KR20200135301A - Lithium ion secondary battery, and its operation method - Google Patents

Abstract

The present invention is a lithium ion secondary battery having a negative electrode made of a yellow-denatured polyacrylonitrile as an active material, wherein the lower charging potential of the negative electrode is 0.1V (vs.Li/Li ⁺ ) or more and 1.0V (vs.Li/Li ⁺ ). It is less than a lithium ion secondary battery. In addition, the present invention is a lithium ion secondary battery having a negative electrode made of yellow-denatured polyacrylonitrile as an active material, wherein the lower charging potential of the negative electrode is at least 0.1V (vs.Li/Li ⁺ ) and 1.0V (vs.Li/Li ⁺⁾ It is a method of operating a lithium ion secondary battery that is less than ). It is preferable that the sulfur content of the yellow-modified polyacrylonitrile is 25% by mass to 60% by mass.

Description

Lithium ion secondary battery, and its operation method

The present invention relates to a lithium ion secondary battery using yellow-modified polyacrylonitrile as an electrode active material, and a method of operating the same.

Non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries are compact and lightweight, have high energy density, and can be repeatedly charged and discharged, and are widely used as power sources for portable electronic devices such as portable computers, handy video cameras, and information terminals. have. In addition, from the viewpoint of environmental issues, electric vehicles using non-aqueous electrolyte secondary batteries or hybrid vehicles using electric power as part of the power have been put to practical use. Therefore, in recent years, further improvement of the performance of the secondary battery is required.

The characteristics of a non-aqueous electrolyte secondary battery depend on an electrode, a separator, an electrolyte, etc., which are constituent members thereof, and research and development of each constituent member is actively conducted. In an electrode, an electrode active material is important along with a binder and a current collector, and research and development of electrode active materials are actively conducted.

Yellow-modified polyacrylonitrile obtained by heat treatment of a mixture of polyacrylonitrile and sulfur in a non-oxidizing atmosphere has a large charge and discharge capacity, and a decrease in charge and discharge capacity due to repeated charge and discharge (hereinafter, sometimes referred to as cycle characteristics) It is known as an electrode active material with few (for example, see Patent Documents 1 to 3). Although yellow-denatured polyacrylonitrile is used as an active material for a positive electrode, it is also being studied as an active material for a negative electrode (see, for example, Patent Document 3).

US2011/200875 A1 US2013/029222 A1 US2014/134485 A1

If the charge lower limit potential for the negative electrode active material is low, the charge/discharge capacity can be increased, but if the charge lower limit potential is too low, there is a risk of precipitation of lithium metal on the negative electrode. For this reason, for example, in Patent Document 3, since the average potential at the time of Li occlusion of yellow-denatured polyacrylonitrile is set to 1.8 V (Li reference), that is, 1.8 V (vs.Li/Li ⁺ ), yellowing The charging lower limit potential of the negative electrode having polyacrylonitrile is estimated to be about 1.0 V (vs. Li/Li ⁺ ) (refer to Patent Document 3).

Non-aqueous electrolyte secondary batteries, particularly non-aqueous electrolyte secondary batteries used in electric vehicles or hybrid vehicles, have high output and excellent cycle characteristics, and furthermore, lightweight and miniaturized lithium ion secondary batteries are required.

The present inventors conducted a thorough review, and as a result, in a non-aqueous electrolyte secondary battery using yellow-denatured polyacrylonitrile as a negative electrode active material, even when the lower charging limit potential of the negative electrode is lower than 1.0 V (vs.Li/Li ⁺ ), the degradation of battery performance is low. , It was found that charging and discharging was possible safely, and that the decrease in cycle characteristics was also low, and the present invention was completed. That is, the present invention is a lithium ion secondary battery having a negative electrode made of yellow-denatured polyacrylonitrile as an active material, and the lower charging potential of the negative electrode is 0.1V (vs.Li/Li ⁺ ) or more and 1.0V (vs.Li/Li). ^It is a lithium ion secondary battery less than ⁺ ).

In addition, the present invention is a method of operating a lithium ion secondary battery having a negative electrode made of yellow-denatured polyacrylonitrile as an active material, wherein the lower charge limit potential of the negative electrode is 0.1V (vs.Li/Li ⁺ ) or more and 1.0V (vs.Li ⁺ ). /Li ⁺ ) to provide a method of operating a lithium ion secondary battery less than.

In the present invention, there is one characteristic in that the charging lower limit potential of the negative electrode of the lithium ion secondary battery is 0.1 V (vs. Li/Li ⁺ ) or more and less than 1.0 V (vs. Li/Li ⁺ ). In the present invention, the unit V(vs.Li/Li ⁺ ) is a lithium metal reference potential. It is preferable that the lower charging limit potential of the negative electrode is lower than 0.1 V (vs. Li/Li ⁺ ), the deterioration of the cycle characteristics of the battery increases. In the present invention, the lower charging potential of the negative electrode is 0.15V (vs.Li/Li ⁺ ) or more and 0.9V (vs.Li/Li ⁺ ) or less is preferable, and 0.17V (vs.Li/Li ⁺ ) or more and 0.85 V (vs.Li/Li ⁺⁾ or less, and more preferably, 0.25V (vs.Li/Li ⁺⁾ or more and is preferably not more than 0.8V (vs.Li/Li ⁺⁾ in particular. In order to make the lower charging limit potential of the negative electrode within the above range, for example, the ratio of the weight of the active material per unit area (mg/cm2) of the positive electrode and the negative electrode may be adjusted by changing the electrode coating thickness or the like. As the ratio of the weight of the active material per unit area of the negative electrode decreases with respect to the positive electrode, the lower charging limit potential of the negative electrode decreases.

The charging/discharging capacity of the negative electrode using yellow-denatured polyacrylonitrile as an active material is slightly different depending on the sulfur content of the yellow-denatured polyacrylonitrile, but for the case where the lower charging potential is 1.0V (vs.Li/Li ⁺ ), 0.5 V (vs.Li/Li ⁺⁾ increased by 30 to 50%, about 60% to 80% at 0.2V (vs.Li/Li ⁺⁾ from, and to reduce the amount of the increase as acrylonitrile, sulfur-modified polyacrylonitrile amount It is possible. By lowering the charge lower limit potential to reduce the active material of the negative electrode, it becomes possible to reduce the weight and size of the lithium ion secondary battery. On the other hand, using a lithium-ion secondary battery with a design with a lower charging limit of less than 1.0V (vs.Li/Li ⁺ ) with a lower charging limit of 1.0V (vs.Li/Li ⁺ ) has sufficient charge and discharge capacity. It is not obtained and is not preferable.

Yellow-modified polyacrylonitrile is a compound obtained by heat-treating polyacrylonitrile and simple sulfur in a non-oxidizing atmosphere. Polyacrylonitrile may be a copolymer of acrylonitrile and other monomers, such as acrylic acid, vinyl acetate, N-vinylformamide, and N-N' methylenebis(acrylamide). However, since battery performance decreases when the content of acrylonitrile is lowered, in the case of a copolymer of acrylonitrile and other monomers, the content of acrylonitrile in the copolymer is preferably 90% by mass or more. The sulfur content of the yellow-denatured polyacrylonitrile is preferably 25% by mass to 60% by mass, and 27% by mass to 50% by mass, since a large charge/discharge capacity is obtained and there is little deterioration in cycle characteristics in the operating method of the present invention. It is more preferable, and 30 mass%-45 mass% are most preferable. On the other hand, the sulfur content of the yellow-modified polyacrylonitrile may be subjected to elemental analysis using a CHN analyzer capable of analyzing sulfur and oxygen to calculate the sulfur content.

The ratio of polyacrylonitrile and simple sulfur in the heat treatment is preferably 100 parts by mass to 1500 parts by mass of simple sulfur, and more preferably 150 parts by mass to 1000 parts by mass per 100 parts by mass of polyacrylonitrile. The heat treatment temperature is preferably 250°C to 550°C, and more preferably 350°C to 450°C. Since unreacted single sulfur becomes a factor of deteriorating the cycle characteristics of the secondary battery, it is preferable to remove it by, for example, heating or washing with a solvent.

When the yellow-denatured polyacrylonitrile of the present invention is used as an electrode active material for an electrode of a secondary battery, for example, according to the intended use, the average particle diameter is preferably 0.5 µm to 100 µm. The average particle diameter means a 50% particle diameter measured by a laser diffraction light scattering method. The particle diameter is a volume-based diameter, and the diameter of the secondary particle is measured in the laser diffraction light scattering method. In order to reduce the average particle diameter of the yellow-denatured polyacrylonitrile of the present invention to less than 0.5 μm, great effort is required for pulverization, etc., but further improvement in battery performance cannot be desired. easy. The average particle diameter of the yellow-modified polyacrylonitrile of the present invention is preferably 0.5 µm to 100 µm, more preferably 1 µm to 50 µm, and still more preferably 2 µm to 30 µm.

A suitable configuration of an electrode using yellow-modified polyacrylonitrile as an electrode active material and a suitable manufacturing method thereof will be described below. The electrode forms an electrode mixture layer having yellow-denatured polyacrylonitrile on the current collector. The electrode mixture layer is formed by, for example, applying a slurry prepared by adding a yellow-denatured polyacrylonitrile, a binder, and a conductive aid to a solvent on a current collector and drying it.

As the binder, a known binder may be used as a binder for the electrode. For example, styrene-butadiene rubber, butadiene rubber, polyethylene, polypropylene, polyamide, polyamideimide, polyimide, polyacrylonitrile, polyurethane, polyfluoride Vinylidene, polytetrafluoroethylene, ethylene-propylene-diene rubber, fluorine rubber, styrene-acrylic acid ester copolymer, ethylene-vinyl alcohol copolymer, acrylonitrile butadiene rubber, styrene-isoprene rubber, polymethylmethacrylate, Polyacrylate, polyvinyl alcohol, polyvinyl ether, carboxymethylcellulose, sodium carboxymethylcellulose, methylcellulose, cellulose nanofiber, polyethylene oxide, starch, polyvinylpyrrolidone, polyvinyl chloride, polyacrylic acid, and the like. .

As the binder, since the environmental load is low and the elution of sulfur is difficult to occur, an aqueous binder is preferred, and styrene-butadiene rubber, sodium carboxymethylcellulose, and polyacrylic acid are more preferred. Only one type of binder may be used, or two or more types may be used in combination. The content of the binder in the slurry is preferably 1 part by mass to 30 parts by mass, and more preferably 1.5 parts by mass to 20 parts by mass based on 100 parts by mass of the yellow-modified polyacrylonitrile of the present invention.

As the conductive aid, a known conductive aid for the electrode can be used. Specifically, natural graphite, artificial graphite, carbon black, ketjen black, acetylene black, channel black, furnace black, lamp black, thermal black, carbon nanotube, Carbon materials such as vapor grown carbon fiber (VGCF), exfoliated graphite, graphene, fullerene, and needle coke; Metal powders such as aluminum powder, nickel powder, and titanium powder; Conductive metal oxides such as zinc oxide and titanium oxide; Sulfides, such as La ₂ S ₃ , Sm ₂ S ₃ , Ce ₂ S ₃ , and TiS ₂ , are mentioned. The particle size of the conductive aid is preferably 0.0001 µm to 100 µm, more preferably 0.01 µm to 50 µm, as for the average particle size. The content of the conductive aid in the slurry is usually 0.1 parts by mass to 50 parts by mass, preferably 1 part by mass to 30 parts by mass, and more preferably 2 parts by mass based on 100 parts by mass of the yellow-modified polyacrylonitrile of the present invention. It is ~20 parts by mass.

As a solvent for preparing the slurry, for example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, acetonitrile, propionitrile , Tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, nitromethane, N-methylpyrrolidone, N,N-dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexa Non, methyl acetate, methyl acrylate, diethyltriamine, N,N-dimethylaminopropylamine, polyethylene oxide, tetrahydrofuran, dimethyl sulfoxide, sulfolane, γ-butyrolactone, water, alcohol, etc. are mentioned. . The amount of solvent used can be adjusted according to the application method of the slurry.For example, in the case of the doctor blade method, 20 parts by mass to 300 parts by mass are preferable based on 100 parts by mass of the total amount of yellow-denatured polyacrylonitrile, binder, and conductive aid. And 30 parts by mass to 200 parts by mass are more preferable.

The slurry may contain other ingredients. Other components include, for example, a viscosity modifier, a reinforcing material, and an antioxidant.

The method of preparing the slurry is not particularly limited, but for example, a conventional ball mill, sand mill, bead mill, pigment disperser, rotator, ultrasonic disperser, homogenizer, rotation/revolution mixer, planetary mixer, Filmmix, Jet Paster, etc. can be used.

Conductive materials such as titanium, titanium alloy, aluminum, aluminum alloy, copper, nickel, stainless steel, and nickel plated steel are used as the current collector. These conductive materials may have their surface coated with carbon. The shape of the current collector includes a foil shape, a plate shape, and a mesh shape. Among these, aluminum, copper, and stainless steel are preferable from the viewpoint of conductivity and cost, and the shape is preferably a thin shape. In the case of a foil shape, the thickness of the foil is usually 1 to 100 μm.

The method of applying the slurry to the current collector is not particularly limited, and the die coater method, comma coater method, curtain coater method, spray coater method, gravure coater method, flexo coater method, knife coater method, doctor blade method, reverse roll method, Each method such as brushing method and dipping method can be used. A die coater method, a doctor blade method, and a knife coater method are preferable because it becomes possible to obtain a satisfactory surface state of the coating layer according to physical properties such as viscosity of the slurry and dryness. Coating may be performed on one side or both sides of the current collector, and when applying to both sides of the current collector, it is possible to apply one by one side by side, and both sides can be applied simultaneously. Further, it can be applied continuously to the surface of the current collector, or can be applied intermittently, and can be applied in a stripe shape. The thickness of the electrode mixture layer is usually 1 to 500 µm, preferably 1 to 300 µm, and more preferably 1 to 150 µm. The thickness, length or width of the coating layer can be appropriately determined according to the size of the battery.

The method of drying the slurry applied on the current collector is not particularly limited, and is irradiated with far-infrared rays, infrared rays, electron beams, etc., which are allowed to stand in a warm air, hot air, low-humidity air, vacuum drying, heating furnace, etc.照射) can be used. By this drying, volatile components such as a solvent are volatilized from the coating film of the slurry, and an electrode mixture layer is formed on the current collector. Thereafter, if necessary, the electrode may be pressed. As a method of press treatment, a die press method and a roll press method are mentioned, for example.

When the amount of yellow-denatured polyacrylonitrile in the negative electrode mixture layer is more than a certain amount, it is easy to obtain sufficient charge/discharge capacity, and by making the amount of yellow-denatured polyacrylonitrile less than a certain amount, conductivity and adhesion to the current collector are sufficient. easy. From these points, the content of the yellow-denatured polyacrylonitrile in the electrode mixture layer of the negative electrode is preferably 30% by mass to 99.5% by mass, more preferably 40% by mass to 99% by mass, and 50% by mass to 98% by mass. It is most preferably %.

The lithium ion secondary battery is composed of a negative electrode, a positive electrode, and a non-aqueous electrolyte using yellow-denatured polyacrylonitrile as an active material, and has a separator between the positive electrode and the negative electrode if necessary. As the positive electrode, an electrode having a known active material may be used as an active material of the positive electrode.

Known positive electrode active materials include, for example, a lithium transition metal composite oxide, a lithium-containing transition metal phosphate compound, and a lithium-containing silicate compound. The transition metal of the lithium transition metal composite oxide is preferably vanadium, titanium, chromium, manganese, iron, cobalt, nickel, copper, and the like. Specific examples of lithium transition metal complex oxides include lithium cobalt complex oxides such as LiCoO ₂ , lithium nickel complex oxides such as LiNiO ₂ , lithium manganese complex oxides such as LiMnO ₂ , LiMn ₂ O ₄ , and Li ₂ MnO ₃ , and these lithium transitions. Some of the transition metal atoms, which are the main body of the metal complex oxide, are substituted with other metals such as aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, lithium, nickel, copper, zinc, magnesium, gallium, zirconium, etc. Can be lifted. Specific examples of the substituted ones include, for example, Li _1.1 Mn _1.8 Mg _0.1 O ₄ , Li _1.1 Mn _1.85 Al _0.05 O ₄ , LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , LiNi _0.5 Mn _0.5 O ₂ , LiNi _0.80 Co _0.17 Al _0.03 O ₂ , LiNi _0.80 Co _0.15 Al _0.05 O ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiMn _1.8 Al _0.2 O ₄ , LiMn _1.5 Ni _0.5 O ₄ , Li ₂ MnO ₃ -LiMO ₂ (M=Co, Ni, Mn), and the like. The transition metal of the lithium-containing transition metal phosphate compound is preferably vanadium, titanium, manganese, iron, cobalt, nickel, etc., and specific examples include phosphoric acid such as LiFePO ₄ and LiMn _x Fe _1-x PO ₄ Iron compounds, cobalt phosphate compounds such as LiCoPO _4, and some of the transition metal atoms that are the main agents of these lithium transition metal phosphate compounds are aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, lithium, nickel, copper, zinc , Magnesium, gallium, zirconium, niobium, and other metals substituted with other metals, and vanadium phosphate compounds such as Li ₃ V ₂ (PO ₄ ) ₃ . Examples of the lithium-containing silicate compound include Li ₂ FeSiO ₄ . These may be used alone, or may be used in combination of two or more.

The configuration of the positive electrode and its manufacturing method include those in which the above-described yellow-denatured polyacrylonitrile is substituted with the known positive electrode active material in the appropriate configuration of the electrode and the appropriate manufacturing method thereof. I can.

Non-aqueous electrolytes include, for example, a liquid electrolyte obtained by dissolving an electrolyte in an organic solvent, a polymer gel electrolyte obtained by dissolving the electrolyte in an organic solvent and gelling it into a polymer, a pure polymer electrolyte in which the electrolyte is dispersed in a polymer without containing an organic solvent, Boron hydride compounds, inorganic solid electrolytes, and the like.

As the electrolyte used in the liquid electrolyte and the polymer gel electrolyte, for example, a conventionally known lithium salt is used. For example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(SO ₂ F) ₂ , LiC(CF ₃ SO ₂ ) ₃ , LiB(CF ₃ SO ₃ ) ₄ , LiB(C ₂ O ₄ ) ₂ , LiBF ₂ (C ₂ O ₄ ), LiSbF ₆ , LiSiF ₅ , LiSCN, LiClO ₄ , LiCl, LiF, LiBr, LiI, LiAlF ₄ , LiAlCl ₄ , LiPO ₂ F ₂ and derivatives thereof, and the like, Among these, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(SO ₂ F) ₂ , and LiC( It is preferable to use at least one selected from the group consisting of a derivative of CF ₃ SO ₂ ) ₃ and LiCF ₃ SO ₃ , and a derivative of LiC(CF ₃ SO ₂ ) ₃ . The content of the electrolyte in the liquid electrolyte and the polymer gel electrolyte is preferably 0.5 to 7 mol/L, more preferably 0.8 to 1.8 mol/L.

Electrolytes used in the pure polymer electrolyte include, for example, LiN(CF ₃ SO ₂ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(SO ₂ F) ₂ , LiC(CF ₃ SO ₂ ) ₃ , LiB(CF ₃ SO ₃ ) ₄ and LiB(C ₂ O ₄ ) ₂ are mentioned. Examples of the boron hydride compound include LiBH ₄ -LiI and LiBH ₄ -P ₂ S ₅ .

As an inorganic solid electrolyte, Li _1+x A _x B _2-y (PO ₄ ) ₃ (x=Al, Ge, Sn, Hf, Zr, Sc, Y, B=Ti, Ge, Zn, 0<x<0.5 ), LiMPO ₄ (M=Mn, Fe, Co, Ni), and phosphoric acid-based materials such as Li ₃ PO ₄ ; Li ₃ XO ₄ (X=As, V), Li _3+x A _x B _1-x O ₄ (A=Si, Ge, Ti, B=P, As, V, 0<x<0.6), Li _{4 +x} A _x Si _1-x O ₄ (A=B, Al, Ga, Cr, Fe, 0<x<0.4)(A=Ni, Co, 0<x<0.1), Li _4-3y Al _y SiO ₄ (0<y<0.06), Li _4-2y Zn _y GeO ₄ (0<y<0.25), LiAlO ₂ , Li ₂ BO ₄ , Li ₄ XO ₄ (X=Si, Ge, Ti), lithium titanate Lithium composite oxides such as (LiTiO ₂ , LiTi ₂ O ₄ , Li ₄ TiO ₄ , Li ₂ TiO ₃ , Li ₂ Ti ₃ O ₇ , Li ₄ Ti ₅ O ₁₂ ); Compounds containing lithium and halogen such as LiBr, LiF, LiCl, LiPF ₆ and LiBF ₄ ; A compound containing lithium and nitrogen such as LiPON, LiN(SO ₂ CF ₃ ) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ , Li ₃ N, and LiN(SO ₂ C ₃ F ₇ ) ₂ ; Crystals having a perovskite structure having lithium ion conductivity such as La _0.55 Li _0.35 TiO ₃ ; Crystals having a garnet-like structure such as Li ₇ -La ₃ Zr ₂ O ₁₃ ; Glass, such as 50Li ₄ SiO ₄ ·50Li ₃ BO ₃ and 90Li ₃ BO ₃ ·10 Li ₂ SO ₄ ; 70Li ₂ S·30P ₂ S ₅ , 75 Li ₂ S·25P ₂ S ₅ , Li ₆ PS ₅ Cl, Li ₁₀ GeP ₂ S ₁₂ , Li _3.25 Ge _0.25 P _0.75 S ₄ Li-phosphorus sulfide-based crystal, 30Li ₂ Lithium such as S·26B ₂ S ₃ 44LiI, 63Li ₂ S 36SiS ₂ 1Li ₃ PO ₄ , 57Li ₂ S 38SiS ₂ 5Li ₄ SiO ₄ , 70Li ₂ S 50GeS ₂ , 50Li ₂ S 50GeS ₂ Phosphorus sulfide glass; Glass ceramics, such as Li ₇ P ₃ S ₁₁ , Li _3.25 P _0.95 S ₄ , Li ₁₀ GeP ₂ S ₁₂ , Li _9.6 P ₃ S ₁₂ , Li _9.54 Si _1.74 P _1.44 S _11.7 Cl _0.3, etc. are mentioned.

As the organic solvent used in the preparation of the liquid non-aqueous electrolyte used in the present invention, one type or a combination of two or more of those commonly used for the liquid non-aqueous electrolyte can be used. Specifically, for example, saturated cyclic carbonate compounds, saturated cyclic ester compounds, sulfoxide compounds, sulfone compounds, amide compounds, saturated chain carbonate compounds, chain ether compounds, cyclic ether compounds, saturated chain ester compounds, etc. Can be lifted.

Among the organic solvents, saturated cyclic carbonate compounds, saturated cyclic ester compounds, sulfoxide compounds, sulfone compounds, and amide compounds have a high relative dielectric constant, and thus serve to increase the dielectric constant of the nonaqueous electrolyte, and saturated cyclic carbonate compounds are particularly preferred. Such saturated cyclic carbonate compounds include, for example, ethylene carbonate, 1,2-propylene carbonate, 1,3-propylene carbonate, 1,2-butylene carbonate, 1,3-butylene carbonate, and 1,1-dimethylethylene. Carbonate, etc. are mentioned. Examples of the saturated cyclic ester compound include γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-hexanolactone, and δ-octanolactone. Examples of the sulfoxide compound include dimethyl sulfoxide, diethyl sulfoxide, dipropyl sulfoxide, diphenyl sulfoxide, and thiophene. As the sulfone compound, for example, dimethyl sulfone, diethyl sulfone, dipropyl sulfone, diphenyl sulfone, sulfolane (also referred to as tetramethylene sulfone), 3-methylsulfolane, 3,4-dimethylsulfolane, 3, 4-diphenymethylsulfolane, sulforene, 3-methylsulforene, 3-ethylsulforene, 3-bromomethylsulforene, etc. are mentioned, and sulfolane and tetramethylsulfolane are preferable. Examples of the amide compound include N-methylpyrrolidone, dimethylformamide, and dimethylacetamide.

Among the organic solvents, saturated chain carbonate compounds, chain ether compounds, cyclic ether compounds, and saturated chain ester compounds can lower the viscosity of the non-aqueous electrolyte and increase the mobility of electrolyte ions. It can be done with excellent things. In addition, since it has a low viscosity, the performance of the non-aqueous electrolyte at a low temperature can be increased, and a saturated chain carbonate compound is particularly preferable. Examples of the saturated chain carbonate compound include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl butyl carbonate, methyl-t-butyl carbonate, diisopropyl carbonate, and t-butylpropyl carbonate. Examples of the chain ether compound or cyclic ether compound include dimethoxyethane, ethoxymethoxyethane, diethoxyethane, tetrahydrofuran, dioxolane, dioxane, 1,2-bis (methoxycarbonyloxy) Ethane, 1,2-bis (ethoxycarbonyloxy) ethane, 1,2-bis (ethoxycarbonyloxy) propane, ethylene glycol bis (trifluoroethyl) ether, propylene glycol bis (trifluoroethyl) Ether, ethylene glycol bis (trifluoromethyl) ether, diethylene glycol bis (trifluoroethyl) ether, and the like. Among these, dioxolane is preferable.

The saturated chain ester compound is preferably a monoester compound and a diester compound having a total number of carbon atoms of 2 to 8 in the molecule, and specific compounds include, for example, methyl formate, ethyl formate, methyl acetate, ethyl acetate, and acetic acid. Propyl, isobutyl acetate, butyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethyl acetate, ethyl trimethyl acetate, methyl malonate, ethyl malonate, methyl succinate, ethyl succinate, 3-methoxy Methyl propionate, ethyl 3-methoxypropionate, ethylene glycol diacetyl, propylene glycol diacetyl, and the like, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, isobutyl acetate, butyl acetate, methyl propionate, And ethyl propionate are preferred.

In addition, as an organic solvent used for preparing the non-aqueous electrolyte, for example, acetonitrile, propionitrile, nitromethane, derivatives thereof, and various ionic liquids may be used.

Polymers used in the polymer gel electrolyte include polyethylene oxide, polypropylene oxide, polyvinyl chloride, polyacrylonitrile, polymethyl methacrylate, polyethylene, polyvinylidene fluoride, and polyhexafluoropropylene. Polymers used in the genuine polymer electrolyte include polyethylene oxide, polypropylene oxide, and polystyrene sulfonic acid. There is no restriction|limiting in particular about the compounding ratio in the gel electrolyte and the compounding method, What is necessary is just to adopt a well-known compounding ratio and a well-known compounding method in the art.

The non-aqueous electrolyte may contain other known additives, such as an electrode film forming agent, an antioxidant, a flame retardant, an overcharge inhibitor, etc. in order to improve battery life and improve safety. When other additives are used, they are usually 0.01 parts by mass to 10 parts by mass, preferably 0.1 parts by mass to 5 parts by mass with respect to the whole non-aqueous electrolyte.

By applying the lithium ion secondary battery and operation method of the present invention, the charge/discharge capacity of a negative electrode using yellow-denatured polyacrylonitrile as an active material can be increased, and the amount of yellow-denatured polyacrylonitrile can be reduced. Furthermore, the battery voltage can be improved, and it becomes possible to reduce the weight and size of the lithium ion secondary battery.

Example

Hereinafter, the present invention will be described in more detail by examples and comparative examples. However, the present invention is not limited at all by the following examples. On the other hand, "parts" and "%" in Examples are by mass unless otherwise noted.

[Production Example 1]

10 parts by mass of polyacrylonitrile powder (manufactured by Sigma-Aldrich, average particle diameter of 200 µm) and 30 parts by mass of sulfur powder (manufactured by Sigma-Aldrich, average particle diameter of 200 µm) were mixed using a mortar. For the yellowing of polyacrylonitrile, a reaction apparatus according to the example of Japanese Unexamined Patent Application Publication No. 2013-054957 was used. This mixture was put in the same cylindrical glass bottle as described in Japanese Patent Application Laid-Open No. 2013-054957, and the lower part of the glass bottle was put in a crucible electric furnace, and hydrogen sulfide generated under a nitrogen stream was removed for 1 hour at 400°C. Heated. After cooling, the product was placed in a glass tube oven and heated at 250° C. for 3 hours while vacuuming to remove simple sulfur. The obtained yellow-denatured polyacrylonitrile was pulverized using a mortar until the average particle diameter became 10 μm, and yellow-denatured polyacrylonitrile powder PANS1 was obtained. The sulfur content of PANS1 is 38% by mass.

[Production Example 2]

The same operation as in Production Example 1 was carried out except for changing the temperature for removing the single sulfur from 250°C to 200°C to obtain yellow-denatured polyacrylonitrile powder PANS2. The sulfur content of PANS2 is 55% by mass. On the other hand, the conditions for removing simple sulfur in Production Example 2 are the same as those described in Patent Document 3, and the sulfur content of PANS2 can be considered to be equivalent to the yellow-modified polyacrylonitrile described in Patent Document 3.

(Examples 1 to 6, Comparative Examples 1 to 4)

[Production of cathodes 1 to 10]

92.0 parts by mass of PANS1 or PANS2 (refer to Table 1) as an electrode active material, 3.5 parts by mass of acetylene black (manufactured by Denki Chemical Industries, Ltd.), 1.5 parts by mass of carbon nanotubes (made by Showa Denko, brand name VGCF), as a binder. 1.5 parts by mass of styrene-butadiene rubber (40% by mass aqueous dispersion, manufactured by Nippon Xeon), 1.5 parts by mass of sodium carboxymethylcellulose (manufactured by Daiseru Fine Chemical), and 100 parts by mass of water were dispersed using a rotating/revolution mixer. A slurry was prepared. This slurry composition was applied to a current collector of stainless steel foil (10 µm in thickness) by a doctor blade method so that the thickness of the electrode mixture layer became the value shown in Table 1, and dried at 90°C for 3 hours. Thereafter, this electrode was cut into a predetermined size, and vacuum-dried at 120° C. for 2 hours to prepare a disc-shaped electrode.

[Production of anode]

90 parts by mass of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (manufactured by Nihon Chemical Industries, hereinafter, abbreviated as NCM) as a positive electrode active material, 5 parts by mass of acetylene black (manufactured by Denki Chemical Industry Co., Ltd.) , 5 parts by mass of polyvinylidene fluoride (Kureha product) as a binder and 100 parts by mass of N-methylpyrrolidone as a solvent were dispersed using a rotating/revolution mixer to prepare a slurry. This slurry composition was applied to an aluminum foil (20 µm thick) current collector by a doctor blade method so that the electrode mixture layer had a thickness of 62 µm, and dried at 90°C for 3 hours. Thereafter, this electrode was cut into a predetermined size, vacuum-dried at 120°C for 2 hours, and a disk-shaped positive electrode was produced. The positive electrode capacity was such that the upper charging limit potential of the positive electrode was 4.2 V (vs.Li/Li ⁺ ) in any case where the lower charging limit potential of the negative electrode was.

〔Preparation of non-aqueous electrolyte〕

LiPF ₆ was dissolved in a mixed solvent comprising 50% by volume of ethylene carbonate and 50% by volume of diethyl carbonate at a concentration of 1.0 mol/L to prepare an electrolyte solution.

〔Battery assembly〕

Negative electrodes 1 to 10 using PANS1 or 2 as an active material, and positive electrodes using NCM as an active material were held in the case by inserting a separator (Cellgard's product, brand name: Celgard 2325). Thereafter, the non-aqueous electrolyte prepared previously was injected into the case, and the case was sealed and sealed to produce batteries 1 to 10 as non-aqueous electrolyte secondary batteries (a coin type having a thickness of 20 mm and a thickness of 3.2 mm). In addition, in order to check the lower potential during the initial charge of the negative electrodes 1 to 10, the negative electrode 1 to 10, NCM positive electrode, glass filter separator, lithium metal reference electrode, and a 3-pole cell kit (manufactured by Toyo Systems) were used. And a three-pole cell composed of the non-aqueous electrolyte prepared above was produced.

(Charge/discharge test method)

Put the triode cell in a thermostat at 25°C, and charge the NCM positive electrode at a charging rate of 0.1C until the potential becomes 4.2V (vs.Li/Li ⁺ ), and the potentials of the negative electrodes 1 to 10 at that time are shown in Table 2. It confirmed that it was the value of.

In addition, a non-aqueous electrolyte secondary battery is placed in a thermostat at 25°C, charged at a charging rate of 0.1C until the lower charge limit potential of the negative electrode reaches the value in Table 2, and the battery voltage is discharged to 0.8V at a discharge rate of 0.1C. The discharge cycle was carried out 50 times. Table 2 shows the discharge capacity per weight of the PANS anode at the 10th charge/discharge cycle and the ratio of the discharge capacity at the 50th to the weight of the PANS cathode at the 10th charge/discharge cycle. The larger the value of this ratio, the better the cycle characteristics.

From Table 2, according to the lithium ion secondary battery of each example in which the lower charge limit potential of the negative electrode is 0.1V (vs.Li/Li ⁺ ) or more and less than 1.0V (vs.Li/Li ⁺ ), the discharge capacity of the lithium ion secondary battery Both the size and cycle characteristics are excellent. On the other hand, Comparative Example 1 in which the lower charging limit potential of the negative electrode was 1.0 V (vs.Li/Li ⁺ ) was inferior to Examples 1, 3 and 5 using PANS of the same sulfur content, and the discharge capacity of the negative electrode was Comparative Example 3 in which the charging lower limit potential was less than 0.1 V had lower cycle characteristics compared to Examples 1, 3 and 5. It can be said that for Comparative Examples 2 and 4, the same can be said for Examples 2, 4 and 6.

According to the present invention, in a lithium-ion secondary battery having a negative electrode made of yellow-denatured polyacrylonitrile as an active material, the lower charge limit potential of the negative electrode is 0.1V (vs.Li/Li ⁺ ) or higher and 1.0V (vs.Li/Li ^{+ ).} ), the charge/discharge capacity and battery voltage per active material increase, and it becomes possible to reduce the amount of use of the active material. This makes it possible to reduce the weight and size of the lithium ion secondary battery.

Claims (4)

As a lithium-ion secondary battery having a negative electrode made of yellow-denatured polyacrylonitrile as an active material, lithium having a lower charge limit potential of the negative electrode is 0.1V (vs.Li/Li ⁺ ) or more and less than 1.0V (vs.Li/Li ⁺ ). Ion secondary battery. The method of claim 1,
A lithium ion secondary battery in which the sulfur content of the yellow-modified polyacrylonitrile is 25% by mass to 60% by mass. As an operating method of a lithium ion secondary battery having a negative electrode made of yellow-denatured polyacrylonitrile as an active material, the lower charging potential of the negative electrode is 0.1V (vs.Li/Li ⁺ ) or higher and 1.0V (vs.Li/Li ⁺ ). How to operate the lithium ion secondary battery to less than. The method of claim 3,
A method of operating a lithium ion secondary battery, wherein the sulfur content of the yellow-modified polyacrylonitrile is 25% by mass to 60% by mass.