary battery capable of producing a lithium ion secondary battery having excellent cycle characteristics, and a lithium ion secondary battery including the negative electrode.

Description

The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery including the negative electrode for the lithium ion secondary battery.

Lithium-ion secondary batteries are used as large-scale stationary power sources for power storage, power sources for electric vehicles, etc., and in recent years, research on miniaturization and thinning of batteries has been progressing. A lithium ion secondary battery is generally provided with both electrodes having an electrode active material layer formed on the surface of a metal foil and a separator arranged between the electrodes. The separator plays a role of preventing a short circuit between both electrodes and holding an electrolytic solution.

As the negative electrode active material of the negative electrode of the lithium ion secondary battery, a carbon-based material is widely used because the reaction potential is low, a high-energy battery can be obtained, and the price is low. It is known that a film called SEI (Solid-Electrolyte-Interphase) is formed on the surface of a negative electrode active material made of a carbon-based material. Although SEI has advantages such as suppressing deterioration of the active material, it is also known to reduce cycle characteristics when a large number of charges and discharges are required to form SEI. It is also known that repeated charging and discharging causes destruction and reconstruction of SEI, which adversely affects performance such as cycle characteristics of a lithium ion secondary battery (for example, Patent Document 1).

Japanese Patent No. 6172309

As described above, the surface condition of the negative electrode active material layer is closely related to the performance of the lithium ion secondary battery. However, at present, there is not much knowledge about what kind of surface condition is good for improving the cycle characteristics of lithium-ion secondary batteries, and the development of lithium-ion secondary batteries with excellent cycle characteristics has been developed. It is desired.
Therefore, it is an object of the present invention to provide a negative electrode for a lithium ion secondary battery capable of producing a lithium ion secondary battery having excellent cycle characteristics, and a lithium ion secondary battery including the negative electrode.

As a result of diligent studies, the present inventors have conducted an insulation provided on the surface of the negative electrode active material layer containing the negative electrode active material and the negative electrode active material layer and containing the insulating fine particles and the binder for the insulating layer. A negative electrode for a lithium ion secondary battery including a layer, wherein the negative electrode for a lithium ion secondary battery in which the coating amount of the insulating fine particles on the surface of the negative electrode active material layer is 20 to 70% can solve the above-mentioned problems. The present invention was completed.
The gist of the present invention is the following [1] to [8].
[1] A lithium ion secondary provided with a negative electrode active material layer containing a negative electrode active material and a negative electrode binder, and an insulating layer provided on the surface of the negative electrode active material layer and containing insulating fine particles and an insulating layer binder. A negative electrode for a lithium ion secondary battery, which is a negative electrode for a battery and has a coating amount of the insulating fine particles on the surface of the negative electrode active material layer of 20 to 70%.
[2] The negative electrode for a lithium ion secondary battery according to the above [1], wherein the insulating layer has a thickness of 1 to 3 μm.
[3] The negative electrode for a lithium ion secondary battery according to the above [1] or [2], wherein the content of the binder for the insulating layer is 10 to 50% by volume based on the total amount of the insulating layer.
[4] The negative electrode for a lithium ion secondary battery according to any one of [1] to [3] above, wherein the negative electrode active material has an average particle size of 10 to 25 μm.
[5] The method for manufacturing a negative electrode for a lithium ion secondary battery according to any one of the above [1] to [4], which is performed on the surface of a negative electrode active material layer containing a negative electrode active material and a negative electrode binder. A step of applying an insulating layer composition containing insulating fine particles, a binder for an insulating layer, and an organic solvent for an insulating layer to form an insulating layer, and the viscosity of the insulating layer composition at 25 ° C. is 700. A method for manufacturing a negative electrode for a lithium ion secondary battery, which is ~ 3000 mPa · s.
[6] A lithium ion secondary comprising the negative electrode according to any one of [1] to [4] above, a positive electrode arranged so as to oppose the negative electrode, and a separator arranged between the negative electrode and the positive electrode. battery.
[7] The lithium ion secondary battery according to the above [6], further comprising an electrolytic solution containing an organic solvent for an electrolytic solution and an electrolyte salt.
[8] The lithium ion secondary battery according to the above [7], wherein the organic solvent for the electrolytic solution contains vinylene carbonate.

According to the present invention, it is possible to provide a negative electrode for a lithium ion secondary battery capable of producing a lithium ion secondary battery having excellent cycle characteristics, and a lithium ion secondary battery including the negative electrode.

It is sectional drawing which shows typically one Embodiment of the negative electrode for a lithium ion secondary battery of this invention. This is an example of an image obtained by observing the surface of the insulating layer with a laser microscope. This is an example of an image (after binarization) of the surface of the insulating layer observed with a laser microscope. It is schematic sectional drawing which shows one Embodiment of the lithium ion secondary battery of this invention schematically.

The present invention includes a negative electrode active material layer containing a negative electrode active material and a negative electrode binder, and an insulating layer provided on the surface of the negative electrode active material layer and containing insulating fine particles and an insulating layer binder. A negative electrode for a secondary battery, which is a negative electrode for a lithium ion secondary battery in which the coating amount of the insulating fine particles on the surface of the negative electrode active material layer is 20 to 70%. In the present specification, the negative electrode for a lithium ion secondary battery may be simply referred to as a negative electrode.

<Negative electrode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery of the present invention includes a negative electrode active material layer and an insulating layer provided on the surface of the negative electrode active material layer.
FIG. 1 is a cross-sectional view schematically showing an embodiment of a negative electrode for a lithium ion secondary battery of the present invention. The negative electrode 10 for a lithium ion secondary battery of the present invention includes a negative electrode active material layer 11, an insulating layer 12 provided on one surface of the negative electrode active material layer 11, and a negative electrode provided on the other surface of the negative electrode active material layer 11. It includes a current collector 13. The insulating layer 12 contains insulating fine particles 14 and a binder (not shown), and the insulating fine particles 14 are bound by a binder. The insulating fine particles 14 cover many regions of the surface 11A of the negative electrode active material layer 11, but an exposed portion 15 having an exposed surface is present on a part of the surface 11A of the negative electrode active material layer 11. ..
In the present invention, the cycle characteristics are improved by adjusting the coating amount of the insulating fine particles 14 within a certain range.
Hereinafter, each layer constituting the negative electrode for the lithium ion secondary battery will be described in detail.

[Insulation layer]
The insulating layer of the present invention is provided on the surface of the negative electrode active material layer described later, and contains insulating fine particles and a binder for the insulating layer. The insulating fine particles cover the surface of the negative electrode active material layer described later.
The coating amount of the insulating fine particles on the surface of the negative electrode active material layer is 20 to 70%. If the coating amount is less than 20%, the cycle characteristics are deteriorated. It is considered that when the amount of coating with the insulating fine particles is small, SEI is likely to be deposited on the surface of the uncoated negative electrode active material, which deteriorates the cycle characteristics. Further, even when the coating amount exceeds 70%, the cycle characteristics are deteriorated. It is considered that this is because lithium is likely to be precipitated due to the decrease in ionic conductivity.

From the viewpoint of improving the cycle characteristics of the lithium ion secondary battery, the coating amount of the insulating fine particles on the surface of the negative electrode active material layer is preferably 40 to 60%.
The amount of insulating fine particles coated on the surface of the negative electrode active material layer can be determined by observing with a laser microscope. Specifically, the surface of the insulating layer is observed with a laser microscope in a constant observation field of view (for example, 800 μm × 800 μm), the area where the insulating fine particles are present is determined, and the ratio of the area where the insulating fine particles are present in the observation field of view. Is calculated, and this is used as the coating amount of the insulating fine particles.
For example, as an example of calculating the coating amount, FIG. 2 shows a diagram in which the surface of the insulating layer is observed with a laser microscope, and FIG. 3 shows a binarized image of FIG. The part that looks white is the part where the insulating fine particles are present, and the part that looks black is the exposed part of the negative electrode active material layer. The amount of the insulating fine particles covered can be obtained by calculating the ratio of the area of the white-looking portion in the observation field of view.

The amount of insulating fine particles coated on the surface of the negative electrode active material layer is the composition of the composition for the insulating layer for forming the insulating layer, the solid content concentration, the viscosity, the thickness of the insulating layer, and the negative electrode activity constituting the negative electrode active material layer. By adjusting the type of substance, particle size, etc., it can be adjusted to a desired value.

The thickness of the insulating layer is not particularly limited, but is preferably 1 to 3 μm, and more preferably 1 to 2 μm. By setting the thickness of the insulating layer to these lower limit values or more, the safety of the lithium ion secondary battery can be enhanced. By setting the thickness of the insulating layer to be equal to or less than these upper limit values, it is possible to suppress the deterioration of the input / output characteristics due to the lengthening of the ion path.
As described above, lithium is excellent in cycle characteristics, input / output characteristics, and safety by appropriately adjusting the thickness of the insulating layer and setting the coating amount of insulating fine particles on the surface of the negative electrode active material layer within a certain range. Ion secondary batteries can be obtained. In the present specification, the term "safety" means a characteristic that the temperature does not rise significantly when the lithium ion secondary battery is placed at a temperature of about 110 ° C.

(Insulating fine particles)
The insulating fine particles are not particularly limited as long as they are insulating, and are preferably inorganic particles. Inorganic particles include silicon dioxide, silicon nitride, alumina, boehmite, titania, zirconia, boron nitride, zinc oxide, tin dioxide, niobium oxide (Nb ₂ O ₅ ), tantalum oxide (Ta ₂ O ₅ ), potassium fluoride, and foot. Examples thereof include particles composed of inorganic compounds such as lithium pentoxide, zeolite, and calcium carbonate. Further, the inorganic particles may be particles composed of known composite oxides such as niobium-tantalum composite oxide and magnesium-tantalum composite oxide. The insulating fine particles may be particles in which one of the above materials is used alone, or particles in which two or more of the above materials are used in combination. Of these, alumina particles are preferred.

The average particle size of the insulating fine particles is not particularly limited as long as it is smaller than the thickness of the insulating layer, and is, for example, 0.001 to 1 μm, preferably 0.05 to 0.8 μm, and more preferably 0.1 to 0.6 μm. is there. By keeping the average particle size of the insulating fine particles within these ranges, it becomes easy to adjust the coating amount within the above ranges.
The average particle size means the particle size (D50) when the volume integration is 50% in the particle size distribution of the insulating fine particles obtained by the laser diffraction / scattering method.
Further, as the insulating fine particles, one type having an average particle diameter within the above range may be used alone, or two types of insulating fine particles having different average particle diameters may be mixed and used.

The content of the insulating fine particles contained in the insulating layer is preferably 10 to 95% by volume, more preferably 40 to 90% by volume, still more preferably 70 to 90% by volume, based on the entire insulating layer. When the content of the insulating fine particles is within the above range, it becomes easy to adjust the coating amount within the above range.

(Binder for insulating layer)
Binders for the insulating layer include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), fluorine-containing resins such as polytetrafluoroethylene (PTFE), polymethylacrylate (PMA), and poly. Acrylic resins such as methyl methacrylate (PMMA) and polybutyl methacrylate (PBMA), polyvinyl acetate, polyimide (PI), polyamide (PA), polyvinylidene fluoride (PVC), polyether nitrile (PEN), polyethylene (PE), Examples thereof include polypropylene (PP), polyacrylonitrile (PAN), acrylonitrile-butadiene rubber, styrene butadiene rubber, poly (meth) acrylic acid, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol and the like. These binders may be used alone or in combination of two or more. Further, carboxymethyl cellulose and the like may be used in the form of a salt such as a sodium salt. Among these, polyvinylidene fluoride (PVDF) and acrylic resin are preferable. The acrylic resin may be a crosslinked acrylic resin or a non-crosslinked acrylic resin, but a crosslinked acrylic resin is preferable, and a crosslinked polybutyl acrylate is more preferable.

The weight average molecular weight of the binder for the insulating layer is preferably 400,000 to 2,000,000, preferably 700,000 to 1,500,000, and 850,000 to 1,300,000. It is more preferable to have. The weight average molecular weight is measured by a gel permeation chromatography method (GPC method).
By adjusting the solid content concentration of the resin composition described later while keeping the weight average molecular weight in the above range, it becomes easy to set the coating amount of the insulating fine particles on the surface of the negative electrode active material layer in a desired range.

The content of the binder for the insulating layer contained in the insulating layer is preferably 10 to 50% by volume, more preferably 10 to 30% by volume, and further preferably 15 to 25% by volume based on the entire insulating layer. .. When the content of the binder for the insulating layer is within the above range, it becomes easy to adjust the coating amount within the above range.

The insulating layer may contain optional components other than the insulating fine particles and the binder for the insulating layer as long as the effects of the present invention are not impaired. However, the total volume of the insulating fine particles and the binder for the insulating layer is preferably 85% by volume or more, more preferably 90% by volume or more, based on the entire insulating layer.

[Negative electrode active material layer]
The negative electrode active material layer of the present invention contains a negative electrode active material and a binder for a negative electrode. Examples of the negative electrode active material include carbon materials such as graphite and hard carbon, a composite of a tin compound, silicon and carbon, and lithium. Among these, a carbon material is preferable, and graphite is more preferable.
The average particle size of the negative electrode active material is not particularly limited, but is preferably 0.5 to 50 μm, more preferably 1 to 30 μm, and even more preferably 10 to 25 μm. The average particle size of the negative electrode active material means the particle size (D50) when the volume integration is 50% in the particle size distribution of the negative electrode active material obtained by the laser diffraction / scattering method.
The content of the negative electrode active material in the negative electrode active material layer is preferably 50 to 99% by mass, more preferably 60 to 98% by mass, based on the total amount of the negative electrode active material layer.

As the negative electrode binder contained in the negative electrode active material layer, the same binder as the above-mentioned insulating layer binder can be used. Of these, styrene-butadiene rubber and carboxymethyl cellulose are preferable.
The content of the negative electrode binder in the negative electrode active material layer is preferably 0.5 to 30% by mass, more preferably 1.0 to 25% by mass, and 1.5 to 10% by mass based on the total amount of the negative electrode active material layer. Mass% is more preferred.
The thickness of the negative electrode active material layer is not particularly limited, but is preferably 10 to 200 μm, and more preferably 50 to 150 μm.

[Negative electrode current collector]
The negative electrode for a lithium ion secondary battery of the present invention preferably includes a negative electrode current collector. The negative electrode current collector is preferably provided on the surface of the negative electrode active material layer opposite to the surface provided with the insulating layer. That is, in this case, the negative electrode for the lithium ion secondary battery has a negative electrode current collector, a negative electrode active material layer, and an insulating layer laminated in this order. Examples of the material constituting the negative electrode current collector include conductive metals such as copper, aluminum, titanium, nickel, and stainless steel. Among these, aluminum or copper is preferable, and copper is more preferable. The negative electrode current collector is generally made of a metal foil, and its thickness is not particularly limited, but is preferably 1 to 50 μm.

<Manufacturing method of negative electrode for lithium ion secondary battery>
Next, an embodiment of a method for manufacturing a negative electrode for a lithium ion secondary battery will be described. In the method for producing a negative electrode for a lithium ion secondary battery of the present invention, a negative electrode active material layer is first formed, and then insulating fine particles are formed on the surface of the negative electrode active material layer containing the negative electrode active material and the negative electrode binder. The present invention comprises a step of applying an insulating layer composition containing an insulating layer binder to form an insulating layer.

(Formation of negative electrode active material layer)
In forming the negative electrode active material layer, first, a composition for the negative electrode active material layer containing the negative electrode active material, the binder for the negative electrode, and the solvent is prepared. The composition for the negative electrode active material layer is a slurry. Water is preferably used as the solvent in the composition for the negative electrode active material layer. The solid content concentration of the composition for the negative electrode active material layer is preferably 7 to 75% by mass, more preferably 20 to 65% by mass.

The negative electrode active material layer may be formed by a known method using the negative electrode active material layer composition. For example, the negative electrode active material layer composition is applied onto a negative electrode current collector and dried. Can be formed by
Further, the negative electrode active material layer may be formed by applying the composition for the negative electrode active material layer on a base material other than the negative electrode current collector and drying it. Examples of the base material other than the negative electrode current collector include known release sheets. The negative electrode active material layer formed on the base material may preferably be an insulating layer, and then the negative electrode active material layer may be peeled off from the base material and transferred onto the negative electrode current collector.
The negative electrode active material layer formed on the negative electrode current collector or the base material is preferably pressure-pressed. By pressurizing, it becomes possible to increase the electrode density. The pressure press may be performed by a roll press or the like.

(Formation of insulating layer)
The composition for an insulating layer used for forming the insulating layer contains insulating fine particles, a binder for the insulating layer, and a solvent. The composition for the insulating layer is a slurry. The insulating fine particles and the binder for the insulating layer are as described above. Examples of the solvent include N-methylpyrrolidone, N-ethylpyrrolidone, dimethylacetamide, and dimethylformamide, and N-methylpyrrolidone is preferable. Only one type of solvent may be used, or two or more types may be used in combination.

The solid content concentration of the composition for the insulating layer is preferably 15 to 60% by mass, more preferably 22 to 40% by mass, and further preferably 25 to 35% by mass. When the solid content concentration is adjusted to such a level, it becomes easy to adjust the coating amount of the insulating fine particles on the surface of the negative electrode active material layer to a desired value while keeping the thickness of the insulating layer within the above range.
The viscosity of the insulating layer composition at 25 ° C. is preferably 700 to 3000 mPa · s, more preferably 1000 to 1500 mPa · s. By setting the viscosity in such a range, it becomes easy to adjust the coating amount of the insulating fine particles to a desired range while keeping the thickness of the insulating layer in the above range. The viscosity is a viscosity measured with a B-type viscometer under the conditions of 60 rpm and 25 ° C.

The insulating layer can be formed by applying the composition for an insulating layer on the negative electrode active material layer and drying it. The method of applying the composition for the insulating layer to the surface of the negative electrode active material layer is not particularly limited, and for example, a dip coating method, a spray coating method, a roll coating method, a doctor blade method, a bar coating method, a gravure coating method, and screen printing. Law etc. can be mentioned. Among these, the gravure coating method is preferable from the viewpoint of uniformly applying the composition for the insulating layer and facilitating the adjustment of the coating amount of the insulating fine particles.
The drying temperature is not particularly limited as long as the solvent can be removed, but is, for example, 40 to 120 ° C, preferably 50 to 100 ° C. The drying time is not particularly limited, but is, for example, 30 seconds to 15 minutes.

<Lithium-ion secondary battery>
FIG. 4 is a cross-sectional view schematically showing an embodiment of the lithium ion secondary battery of the present invention. The lithium ion secondary battery 20 includes a positive electrode 29 and a negative electrode 28 arranged so as to face the positive electrode 29. The negative electrode 28 is the negative electrode for the lithium ion secondary battery of the present invention.
The negative electrode 28 includes a negative electrode active material layer 21, an insulating layer 22 provided on one surface of the negative electrode active material layer 21, and a negative electrode current collector 23 provided on the other surface of the negative electrode active material layer 21. .. The positive electrode 29 includes a positive electrode current collector 27 and a positive electrode active material layer 26 laminated on the positive electrode current collector 27. The insulating layer 22 in the negative electrode 28 is provided between the negative electrode active material layer 21 and the positive electrode active material layer 26.
The lithium ion secondary battery of the present invention has excellent cycle characteristics because the coating amount of insulating fine particles on the surface of the negative electrode active material layer 21 is within a specific range.
A separator (not shown) may be provided between the insulating layer 22 and the positive electrode active material layer 26, that is, between the positive electrode 29 and the negative electrode 28. That is, a lithium ion secondary battery may include a negative electrode, a positive electrode arranged so as to face the negative electrode, and a separator arranged between the negative electrode and the positive electrode.
By providing the separator, a short circuit between the positive electrode and the negative electrode is more effectively prevented. Further, the lithium ion secondary battery may be provided with an electrolytic solution (not shown).

<Positive electrode>
The positive electrode in the lithium ion secondary battery of the present invention has a positive electrode active material layer, preferably has a positive electrode current collector and a positive electrode active material layer laminated on the positive electrode current collector. The positive electrode active material layer typically includes a positive electrode active material and a binder for the positive electrode.
Examples of the positive electrode active material include lithium metallic acid compounds. Examples of the lithium metal acid compound include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and the like. Further, it may be olivine type lithium iron phosphate (LiFePO ₄ ) or the like. Further, a plurality of metals other than lithium may be used, and an NCM (nickel cobalt manganese) oxide, an NCA (nickel cobalt aluminum) oxide, or the like, which is called a ternary system, may be used. Of these, NCA-based oxides are preferable from the viewpoint of improving output characteristics and the like.
The average particle size of the positive electrode active material is not particularly limited, but is preferably 0.5 to 50 μm, and more preferably 1 to 30 μm. The average particle size of the positive electrode active material means the particle size (D50) when the volume integration is 50% in the particle size distribution of the positive electrode active material obtained by the laser diffraction / scattering method.
The content of the positive electrode active material in the positive electrode active material layer is preferably 50 to 99% by mass, more preferably 60 to 95% by mass, based on the total amount of the positive electrode active material layer.

The positive electrode active material layer may contain a conductive auxiliary agent. As the conductive auxiliary agent, a material having higher conductivity than the positive electrode active material is used, and specific examples thereof include carbon materials such as Ketjen black, acetylene black, carbon nanotubes, and rod-shaped carbon.
When the positive electrode active material layer contains a conductive auxiliary agent, the content of the conductive auxiliary agent is preferably 0.5 to 30% by mass, preferably 1 to 25% by mass, based on the total amount of the positive electrode active material layer. More preferably, it is more preferably 1.5 to 10% by mass.

The binder for the positive electrode is not particularly limited, but the same binder as described for the binder for the insulating layer can be used.
The content of the binder in the positive electrode active material layer is preferably 0.5 to 30% by mass, more preferably 1.0 to 25% by mass, and 1.5 to 10% by mass based on the total amount of the positive electrode active material layer. Is more preferable.
The thickness of the positive electrode active material layer is not particularly limited, but is preferably 10 to 200 μm, and more preferably 50 to 150 μm.

The material used as the positive electrode current collector is the same as that of the compound used for the negative electrode current collector, but aluminum or copper is preferably used, and aluminum is more preferable. The positive electrode current collector is generally made of a metal foil, and its thickness is not particularly limited, but is preferably 1 to 50 μm.

(Separator)
The lithium ion secondary battery of the present invention may include a separator arranged between the negative electrode and the positive electrode. The separator more effectively prevents short circuits between the positive and negative electrodes.
Examples of the separator include a porous polymer film, a non-woven fabric, and glass fiber, and among these, a porous polymer film is preferable. Examples of the porous polymer film include an olefin-based porous film such as an ethylene-based porous film.

(Electrolytic solution)
The lithium ion secondary battery of the present invention may include an electrolytic solution. The electrolytic solution is filled in the negative electrode and the positive electrode, or in the battery cell in which the negative electrode, the positive electrode, and the separator are housed.
Examples of the electrolytic solution include those containing an organic solvent for an electrolytic solution and an electrolyte salt. Examples of the organic solvent for the electrolytic solution used in the electrolytic solution include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and vinylene carbonate (VC). Carbonate-based organic solvents such as γ-butyrolactone, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3- Examples thereof include dioxolane and methyl acetate. As the electrolytic solution organic solvent, one type may be used alone, or a plurality of types may be used in combination.

The organic solvent for the electrolytic solution preferably contains a carbonate-based organic solvent, and more preferably vinylene carbonate (VC). By using vinylene carbonate (VC), an oxide film can be formed on the surface of the negative electrode at the time of initial charging of the lithium ion secondary battery, and deterioration of the negative electrode active material can be suppressed. Vinylene carbonate (VC) is known as a substance that causes the formation of SEI, and there is usually concern that this may deteriorate the cycle characteristics. However, the lithium ion secondary battery of the present invention has a negative electrode. Since the surface of the active material is appropriately coated with insulating fine particles, the cycle characteristics are improved.
The content of vinylene carbonate (VC) is preferably 0.1 to 5% by mass, preferably 0.5 to 2% by mass, based on the total amount of the electrolytic solution.

As the electrolyte salt contained in the electrolytic _{solution, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 CO 2, LiPF 6 SO 3, LiN (SO 2 CF 3) 2, LiN (SO 2 CF 2 Examples thereof include salts containing lithium such as CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ and LiN (COCF ₂ CF ₃ ) ₂ , and lithium bisoxalate boronate (LiB (C ₂ O ₄ ) ₂ ). Further, examples thereof include a reaction product of an organic acid lithium salt and a boron trifluoride complex, and a complex such as a complex hydride such as LiBH ₄ . One type of electrolyte salt may be used alone, or a plurality of types may be used in combination.

Among these electrolyte salts, a reaction product of an organic acid lithium salt and a boron trifluoride complex, LiPF _{6 and the} like are preferable, and a reaction product of an organic acid lithium salt and a boron trifluoride complex is more preferable. By using a reaction product of an organic acid lithium salt and a boron trifluoride complex, the cycle characteristics can be easily improved. It is considered that this is because the formation of SEI on the surface of the negative electrode active material can be suppressed by using the reaction product of the lithium organic acid salt and the boron trifluoride complex.

<Reactant of lithium organic acid salt and boron trifluoride complex>
The reaction product of the lithium organic acid salt and the boron trifluoride complex is obtained by mixing the lithium organic acid salt and the boron trifluoride complex and reacting the two, preferably the following mixing step and purification step. It is obtained by going through.

The mixing step is a step of preparing each of the lithium organic acid salt and the boron trifluoride complex and mixing them by a known means such as a stirrer and a stirrer blade.
The compounding ratio of the organic acid lithium salt and the boron trifluoride complex (organic acid lithium salt: boron trifluoride complex) is preferably 1: 0.5 to 1:20 in terms of mass ratio, and more preferably 1. : 1 to 1:10, more preferably 1: 2 to 1: 5.

Since the boron trifluoride complex is liquid, it is not necessary to use a solvent in the mixing step, but a solvent may be used as appropriate.
Preferred solvents include a chain carbonate compound such as dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate (a compound having a carbonate bond in the chain structure), a nitrile compound such as acetonitrile, and a cyclic ether compound such as tetrahydrofuran (the compound). Chain ether compounds such as diethyl ether and 1,2-dimethoxyethane (compounds having an ether bond in the cyclic structure), carboxylic acid ester compounds such as ethyl acetate and isopropyl acetate Can be exemplified.
Among these, as the organic solvent, a chain carbonate ester compound, a nitrile compound, and a cyclic ether compound are preferable.

The mixing time in the mixing step may be longer than or equal to the time required for the completion of the reaction, and is not particularly limited, but is preferably 1 to 48 hours, more preferably 2 to 24 hours.
The temperature at the time of mixing is preferably 10 to 40 ° C, more preferably 20 to 30 ° C.
In the mixing step, the formation of the reaction product of the lithium organic acid salt and the boron trifluoride complex can be confirmed by a known means such as analysis by chromatography.

The purification step is a step performed for the purpose of removing components other than the reactants produced by the mixing step. Examples of the components other than the reactant include unreacted lithium organic acid salt, unreacted boron trifluoride complex, and a solvent used as necessary in the mixing step. The specific purification means is not particularly limited as long as it can remove components other than the reactants, but can be obtained by washing with an organic solvent capable of dissolving an organic acid lithium salt and a boron trifluoride complex. It is preferable to dry the washed product by a known means.
Examples of the organic solvent include diethyl ether, din-butyl ether, tetrahydrofuran and the like, and more preferably diethyl ether.
Drying may be carried out at room temperature or under heating (for example, 40 to 100 ° C.). Further, the drying may be carried out under normal pressure or under reduced pressure.

In the purification step, the unreacted lithium organic acid salt and the unreacted boron trifluoride complex are preferably completely removed, but some of them may remain. Therefore, the electrolytic solution may contain either or both of the lithium organic acid salt and the boron trifluoride complex. The total amount of the lithium organic acid salt and the boron trifluoride complex is preferably 5% by mass or less, more preferably 1% by mass or less, and further preferably 0% by mass based on the total amount of the electrolytic solution.

As the lithium organic acid salt in the present invention, a lithium carboxylic acid salt is preferable.
Preferred lithium carboxylic acid salts include lithium formate (HCOOLi), lithium acetate (CH ₃ COOLi), lithium propionate (CH ₃ CH ₂ COOLi), lithium butyrate (CH ₃ (CH ₂ ) ₂ COOLi), and the like. Lithium isobutyrate ((CH ₃ ) ₂ CHCOOLi), Lithium valerate (CH ₃ (CH ₂ ) ₃ COOLi), Lithium isovaleric acid ((CH ₃ ) ₂ CHCH ₂ COOLi), Lithium caproate (CH) ₃ (CH ₂ ) ₄ COOLi) and other monovalent carboxylic acid lithium salts, lithium oxalate salt ((COOLi) ₂ ), lithium malonic acid salt (LiOOCCH ₂ COOLi), lithium succinate ((CH ₂ COOLi) ₂ ) ), Lithium glutarate (LiOOC (CH ₂ ) ₃ COOLi), lithium adipate ((CH ₂ CH ₂ COOLi) ₂ ) and other divalent carboxylic acid lithium salts, lithium lactate (CH ₃ CH (OH)) Lithium salt of monovalent carboxylic acid having a hydroxyl group such as COOLi), divalent carboxylic acid having a hydroxyl group such as lithium tartrate ((CH (OH) COOLi) ₂ ), lithium malate (LiOOCCH ₂ CH (OH) COOLi) Lithium salt of acid, lithium maleic acid salt (LiOOCCH = CHCOOLi, cis form), lithium fumaric acid salt (LiOOCCH = CHCOOLi, trans form) and other unsaturated divalent carboxylic acid lithium salt, lithium citrate salt (LiOOCCH ₂ C) An example thereof is a lithium salt of a trivalent carboxylic acid having a hydroxyl group such as (COOLi) (OH) CH ₂ COOLi). Among these, lithium formate, lithium acetate, lithium oxalate, and lithium succinate are more preferable, and lithium oxalate is even more preferable.
That is, the reaction product of the organic acid lithium salt and the boron trifluoride complex contained in the electrolyte layer includes a reaction product of the lithium formate and the boron trifluoride complex, and a reaction product of the lithium acetate salt and the boron trifluoride complex. A reaction product, a reaction product of a lithium oxalate salt and a boron trifluoride complex, a reaction product of a lithium succinate salt and a boron trifluoride complex is preferable, and a reaction product of a lithium oxalate salt and a boron trifluoride complex is preferable. More preferred.

Preferred examples of the boron trifluoride complex include a boron trifluoride alkyl ether complex and a boron trifluoride alcohol complex.
Examples of the above-mentioned boron trifluoride alkyl ether complex include boron trifluoride dimethyl ether complex (BF ₃ · O (CH ₃ ) ₂ ), boron trifluoride diethyl ether complex (BF ₃ · O (C ₂ H ₅ ) ₂ ), and the like. Boron trifluoride di n-butyl ether complex (BF ₃ · O (C ₄ H ₉ ) ₂ ), Boron trifluoride ditert-butyl ether complex (BF ₃ · O ((CH ₃ ) ₃ C) ₂ ), Trifluoride Boron trifluoride tert-butyl methyl ether complex (BF ₃ · O ((CH ₃ ) ₃ C) (CH ₃ )), boron trifluoride tetrahydrofuran complex (BF ₃ · OC ₄ H ₈ ) and the like can be mentioned.
Examples of the boron trifluoride alcohol complex include a boron trifluoride methanol complex (BF ₃ , HOCH ₃ ), a boron trifluoride propanol complex (BF ₃ , HOC ₃ H ₇ ), and a boron trifluoride phenol complex (BF ₃ , BF ₃ ,). HOC ₆ H ₅ ) and the like can be mentioned.
Among these boron trifluoride complexes, the boron trifluoride alkyl ether complex is preferable, and the boron trifluoride diethyl ether complex is more preferable, considering the reactivity in the mixing step and the like.

The content of the reaction product of the lithium organic acid salt and the boron trifluoride complex on the basis of the total amount of the electrolytic solution is preferably 0.1 to 3% by mass, more preferably 1.5 to 2.5% by mass. ..

When a reaction product of an organic acid lithium salt and a boron trifluoride complex is used, it is preferable to use it in combination with LiPF _6, and in this case, the content of LiPF ₆ is preferably 1 to 15 based on the total amount of the electrolytic solution. It is by mass, more preferably 10 to 13% by mass.

The lithium ion secondary battery may have a multilayer structure in which a plurality of negative electrodes and a plurality of positive electrodes are laminated. In this case, the negative electrode and the positive electrode may be provided alternately along the stacking direction. When a separator is used, the separator may be arranged between each negative electrode and each positive electrode.
In the lithium ion secondary battery, the negative electrode and the positive electrode, or the negative electrode, the positive electrode, and the separator described above are housed in the battery cell. The battery cell may be a square type, a cylindrical type, a laminated type, or the like.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The obtained lithium ion secondary battery was evaluated by the following evaluation method.
(Coating amount of insulating fine particles)
The insulating layer formed by applying the composition for an insulating layer to the surface of the negative electrode active material layer was observed with a laser microscope (manufactured by Olympus Corporation, trade name: OLS-4500). The observation was performed in a field of view of 800 μm × 800 μm, and the surface of the negative electrode active material was binarized so that the portion covered with the insulating fine particles and the exposed portion of the negative electrode active material layer could be visually separated. Alumina, which is an insulating fine particle, was observed to be white, and the exposed portion of the negative electrode active material was observed to be black. The ratio of the insulating fine particles to the entire field of view was defined as the coating amount of the insulating fine particles.

(Cycle characteristics)
The charge / discharge cycle was repeated with the lithium ion secondary batteries produced in each Example and Comparative Example at 60 ° C., a charge rate of 2C, and a discharge rate of 3C.
The discharge capacity after 1000 cycles was compared with the discharge capacity after 10 cycles to determine the capacity retention rate. The cycle characteristics were evaluated according to the following evaluation criteria. The higher the capacity retention rate, the better the cycle characteristics.
A: Capacity retention rate 70% or more B: Capacity retention rate 65% or more and less than 70% C: Capacity retention rate 60% or more and less than 65% D: Capacity retention rate less than 60%

(Evaluation of output characteristics)
The lithium ion secondary batteries produced in each Example and Comparative Example were evaluated by determining the discharge capacity as follows.
Constant current charging of 1C was performed, and then constant voltage charging was performed to complete charging when the current was reduced to 0.05A as soon as 4.2V was reached. After that, a constant current discharge of 10C was performed, and when the discharge was made to 2.5V, the discharge was completed, and the discharge capacity was calculated. The output characteristics were evaluated according to the following criteria.
A: 1C constant current discharge capacity is 30% or more 10C discharge capacity B: 1C constant current discharge capacity is 20% or more and less than 30% C: 1C constant current discharge capacity Compared to the discharge capacity, the discharge capacity of 10C is 10% or more and less than 20%. Compared to the constant current discharge capacity of D: 1C, the discharge capacity of 10C is less than 10%.

(Safety evaluation)
The lithium-ion secondary batteries produced in each example and comparative example are charged with a constant current of 0.1 A, and then the current is reduced as soon as 4.2 V is reached, and charging is completed when the current reaches 0.02 A. Voltage charging was performed. The battery was then heated and stored at 110 ° C. The maximum temperature of the battery when it was held for 1 hour after reaching 110 ° C. was measured, and the safety was evaluated according to the following evaluation criteria.
A: Maximum temperature less than 115 ° C B: Maximum temperature 115 ° C or more and less than 140 ° C C: Maximum temperature 140 ° C or more and less than 200 ° C D: Maximum temperature 200 ° C or more

(Reactant of lithium organic acid salt and boron trifluoride complex)
After mixing the lithium oxalate salt and the boron trifluoride diethyl ether complex at a ratio of 1: 3.5 (mass ratio), the mixture was stirred at 50 ° C. for 12 hours. As a result of analysis by ion chromatography, it was confirmed that a reactant obtained by reacting the raw material lithium oxalate salt with boron trifluoride diethyl ether complex was produced. Then, it was washed with diethyl ether and vacuum dried to obtain a reaction product of a lithium oxalate salt and a boron trifluoride diethyl ether complex.

(Example 1)
(Preparation of composition for insulating layer)
Alumina particles (manufactured by Nippon Light Metal Co., Ltd., product name: AHP200, average particle diameter 0.4 μm) were used as the insulating fine particles. A crosslinked polybutyl acrylate was used as a binder, and a binder solution prepared by dissolving this in N-methylpyrrolidone (NMP) was prepared. While moderately shearing 100 parts by volume of the alumina particles so that the binder became 25 parts by volume, the alumina particles and the binder solution were mixed to obtain a slurry-like composition for an insulating layer. The solid content concentration of the insulating layer composition was 30% by mass.

(Manufacture of negative electrode for lithium ion secondary battery)
100 parts by mass of graphite (average particle diameter 10 μm) as a negative electrode active material, 1.5 parts by mass of styrene-butadiene rubber as a binder for a negative electrode, 1.5 parts by mass of a sodium salt of carboxymethyl cellulose (CMC), and water as a solvent. The mixture was mixed to obtain a composition for a negative electrode active material layer adjusted to have a solid content of 50% by mass. This composition for the negative electrode active material layer was applied to both sides of a copper foil having a thickness of 12 μm as a negative electrode current collector and vacuum dried at 100 ° C. Then, the negative electrode current collector coated with the composition for the negative electrode active material layer on both sides is pressure-pressed at a linear pressure of 500 kN / m to obtain a laminated body in which the negative electrode active material layers are formed on both sides of the negative electrode current collector. (The laminate is called a negative electrode for a lithium ion battery without an insulating layer). The thickness of each of the negative electrode active material layers was 50 μm, and the density of the negative electrode active material layer was 1.55 g / cc. The size of the negative electrode was 45 mm × 55 mm, and the area on which the negative electrode active material layer was applied was 45 mm × 50 mm.
The insulating layer composition prepared as described above was applied to the surfaces of the negative electrode active material layers on both sides of the laminate to form a coating film. The coating was carried out by gravure coating while applying a shearing force. The viscosity of the insulating layer composition at the time of coating was 1300 mPa · s. The coating film was dried at 90 ° C. for 10 minutes using an oven to form an insulating layer to obtain a negative electrode for a lithium ion secondary battery. The thickness of the insulating layer after drying was 1.8 μm per side.

(Preparation of positive electrode)
100 parts by mass of Li (Ni-Co-Al) O ₂ (NCA-based oxide) having an average particle diameter of 10 μm as the positive electrode active material, 4 parts by mass of acetylene black as the conductive auxiliary agent, and polyvinylidene fluoride as the positive electrode binder. 4 parts by mass of (PVDF) and N-methylpyrrolidone (NMP) as a solvent were mixed. As a result, a composition for a positive electrode active material layer adjusted to a solid content concentration of 60% by mass was obtained. This composition for the positive electrode active material layer was applied to both sides of an aluminum foil having a thickness of 15 μm as a positive electrode current collector, pre-dried, and then vacuum dried at 120 ° C. Then, a positive electrode current collector coated with the composition for a positive electrode active material layer on both sides is pressure-pressed at 400 kN / m and further punched into an electrode size of 40 mm × 50 mm square to have a positive electrode active material layer on both sides. And said. Of the dimensions, the area to which the positive electrode active material was applied was 40 mm × 45 mm. The thickness of the positive electrode active material was 50 μm, respectively.

(Preparation of electrolyte)
Ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), and vinylene carbonate (VC) as organic solvents, lithium hexafluorophosphate (LiPF ₆ ) as electrolyte salts, lithium oxalate and boron trifluoride. The reaction product with the diethyl ether complex was mixed in the mass shown in Table 1 to prepare an electrolytic solution.

(Manufacturing of lithium-ion secondary batteries)
The 25 negative electrodes for the lithium ion secondary battery obtained above and 24 positive electrodes were alternately laminated to obtain a temporary laminate. Using a flat plate type hot press machine, the temporary laminate was pressed at 80 ° C. and 0.6 MPa for 1 minute to obtain a laminate.
The exposed ends of the positive electrode current collectors of each positive electrode were joined together by ultrasonic fusion, and the terminal tabs protruding to the outside were joined. Similarly, the exposed ends of the negative electrode current collectors of each negative electrode were joined together by ultrasonic fusion, and the terminal tabs protruding to the outside were joined.
Next, the laminate was sandwiched between aluminum laminate films, the terminal tabs were projected to the outside, and the three sides were sealed by laminating. A laminated cell was manufactured by injecting the electrolytic solution obtained above from one side left unsealed and vacuum-sealing. The results are shown in Table 1.

(Examples 2 to 6, Comparative Examples 1 to 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the compositions of the insulating layer composition, the insulating layer, and the electrolytic solution were changed as shown in Table 1. The results are shown in Table 1.

(Comparative Example 3)
In the production of the lithium ion secondary battery of Example 1, the lithium ion secondary battery is used in the same manner as in Example 1 except that the negative electrode for the lithium ion battery without an insulating layer is used as the negative electrode for the lithium ion secondary battery. Made. The results are shown in Table 1.

As shown in Examples 1 to 6 above, the lithium ion secondary battery of the present invention in which the coating amount of the insulating fine particles on the surface of the negative electrode active material layer was within a specific range was excellent in cycle characteristics. On the other hand, as shown in Comparative Examples 1 and 2, the lithium ion secondary battery having a coating amount outside the specific range was inferior in cycle characteristics. Further, the lithium ion secondary battery of Comparative Example 3 not provided with the insulating layer gave poor results in all items of cycle characteristics, output characteristics, and safety.

10 Negative electrode for lithium ion secondary battery 11 Negative electrode active material layer 12 Insulation layer 13 Negative electrode current collector 14 Insulating fine particles 15 Exposed part 20 Lithium ion secondary battery 21 Negative electrode active material layer 22 Insulation layer 23 Negative electrode current collector 26 Positive electrode activity Material layer 27 Positive electrode Current collector 28 Negative electrode (negative electrode for lithium ion secondary battery)
29 Positive electrode

Claims (8)

A negative electrode for a lithium ion secondary battery including a negative electrode active material layer containing a negative electrode active material and a negative electrode binder, and an insulating layer provided on the surface of the negative electrode active material layer and containing insulating fine particles and an insulating layer binder. And
A negative electrode for a lithium ion secondary battery, wherein the coating amount of the insulating fine particles on the surface of the negative electrode active material layer is 20 to 70%. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the insulating layer has a thickness of 1 to 3 μm. The negative electrode for a lithium ion secondary battery according to claim 1 or 2, wherein the content of the binder for the insulating layer is 10 to 50% by volume based on the total amount of the insulating layer. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the negative electrode active material has an average particle size of 10 to 25 μm. The method for manufacturing a negative electrode for a lithium ion secondary battery according to any one of claims 1 to 4.
An insulating layer composition containing insulating fine particles, an insulating layer binder, and an insulating layer organic solvent is applied onto the surface of the negative electrode active material layer containing the negative electrode active material and the negative electrode active material to form an insulating layer. With a process to form
A method for producing a negative electrode for a lithium ion secondary battery, wherein the insulating layer composition has a viscosity at 25 ° C. of 700 to 3000 mPa · s. A lithium ion secondary battery comprising the negative electrode according to any one of claims 1 to 4, a positive electrode arranged so as to oppose the negative electrode, and a separator arranged between the negative electrode and the positive electrode. The lithium ion secondary battery according to claim 6, further comprising an electrolytic solution containing an organic solvent for an electrolytic solution and an electrolyte salt. The lithium ion secondary battery according to claim 7, wherein the organic solvent for the electrolytic solution contains vinylene carbonate.

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