JP7053130B2 - Negative electrode for lithium-ion secondary battery, and lithium-ion secondary battery - Google Patents

Description

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

In recent years, a lithium ion secondary battery having a high voltage and a high capacity has been widely adopted as a power source for mobile devices and the like. In addition, lithium-ion secondary batteries are required to have higher capacities in order to further extend the operating time of such mobile devices.

For example, a technique for increasing the discharge capacity of a lithium ion secondary battery by using a silicon (Si) -based active material or a tin (Sn) -based active material capable of storing and releasing more lithium ions as a negative electrode active material. Proposed.

However, since such a negative electrode active material has a large volume change due to occlusion and release of lithium ions, the conductive network (newwork) between the negative electrode active materials is cut by repeating charging and discharging. Therefore, the lithium ion secondary battery using the Si-based active material or the Sn-based active material did not have good cycle characteristics.

Therefore, Patent Document 1 studies a technique for improving the cycle characteristics of a lithium ion secondary battery by coating a Si-based active material or a Sn-based active material with a conductive protective film.

Japanese Unexamined Patent Publication No. 2008-115744

However, the technique disclosed in Patent Document 1 has not sufficiently improved the cycle characteristics. Therefore, the present invention has been made in view of the above problems, and an object of the present invention is a novel and improved one capable of improving the cycle characteristics of a lithium ion secondary battery to the same or higher level. It is an object of the present invention to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the negative electrode.

In order to solve the above problems, according to a certain viewpoint of the present invention, a silicon-based active material and carbon nanotubes having an average diameter of 10 nm or more and 120 nm or less and an average length of 0.5 μm or more and 20 μm or less are used. Provided by the negative electrode for a lithium ion secondary battery, the negative electrode for a lithium ion secondary battery, comprising a negative electrode active material layer containing the carbon nanotubes, wherein the content of the carbon nanotubes is 0.1% by mass or more and 2% by mass or less with respect to the total mass of the negative electrode active material layer. Will be done.

From this point of view, it is possible to improve the cycle characteristics of the lithium ion secondary battery using the negative electrode.

The average particle size of the silicon-based active material may be 0.5 μm or more and 5 μm or less.

From this point of view, it is possible to further improve the cycle characteristics of the lithium ion secondary battery using the negative electrode.

The content of the carbon nanotubes may be 0.2% by mass or more and 1% by mass or less with respect to the total mass of the negative electrode active material layer.

From this point of view, it is possible to further improve the cycle characteristics of the lithium ion secondary battery using the negative electrode.

The negative electrode active material layer may further contain a graphite active material.

From this point of view, it is possible to further improve the cycle characteristics of the lithium ion secondary battery using the negative electrode.

The silicon-based active material may be contained in the negative electrode active material layer at a content such that 10% or more of the reversible capacity from 0 V to 1.2 V in the negative electrode is derived from the silicon-based active material.

From this point of view, it is possible to further improve the battery characteristics of the lithium ion secondary battery using the negative electrode.

Further, in order to solve the above-mentioned problems, according to another aspect of the present invention, a lithium ion secondary battery including the above-mentioned negative electrode is provided.

From this point of view, it is possible to improve the cycle characteristics of the lithium ion secondary battery.

As described above, according to the present invention, it is possible to improve the cycle characteristics of the lithium ion secondary battery to the same or higher level.

It is a schematic diagram which shows the structure of the lithium ion secondary battery which concerns on one Embodiment of this invention. It is a scatter diagram which plotted the result of Table 1 with the horizontal axis as a CNT average diameter and the vertical axis as a capacity retention rate. It is a scatter diagram which plotted the result of Table 1 with the horizontal axis as the CNT average diameter and the vertical axis as the initial efficiency.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the present specification and the drawings, components having substantially the same functional configuration are designated by the same reference numerals, so that duplicate description will be omitted.

<1. Lithium-ion secondary battery configuration>
First, with reference to FIG. 1, a specific configuration of the lithium ion secondary battery 10 according to the embodiment of the present invention will be described. FIG. 1 is a schematic diagram showing the configuration of the lithium ion secondary battery 10 according to the present embodiment.

As shown in FIG. 1, the lithium ion secondary battery 10 includes a positive electrode 20, a negative electrode 30, and a separator layer 40. The form of the lithium ion secondary battery 10 is not particularly limited, and may be, for example, any of a cylindrical shape, a square shape, a laminate type, a button type, and the like.

The positive electrode 20 includes a current collector 21 and a positive electrode active material layer 22. The current collector 21 may be any conductive material, and may be, for example, aluminum (Al), stainless steel (stainless steel), nickel-plated steel (nickel-plated steel), or the like.

The positive electrode active material layer 22 contains at least a positive electrode active material and a conductive agent, and may further contain a binder. The content of the positive electrode active material, the conductive agent, and the binder is not particularly limited, and may be any content as long as it is applicable to the conventional lithium ion secondary battery.

The positive electrode active material is, for example, a transition metal oxide containing lithium or a solid solution oxide, and is not particularly limited as long as it is a substance capable of electrochemically occluding and releasing lithium ions. _Examples of the transition metal oxide containing lithium include Li / Co composite oxides such as LiCoO ₂ , Li / Ni / Co / Mn composite oxides such as LiNi _x Coy Mn _z O ₂ , and Li / Co / Mn composite oxides such as LiNiO ₂ . Examples thereof include Ni-based composite oxides and Li-Mn-based composite oxides such as LiMn _{2 O4} . _Examples of the solid solution oxide include Li _a Mn _x Coy _Niz O ₂ (1.150 ≦ a ≦ 1.430, 0.45 ≦ x ≦ 0.6, 0.10 ≦ y ≦ 0.15, 0.20. ≦ _z ≦ 0.28), LiMn _x Coy Ni _z O ₂ (0.3 ≦ x ≦ 0.85, 0.10 ≦ y ≦ 0.3, 0.10 ≦ z ≦ 0.3), LiMn _{1 .5} Ni _0.5 O ₄ and the like can be exemplified. The content (content ratio) of the positive electrode active material is not particularly limited, and may be any content as long as it is applicable to the positive electrode active material layer of the lithium ion secondary battery. In addition, these compounds may be used alone or in combination of two or more.

The conductive agent is, for example, carbon black such as ketjen black or acetylene black, natural graphite, artificial graphite, carbon nanotubes, graphene, and carbon nanofibers. It is a fibrous carbon such as (carbon nanofivers), or a composite of these fibrous carbons and carbon black (carbon black) or the like. However, the conductive agent can be used without particular limitation as long as it is for increasing the conductivity of the positive electrode. The content of the conductive agent is not particularly limited as long as it is applicable to the positive electrode active material layer of the lithium ion secondary battery.

The binder is, for example, polyvinylidene fluoride (polyvinylidene fluoride), ethylene propylene diene ternary copolymer (ethylene-polyethylene-diene terpolymer), styrene butadiene rubber (styrene-butadiene rubber), acrylic nitrile rubber. Fluoroelastorer, polyvinylacetate, polymethylmethacrylate, polyethylene, nitrocellulose and the like. However, the binder is not particularly limited as long as it can bind the positive electrode active material and the conductive agent on the current collector 21 and has oxidation resistance and electrolyte stability to withstand the high potential of the positive electrode. .. Further, the content of the binder is not particularly limited, and may be any content as long as it can be applied to the positive electrode active material layer of the lithium ion secondary battery.

In the positive electrode active material layer 22, for example, the positive electrode active material, the conductive agent, and the binder are dispersed in an appropriate organic solvent (for example, N-methyl-2-pyrrolidone (N-methyl-2-pyrrolidone)) to disperse the positive electrode slurry. (Slurry) can be formed, and the positive electrode slurry can be formed by applying the positive electrode slurry on the current collector 21, drying, and rolling. The density of the positive electrode active material layer 22 after rolling is not particularly limited, and may be any density as long as it can be applied to the positive electrode active material layer of the lithium ion secondary battery.

The negative electrode 30 includes a current collector 31 and a negative electrode active material layer 32. The current collector 31 may be any conductor as long as it is a conductor, and may be, for example, copper, a copper alloy, aluminum, stainless steel, nickel-plated steel, or the like.

In the present embodiment, the negative electrode active material layer 32 further contains a silicon (Si) -based active material, and further contains carbon nanotubes having an average diameter of 10 nm or more and 120 nm or less and an average length of 0.5 μm or more and 20 μm or less. Here, the content of the carbon nanotubes in the negative electrode active material layer 32 is 0.1% by mass or more and 2% by mass or less with respect to the total mass of the negative electrode active material layer. The negative electrode active material layer 32 may further contain a binder.

By using such a negative electrode active material layer 32, the lithium ion secondary battery 10 according to the present embodiment can improve the cycle characteristics.

Specifically, since the volume of the Si-based active material changes significantly due to the insertion and desorption of lithium ions, the conductive network between the Si-based active materials is cut during contraction (during the desorption of lithium ions), and electrical I was sometimes isolated. As a result, in the negative electrode active material layer 32, the electrically isolated Si-based active material increases each time charging and discharging are repeated, so that the discharge capacity is lowered and the cycle characteristics are lowered.

Since the negative electrode 30 according to the present embodiment contains carbon nanotubes having a specific shape in the negative electrode active material layer 32, even when the Si-based active material shrinks, the Si-based active material is made of highly conductive carbon nanotubes. It is possible to bridge between them and maintain a conductive network. As a result, in the lithium ion secondary battery 10 according to the present embodiment, even when charging and discharging are repeated, the generation of electrically isolated Si-based active material is suppressed, so that a decrease in discharge capacity is suppressed and a cycle is performed. The characteristics can be improved.

Here, the average diameter of the carbon nanotubes contained in the negative electrode active material layer 32 is 10 nm or more and 120 nm or less. As will be demonstrated in the examples described later, when the average diameter of the carbon nanotubes is within these ranges, the carbon nanotubes maintain the conductivity between the Si-based active materials, and the lithium ion secondary battery 10 is used. The cycle characteristics can be improved.

Specifically, when the average diameter of the carbon nanotubes is less than 10 nm, the number of carbon nanotubes contained in the negative electrode active material layer 32 increases, so that the side reaction with lithium ions increases and the initial efficiency decreases. Therefore, it is not preferable. The initial efficiency is a value obtained by dividing the discharge capacity at the time of the first charge / discharge by the charge capacity at the time of the first charge / discharge, and a higher value is more preferable. Further, by increasing the number of carbon nanotubes contained in the negative electrode active material layer 32, the total surface area is increased, the carbon nanotubes take in more binder, and the binding property of the negative electrode active material layer 32 is lowered. do. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 are deteriorated.

On the other hand, when the average diameter of the carbon nanotubes exceeds 120 nm, the rigidity of the carbon nanotubes increases and the carbon nanotubes cannot be flexibly deformed when the Si-based active material shrinks, so that the conductivity between the Si-based active materials becomes conductive. Can no longer be maintained. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 are deteriorated.

The average length of the carbon nanotubes contained in the negative electrode active material layer 32 is 0.5 μm or more and 10 μm or less. As will be demonstrated in the examples described later, when the average length of the carbon nanotubes is within these ranges, the carbon nanotubes maintain the conductivity between the Si-based active materials, and the lithium ion secondary battery 10 Cycle characteristics can be improved.

Specifically, when the average length of the carbon nanotubes is less than 0.5 μm, the length of the carbon nanotubes is short, so that when the Si-based active material shrinks, it becomes impossible to bridge between the Si-based active materials. It becomes impossible to maintain conductivity. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 are deteriorated. Further, the longer the average length of the carbon nanotubes is, the more preferable it is, and the upper limit value is not particularly limited, but for example, the upper limit value is 10 μm.

The average diameter of the carbon nanotubes is an arithmetic average value of the fiber diameter (outer diameter) of the carbon nanotubes, and the average length of the carbon nanotubes is an arithmetic average value of the fiber lengths of the carbon nanotubes. These can be obtained, for example, from images observed with a scanning electron microscope (SEM). That is, the average diameter and average length of carbon nanotubes can be obtained by extracting some samples from the SEM observation image of carbon nanotubes, measuring the diameter and length of these samples, and arithmetically averaging them.

Further, the content of the carbon nanotubes in the negative electrode active material layer 32 in the present embodiment is 0.1% by mass or more and 2% by mass or less with respect to the total mass of the negative electrode active material layer. As will be demonstrated in the examples described later, when the content of the carbon nanotubes is within these ranges, the carbon nanotubes maintain the conductivity between the Si-based active materials, and the lithium ion secondary battery 10 is used. The cycle characteristics can be improved.

Specifically, when the content of carbon nanotubes is less than 0.1% by mass, the amount of carbon nanotubes is too small, so that the conductivity between the Si-based active materials cannot be maintained when the Si-based active material shrinks. .. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 are deteriorated. On the other hand, when the content of the carbon nanotubes exceeds 2% by mass, the amount of the binder taken up by the carbon nanotubes increases, so that the binding property of the negative electrode active material layer 32 decreases. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 are deteriorated.

The content of the carbon nanotubes in the negative electrode active material layer 32 in the present embodiment is more preferably 0.2% by mass or more and 1% by mass or less with respect to the total mass of the negative electrode active material layer. As will be demonstrated in the examples described later, when the content of the carbon nanotubes is within these ranges, the lithium ion secondary battery 10 according to the present embodiment further improves the cycle characteristics.

Further, the Si-based active material contained in the negative electrode active material layer 32 is specifically Si, a Si alloy, or a Si oxide.

For example, a Si alloy consists of a Si phase and a phase of an intermetallic compound of Si and one or more other metal elements. The Si phase is a phase in which lithium ions can be occluded and released by reversibly inserting and removing lithium ions. Further, the phase of the intermetallic compound of Si and other elements (Si-containing intermetallic compound phase) retains the Si phase against the volume change of the Si phase due to charging and discharging by adhering to the Si phase which is the active material. It is a phase to do.

The element forming the metalloid compound with Si may be, for example, one or more selected from alkaline earth metal elements and transition metal elements capable of forming a stable metalloid compound with Si. The element forming the intermetallic compound with Si is preferably one or more selected from Mg, Ti, V, Cr, Mn, Co, Cu, Fe and Ni.

The Si-based active material such as the above-mentioned Si alloy may be obtained by jet milling or ball milling an amorphous Si compound formed by, for example, an atomizing method, a roll quenching method, or a rotary electrode method. ) And the like, it can be obtained. Further, the Si-based active material can also be obtained by pulverizing Si simple substance powder and other compound powder, mixing them, and subjecting them to mechanical alloying treatment.

Here, the average particle size of the Si-based active material is preferably 0.5 μm or more and 5 μm or less. As will be demonstrated in the examples described later, when the average particle size of the Si-based active material is within these ranges, the cycle characteristics of the lithium ion secondary battery 10 can be further improved.

Specifically, when the average particle size of the Si-based active material is less than 0.5 μm, the total surface area of the Si-based active material increases by increasing the number of Si-based active materials, and more binder is taken in. Therefore, the binding property of the negative electrode active material layer 32 is lowered. Further, as the number of Si-based active materials increases, carbon nanotubes cannot sufficiently form a conductive network between Si-based active materials. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 are deteriorated.

Further, when the average particle size of the Si-based active material exceeds 5 μm, the volume change of the Si-based active material when lithium ions are inserted and desorbed becomes large, so that carbon nanotubes are generated when the Si-based active material shrinks. The conductive network between Si-based active materials cannot be maintained, and the conductivity cannot be maintained. This is not preferable because the cycle characteristics of the lithium ion secondary battery 10 are deteriorated.

The average particle size of the Si-based active material represents an arithmetic average value of the diameter when the Si-based active material is approximated to a sphere. The average particle size of the Si-based active material can be measured by, for example, a particle size distribution measuring device using a laser diffraction / scattering method.

Here, it is preferable that the negative electrode 30 according to the present embodiment further contains a graphite active material in the negative electrode active material layer 32. The graphite active material is a substance that contains carbon atoms and is capable of electrochemically storing and releasing lithium ions, and is, for example, coated with artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, or artificial graphite. Examples thereof include natural graphite and the like. The lithium ion secondary battery 10 according to the present embodiment can improve the battery characteristics by further containing the graphite active material in the negative electrode active material layer 32.

However, when the negative electrode active material layer 32 contains a graphite active material, the negative electrode active material layer 32 has a content such that 10% or more of the reversible capacity from 0 V to 1.2 V in the negative electrode 30 is derived from the Si-based active material. It is preferable to contain a Si-based active material. As will be demonstrated in the examples described later, when the content of the Si-based active material satisfies these conditions, the lithium ion secondary battery 10 according to the present embodiment can further improve the discharge capacity.

The ratio of the reversible capacity derived from the Si-based active material is the capacity A of the negative electrode 30 using only the graphite active material as the active material and the capacity B of the negative electrode 30 using the Si-based active material and the graphite active material as the active material. It is a ratio obtained by dividing the difference (increase) from and by the capacity B. That is, in the above, it is the ratio (%) calculated by (BA) / B × 100 (for example, A = 360 mAh / g).

The ratio of the reversible capacity derived from the Si-based active material can be controlled by changing the contents of the Si-based active material and the graphite active material in the negative electrode active material layer 32. Specifically, the ratio of the reversible capacity derived from the Si-based active material can be increased by increasing the content of the Si-based active material in the negative electrode active material layer 32.

Here, when the ratio of the Si-based active material-derived material in the reversible capacity from 0 V to 1.2 V in the negative electrode 30 is less than 10%, the discharge capacity is increased by containing the Si-based active material in the negative electrode active material layer 32. It is not preferable because the amount is small. Further, the higher the ratio of the Si-based active material-derived material in the reversible capacity from 0 V to 1.2 V in the negative electrode 30, the more preferable, and the upper limit is not particularly limited. However, when the content of the Si-based active material in the negative electrode active material layer 32 is limited by requirements other than the negative electrode active material such as a binder, the upper limit may be, for example, 50%.

As the binder, the same binder as that constituting the positive electrode active material layer 22 can be used. The contents of the negative electrode active material and the binder are not particularly limited, and the contents adopted in the conventional lithium ion secondary battery can be applied in the present invention.

In the negative electrode active material layer 32, for example, the above-mentioned Si-based active material, carbon nanotubes, and binder are dispersed in an appropriate solvent (for example, water) to form a negative electrode slurry, and the negative electrode slurry is placed on the current collector 31. It can be formed by coating, drying and rolling. The thickness of the negative electrode active material layer 32 after rolling is not particularly limited, and may be any thickness as long as it can be applied to the negative electrode active material layer of the lithium ion secondary battery. Further, the negative electrode active material layer 32 may be formed by selectively containing a graphite active material.

According to the negative electrode 30 according to the present embodiment as described above, the conductivity between the Si-based active materials is maintained even when charging and discharging are repeated, and the generation of electrically isolated Si-based active materials is suppressed. , The cycle characteristics of the lithium ion secondary battery 10 can be improved.

The separator layer 40 contains a separator and an electrolytic solution. The separator is not particularly limited as long as it is used as a separator for a lithium ion secondary battery, and any separator can be used. As the separator, it is preferable to use a porous membrane, a non-woven fabric, or the like exhibiting excellent high-rate discharge performance alone or in combination. Further, the separator may be coated with an inorganic substance such as Al ₂ O ₃ , Mg (OH) ₂ , SiO ₂ , etc., and the above-mentioned inorganic substance may be contained as a filler.

Examples of the material constituting such a separator include a polyolefin resin typified by polyethylene, polymer, polymer, polyethylene terephthalate, polybutylene terephate, and the like. Representative polyester-based resins, polyvinylidene difluoride, vinylidene fluoride-hexafluoropropylene copolymer (vinylidene difluoride-hexafluoropolylone copolymer), fluorovinylidene polymer, fluorovinylidene polymer, fluorovinylidene polymer, fluorovinylidene polymer. perfluorovinylester polymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene-trifluoroethylene copolymer (vinylidene-trifluoroethylene copolymer), vinylidene-trifluoroethylene copolymer (vinylidene-difluoroethylene copolymer), vinylidene-trifluoroethylene copolymer (vinylidene) Vinylidene difluoride-fluoroethylene polymer, vinylidene fluoride-hexafluoroacetone copolymer (vinylidene difluoride-hexafluoroacetonene polymer), vinylidene fluoride-ethylene copolymer (vinylidene-ethylene copolymer) Vinylidene difluoride-propylene polymer, vinylidene fluoride-trifluoropropylene copolymer (vinylidene difluoride-trifluoropolylone polymer), vinylidene fluoride-tetrafluoroethylene-hexafluoropolymer Threene-hexafluoropolylone polymer), vinylidene fluoride-ethylene-tetrafluoroethylene copolymer (vinylidene difluoride-ethylene-tetrafluoroethylene copolymer) and the like can be used. The porosity of the separator is not particularly limited, and the porosity of the separator of the conventional lithium ion secondary battery can be arbitrarily applied.

The electrolytic solution contains an electrolyte salt and a solvent.

The electrolyte salt is an electrolyte such as a lithium salt. Electrolyte salts include, for example, LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN and the like. Inorganic ion salt containing one of lithium (Li), sodium (Na) or potassium (K), LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , (CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (n-C ₄ H ₉ ) ₄ NClO ₄ , (n-C ₄ H ₉ ) ₄ NI , (C ₂ H ₅ ) ₄ N-malate, (C ₂ H ₅ ) ₄ N-benzoate, (C ₂ H ₅ ) ₄ N-phtalate, lithium stearylsulfate, lithium octyl sulphate ), Organic ion salts such as lithium dodecylbenzenesulphonate can be used. In addition, these electrolyte salts may be used individually or in mixture of 2 or more types. The concentration of the electrolyte salt is not particularly limited, but for example, a concentration of about 0.5 to 2.0 mol / L can be used.

The solvent is a non-aqueous solvent that dissolves the electrolyte salt. The solvent may be, for example, propylene carbonate (propylene carbononate), ethylene carbonate (ethylene carbide), butylene carbonate (butyrene carbononate), chloroethylene carbonate (chloroethylene carbononate), vinylene carbonate (vinyrene carbonate), or the like. Cyclic esters such as (γ-butyrolactone) and γ-valerolactone (γ-valerolactone), chain carbonates such as dimethyl carbonate (dimethyl carbonone), diethyl carbonate (diethyl carbonone), ethylmethyl carbonate (ethylmethyl carbonone) and the like. Chain esters such as methyl formate, methyl acetate, methyl butyrate, tetrahydrofuran or derivatives thereof, 1,3-dioxane, 1,4 -Esters such as dioxane (1,4-dioxane), 1,2-dimethoxyethane (1,2-dimethoxyethane), 1,4-dibutoxyethane (1,4-dibutoxyethane), or methyldiglycyme. , Nitriles such as acetonitrile, benzonitrile, dioxolane or derivatives thereof, ethylene sulfide, sulfolane, sulfone or derivatives thereof, etc., alone or Two or more of them can be mixed and used. When two or more kinds of solvents are mixed and used, the mixing ratio used in the conventional lithium ion secondary battery can be applied as the mixing ratio of each solvent.

In addition, various additives such as a negative electrode SEI (Solid Electrolyte Interface) forming agent and a surfactant may be added to the electrolytic solution.

Examples of such additives include succinic anhydride, lithium bis oxalate boronate, lithium tetrafluoroborate, dinitrile compounds, and propane. Uses sultone (propane sultone), butane sultone (butane sultone), propene sultone (propene sultone), 3-sulfurene, fluorinated allylether, fluorinated acrylate, etc. can. Further, as the content concentration of such an additive, the content concentration of the additive in a general lithium ion secondary battery can be used.

<2. Manufacturing method of lithium-ion secondary battery ＞
Subsequently, a method for manufacturing the lithium ion secondary battery 10 will be described. However, the method for manufacturing the lithium ion secondary battery 10 is not limited to the following methods, and any manufacturing method can be applied.

The positive electrode 20 is manufactured as follows. First, a mixture of a positive electrode active material, a conductive agent, and a binder in a desired ratio is dispersed in an organic solvent (for example, N-methyl-2-pyrrolidone) to form a positive electrode slurry. Next, the positive electrode slurry is formed (for example, coated) on the current collector 21 and dried to form the positive electrode active material layer 22.

The coating method is not particularly limited, but for example, a knife coater method, a gravure coater method, or the like may be used. Each of the following coating steps is also performed by the same method.

Further, the positive electrode active material layer 22 is compressed to a desired thickness by a compressor. As a result, the positive electrode 20 is manufactured. Here, the thickness of the positive electrode active material layer 22 is not particularly limited as long as it is the thickness of the positive electrode active material layer of the conventional lithium ion secondary battery.

The negative electrode 30 is also manufactured in the same manner as the positive electrode 20. First, a negative electrode slurry is formed by dispersing a mixture of Si-based active material, carbon nanotubes, and binder in a desired ratio in a solvent (for example, water). The negative electrode slurry may be selectively mixed with a graphite active material.

Next, the negative electrode slurry is formed (for example, coated) on the current collector 31 and dried to form the negative electrode active material layer 32. Further, the negative electrode active material layer 32 is compressed to a desired thickness by a compressor. As a result, the negative electrode 30 is manufactured. Here, the thickness of the negative electrode active material layer 32 is not particularly limited, and may be any thickness as long as it has the negative electrode active material layer of the conventional lithium ion secondary battery. When metallic lithium is used as the negative electrode active material layer 32, a metallic lithium foil may be laminated on the current collector 31.

Here, for the above-mentioned Si-based active material, for example, Si alone and other compound powder are mixed to obtain an amorphous Si compound by an atomizing method, a roll quenching method, a rotary electrode method, or the like, and then a jet mill or a jet mill or a method is used. It can be obtained by pulverizing the Si compound with a ball mill or the like.

Subsequently, the electrode structure is manufactured by sandwiching the separator between the positive electrode 20 and the negative electrode 30. Next, the manufactured electrode structure is processed into a desired shape (for example, cylindrical shape, square shape, laminated shape, button shape, etc.) and inserted into a container of the desired shape. Further, by injecting a desired electrolytic solution into the container, each pore in the separator is impregnated with the electrolytic solution. As a result, the lithium ion secondary battery 10 is manufactured.

<3. Example>
Hereinafter, the lithium ion secondary battery according to the present embodiment will be specifically described with reference to Examples and Comparative Examples. The examples shown below are merely examples, and the lithium ion secondary battery according to the present embodiment is not limited to the following examples.

[3.1. Examination of average diameter and average length of carbon nanotubes]
(Manufacturing of lithium-ion secondary batteries)
First, the lithium ion secondary batteries according to Examples 1 to 11 and Comparative Examples 1 to 9 were manufactured by the following methods.

Negative electrode by dispersing 20% by mass of Si-based active material (Si alloy: Cr / Si = 20atom% / 80atom%), 74% by mass of graphite, 5% by mass of polyfluorinated bilinidene, and 1% by mass of carbon nanotubes in water. A slurry was formed. Subsequently, the formed negative electrode slurry was applied onto a copper foil as a current collector and dried to form a negative electrode active material layer. Further, the negative electrode was manufactured by compressing the negative electrode active material layer with a compressor.

The average particle size of the Si-based active material was 2 μm, and the average diameter and average length of the carbon nanotubes (CNTs) in each Example and Comparative Example were the values shown in Table 1 below. Here, the average particle size of the Si-based active material is measured by a laser diffraction type particle size distribution measuring device (manufactured by Shimadzu Corporation), and the average diameter and average length of the carbon nanotubes are measured by a scanning electron microscope (manufactured by JEOL Ltd.). Measured from the image.

The positive electrode was manufactured by attaching metallic lithium to a current collector.

A porous polyethylene film (thickness 25 μm) was prepared as a separator, and an electrode structure was manufactured by sandwiching the separator between a positive electrode and a negative electrode, and stored in a coin half cell.

On the other hand, lithium hexafluorophosphate (LiPF ₆ ) was dissolved at a concentration of 1.0 mol / L in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of EC: DEC = 1: 1. Then, an electrolytic solution was produced.

Further, by injecting the electrolytic solution having the above composition into the coin half cell, each pore in the separator was impregnated with the electrolytic solution. As a result, the lithium ion secondary batteries according to Examples 1 to 11 and Comparative Examples 1 to 9 were manufactured.

(Evaluation results)
The cycle characteristics and initial efficiency of the lithium ion secondary batteries produced in Examples 1 to 11 and Comparative Examples 1 to 9 manufactured above were evaluated.

Specifically, first, each lithium ion secondary battery is charged with a constant current constant voltage (CCCV charge, specifically 1C, 10 mV charge) until the current value reaches 0.01 C. , It was discharged to 1.2V with a constant current (CC discharge, specifically, 1C discharge). This was repeated as one cycle, and 50 cycles were carried out.

Here, the cycle characteristics were evaluated by the capacity retention rate obtained by dividing the discharge capacity after 50 cycles of charge / discharge by the discharge capacity of the first cycle. The initial efficiency is a value obtained by dividing the discharge capacity of the first cycle by the charge capacity of the first cycle. The results are shown in Table 1 and FIGS. 2A and 2B below.

2A is a scatter plot in which the results of Table 1 are plotted with the horizontal axis as the CNT average diameter and the vertical axis as the capacity retention rate, and FIG. 2B shows the horizontal axis as the CNT average diameter and the vertical axis. It is a scatter diagram plotting the results of Table 1 with the above as the initial efficiency. In FIGS. 2A and 2B, Examples 1 to 11 are shown in diamond-shaped plots, and Comparative Examples 1 to 9 are shown in round-shaped plots.

Referring to Table 1 and FIGS. 2A and 2B, in Examples 1 to 11, since the average diameter and the average length of the carbon nanotubes are included in the range of the present invention, the cycle is compared with Comparative Examples 1 to 9. It can be seen that the characteristics (capacity retention rate) are improved.

On the other hand, in Comparative Example 1, since the average length of the carbon nanotubes is less than 0.5 μm, the cycle characteristics are deteriorated. Further, in Comparative Examples 2 to 9, since the average diameter of the carbon nanotubes exceeds 120 nm, the cycle characteristics are deteriorated.

Therefore, according to the present embodiment, it has been found that the cycle characteristics of the lithium ion secondary battery can be improved by setting the average diameter and the average length of the carbon nanotubes within the scope of the present invention.

[3.2. Examination of carbon nanotube content]
(Manufacturing of lithium-ion secondary batteries)
Next, the lithium ion secondary batteries according to Examples 12 to 17 and Comparative Examples 10 to 12 were manufactured by the following methods.

By dispersing 20% by mass of Si-based active material (Si alloy: Cr / Si = 20atom% / 80atom%), 72 to 77% by mass of graphite, 3% by mass of polyvinylidene fluoride, and 0 to 5% by mass of carbon nanotubes in water. A negative electrode slurry was formed. Subsequently, the formed negative electrode slurry was applied onto a copper foil as a current collector and dried to form a negative electrode active material layer. Further, the negative electrode was manufactured by compressing the negative electrode active material layer with a compressor.

The carbon nanotube content in each Example and Comparative Example was set to the value shown in Table 2 below. The average particle size of the Si-based active material was 2 μm, the average diameter of the carbon nanotube (CNT) was 20 nm, and the average length was 5 μm. Here, the average particle size of the Si-based active material, the average diameter and the average length of the carbon nanotubes were measured by the same measuring method as in Example 1.

Further, the positive electrode, the separator, and the electrolytic solution are the same as those used in Example 1, and the lithium ion secondary batteries (coin half cells) according to Examples 12 to 17 and Comparative Examples 10 to 12 are used in the same manner as in Example 1. ) Was manufactured.

(Evaluation results)
Evaluation of cycle characteristics and initial efficiency of the lithium ion secondary batteries according to Examples 12 to 17 and Comparative Examples 10 to 12 manufactured above by the same evaluation method as that carried out for Example 1. Was carried out. The results are shown in Table 2 below.

Referring to the results in Table 2, in Examples 12 to 17, since the content of carbon nanotubes is within the range of the present invention, the cycle characteristics are improved as compared with Comparative Examples 10 to 12. Understand.

In particular, in Examples 13 to 16, the cycle characteristics and the initial efficiency are further improved as compared with Examples 12 and 17, and the carbon nanotube content is more preferably 0.2% by mass or more and 1.0% by mass or less. You can see that. Further, in Comparative Examples 10 to 12, it can be seen that the content of the carbon nanotubes is out of the range of the present invention, so that the cycle characteristics and the initial efficiency are deteriorated.

[3.3. Examination of average particle size of Si-based active material]
(Manufacturing of lithium-ion secondary batteries)
Next, the lithium ion secondary batteries according to Examples 18 to 24 were manufactured by the following methods.

Negative electrode slurry is formed by dispersing 20% by mass of Si-based active material (Si alloy: Cr / Si = 20atom% / 80atom%), 76% by mass of graphite, 3% by mass of polyvinylidene fluoride, and 1% by mass of carbon nanotubes in water. bottom. Subsequently, the formed negative electrode slurry was applied onto a copper foil as a current collector and dried to form a negative electrode active material layer. Further, the negative electrode was manufactured by compressing the negative electrode active material layer with a compressor.

The average particle size of the Si-based active material in each example was the value shown in Table 3 below. The average diameter of carbon nanotubes (CNTs) was 20 nm, and the average length was 5 μm. Here, the average particle size of the Si-based active material, the average diameter and the average length of the carbon nanotubes were measured by the same measuring method as in Example 1.

Further, the positive electrode, the separator, and the electrolytic solution were the same as in Example 1, and the lithium ion secondary batteries (coin half cells) according to Examples 18 to 24 were manufactured by the same method as in Example 1.

(Evaluation results)
For the lithium ion secondary batteries manufactured above according to Examples 18 to 24, the cycle characteristics and the initial efficiency were evaluated by the same evaluation method as that carried out for Example 1. The results are shown in Table 3 below.

With reference to the results in Table 3, Examples 19 to 22 have further improved cycle characteristics and initial efficiency as compared with Examples 18, 23 and 24, and the average particle size of the Si-based active material is 0. It can be seen that 5 μm or more and 5 μm or less are more preferable.

[3.4. Examination of the ratio of reversible capacity derived from Si-based active material]
(Manufacturing of lithium-ion secondary batteries)
Next, the lithium ion secondary batteries according to Examples 25 to 30 and Comparative Examples 13 to 18 were manufactured by the following methods.

By dispersing 0 to 20% by mass of Si-based active material (Si alloy: Cr / Si = 20 atom% / 80 atm%), 76 to 96% by mass of graphite, 3% by mass of polyvinylidene fluoride, and 1% by mass of carbon nanotubes in water. A negative electrode slurry was formed. Subsequently, the formed negative electrode slurry was applied onto a copper foil as a current collector and dried to form a negative electrode active material layer. Further, the negative electrode was manufactured by compressing the negative electrode active material layer with a compressor. However, in Comparative Examples 13 to 18, the negative electrode slurry was formed by increasing graphite by 1% by mass without containing carbon nanotubes.

The contents of the Si-based active material in each Example and Comparative Example were set to the values shown in Table 4 below. The average particle size of the Si-based active material in each Example and Comparative Example was 2 μm, the average diameter of the carbon nanotube (CNT) was 20 nm, and the average length was 5 μm. Here, the average particle size of the Si-based active material, the average diameter and the average length of the carbon nanotubes were measured by the same measuring method as in Example 1.

Further, the positive electrode, the separator, and the electrolytic solution are the same as those used in Example 1, and the lithium ion secondary batteries (coin half cells) according to Examples 25 to 30 and Comparative Examples 13 to 18 are used in the same manner as in Example 1. ) Was manufactured.

(Evaluation results)
Evaluation of cycle characteristics and initial efficiency of the lithium ion secondary batteries according to Examples 25 to 30 and Comparative Examples 13 to 18 manufactured above by the same evaluation method as that carried out for Example 1. Was carried out. The results are shown in Table 4 below.

With reference to the results in Table 4, it can be seen that in Examples 25 to 30, the cycle characteristics are improved to the same extent or more as compared with Comparative Examples 13 to 18.

Specifically, Examples 25 to 29 in which the ratio of the reversible capacity derived from the Si-based active material is 10% or more are higher than those in Comparative Examples 13 to 17 in which carbon nanotubes are not contained in Examples 25 to 29. , It can be seen that the cycle characteristics are improved. Further, it can be seen that Example 30 in which the ratio of the reversible capacity derived from the Si-based active material is less than 10% has the same cycle characteristics as Comparative Example 18 in which carbon nanotubes are not contained in Example 30.

This is because when the ratio of the reversible capacity derived from the Si-based active material is less than 10%, the ratio of the Si-based active material in the negative electrode active material is excessively small, so that the effect of the present invention is not effectively exhibited. be. That is, the present invention can more effectively improve the cycle characteristics when the ratio of the reversible capacity derived from the Si-based active material is 10% or more.

The higher the ratio of the reversible capacity derived from the Si-based active material, the higher the discharge capacity of the lithium ion secondary battery, which is preferable. That is, the larger the content of the Si-based active material in the negative electrode active material layer, the more the discharge capacity of the lithium ion secondary battery can be increased, and the effect of improving the cycle characteristics according to the present invention is enhanced, which is preferable. .. Further, the upper limit of the content of the Si-based active material in the negative electrode active material layer is not particularly limited (that is, it may be 100% by mass).

As can be seen from the above evaluation results, according to the present embodiment, the cycle of the lithium ion secondary battery is obtained by further containing an appropriate amount of carbon nanotubes having a specific shape in the negative electrode active material layer containing the Si-based active material. It is possible to improve the characteristics. Further, the average diameter of carbon nanotubes having such a specific shape is 10 nm or more and 120 nm or less, and the average length is 0.5 μm or more and 20 μm or less. Further, the content of such carbon nanotubes is 0.1% by mass or more and 2% by mass or less with respect to the total mass of the negative electrode active material layer.

Although the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person having ordinary knowledge in the field of technology to which the present invention belongs can come up with various modifications or modifications within the scope of the technical ideas described in the claims. , These are also naturally understood to belong to the technical scope of the present invention.

10 Lithium-ion secondary battery 20 Positive electrode 21 Current collector 22 Positive electrode active material layer 30 Negative electrode 31 Current collector 32 Negative electrode active material layer 40 Separator layer

Claims (4)

Graphite active material and
Silicon-based active material and
Carbon nanotubes with an average diameter of 10 nm or more and 120 nm or less and an average length of 2 μm or more and 10 μm or less.
Equipped with a negative electrode active material layer consisting of a binder,
The silicon-based active material is Si or a Si alloy, and is
The content of the carbon nanotubes is 0.1% by mass or more and 2% by mass or less with respect to the total mass of the negative electrode active material layer .
A negative electrode for a lithium ion secondary battery having an average particle size of 0.5 μm or more and 5 μm or less of the silicon-based active material . The negative electrode for a lithium ion secondary battery according to claim 1 , wherein the content of the carbon nanotubes is 0.2% by mass or more and 1% by mass or less with respect to the total mass of the negative electrode active material layer. The negative electrode active material layer is contained in the negative electrode active material layer in such a content that 10% or more of the reversible capacity from 0V to 1.2V in the negative electrode is derived from the silicon-based active material. Alternatively , the negative electrode for a lithium ion secondary battery according to 2. A lithium ion secondary battery comprising the negative electrode according to any one of claims 1 to 3 .

Images