JP7304339B2 - Lithium ion secondary battery and operating method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a lithium ion secondary battery using sulfur-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, lightweight, have high energy density, and can be repeatedly charged and discharged. Widely used as a power source. In addition, from the viewpoint of environmental problems, electric vehicles using non-aqueous electrolyte secondary batteries and hybrid vehicles using electric power as a part of power are being put to practical use. Therefore, in recent years, there has been a demand for further improvement in the performance of secondary batteries.

The characteristics of non-aqueous electrolyte secondary batteries depend on their constituent members such as electrodes, separators, electrolytes, and the like, and research and development of each constituent member is being actively carried out. In the electrode, the electrode active material is important as well as the binder, current collector, etc., and research and development of the electrode active material are actively carried out.

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

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

If the lower limit charging potential of the negative electrode active material is low, the charge/discharge capacity can be increased. For this reason, for example, in Patent Document 3, the average potential of sulfur-modified polyacrylonitrile during Li absorption is set to 1.8 V (based on Li), that is, 1.8 V (vs. Li/Li ⁺ ). The lower limit charging potential of a negative electrode containing acrylonitrile is estimated to be about 1.0 V (vs. Li/Li ⁺ ) (see Patent Document 3).

BACKGROUND ART Non-aqueous electrolyte secondary batteries, particularly non-aqueous electrolyte secondary batteries used in electric vehicles and hybrid vehicles, are required to have high output, excellent cycle characteristics, and lightweight and compact lithium-ion secondary batteries.

As a result of extensive studies, the present inventors have found that in a non-aqueous electrolyte secondary battery using sulfur-modified polyacrylonitrile as a negative electrode active material, the lower limit charging potential of the negative electrode is lowered below 1.0 V (vs. Li/Li ⁺ ). However, the inventors have found that the deterioration of battery performance is low, safe charging and discharging is possible, and the deterioration of cycle characteristics is also low, and the present invention has been completed. That is, the present invention provides a lithium ion secondary battery having a negative electrode using sulfur-modified polyacrylonitrile as an active material, wherein the negative electrode has a lower charging limit potential of 0.1 V (vs. Li/Li ⁺ ) or more, and 1. It is a lithium ion secondary battery that is less than 0 V (vs. Li/Li ⁺ ).
The present invention also provides a method for operating a lithium ion secondary battery having a negative electrode using sulfur-modified polyacrylonitrile as an active material, wherein the lower limit charging potential of the negative electrode is 0.1 V (vs. Li/Li ⁺ ) or more, A method for operating a lithium-ion secondary battery is provided, wherein the voltage is less than 1.0 V (vs. Li/Li ⁺ ).

One of the characteristics of the present invention is that the lower limit charging 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 ⁺ ). have In the present invention, the unit V (vs. Li/Li ⁺ ) is the potential relative to lithium metal. Although it is preferable that the lower limit charge potential of the negative electrode is low, if it is lower than 0.1 V (vs. Li/Li ⁺ ), the cycle characteristics of the battery are significantly deteriorated. The lower charging limit potential of the negative electrode in the present invention is 0.15 V (vs. Li/Li ⁺ ) or more, preferably 0.9 V (vs. Li/Li ⁺ ) or less, and 0.17 V (vs. Li/Li ⁺ ). Above, 0.85 V (vs. Li/Li ⁺ ) or less is more preferable, and 0.25 V (vs. Li/Li ⁺ ) or more and 0.8 V (vs. Li/Li ⁺ ) or less is particularly preferable. In order to set the lower charge potential of the negative electrode within the above range, for example, the ratio of the active material weight per unit area (mg/cm ² ) of the positive electrode and the negative electrode can be adjusted by changing the electrode coating thickness. good. As the ratio of the weight of the active material per unit area of the negative electrode to that of the positive electrode decreases, the lower limit charging potential of the negative electrode decreases.

The charge-discharge capacity of the negative electrode using sulfur-modified polyacrylonitrile as an active material varies slightly depending on the sulfur content of the sulfur-modified polyacrylonitrile, but when the lower limit charging potential is 1.0 V (vs. Li/Li ⁺ ), it is 0. .5 V (vs. Li/Li ⁺ ) increases by about 30 to 50%, and 0.2 V (vs. Li/Li ⁺ ) increases by about 60 to 80%, and the amount of sulfur-modified polyacrylonitrile used is equal to the increase. can be reduced. By lowering the lower limit charging potential and reducing the active material of the negative electrode, it becomes possible to reduce the weight and size of the lithium ion secondary battery. In addition, a lithium-ion secondary battery designed with a lower charging potential of less than 1.0 V (vs. Li/Li ⁺ ) should be used with a lower charging potential of 1.0 V (vs. Li/Li ⁺ ) or higher. is not preferable because sufficient charge/discharge capacity cannot be obtained.

Sulfur-modified polyacrylonitrile is a compound obtained by heat-treating polyacrylonitrile and elemental sulfur in a non-oxidizing atmosphere. Polyacrylonitrile may be a copolymer of acrylonitrile with other monomers such as acrylic acid, vinyl acetate, N-vinylformamide, N—N' methylenebis(acrylamide). However, if the acrylonitrile content is low, the battery performance will be low, so in the case of copolymers of acrylonitrile and other monomers, the acrylonitrile content in the copolymer is preferably 90% by mass or more. The sulfur content of the sulfur-modified polyacrylonitrile is preferably 25% by mass to 60% by mass, and 27% by mass to 50% by mass, because a large charge-discharge capacity is obtained and the deterioration of cycle characteristics in the operating method of the present invention is small. More preferably, 30% by mass to 45% by mass is most preferable. The sulfur content of the sulfur-modified polyacrylonitrile may be calculated by performing elemental analysis using, for example, a CHN analyzer capable of analyzing sulfur and oxygen.

The ratio of polyacrylonitrile and elemental sulfur in the heat treatment is preferably 100 parts by mass to 1500 parts by mass, more preferably 150 parts by mass to 1000 parts by mass, per 100 parts by mass of polyacrylonitrile. The temperature of the heat treatment is preferably 250°C to 550°C, more preferably 350°C to 450°C. Since unreacted elemental sulfur causes deterioration of the cycle characteristics of the secondary battery, it is preferably removed by, for example, heating or solvent washing.

The sulfur-modified polyacrylonitrile of the present invention preferably has an average particle size of 0.5 μm to 100 μm depending on the application, for example, when used as an electrode active material for electrodes of secondary batteries. The average particle size refers to a 50% particle size measured by a laser diffraction light scattering method. The particle size is a volume-based diameter, and the diameter of secondary particles is measured by a laser diffraction light scattering method. In order to reduce the average particle size of the sulfur-modified polyacrylonitrile of the present invention to less than 0.5 μm, a large amount of labor is required for pulverization, etc., but further improvement in battery performance cannot be expected. It is easy to obtain an electrode mixture layer. The average particle size of the sulfur-modified polyacrylonitrile of the present invention is preferably 0.5 μm to 100 μm, more preferably 1 μm to 50 μm, even more preferably 2 μm to 30 μm.

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

As the binder, one known as an electrode binder can be used, and examples thereof include styrene-butadiene rubber, butadiene rubber, polyethylene, polypropylene, polyamide, polyamideimide, polyimide, polyacrylonitrile, polyurethane, polyvinylidene fluoride, and polytetrafluoroethylene. , ethylene-propylene-diene rubber, fluorine rubber, styrene-acrylate copolymer, ethylene-vinyl alcohol copolymer, acrylonitrile-butadiene rubber, styrene-isoprene rubber, polymethyl methacrylate, polyacrylate, polyvinyl alcohol, polyvinyl ether, carboxy Methyl cellulose, carboxymethyl cellulose sodium, methyl cellulose, cellulose nanofiber, polyethylene oxide, starch, polyvinylpyrrolidone, polyvinyl chloride, polyacrylic acid and the like.

As the binder, a water-based binder is preferable, and styrene-butadiene rubber, sodium carboxymethylcellulose, and polyacrylic acid are more preferable, since they have a low environmental load and are less likely to cause sulfur elution. Only one type of binder can be used, or two or more types can be used in combination. The content of the binder in the slurry is preferably 1 part by mass to 30 parts by mass, more preferably 1.5 parts by mass to 20 parts by mass, relative to 100 parts by mass of the sulfur-modified polyacrylonitrile of the present invention.

As the conductive aid, one known as a conductive aid for electrodes can be used. Specific examples include natural graphite, artificial graphite, carbon black, ketjen black, acetylene black, channel black, furnace black, and lamp black. , Thermal black, carbon nanotube, vapor grown carbon fiber (VGCF), exfoliated graphite, graphene, fullerene, carbon materials such as needle coke; metal powder such as aluminum powder, nickel powder, titanium powder; oxidation conductive metal oxides such _as zinc and _titanium oxide; and sulfides such as _La2S3 , _{Sm2S3 , Ce2S3} and _TiS2 . As for the particle size of the conductive aid, the average particle size is preferably 0.0001 μm to 100 μm, more preferably 0.01 μm to 50 μm. 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, more preferably 2 parts by mass with respect to 100 parts by mass of the sulfur-modified polyacrylonitrile of the present invention. parts by mass to 20 parts by mass.

Solvents for preparing the slurry include, for example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl 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, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N,N-dimethylamino Propylamine, polyethylene oxide, tetrahydrofuran, dimethylsulfoxide, sulfolane, γ-butyrolactone, water, alcohol and the like. The amount of the solvent used can be adjusted according to the slurry coating method. For example, in the case of the doctor blade method, 20 parts by mass per 100 parts by mass of the total amount of sulfur-modified polyacrylonitrile, binder and conductive aid. 300 parts by mass is preferable, and 30 parts by mass to 200 parts by mass is more preferable.

The slurry may also contain other ingredients. Other components include, for example, viscosity modifiers, reinforcing agents, antioxidants, and the like.

The method for preparing the slurry is not particularly limited, but examples include ordinary ball mills, sand mills, bead mills, pigment dispersers, crushers, ultrasonic dispersers, homogenizers, rotation/revolution mixers, planetary mixers, film mixes, jet A paste or the like can be used.

Conductive materials such as titanium, titanium alloys, aluminum, aluminum alloys, copper, nickel, stainless steel, and nickel-plated steel are used as current collectors. The surfaces of these conductive materials are sometimes coated with carbon. Examples of the shape of the current collector include a foil shape, a plate shape, and a mesh shape. Among these, aluminum, copper, and stainless steel are preferred from the viewpoint of conductivity and price, and the shape is preferably foil-like. When in foil form, 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 includes a die coater method, a comma coater method, a curtain coater method, a spray coater method, a gravure coater method, a flexo coater method, a knife coater method, a doctor blade method, and a reverse roll. method, brush coating method, dipping method, and the like can be used. A die coater method, a doctor blade method, and a knife coater method are preferable because they make it possible to obtain a good surface condition of the coating layer according to physical properties such as viscosity and drying property of the slurry. The coating may be applied to one side or both sides of the current collector, and when both sides of the current collector are to be coated, the coating can be applied one by one successively or can be applied to both sides at the same time. In addition, it can be applied continuously or intermittently on the surface of the current collector, and can be applied in a stripe pattern. The thickness of the electrode mixture layer is generally 1-500 μm, preferably 1-300 μm, more preferably 1-150 μm. The thickness, length and width of the coating layer can be appropriately determined according to the size of the battery.

The method for drying the slurry applied on the current collector is not particularly limited, and includes drying with warm air, hot air, low humidity air, vacuum drying, standing still in a heating furnace, etc., far infrared rays, infrared rays, electron beams, etc. Each technique such as irradiation can be used. By this drying, volatile components such as a solvent volatilize from the coating film of the slurry, and an electrode mixture layer is formed on the current collector. After this, the electrodes may be pressed if necessary. The method of press treatment includes, for example, a mold press method and a roll press method.

When the sulfur-modified polyacrylonitrile in the electrode mixture layer of the negative electrode is set to a certain amount or more, it is easy to obtain a sufficient charge-discharge capacity, and the sulfur-modified polyacrylonitrile is set to a certain amount or less to improve conductivity and adhesion with the current collector. easy enough. From these points, the content of the sulfur-modified 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. Most preferably, it is between weight percent and 98 weight percent.

A lithium ion secondary battery comprises a negative electrode using sulfur-modified polyacrylonitrile as an active material, a positive electrode, and a non-aqueous electrolyte, and optionally has a separator between the positive electrode and the negative electrode. For the positive electrode, an electrode having a known active material as a positive electrode active material may be used.

Examples of known positive electrode active materials include lithium-transition metal composite oxides, lithium-containing transition metal phosphate compounds, lithium-containing silicate compounds, and the like. Preferred transition metals for the lithium-transition metal composite oxide include vanadium, titanium, chromium, manganese, iron, cobalt, nickel, and copper. Specific examples of lithium transition metal composite oxides include lithium cobalt composite oxides such as LiCoO ₂ , lithium nickel composite oxides such as LiNiO ₂ , and lithium manganese composite oxides such as LiMnO ₂ , LiMn ₂ O ₄ and Li ₂ MnO ₃ . Some of the transition metal atoms that are the main constituent of these lithium-transition metal composite oxides are aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, lithium, nickel, copper, zinc, magnesium, gallium, zirconium, etc. Examples include those substituted with other metals.置換されたものの具体例としては、例えば、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 _{0.5O2 , LiNi0.80Co0.17Al0.03O2 , LiNi0.80Co0.15Al0.05O2 , LiNi1 / 3Co1 / 3Mn1 / 3O2 , LiNi0.6Co0.2Mn0.2O2 , LiMn1.8 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. As the transition metal of the lithium-containing transition metal phosphate compound _, vanadium, titanium, manganese, iron _{, cobalt} , _nickel and the like are preferable. Acidic iron compounds, cobalt phosphate compounds such as _LiCoPO4 , and some of the transition metal atoms that are the main component of these lithium transition metal phosphate compounds are aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, lithium, Examples thereof include nickel, copper, zinc _, magnesium, gallium, zirconium, niobium and other metal-substituted compounds, and vanadium phosphate compounds such as _Li3V2 ( _PO4 ) ₃ . Lithium-containing silicate compounds include Li ₂ FeSiO ₄ and the like. These may be used alone or in combination of two or more.

As the configuration of the positive electrode and the manufacturing method thereof, in the above-described suitable configuration of the electrode using the sulfur-modified polyacrylonitrile as the electrode active material and the preferable manufacturing method thereof, the sulfur-modified polyacrylonitrile is replaced with the known positive electrode active material. There are other things.

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 an electrolyte in an organic solvent and gelled with a polymer, and an electrolyte that does not contain an organic solvent and is a polymer. Dispersed pure polymer electrolytes, borohydride compounds, inorganic solid electrolytes and the like can be used.

As electrolytes used for liquid electrolytes and polymer gel electrolytes, conventionally known lithium salts are used, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiN(CF ₃ SO ₂ ) ₂ , LiN( _{C2F5SO2 ) 2} , LiN( _SO2F ) _{2 ,} LiC( _{CF3SO2 ) 3} , LiB( _{CF3SO3 ) 4} , LiB _{( C2O4} ) 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, among others. , _{LiPF6 , LiBF4} , _LiClO4 , _LiAsF6 , _LiCF3SO3 , LiN _{( CF3SO2 ) 2} , LiN( _C2F5SO2 ) ₂ , _LiN ( _SO2F ) ₂ , and LiC(CF ₃ SO ₂ ) ₃ , LiCF ₃ SO ₃ derivatives, and LiC(CF ₃ SO ₂ ) ₃ derivatives are preferably used. The electrolyte content in the liquid electrolyte and polymer gel electrolyte is preferably 0.5 to 7 mol/L, more preferably 0.8 to 1.8 mol/L.

Examples of electrolytes used for pure polymer electrolytes include LiN( _{CF3SO2 ) 2 ,} LiN( _C2F5SO2 ) ₂ , LiN( _{SO2F ) 2} , LiC( _{CF3SO2 ) 3} , LiB _{( CF3SO3} ) ₄ , LiB _{( C2O4} ) ₂ . Borohydride compounds include LiBH ₄ -LiI and LiBH ₄ -P ₂ S ₅ .

As an inorganic solid electrolyte, Li1 _{+ xAxB2 -y} ( _PO4 ) ₃ (x=Al, Ge, Sn, Hf, Zr, Sc, Y, B=Ti, Ge, Zn, 0<x< 0.5), _{LiMPO4 (} M=Mn _{, Fe, Co, Ni), phosphoric acid materials such as Li3PO4; Li3XO4 ( X} =As, _V ), Li3 _{+ xAxB1 -xO4} (A=Si, Ge, Ti, B=P, As, V, 0<x _< 0.6), Li4 _{+ xAxSi1 - xO4} (A=B, Al, Ga , Cr, Fe, 0<x<0.4) ₍ A=Ni, Co, 0 _< x<0.1), _{Li4-3yAlySiO4} (0<y<0.06), _{Li4- 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 (LiTiO ₂ , LiTi ₂ O ₄ , Li ₄ Lithium composite oxides such _{as TiO4} , _Li2TiO3 , _Li2Ti3O7 , _Li4Ti5O12 ) _; compounds containing lithium and halogen such _as LiBr _{, LiF} , LiCl, _LiPF6 and _LiBF4 ; LiPON , LiN( _{SO2CF3 ) 2} , LiN( _SO2C2F5 ) _{2 , Li3N} , _{LiN ( SO2C3F7 ) 2 and} other compounds containing lithium and nitrogen _{; La0.55Li0.35TiO3} crystals having _{a perovskite} structure with lithium ion conductivity such as _; crystals having _{a garnet structure} such as _{Li7 - La3Zr2O13} ; _{50Li4SiO4.50Li3BO3} , _{90Li3BO3.10Li2SO} Glass such as ₄ ; Lithium phosphorous sulfide such as 70Li ₂ S.30P ₂ S ₅ , 75Li 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 30Li2S.26B2S3.44LiI , 63Li2S.36SiS2.1Li3PO4 , 57Li2S.38SiS2.5Li4SiO4 , 70Li2S.50GeS2 , 50Li2S _ _ _ _ _} Lithium _{- phosphorus} sulfide _{- based glass such as} 50GeS2; _{Li7P3S11 , Li3.25P0.95S4 , Li10GeP2S12 , Li9.6P3S12} , _Li9.54Si1 and glass ceramics such as _.74 P _1.44 S _11.7 Cl _0.3 .

As the organic solvent used in the preparation of the liquid non-aqueous electrolyte used in the present invention, one or a combination of two or more of those commonly used in liquid non-aqueous electrolytes can be used. Specific examples include 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, and the like. .

Among the organic solvents, saturated cyclic carbonate compounds, saturated cyclic ester compounds, sulfoxide compounds, sulfone compounds, and amide compounds have high dielectric constants, and thus play a role in increasing the dielectric constant of the non-aqueous electrolyte. is preferred. Examples of such saturated cyclic carbonate compounds include ethylene carbonate, 1,2-propylene carbonate, 1,3-propylene carbonate, 1,2-butylene carbonate, 1,3-butylene carbonate, 1,1-dimethylethylene carbonate, and the like. are mentioned. Examples of the saturated cyclic ester compounds include γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-hexanolactone, δ-octanolactone and the like. Examples of the sulfoxide compounds include dimethylsulfoxide, diethylsulfoxide, dipropylsulfoxide, diphenylsulfoxide, thiophene, and the like. Examples of the sulfone compounds include dimethylsulfone, diethylsulfone, dipropylsulfone, diphenylsulfone, sulfolane (also referred to as tetramethylenesulfone), 3-methylsulfolane, 3,4-dimethylsulfolane, and 3,4-diphenylmethylsulfolane. , sulfolene, 3-methylsulfolene, 3-ethylsulfolene, 3-bromomethylsulfolene and the like, preferably sulfolane and tetramethylsulfolane. Examples of the amide compound include N-methylpyrrolidone, dimethylformamide, dimethylacetamide and the like.

Among the organic solvents, the saturated chain carbonate compound, the chain ether compound, the cyclic ether compound, and the saturated chain ester compound can lower the viscosity of the non-aqueous electrolyte and increase the mobility of electrolyte ions. It is possible to improve battery characteristics such as output density. Moreover, since the viscosity is low, the performance of the non-aqueous electrolyte can be enhanced at low temperatures, and a saturated chain carbonate compound is particularly preferred. Examples of saturated chain carbonate compounds include dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, ethylbutyl carbonate, methyl-t-butyl carbonate, diisopropyl carbonate, t-butylpropyl carbonate and the like. 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, and among these, dioxolane is preferred.

As the saturated chain ester compound, monoester compounds and diester compounds having a total number of carbon atoms in the molecule of 2 to 8 are preferable. Specific compounds include, for example, methyl formate, ethyl formate, methyl acetate, and acetic acid. ethyl, propyl acetate, 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, methyl 3-methoxypropionate, 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, acetonitrile, propionitrile, nitromethane, derivatives thereof, and various ionic liquids can also be used as the organic solvent used for preparing the non-aqueous electrolyte.

Polymers used for polymer gel electrolytes include polyethylene oxide, polypropylene oxide, polyvinyl chloride, polyacrylonitrile, polymethyl methacrylate, polyethylene, polyvinylidene fluoride, polyhexafluoropropylene, and the like. Polymers used for pure polymer electrolytes include polyethylene oxide, polypropylene oxide, and polystyrene sulfonic acid. The mixing ratio in the gel electrolyte and the method of compositing are not particularly limited, and a known mixing ratio and a known compositing method in the technical field may be adopted.

The non-aqueous electrolyte may contain other known additives such as electrode film-forming agents, antioxidants, flame retardants, overcharge inhibitors, etc., for the purpose of improving battery life and safety. When other additives are used, they are usually 0.01 to 10 parts by mass, preferably 0.1 to 5 parts by mass, based on the total non-aqueous electrolyte.

By applying the lithium ion secondary battery and operating method of the present invention, the charge/discharge capacity of the negative electrode using sulfur-modified polyacrylonitrile as an active material can be increased, and the amount of sulfur-modified polyacrylonitrile used can be reduced. Furthermore, the battery voltage can be improved, and the weight and size of the lithium ion secondary battery can be reduced.

The present invention will be described in more detail below with reference to examples and comparative examples. However, the present invention is not limited in any way by the following examples and the like. "Parts" and "%" in the examples are by mass unless otherwise specified.

[Production Example 1]
10 parts by mass of polyacrylonitrile powder (manufactured by Sigma-Aldrich, average particle size 200 μm) and 30 parts by mass of sulfur powder (manufactured by Sigma-Aldrich, average particle size 200 μm) were mixed using a mortar. For the sulfur modification of polyacrylonitrile, a reactor according to the examples of JP-A-2013-054957 was used. This mixture is placed in a cylindrical glass bottle similar to that described in the example of JP 2013-054957 A, the lower part of the glass bottle is placed in a crucible type electric furnace, and hydrogen sulfide generated under a nitrogen stream is removed. Heated at 400° C. for 1 hour. After cooling, the product was placed in a glass tube oven and heated at 250° C. for 3 hours under vacuum to remove elemental sulfur. The resulting sulfur-modified polyacrylonitrile was pulverized using a mortar to an average particle size of 10 μm to obtain sulfur-modified polyacrylonitrile powder PANS1. The sulfur content of PANS1 is 38% by weight.

[Production Example 2]
A sulfur-modified polyacrylonitrile powder PANS2 was obtained in the same manner as in Production Example 1, except that the temperature for removing elemental sulfur was changed from 250°C to 200°C. The sulfur content of PANS2 is 55% by weight. The conditions for removing elemental sulfur in Production Example 2 are the same as those described in Patent Document 3, and the sulfur content of PANS2 is considered to be equivalent to that of sulfur-modified polyacrylonitrile described in Patent Document 3.

[Examples 1 to 6, Comparative Examples 1 to 4]
[Production of negative electrodes 1 to 10]
As an electrode active material, 92.0 parts by mass of PANS1 or PANS2 (see Table 1), 3.5 parts by mass of acetylene black (manufactured by Denki Kagaku Kogyo) as a conductive agent, 1.5 parts by mass of carbon nanotubes (Showa Denko (product name: VGCF), 1.5 parts by mass of styrene-butadiene rubber (40% by mass aqueous dispersion, manufactured by Nippon Zeon) as a binder, 1.5 parts by mass of carboxymethylcellulose sodium (manufactured by Daicel Finechem), and 100 parts by mass. A portion of the water was dispersed using a rotation/revolution mixer to prepare a slurry. This slurry composition was applied to a current collector made of stainless steel foil (thickness 10 μm) by a doctor blade method so that the thickness of the electrode mixture layer would be the value shown in Table 1 below, and dried at 90° C. for 3 hours. After that, this electrode was cut into a predetermined size and vacuum-dried at 120° C. for 2 hours to prepare a disk-shaped electrode.

[Manufacturing of positive electrode]
90 parts by mass of LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (manufactured by Nippon Kagaku Sangyo, hereinafter abbreviated as NCM) as a positive electrode active material, and 5 parts by mass of acetylene black (Denki Kagaku Kogyo Co., Ltd.) as a conductive aid. ), 5 parts by mass of polyvinylidene fluoride (manufactured by Kureha) as a binder, and 100 parts by mass of N-methylpyrrolidone as a solvent were dispersed using a rotation/revolution mixer to prepare a slurry. This slurry composition was applied to an aluminum foil (thickness: 20 μm) 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 and vacuum-dried at 120° C. for 2 hours to prepare a disk-shaped positive electrode. The positive electrode capacity was set so that the upper limit charge potential of the positive electrode was 4.2 V (vs. Li/Li ⁺ ) regardless of the lower limit charge potential of the negative electrode.

[Preparation of non-aqueous electrolyte]
An electrolyte solution was prepared by dissolving LiPF ₆ at a concentration of 1.0 mol/L in a mixed solvent consisting of 50% by volume of ethylene carbonate and 50% by volume of diethyl carbonate.

[Battery assembly]
Negative electrodes 1 to 10 using PANS 1 or 2 as an active material and a positive electrode using NCM as an active material were held in a case with a separator (manufactured by Celgard, trade name: Celgard 2325) sandwiched therebetween. After that, the previously prepared non-aqueous electrolyte is injected into the case, and the case is hermetically sealed to obtain Batteries 1 to 10 which are non-aqueous electrolyte secondary batteries (φ20 mm, 3.2 mm thick coin type). made. In addition, in order to confirm the lower potential of the negative electrodes 1 to 10 during the initial charge, a triode cell kit (manufactured by Toyo System Co., Ltd.) was used to prepare the negative electrodes 1 to 10, the NCM positive electrode, the glass filter separator, the lithium metal reference electrode, and the , a three-electrode cell composed of the previously prepared non-aqueous electrolyte was fabricated.

(Charging and discharging test method)
The three-electrode cell is placed in a constant temperature bath at 25° C. and charged at a charge rate of 0.1 C until the potential of the NCM positive electrode reaches 4.2 V (vs. Li/Li ⁺ ). The values in Table 2 were confirmed.
In addition, the non-aqueous electrolyte secondary battery was placed in a constant temperature bath at 25° C., charged at a charge rate of 0.1 C until the lower limit of charge potential of the negative electrode reached the value shown in Table 2, and discharged at a discharge rate of 0.1 C so that the battery voltage was 0. 50 charge/discharge cycles discharging to 0.8V were performed. Table 2 shows the discharge capacity per PANS negative electrode weight after the 10th charge/discharge cycle and the ratio of the discharge capacity after the 50th charge/discharge cycle to the discharge capacity per PANS negative electrode weight after the 10th charge/discharge cycle. A larger value of this ratio indicates better cycle characteristics.

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

According to the present invention, in a lithium ion secondary battery having a negative electrode using sulfur-modified polyacrylonitrile as an active material, the lower limit charging potential of the negative electrode is 0.1 V (vs. Li/Li ⁺ ) or more and 1.0 V ( vs. Li/Li ⁺ ), the charge/discharge capacity and battery voltage per active material increase, and the amount of active material used can be reduced. This makes it possible to reduce the weight and size of the lithium-ion secondary battery.

Claims (8)

A lithium ion secondary battery having a negative electrode using sulfur-modified polyacrylonitrile as an active material, wherein the lower limit charging potential of the negative electrode is 0.17 V (vs. Li/Li ⁺ ) or more and 1.0 V (vs. Li/Li ⁺ ), a lithium ion secondary battery. The charge lower limit potential of the negative electrode is 0.2 V (vs. Li/Li ⁺ ) above, the lithium ion secondary battery according to claim 1 . The charge lower limit potential of the negative electrode is 0.85 (vs. Li/Li ⁺ ) The lithium ion secondary battery according to claim 1 or 2, wherein: The lithium ion secondary battery according to any one of claims 1 to 3, wherein the sulfur content of the sulfur-modified polyacrylonitrile is 25% by mass to 60% by mass. A method for operating a lithium ion secondary battery having a negative electrode using sulfur-modified polyacrylonitrile as an active material, wherein the lower limit charging potential of the negative electrode is 0.17 V (vs. Li/Li ⁺ ) or more and 1.0 V (vs. Li/Li ⁺ ), a method of operating a lithium ion secondary battery. The charge lower limit potential of the negative electrode is 0.2 V (vs. Li/Li ⁺ ). The charge lower limit potential of the negative electrode is 0.85 (vs. Li/Li ⁺ ) The method for operating a lithium ion secondary battery according to claim 5 or 6, wherein: The method for operating a lithium ion secondary battery according to any one of claims 5 to 7, wherein the sulfur content of the sulfur-modified polyacrylonitrile is 25% by mass to 60% by mass.