JP7048345B2 - Lithium ion secondary battery - Google Patents

Description

The present invention relates to a lithium ion secondary battery.

Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are widely used as a power source for portable devices such as notebook personal computers (note PCs) and mobile phones. In addition to these applications, lithium-ion secondary batteries are also attracting attention as a power source for xEVs such as electric vehicles and hybrid vehicles. Therefore, in recent years, the demand for lithium-ion secondary batteries is expected to grow even more actively.

Here, the lithium ion secondary battery for xEV is required to have a high capacity and a long life in order to secure the same performance as a conventional gasoline engine (gasoline engine) vehicle. Further, in the lithium ion secondary battery for xEV, there is a strong demand for a quick charge characteristic for completing charging within a time equivalent to the refueling time of a gasoline engine vehicle.

However, although the development of lithium-ion secondary batteries with higher capacity or longer life is progressing, the development of quick charge characteristics (that is, charge / discharge characteristics at high rates) has not progressed sufficiently. ..

On the other hand, in other secondary batteries, research on charging characteristics is being conducted from various viewpoints. For example, Non-Patent Document 1 below discloses that the charge / discharge characteristics of a negative electrode are improved by adding barium sulfate or the like to an electrolytic solution of a lead storage battery.

H. Vermesan, et al., "Effect of barium sulfate and strontium sulfate on charging and discharging of the negative electrode in a lead-acid battery" Journal of Power Sources, 28 May 2004, Volume 133, Issue 1, Pages 52-58

However, the technique disclosed in Non-Patent Document 1 described above has only been related to a lead storage battery. Therefore, it has not been sufficiently verified whether or not the technique disclosed in Non-Patent Document 1 can be applied to a lithium ion secondary battery having a different structure and mechanism as a secondary battery. In addition, no study has been made on a suitable configuration when the technique disclosed in Non-Patent Document 1 is applied to a lithium ion secondary battery.

Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is a new and improved lithium capable of improving charge / discharge characteristics at a high rate (rate). The purpose is to provide an ion secondary battery.

In order to solve the above problems, according to a certain aspect of the present invention, the SEI film is provided with a negative electrode having an SEI film containing a barium compound, and the barium compound is formed into the SEI film as fine particles having a particle size of 0.8 μm or less. Provided is a lithium ion secondary battery that is contained and the content of the barium compound in the SEI film is 0.3 atomic% or less in terms of barium.

According to this viewpoint, the charge / discharge characteristics of the lithium ion secondary battery at a high rate are improved.

The barium compound may be barium sulfate.

From this viewpoint, it is possible to provide a lithium ion secondary battery having improved charge / discharge characteristics at a high rate.

The SEI film may be provided on the surface of the negative electrode.

From this viewpoint, it is possible to provide a lithium ion secondary battery having improved charge / discharge characteristics at a high rate.

The SEI film may be composed of a decomposition product of a component contained in the electrolytic solution impregnated with the negative electrode.

From this viewpoint, it is possible to provide a lithium ion secondary battery having improved charge / discharge characteristics at a high rate.

As described above, according to the present invention, it is possible to provide a lithium ion secondary battery having improved charge / discharge characteristics at a high rate.

It is a side sectional view schematically showing the structure of a lithium ion secondary battery.

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, the configuration of the lithium ion secondary battery 10 according to the embodiment of the present invention will be described. FIG. 1 is a side sectional view schematically showing the configuration of a lithium ion secondary battery.

As shown in FIG. 1, the lithium ion secondary battery 10 includes a positive electrode 20, a negative electrode 30, a separator 40, and an electrolytic solution. The form of the lithium ion secondary battery 10 is not particularly limited. That is, the lithium ion secondary battery 10 may be in any form such as a cylindrical shape, a square shape, a laminated shape, or a button shape.

(Positive electrode 20)
The positive electrode 20 includes a current collector 21 and a positive electrode active material layer 22. The current collector 21 may be any conductor as long as it is a conductor. The current collector 21 is made of, for example, aluminum, stainless steel, nickel-plated steel, or the like.

The positive electrode active material layer 22 contains at least the positive electrode active material, and may further contain a conductive agent and a binder. The ratio of the contents of the positive electrode active material, the conductive agent and the binder is not particularly limited, and the ratio of the contents used in a general lithium ion secondary battery can be used.

The positive electrode active material is, for example, a solid solution oxide containing lithium. However, the positive electrode active material is not particularly limited as long as it is a substance that can electrochemically occlude and release lithium ions. The solid solution oxide is, for example, 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), or It may be LiMn _1.5 Ni _0.5 O ₄ or the like.

The conductive agent is, for example, carbon black such as ketjen black or acetylene black, natural graphite, artificial graphite, carbon nanotubes, graphene or carbon nanofibers. Fibrous carbon such as (carbon nanofibers), or a composite of these fibrous carbon and carbon black (carbon black) or the like can be used. However, the conductive agent is not particularly limited as long as it is for increasing the conductivity of the positive electrode.

The binder is, for example, polyvinylidene fluoride, ethylene propylene diene (ethylene-polyethylene-diene) ternary copolymer, styrene butadiene rubber (Stylene-butadiene rubber), acrylonitrile butadiene rubber (acrylloburi) rubber (acrylone). (Fluororubber), polyvinyl acetate, 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 onto the current collector 21.

(Negative electrode 30)
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. The current collector 31 may be, for example, copper, a copper alloy, aluminum, stainless steel, nickel-plated steel, or the like.

The negative electrode active material layer 32 contains at least the negative electrode active material, and may further contain a conductive agent and a binder. The ratio of the contents of the negative electrode active material, the conductive agent and the binder is not particularly limited, and the ratio of the contents used in a general lithium ion secondary battery can be used.

The negative electrode active material is, for example, a graphite active material, a silicon (Si) or tin (Sn) -based active material, a titanium oxide (TiO _x ) -based active material, or the like. However, the negative electrode active material is not particularly limited as long as it is a substance that can electrochemically occlude and release lithium ions. The graphite-based active material may be artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, or the like. The silicon or tin-based active material may be fine particles of silicon or tin, fine particles of an oxide of silicon or tin, an alloy of silicon or tin, or the like. The titanium oxide-based active material may be Li ₄ Ti ₅ O ₁₂ or the like. Further, the negative electrode active material may be, for example, metallic lithium (Li) or the like in addition to these.

As the conductive agent, the same conductive agent as that used in the positive electrode active material layer 22 can be used.

As the binder, for example, styrene-butadiene rubber (Stylene-Butagene Rubber: SBR) or the like can be used. However, the binder is not particularly limited as long as it can bind the negative electrode active material and the conductive agent onto the current collector 31.

Here, in the present embodiment, a SEI (Solid Electrolyte Interphase) film containing a barium compound is formed on the surface of the negative electrode 30.

The SEI film is formed by decomposing the electrolytic solution on the surface of the negative electrode 30 at the time of initial charging / discharging. Specifically, the SEI film is an ultrathin film formed by an electrolytic solution impregnating the negative electrode 30 and a decomposition product of an additive of the electrolytic solution. The SEI film can suppress further decomposition reaction of the electrolytic solution on the negative electrode 30 while playing a role of inserting or removing lithium ions into the negative electrode 30.

The barium compound contained in the SEI film may be a salt of barium, an oxide, a sulfide, a hydroxide, a halide or the like. The barium compound may be, for example, barium sulfate (BaSO ₄ ). Barium sulfate (BaSO ₄ ) has low solubility in water and is stable, so that it can be easily handled.

It is clear from the examples and comparative examples described below that the inclusion of the barium compound in the SEI film improves the charge / discharge characteristics of the lithium ion secondary battery at a high rate. I'm not sure. It is presumed that the barium compound, which is a dielectric, is polarized on the surface of the negative electrode 30 to support the insertion or desorption of lithium ions into the negative electrode 30.

In the lithium ion secondary battery 10 according to the present embodiment, the barium compound is contained in the SEI film on the surface of the negative electrode 30 by spraying the barium compound on the surface of the negative electrode 30 at the time of manufacturing the negative electrode 30. According to this method, it becomes easy to control the amount and particle size of the barium compound contained in the SEI film within the range described later.

The content of the barium compound in the SEI film is determined by disassembling the fully charged lithium ion secondary battery 10 after initial charging and discharging, and analyzing the SEI film on the surface of the negative electrode 30 by X-ray photoelectron spectroscopy. It can be measured by. The content of the barium compound in the SEI film is 0.3 atomic% or less in terms of barium, and preferably 0.2 atomic% or less.

When the content of the barium compound is more than 0.3 atomic%, the excess amount of the barium compound in the SEI film deteriorates the charge / discharge characteristics of the lithium ion secondary battery at a high rate. On the other hand, the content of the barium compound in the SEI film is not particularly limited, but may be more than 0.05 atomic%. In other words, the lower limit of the content of the barium compound in the SEI film may be such that the peak of barium can be confirmed in the spectrum of X-ray photoelectron spectroscopy.

The content of the barium compound in the SEI film does not match the amount of the barium compound sprayed on the negative electrode 30. However, the content of the barium compound in the SEI film can be increased by increasing the amount of the barium compound sprayed on the negative electrode 30.

Further, the barium compound is contained in the SEI film as fine particles. The particle size of the barium compound contained in the SEI film is 0.8 μm or less, preferably 0.5 μm or less. When the particle size of the barium compound is more than 0.8 μm, the region occupied by the barium compound in the SEI film becomes excessive, and the charge / discharge characteristics of the lithium ion secondary battery deteriorate at a high rate. On the other hand, the particle size of the barium compound is not particularly limited, but may be 0.01 μm or more in the production of fine particles of the barium compound.

The particle size of the barium compound is the particle size when sprayed on the negative electrode 30. The particle size of the barium compound is calculated as, for example, the average value of the diameters when the barium compound before spraying is observed with a scanning electron microscope (SEM) and the shape of the barium compound is approximated to a spherical shape. Can be done.

(Separator 40)
The separator 40 can be used without particular limitation as long as it is used as a separator for a lithium ion secondary battery. As the separator, it is preferable to use a porous membrane or a non-woven fabric showing excellent high rate discharge performance alone or in combination. The resin constituting the separator is, for example, a polyolefin-based resin typified by polyethylene, polymer, or the like, a polyethylene terephthalate, a polybutylene terephthalate, or the like. Polymer) -based resin, polyvinylidene fluoride (PVDF), vinylidene fluoride (VDF) -hexafluoropropylene (HFP) copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetra Fluoroethylene polymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone, polymer. Vinylidene fluoride-ethylene (ethylene) polymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene- Hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer and the like may be used.

(Electrolytic solution)
The electrolytic solution is not particularly limited as long as it is used as an electrolytic solution for a lithium ion secondary battery. The electrolytic solution has a composition in which an electrolyte salt is contained in a non-aqueous solvent.

Examples of the non-aqueous solvent include propylene carbonate (propylene carbonate), ethylene carbonate (ethylene carbonate), butylene carbonate (ethylene carbononate), chloroethylene carbonate (chloroethylene carbonate), vinylene carbonate (vinylene carbonate), and the like. Kind: Cyclic esters such as γ-butyrolactone and γ-valerolactone; chains of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and the like. Chain esters such as methyl format, methyl acetate, butyric acid methyl; tetrahydrofuran (Tetrahydrofuran) or derivatives thereof; 1,3-dioxane, 1,4- Ethers such as dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyldilyme; acetonitrile (acetylene), benzonitrile (benez). ) Etc. nitriles; dioxolane or its derivatives; ethylene sulfide (estersulfide), sulfolane, sultone (sultone) or its derivatives, etc. may be used alone or in admixture of two or more. can.

The electrolyte salts are, for example, LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiPF _6-x (Cn F _{2n + 1} ) _x (where 1 <x <6, n = 1 or 2), LiSCN, LiBr, LiI, Li. ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr, KClO ₄ , KSCN and other inorganic ion salts 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 sulphonic acid lithium, lithium sulphonic acid, lithium dodecylbenzene sulphonate, etc., organic salts, etc. May be good. The electrolyte salt can be used alone or in combination of two or more of these ionic salts or ionic compounds. The concentration of the electrolyte salt can be the same as the concentration used in a general lithium ion secondary battery (about 0.8 mol / L to 2.0 mol / L).

In addition, various additives may be added to the electrolytic solution. Examples of such additives include negative-negative action additives, positive-side action additives, ester-based additives, carbonic acid ester-based additives, sulfuric acid ester-based additives, phosphate ester-based additives, borate ester-based additives, and acids. Examples thereof include an anhydride-based additive and an electrolyte-based additive. One or more of these additives may be added to the electrolytic solution.

<2. Manufacturing method of lithium-ion secondary battery ＞
Next, a method for manufacturing the lithium ion secondary battery 10 will be described.

The positive electrode 20 is manufactured by the following method. First, a slurry is formed by dispersing a mixture of a positive electrode active material, a conductive agent and a binder in a predetermined ratio in a solvent (for example, N-methyl-2-pyrrolidone). Next, the slurry is applied onto the current collector 21 and dried to form the positive electrode active material layer 22. The method of application is not particularly limited. As a method of coating, for example, a knife coater method, a gravure coater method, or the like can be used. Subsequently, the positive electrode active material layer 22 is compressed to a predetermined density by a press machine. As a result, the positive electrode 20 is manufactured.

The negative electrode 30 is also manufactured by the same method as that of the positive electrode 20. First, a slurry is formed by dispersing a mixture of a negative electrode active material and a binder in a solvent (for example, water). Next, the slurry is applied onto the current collector 31 and dried to form the negative electrode active material layer 32. Subsequently, the negative electrode active material layer 32 is compressed to a predetermined density by a press machine. Further, by spraying fine particles of the barium compound having a predetermined particle size on the surface of the negative electrode 30 after compression, the fine particles of the barium compound were arranged on the surface of the negative electrode 30. As a result, the negative electrode 30 is manufactured.

Next, an electrode structure is manufactured by sandwiching the separator 40 between the positive electrode 20 and the negative electrode 30. Subsequently, the prepared electrode structure is processed into a desired shape (for example, cylindrical shape, square shape, laminated shape, button shape, etc.) and inserted into the container having the desired shape. Then, by injecting the electrolytic solution into the container, each pore in the separator 40 is impregnated with the electrolytic solution. As a result, the lithium ion secondary battery 10 is manufactured.

Further, the produced lithium ion secondary battery 10 is initially charged and discharged to form an SEI film containing a barium compound on the surface of the negative electrode 30.

As described above, according to the present embodiment, by spraying fine particles of the barium compound on the surface of the negative electrode 30 in advance, an SEI film containing a suitable amount of the barium compound is applied to the surface of the negative electrode 30. Can be formed. According to this, the charge / discharge characteristics of the lithium ion secondary battery 10 at a high rate can be improved.

Hereinafter, the lithium ion secondary battery and the method for manufacturing 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 method for manufacturing the lithium ion secondary battery and the lithium ion secondary battery according to the present embodiment is not limited to the following examples.

(Manufacturing of lithium-ion secondary battery)
Lithium nickel-cobalt oxide represented by LiNi _0.88 Co _0.1 Al _0.02 O ₂ was used as the positive electrode active material, carbon powder was used as the conductive agent, and polyvinylidene fluoride was used as the binder. The positive electrode slurry (positive electrode mixture) was prepared by mixing the positive electrode active material, the conductive agent, and the binder in a mass ratio of 94: 4: 2, and adding N-methyl-2-pyrrolidone and kneading. ..

Next, on a current collector made of aluminum foil having a thickness of 12 μm, a length of 238 mm and a width of 29 mm, the above positive positive slurry is applied to one surface of the current collector with a length of 222 mm and a width of 29 mm to face one surface of the current collector. The other surface was coated with a length of 172 mm and a width of 29 mm. The current collector after applying the positive electrode slurry was dried and then rolled to prepare a positive electrode plate. At this time, the thickness of the positive electrode was 125 μm on both sides, the amount of the positive electrode mixture on the current collector was 42.5 mg / cm ² , and the packing density of the positive electrode mixture was 3.75 g / cm ³ . ..

Then, a current collecting tab made of an aluminum flat plate having a thickness of 70 μm, a length of 40 mm, and a width of 4 mm was attached to the portion of the positive electrode electrode plate to which the positive electrode mixture was not applied.

Artificial graphite and silicon-containing carbon were used as the negative electrode active material, and carboxymethyl cellulose and styrene-butadiene rubber were used as the binder. Artificial graphite, silicon-containing carbon, carboxymethyl cellulose, and styrene-butadiene rubber are mixed so as to have a mass ratio of 92.2: 5.3: 1.0: 1.5, and water is added and kneaded to obtain a negative electrode. The slurry (negative electrode mixture) was adjusted.

Next, the above negative electrode slurry was applied to one surface of the current collector having a length of 235 mm and a width of 30 mm on a current collector made of an aluminum foil having a thickness of 8 μm, a length of 271 mm and a width of 30 mm, and opposed to one surface of the current collector. The other surface was coated with a length of 178 mm and a width of 30 mm. The current collector after applying the negative electrode slurry was dried and then rolled to prepare a negative electrode electrode plate. At this time, the thickness of the negative electrode was 152 μm on both sides, the amount of the negative electrode mixture on the current collector was 23.0 mg / cm ² , and the packing density of the negative electrode mixture was 1.6 g / cm ³ .

Then, barium sulfate powder having a particle size shown in Table 1 below was sprayed on the surface of the prepared negative electrode plate in different amounts. Further, a current collecting tab made of a nickel flat plate having a thickness of 70 μm, a length of 40 mm and a width of 4 mm was attached to the portion of the negative electrode electrode plate not coated with the negative electrode mixture.

A non-aqueous solvent was prepared by mixing ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate in a volume ratio of 20:40:40. LiPF ₆ as an electrolyte salt was dissolved in the prepared non-aqueous solvent at a concentration of 1.15 mol / L to prepare an electrolytic solution. Further, vinylene carbonate was added to the prepared electrolytic solution in an amount of 1.5% by mass based on the total mass of the electrolytic solution.

A lithium secondary battery was manufactured using the positive electrode, the negative electrode, and the electrolytic solution prepared above. The positive electrode and the negative electrode were arranged so as to face each other via a separator, and these were folded back at a predetermined position, wound, and then pressed to prepare a flat electrode assembly. As the separator, two separators made of a polyethylene porous body having a length of 350 mm and a width of 32 mm were used.

The prepared electrode assembly was housed in a battery container made of aluminum laminate, and the electrolytic solution was injected into the battery container. At this time, the current collecting tabs of the positive electrode and the negative electrode were arranged so that they could be taken out. The design capacity of the manufactured lithium ion secondary battery is 480 mAh.

(Evaluation of lithium-ion secondary battery)
Subsequently, the lithium ion secondary battery produced above was initially charged and discharged. Specifically, in an environment of 25 ° C., the lithium ion secondary battery is charged with a constant current of 48 mA until it reaches 4.3 V, and then charged with a constant voltage of 4.3 V until the current value reaches 24 mA. , A constant current of 48 mA was discharged until it reached 2.8 V. The discharge capacity at this time was set to the initial capacity Q1.

Next, the lithium secondary battery that has been initially charged and discharged as described above is charged in an environment of 25 ° C. with a constant current of 240 mA until it reaches 4.3 V, and further, the current value is a constant voltage of 4.3 V. Was charged until the voltage reached 24 mA. Then, the charged lithium ion secondary battery was disassembled, and the negative electrode plate was taken out.

The SEI film formed on the surface of the negative electrode plate taken out was analyzed by X-ray photoelectron spectroscopy. The X-ray source was monochromatic AlKα (1486.6 eV), and the analysis area of the negative electrode plate was 700 μm × 300 μm. From the spectrum obtained by X-ray photoelectron spectroscopy, the composition ratio of barium was calculated in atomic%. The results are shown in Table 1.

Further, the lithium secondary battery that has been initially charged and discharged as described above is charged in an environment of 25 ° C. with a constant current of 240 mA until it reaches 4.3 V, and then the current value changes at a constant voltage of 4.3 V. The battery was charged to 24 mA and then discharged to 2.8 V with a current of 240 mA. With this as one cycle, 100 cycles of charging and discharging were repeated. Then, from the initial capacity Q1 and the discharge capacity Q [0.5C] at the 100th cycle, the capacity retention rate in the 0.5C cycle was determined using the following formula.
Capacity retention rate (%) in 0.5C cycle = (Q [0.5C] / Q1) × 100

Further, the lithium secondary battery that has been initially charged and discharged as described above is charged at a constant current of 960 mA until it reaches 4.3 V in an environment of 25 ° C., and then the current value changes at a constant voltage of 4.3 V. The battery was charged to 24 mA and then discharged to 2.8 V with a current of 240 mA. With this as one cycle, 100 cycles of charging and discharging were repeated. Then, from the initial capacity Q1 and the discharge capacity Q [2C] at the 100th cycle, the capacity retention rate in the 2.0C cycle was obtained using the following formula.
Capacity retention rate (%) in 2.0C cycle = (Q [2C] / Q1) × 100

The particle size and content of each barium compound in Examples and Comparative Examples, and the evaluation results are shown in Table 1 below. Comparative Example 1 is a comparative example in which the barium compound is not sprayed on the surface of the negative electrode.

Referring to Table 1, in Examples 1 to 4, since the barium compound is contained in the SEI film on the surface of the negative electrode, the capacity retention rates of 0.5 C cycle and 2.0 C cycle are improved as compared with Comparative Example 1. You can see that it is doing.

Further, in Comparative Example 2, since the content of the barium compound in the SEI film on the surface of the negative electrode exceeds the range according to the present embodiment, the capacity retention rate of the 0.5C cycle and the 2.0C cycle is lowered. I understand. Further, in Comparative Example 3, since the particle size of the barium compound sprayed on the surface of the negative electrode exceeds the range according to the present embodiment, the capacity retention rate of the 0.5C cycle and the 2.0C cycle is lowered. Understand.

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

Claims (3)

A negative electrode having an SEI film containing a barium compound is provided.
The barium compound is contained in the SEI film as fine particles having a particle size of 0.8 μm or less, and the content of the barium compound in the SEI film is 0.3 atomic% or less in terms of barium .
The barium compound is a lithium ion secondary battery, which is barium sulfate . The lithium ion secondary battery according to claim 1 , wherein the SEI film is provided on the surface of the negative electrode. The lithium ion secondary battery according to claim 1 or 2 , wherein the SEI film is composed of a decomposition product of a component contained in an electrolytic solution impregnating the negative electrode.

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