JP2018156751A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having a silicon material used therein, which is superior in output under a low-temperature condition.SOLUTION: A lithium ion secondary battery comprises: a positive electrode; a negative electrode with a silicon material, having a structure in which a plurality of platy silicon bodies are laminated in a thickness direction; and an electrolyte solution containing LiPOF.SELECTED DRAWING: None

Description

  The present invention relates to a lithium ion secondary battery.

  Generally, a lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution as main components. As the negative electrode active material provided in the negative electrode, graphite is generally used. As negative electrode active materials other than graphite, negative electrode active materials containing Si having a high lithium ion storage capability are known.

  As a lithium ion secondary battery including a negative electrode active material containing Si, for example, Patent Document 1 and Patent Document 2 describe a lithium ion secondary battery in which the negative electrode active material is silicon. Patent Literature 3 and Patent Literature 4 describe lithium ion secondary batteries in which the negative electrode active material is SiO.

In Patent Document 5, a layered silicon compound mainly composed of layered polysilane obtained by reacting CaSi ₂ with an acid to remove Ca is synthesized, and the layered silicon compound is heated at 300 ° C. or more to release hydrogen. A material is manufactured and a lithium ion secondary battery including the silicon material as a negative electrode active material is described.

JP 2014-203595 A Japanese Patent Laying-Open No. 2015-57767 JP 2015-185509 A JP-A-2015-179625 International Publication No. 2014/080608

  The present inventor made an earnest study after actually manufacturing a lithium ion secondary battery having a silicon material described in Patent Document 5 as a negative electrode active material. As a result, improvement in terms of output under low temperature conditions was achieved. I found that there was room.

  This invention is made | formed in view of this situation, and it aims at providing the lithium ion secondary battery which comprises the silicon material described in patent document 5 which is excellent in the output under low temperature conditions.

As a result of intensive studies, the present inventor has found that the output of the lithium ion secondary battery including the silicon material under a low temperature condition is improved by using an electrolyte solution to which LiPO ₂ F ₂ is added. . Based on this discovery, the present inventor has completed the present invention.

The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode comprising a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction, an electrolyte containing LiPO ₂ F ₂ , It is characterized by providing.

  The lithium ion secondary battery of the present invention is excellent in output under low temperature conditions.

  Below, the form for implementing this invention is demonstrated. Unless otherwise specified, the numerical range “ab” described herein includes the lower limit “a” and the upper limit “b”. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from these numerical ranges can be used as new upper and lower numerical values.

The lithium ion secondary battery of the present invention includes a positive electrode, a negative electrode comprising a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction, an electrolyte containing LiPO ₂ F ₂ , It is characterized by providing.

  The silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction is the silicon material described in Patent Document 5, and as a negative electrode active material in the lithium ion secondary battery of the present invention. Function. Hereinafter, the silicon material may be simply referred to as “silicon material”.

The silicon material has a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction. In order for lithium ions to efficiently occlude and release, the plate-like silicon body preferably has a thickness in the range of 10 nm to 100 nm, more preferably in the range of 20 nm to 50 nm. The length in the longitudinal direction of the plate-like silicon body is preferably within a range of 0.1 μm to 50 μm. The plate-like silicon body preferably has (length in the longitudinal direction) / (thickness) in the range of 2 to 1000. The laminated structure of the plate-like silicon body can be confirmed by observation with a scanning electron microscope or the like. Moreover, this laminated structure is considered to be a remnant of the Si layer in CaSi _{2 which} is a raw material of silicon material.

  The silicon material preferably includes amorphous silicon and / or silicon crystallites. In particular, the above plate-like silicon body is preferably in a state where amorphous silicon is used as a matrix and silicon crystallites are scattered in the matrix. The size of the silicon crystallite is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, still more preferably in the range of 1 nm to 50 nm, and particularly preferably in the range of 1 nm to 10 nm. The size of the silicon crystallite is calculated from the Scherrer equation using X-ray diffraction measurement on the silicon material and using the half-value width of the diffraction peak of the Si (111) plane of the obtained X-ray diffraction chart. .

For example, the silicon material is subjected to a process of synthesizing a layered silicon compound containing polysilane as a main component by reacting CaSi ₂ with an acid, and further a process of heating the layered silicon compound at 300 ° C. or higher to desorb hydrogen. It is manufactured.

The method for producing a silicon material described in Patent Document 5 is represented by the following ideal reaction formula when hydrogen chloride is used as an acid.
3CaSi ₂ + 6HCl → Si ₆ H ₆ + 3CaCl ₂
Si ₆ H ₆ → 6Si + 3H ₂ ↑

However, in the upper reaction for synthesizing Si ₆ H ₆ which is polysilane, water is usually used as a reaction solvent from the viewpoint of removing by-products and impurities. Since Si ₆ H ₆ can react with water, in the step of synthesizing the layered silicon compound including the upper reaction, the layered silicon compound is hardly manufactured as containing only Si ₆ H ₆ , and the layered The silicon compound is represented by Si ₆ H _s (OH) _t X _u (X is an element or group derived from an acid anion, s + t + u = 6, 0 <s <6, 0 <t <6, 0 <u <6). Manufactured as a product. In the above chemical formula, inevitable impurities such as Ca that can remain are not taken into consideration. A silicon material obtained by heating the layered silicon compound also contains an element derived from an anion of oxygen or acid.

  The abundance and size of the plate-like silicon body, amorphous silicon and silicon crystallite contained in the silicon material mainly depend on the heating temperature and the heating time. The heating temperature is preferably in the range of 350 ° C to 950 ° C, more preferably in the range of 400 ° C to 900 ° C.

  The silicon material may be coated with carbon. A silicon material coated with carbon is excellent in conductivity.

  The average particle diameter of the silicon material is preferably in the range of 0.6 to 30 μm, more preferably in the range of 1 to 20 μm, further preferably in the range of 2 to 10 μm, and particularly preferably in the range of 3 to 8 μm. In the present specification, the average particle diameter means D50 when a sample is measured with a general laser diffraction particle size distribution analyzer.

  As the negative electrode active material in the negative electrode, only a silicon material may be used, or a known negative electrode active material such as graphite and a silicon material may be used in combination.

  The negative electrode has a current collector and a negative electrode active material layer bound to the surface of the current collector.

  The current collector refers to a chemically inert electronic conductor that keeps a current flowing through an electrode during discharge or charging of a lithium ion secondary battery. As the current collector, at least one selected from silver, copper, gold, aluminum, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel, etc. Metal materials can be exemplified. The current collector may be covered with a known protective layer. What collected the surface of the electrical power collector by the well-known method may be used as an electrical power collector.

  The current collector can take the form of a foil, a sheet, a film, a linear shape, a rod shape, a mesh, or the like. Therefore, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, and a stainless steel foil can be suitably used as the current collector. When the current collector is in the form of foil, sheet or film, the thickness is preferably in the range of 1 μm to 100 μm.

  The negative electrode active material layer includes a negative electrode active material and, if necessary, a binder and / or a conductive aid.

  The binder plays a role of tying an active material, a conductive aid, and the like to the surface of the current collector.

  Binders include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene, and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, alkoxysilyl group-containing resins, and styrene butadiene. What is necessary is just to employ | adopt well-known things, such as rubber | gum.

  Moreover, you may employ | adopt the polymer which has a hydrophilic group as a binder. The lithium ion secondary battery of the present invention comprising a polymer having a hydrophilic group as a binder can maintain the capacity more suitably. Examples of the hydrophilic group of the polymer having a hydrophilic group include a phosphate group such as a carboxyl group, a sulfo group, a silanol group, an amino group, a hydroxyl group, and a phosphate group. Among them, a polymer containing a carboxyl group in a molecule such as polyacrylic acid, carboxymethylcellulose, or polymethacrylic acid, or a polymer containing a sulfo group such as poly (p-styrenesulfonic acid) is preferable.

  A polymer containing many carboxyl groups and / or sulfo groups such as polyacrylic acid or a copolymer of acrylic acid and vinyl sulfonic acid becomes water-soluble. The polymer having a hydrophilic group is preferably a water-soluble polymer, and in terms of chemical structure, a polymer containing a plurality of carboxyl groups and / or sulfo groups in one molecule is preferable.

  The polymer containing a carboxyl group in the molecule can be produced by, for example, a method of polymerizing an acid monomer or a method of imparting a carboxyl group to the polymer. Acid monomers include acrylic acid, methacrylic acid, vinyl benzoic acid, crotonic acid, pentenoic acid, angelic acid, tiglic acid, etc., acid monomers having one carboxyl group in the molecule, itaconic acid, mesaconic acid, citraconic acid, fumaric acid , Maleic acid, 2-pentenedioic acid, methylene succinic acid, allyl malonic acid, isopropylidene succinic acid, 2,4-hexadiene diacid, acetylenedicarboxylic acid, etc., acid monomers having two or more carboxyl groups in the molecule Is done.

  A copolymer obtained by polymerizing two or more acid monomers selected from the above acid monomers may be used as a binder.

  Further, for example, a polymer containing an acid anhydride group formed by condensation of carboxyl groups of a copolymer of acrylic acid and itaconic acid, as described in JP 2013-065493 A, in the molecule It is also preferable to use as a binder. When the binder has a structure derived from a highly acidic monomer having two or more carboxyl groups in one molecule, it becomes easier for the binder to trap lithium ions etc. before the electrolyte decomposition reaction occurs during charging. It is considered. Furthermore, since the polymer has more carboxyl groups per monomer than polyacrylic acid or polymethacrylic acid, the acidity is increased, but since the predetermined amount of carboxyl groups is changed to acid anhydride groups, the acidity is increased. Is not too high. Therefore, a secondary battery including a negative electrode using the polymer as a binder has improved initial efficiency and improved input / output characteristics.

  Further, a crosslinked polymer obtained by crosslinking a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid with diamine may be used as a binder.

  Examples of the diamine used in the crosslinked polymer include alkylene diamines such as ethylene diamine, propylene diamine, and hexamethylene diamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophorone diamine, and bis (4-aminocyclohexyl) methane. Saturated carbocyclic diamine, m-phenylenediamine, p-phenylenediamine, 4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenyl ether, bis (4-aminophenyl) sulfone, benzidine, o-tolidine, 2,4- Aromatic diamines such as tolylenediamine, 2,6-tolylenediamine, xylylenediamine, and naphthalenediamine are exemplified.

  Further, a mixture or a reaction product of a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid and polyamideimide may be used as a binder.

  Polyamideimide means a compound having two or more amide bonds and imide bonds in the molecule. Polyamideimide is produced by reacting an acid component that becomes a carbonyl moiety in an amide bond and an imide bond with a diamine component or diisocyanate component that becomes a nitrogen moiety in the amide bond and imide bond. In order to obtain a polyamideimide, it may be produced by the method, or a commercially available polyamideimide may be purchased.

  Acid components used for the production of polyamideimide include trimellitic acid, pyromellitic acid, biphenyltetracarboxylic acid, diphenylsulfonetetracarboxylic acid, benzophenonetetracarboxylic acid, diphenylethertetracarboxylic acid, oxalic acid, adipic acid, malonic acid, Sebacic acid, azelaic acid, dodecanedicarboxylic acid, dicarboxypolybutadiene, dicarboxypoly (acrylonitrile-butadiene), dicarboxypoly (styrene-butadiene), cyclohexane-1,4-dicarboxylic acid, cyclohexane-1,3-dicarboxylic acid, Dicyclohexylmethane-4,4′-dicarboxylic acid, terephthalic acid, isophthalic acid, bis (carboxyphenyl) sulfone, bis (carboxyphenyl) ether, naphthalenedicarboxylic acid, and These anhydrides, acid halides, mention may be made of derivatives. As the acid component, the above compounds may be employed singly or in combination, but from the point of forming an imide bond, an acid component in which a carboxyl group is present on the carbon adjacent to the carbon to which the carboxyl group is bonded or Its equivalent is essential. As the acid component, trimellitic anhydride is preferable from the viewpoints of reactivity and heat resistance. In addition to trimellitic acid anhydride, pyromellitic acid anhydride, benzophenonetetracarboxylic acid anhydride, biphenyltetra, as part of the acid component in addition to trimellitic acid anhydride, from the viewpoint of tensile strength, tensile modulus, and electrolyte resistance of polyamideimide It is preferable to employ a carboxylic acid anhydride.

  What is necessary is just to employ | adopt the diamine used for the crosslinked polymer mentioned above as a diamine component used for manufacture of a polyamideimide. From the viewpoint of heat resistance and solubility, 4,4'-diaminodiphenylmethane, 2,4-tolylenediamine, o-tolidine, naphthalenediamine, and isophoronediamine are preferable. From the viewpoint of tensile strength and tensile modulus of polyamideimide, o-tolidine and naphthalenediamine are preferable.

  Examples of the diisocyanate component used for producing the polyamideimide include those obtained by replacing the amine of the diamine component with an isocyanate.

  The blending ratio of the binder in the negative electrode active material layer is preferably a mass ratio of negative electrode active material: binder = 1: 0.005 to 1: 0.3. This is because when the amount of the binder is too small, the moldability of the electrode is lowered, and when the amount of the binder is too large, the energy density of the electrode is lowered.

  The conductive assistant is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be added arbitrarily when the electrode conductivity is insufficient, and may not be added when the electrode conductivity is sufficiently excellent. The conductive auxiliary agent may be any chemically inert electronic high conductor such as carbon black, graphite, vapor grown carbon fiber (VGCF), and various metal particles. Illustrated. Examples of carbon black include acetylene black, ketjen black (registered trademark), furnace black, and channel black. These conductive assistants can be added to the active material layer alone or in combination of two or more.

  The blending ratio of the conductive additive in the negative electrode active material layer is preferably a negative electrode active material: conductive additive = 1: 0.01 to 1: 0.5 in mass ratio. This is because if the amount of the conductive aid is too small, an efficient conductive path cannot be formed, and if the amount of the conductive aid is too large, the formability of the negative electrode active material layer is deteriorated and the energy density of the electrode is lowered.

  The positive electrode has a current collector and a positive electrode active material layer bound to the surface of the current collector. What was demonstrated in the negative electrode should just be employ | adopted for the collector of a positive electrode suitably. The positive electrode active material layer includes a positive electrode active material and, if necessary, a binder and / or a conductive aid. What was demonstrated with the negative electrode should just be employ | adopted for the binder and the conductive support agent of a positive electrode by the same compounding ratio.

As the positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c Mn} d D e O f (0.2 ≦ a ≦ 2, b + c + d + e = 1,0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, At least one element selected from Sc, Sn, In, Y, Bi, S, Si, Na, K, P, and V, 1.7 ≦ f ≦ 3), a lithium mixed metal oxide represented by Li ₂ MnO ₃ can be mentioned. Further, as a positive electrode active material, a spinel structure metal oxide such as LiMn ₂ O ₄ , a solid solution composed of a mixture of a spinel structure metal oxide and a layered compound, LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (wherein M is selected from at least one of Co, Ni, Mn, and Fe). Furthermore, as the positive electrode active material, tavorite compound (the M a transition metal) LiMPO ₄ F, such as LiFePO ₄ F represented by, Limbo ₃ such LiFeBO ₃ (M is a transition metal) include borate-based compound represented by be able to. Any metal oxide used as the positive electrode active material may have the above composition formula as a basic composition, and a metal element contained in the basic composition may be substituted with another metal element. Moreover, you may use as a positive electrode active material the thing which does not contain the lithium ion used as a charge carrier. For example, sulfur alone, compounds in which sulfur and carbon are compounded, metal sulfides such as TiS ₂ , oxides such as V ₂ O ₅ and MnO ₂ , compounds containing polyaniline and anthraquinone, and aromatics in their chemical structures, conjugated two Conjugated materials such as acetic acid organic materials and other known materials can also be used. Further, a compound having a stable radical such as nitroxide, nitronyl nitroxide, galvinoxyl, phenoxyl, etc. may be adopted as the positive electrode active material. When using a positive electrode active material that does not include a charge carrier, it is necessary to add the charge carrier to the positive electrode and / or the negative electrode in advance by a known method. The charge carrier may be added in an ionic state or in a non-ionic state such as a metal. For example, it may be integrated by attaching lithium foil to the positive electrode and / or the negative electrode.

From the viewpoint of excellent and high capacity, and durability, as the positive electrode active material, the general formula of the layered rock salt _{structure: Li a Ni b Co c Mn} d D e O f (0.2 ≦ a ≦ 2, b + c + d + e = 1,0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, At least one element selected from Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V, 1.7 ≦ f ≦ 3) It is preferable to employ a lithium composite metal oxide.

  In the above general formula, the values of b, c, and d are not particularly limited as long as the above conditions are satisfied, but those in which 0 <b <1, 0 <c <1, 0 <d <1 are satisfied. And at least one of b, c, and d is in the range of 10/100 <b <90/100, 10/100 <c <90/100, 5/100 <d <70/100. More preferably, the ranges are 20/100 <b <80/100, 12/100 <c <70/100, 10/100 <d <60/100, 30/100 <b <70/100, More preferably, the ranges are 15/100 <c <50/100 and 12/100 <d <50/100.

  About a, e, and f, what is necessary is just a numerical value within the range prescribed | regulated by the said general formula, Preferably 0.5 <= a <= 1.5, 0 <= e <0.2, 1.8 <= f <= 2 0.5, more preferably 0.8 ≦ a ≦ 1.3, 0 ≦ e <0.1, 1.9 ≦ f ≦ 2.1, respectively.

Specific positive electrode active _{material, Li x A y Mn 2-} y O 4 (A spinel structure, Ca, Mg, S, Si , Na, K, Al, P, Ga, at least one selected from Ge Examples include at least one metal element selected from an element and a transition metal element, 0 <x ≦ 2.2, 0 ≦ y ≦ 1). More specifically, LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ can be exemplified.

Specific examples of the positive electrode active material include LiFePO ₄ , Li ₂ FeSiO ₄ , LiCoPO ₄ , Li ₂ CoPO ₄ , Li ₂ MnPO ₄ , Li ₂ MnSiO ₄ , and Li ₂ CoPO ₄ F. As another specific positive electrode active material, Li ₂ MnO ₃ —LiCoO ₂ can be exemplified.

One type of positive electrode active material may be used, or a plurality of types may be used in combination. As a combination example of the positive electrode active material, the general formula of the layered rock salt structure: Li _a Ni _b Co _c Mn _{d De} O _f (0.2 ≦ a ≦ 2, b + c + d + e = 1, 0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, At least one element selected from Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V, and a lithium composite metal oxide represented by 1.7 ≦ f ≦ 3); A combined use with a polyanionic compound represented by LiMPO ₄ , LiMVO ₄ or Li ₂ MSiO ₄ (wherein M is selected from at least one of Co, Ni, Mn, and Fe) can be given. In the combined use of the lithium composite metal oxide and the polyanion compound, these mass ratios are preferably in the range of lithium composite metal oxide: polyanion compound = 90: 10 to 50:50, and 80:20 to 60:40. The range of is more preferable. The positive electrode active material may be coated with carbon.

  In order to form an active material layer on the surface of the current collector, a current collecting method such as a roll coating method, a die coating method, a dip coating method, a doctor blade method, a spray coating method, or a curtain coating method can be used. An active material may be applied to the surface of the body. Specifically, an active material, a solvent, and if necessary, a binder and a conductive additive are mixed to form a slurry, and the slurry is applied to the surface of the current collector and then dried. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone, and water. In order to increase the electrode density, the dried product may be compressed.

Next, the electrolytic solution will be described. The electrolytic solution provided in the lithium ion secondary battery of the present invention contains LiPO ₂ F ₂ which is a lithium salt. Due to the presence of LiPO ₂ F ₂ , the lithium ion secondary battery of the present invention is excellent in output under low temperature conditions. The proportion of LiPO ₂ F ₂ to the total electrolyte, 0.1 to 5 mass%, 0.2 to 4 wt%, 0.3 to 3 wt%, the range of 0.4 to 2 wt% can be exemplified .

The electrolytic solution provided in the lithium ion secondary battery of the present invention may be an electrolytic solution containing LiPO ₂ F ₂ and other lithium salts. Any other lithium salt may be used as long as it can be used as an electrolyte of a lithium ion secondary battery. LiPF ₆ , LiBF ₄ , LiAsF ₆ , Li ₂ SiF ₆ , (FSO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, FSO ₂ (CF ₃ SO ₂ ) NLi, (SO ₂ CF ₂ CF ₂ SO ₂ ) NLi, (SO ₂ CF ₂ CF ₂ CF ₂ SO ₂ ) NLi , FSO ₂ (CH ₃ SO ₂ ) NLi, FSO ₂ (C ₂ F ₅ SO ₂ ) NLi, or FSO ₂ (C ₂ H ₅ SO ₂ ) NLi, (OCOCO ₂ ) ₂ BLi, (OCOCO ₂ ) BF ₂ Li Can be illustrated.

Examples of the concentration of the lithium salt including LiPO ₂ F ₂ and other lithium salts in the electrolyte include ranges of 0.5 to 4 mol / l, 0.9 to 3 mol / l, and 1 to 2.5 mol / l. it can. As a density | concentration of the other lithium salt in electrolyte solution, the range of 0.5-3.5 mol / l, 0.9-2.5 mol / l, 1-1.5 mol / l can be illustrated.

  The electrolytic solution contains a nonaqueous solvent in addition to the lithium salt.

  As the non-aqueous solvent, cyclic esters, chain esters, ethers and the like can be used. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, ethyl methyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester, and acetic acid alkyl ester. Examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane.

  As the electrolytic solution, an electrolytic solution containing a fluorine-containing cyclic carbonate is preferable. The fluorine-containing cyclic carbonate means a cyclic carbonate having fluorine in the molecule. Fluorine-containing cyclic carbonate is excellent in oxidation resistance, but easily decomposes under reducing conditions. Accordingly, the fluorine-containing cyclic carbonate is preferentially decomposed at the interface between the negative electrode and the electrolytic solution under the charge / discharge conditions of the lithium ion secondary battery. As a result, a film derived from the decomposition product of the fluorine-containing cyclic carbonate is formed on the surface of the silicon material. Since such a film serves as a protective film against the silicon material, it is considered that deterioration of the silicon material is suppressed.

  The amount of the fluorine-containing cyclic carbonate with respect to the entire non-aqueous solvent contained in the electrolytic solution is preferably in the range of 0.1 to 40% by volume, more preferably in the range of 1 to 30% by volume, and 5 to 25% by volume. Within the range is more preferable.

  Specific examples of the fluorine-containing cyclic carbonate include compounds represented by the following general formula (A).

(R ¹ and R ² are each independently hydrogen, an alkyl group, a halogen-substituted alkyl group or halogen. However, at least one of each R ¹ and each R ² includes F.)

  Specific examples of the fluorine-containing cyclic carbonate represented by the general formula (A) include fluoroethylene carbonate, 4- (trifluoromethyl) -1,3-dioxolan-2-one, and 4,4-difluoro. -1,3-dioxolan-2-one, 4-fluoro-4-methyl-1,3-dioxolan-2-one, 4- (fluoromethyl) -1,3-dioxolan-2-one, 4,5- Difluoro-1,3-dioxolan-2-one, 4-fluoro-5-methyl-1,3-dioxolan-2-one, 4,5-difluoro-4,5-dimethyl-1,3-dioxolane-2- ON can be mentioned, and among these, fluoroethylene carbonate is preferred.

  Moreover, you may add a well-known additive to electrolyte solution in the range which does not deviate from the meaning of this invention. Examples of known additives include carbonate compounds represented by phenylethylene carbonate and erythritan carbonate; succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride , Cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, carboxylic anhydrides represented by phenylsuccinic anhydride; γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, δ- Caprolactone, lactone represented by ε-caprolactone; cyclic ether represented by 1,4-dioxane; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfo , Sulfur compounds represented by dimethylsulfone, tetramethylthiuram monosulfide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2- Nitrogen compounds represented by imidazolidinone and N-methylsuccinimide; phosphates represented by monofluorophosphate and difluorophosphate; saturated hydrocarbon compounds represented by heptane, octane and cycloheptane; biphenyl , Alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, unsaturated hydrocarbon compounds represented by dibenzofuran, vinylene carbonate, fluorovinylene carbonate, methyl vinylene -Unsaturated cyclic carbonates typified by boronate, fluoromethyl vinylene carbonate, ethyl vinylene carbonate, propyl vinylene carbonate, butyl vinylene carbonate, dimethyl vinylene carbonate, diethyl vinylene carbonate, dipropyl vinylene carbonate, trifluoromethyl vinylene carbonate, vinyl ethylene carbonate Can be mentioned.

  A separator is used in the lithium ion secondary battery of the present invention as necessary. The separator separates the positive electrode and the negative electrode and allows lithium ions to pass while preventing a short circuit due to contact between the two electrodes. A known separator may be employed, such as polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polyamide), polyester, polyacrylonitrile or other synthetic resin, cellulose, amylose or other polysaccharide, fibroin. And porous materials, nonwoven fabrics, woven fabrics, and the like using one or more electrical insulating materials such as natural polymers such as keratin, lignin, and suberin, and ceramics. The separator may have a multilayer structure.

  A specific method for manufacturing the lithium ion secondary battery will be described. For example, a separator is sandwiched between a positive electrode and a negative electrode to form an electrode body. The electrode body may be any of a stacked type in which a positive electrode, a separator and a negative electrode are stacked, or a wound type in which a positive electrode, a separator and a negative electrode are stacked. After connecting the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal connected to the outside using a current collecting lead or the like, an electrolyte is added to the electrode body to form a lithium ion secondary battery. Good.

  The shape of the lithium ion secondary battery of the present invention is not particularly limited, and various shapes such as a cylindrical shape, a square shape, a coin shape, and a laminate shape can be adopted.

  The lithium ion secondary battery of the present invention may be mounted on a vehicle. The vehicle may be a vehicle that uses electric energy generated by a lithium ion secondary battery for all or a part of its power source. For example, the vehicle may be an electric vehicle or a hybrid vehicle. When a lithium ion secondary battery is mounted on a vehicle, a plurality of lithium ion secondary batteries may be connected in series to form an assembled battery. Examples of devices equipped with lithium ion secondary batteries include various home appliances driven by batteries such as personal computers and portable communication devices, office devices, and industrial devices in addition to vehicles. Furthermore, the lithium ion secondary battery of the present invention includes wind power generation, solar power generation, hydroelectric power generation and other power system power storage devices and power smoothing devices, power supplies for ships and / or auxiliary power supply sources, aircraft, Power supply for spacecraft and / or auxiliary equipment, auxiliary power supply for vehicles that do not use electricity as a power source, power supply for mobile home robots, power supply for system backup, power supply for uninterruptible power supply, You may use for the electrical storage apparatus which stores temporarily the electric power required for charge in the charging station for electric vehicles.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. In addition, this invention is not limited by these Examples.

Example 1
Fluoroethylene carbonate, ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate 15:15: 30: 40 mixed solvent were mixed at a volume ratio of, was added and dissolved to prepare LiPO ₂ F ₂ and LiPF _6, and LiPO ₂ F ₂ An electrolytic solution of Example 1 was manufactured that contained 0.2% by mass and the concentration of LiPF ₆ was 1 mol / L.

69 parts by mass of Li _1.1 Ni _5/10 Co _3/10 Mn _2/10 O ₂ as a positive electrode active material, 25 parts by mass of LiFePO ₄ coated with carbon as a positive electrode active material, and 3 parts by mass of acetylene black as a conductive auxiliary agent , And 3 parts by weight of polyvinylidene fluoride as a binder were mixed to obtain a mixture. This mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone to prepare a slurry. An aluminum foil was prepared as a positive electrode current collector. The slurry was applied to the surface of the aluminum foil using a doctor blade so as to form a film. The aluminum foil coated with the slurry was dried to remove N-methyl-2-pyrrolidone. Thereafter, the aluminum foil was pressed to obtain a bonded product. The obtained joined product was heated and dried with a vacuum dryer to produce a positive electrode made of an aluminum foil on which a positive electrode active material layer was formed.

  72.5 parts by mass of a carbon-coated silicon material as a negative electrode active material, 13.5 parts by mass of acetylene black as a conductive additive, and 14 mixtures of polyacrylic acid and 4,4′-diaminodiphenylmethane as a binder A slurry was prepared by mixing parts by mass and an appropriate amount of N-methyl-2-pyrrolidone. A copper foil was prepared as a negative electrode current collector. The slurry was applied in a film form on the surface of the copper foil using a doctor blade. The copper foil coated with the slurry was dried to remove N-methyl-2-pyrrolidone to produce a negative electrode on which a negative electrode active material layer was formed. The mixture of polyacrylic acid and 4,4′-diaminodiphenylmethane used as the binder is a crosslinked polymer in which the dehydration reaction proceeds by drying and the polyacrylic acid is crosslinked with 4,4′-diaminodiphenylmethane. To change.

  A polyolefin porous membrane was prepared as a separator. A separator was sandwiched between the positive electrode and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed. Then, the electrolyte solution of Example 1 was injected into the bag-like laminated film. Thereafter, the remaining one side was sealed, whereby the four sides were hermetically sealed, and the lithium ion secondary battery of Example 1 in which the electrode plate group and the electrolyte solution were sealed was manufactured.

(Example 2)
The electrolysis of Example 2 containing LiPO ₂ F ₂ at 0.5 mass% and having a concentration of LiPF ₆ of 1 mol / L in the same manner as in Example 1 except that the amount of LiPO ₂ F ₂ added was increased. A liquid was produced. And the lithium ion secondary battery of Example 2 was manufactured by the method similar to Example 1 except having used the electrolyte solution of Example 2.

(Example 3)
The electrolytic solution of Example 3 containing 1% by mass of LiPO ₂ F _{2 and} having a concentration of LiPF ₆ of 1 mol / L was obtained in the same manner as in Example 1 except that the amount of LiPO ₂ F ₂ added was increased. Manufactured. And the lithium ion secondary battery of Example 3 was manufactured by the same method as Example 1 except having used the electrolyte solution of Example 3.

Example 4
The electrolysis of Example 4 including LiPO ₂ F ₂ at 1.8% by mass and the concentration of LiPF ₆ being 1 mol / L in the same manner as in Example 1 except that the addition amount of LiPO ₂ F ₂ was increased. A liquid was produced. And the lithium ion secondary battery of Example 4 was manufactured by the method similar to Example 1 except having used the electrolyte solution of Example 4.

(Comparative Example 1)
An electrolytic solution of Comparative Example 1 having a LiPF ₆ concentration of 1 mol / L was produced in the same manner as in Example 1 except that LiPO ₂ F ₂ was not added. And the lithium ion secondary battery of the comparative example 1 was manufactured by the method similar to Example 1 except having used the electrolyte solution of the comparative example 1.

(Comparative Example 2)
An electrolytic solution of Comparative Example 2 in which the concentration of LiPF ₆ was 1.1 mol / L was produced in the same manner as in Example 1 except that LiPO ₂ F ₂ was not added and the amount of LiPF ₆ added was increased. . And the lithium ion secondary battery of the comparative example 2 was manufactured by the method similar to Example 1 except having used the electrolyte solution of the comparative example 2.

(Comparative Example 3)
An electrolytic solution of Comparative Example 3 having a LiPF ₆ concentration of 1.2 mol / L was produced in the same manner as in Example 1, except that LiPO ₂ F ₂ was not added and the amount of LiPF ₆ added was increased. . And the lithium ion secondary battery of the comparative example 3 was manufactured by the method similar to Example 1 except having used the electrolyte solution of the comparative example 3.

(Comparative Example 4)
An electrolytic solution of Comparative Example 4 having a LiPF ₆ concentration of 1.3 mol / L was produced in the same manner as in Example 1, except that LiPO ₂ F ₂ was not added and the amount of LiPF ₆ added was increased. . And the lithium ion secondary battery of the comparative example 4 was manufactured by the method similar to Example 1 except having used the electrolyte solution of the comparative example 4.

(Comparative Example 5)
An electrolyte solution of Comparative Example 5 having a LiPF ₆ concentration of 1.4 mol / L was produced in the same manner as in Example 1 except that LiPO ₂ F ₂ was not added and the amount of LiPF ₆ added was increased. . And the lithium ion secondary battery of the comparative example 5 was manufactured by the method similar to Example 1 except having used the electrolyte solution of the comparative example 5.

(Evaluation example 1)
The lithium ion secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 5 adjusted to a voltage of 3.05 V were discharged at a constant output up to a voltage of 2.8 V under the condition at −20 ° C. The discharge time was measured. The measurement was performed under a plurality of conditions with different outputs. From the obtained results, for each lithium ion secondary battery, a constant output (W) in which the discharge time from 3.05 V to 2.8 V was 3 minutes was calculated, and normalized as Comparative Example 1 = 100. The results are shown in Table 1.

From the results in Table 1, it can be seen that the outputs of the lithium ion secondary batteries of Examples 1 to 4 at −20 ° C. are superior to the outputs of the lithium ion secondary batteries of Comparative Examples 1 to 5. . From the evaluation results of the lithium ion secondary batteries of Examples 1 to 4 and the lithium ion secondary battery of Comparative Example 1, it can be said that the output increases as the amount of LiPO ₂ F ₂ increases. On the other hand, it can be seen from the results of the lithium ion secondary batteries of Comparative Examples 1 to 5 that even if the concentration of LiPF ₆ is increased, the output does not increase, but tends to decrease. From these results, it can be said that the output of the lithium ion secondary battery under the low temperature condition at −20 ° C. is influenced by the type of the lithium salt. Further, LiPO ₂ F ₂ are said to be suitable as a lithium salt.

(Evaluation example 2)
The lithium ion secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 5 adjusted to a voltage of 3.05 V were discharged at a constant output up to a voltage of 2.8 V under the condition at 0 ° C. The case discharge time was measured. The measurement was performed under a plurality of conditions with different outputs. From the obtained results, a constant output (W) with a discharge time of 5 seconds was calculated for each lithium ion secondary battery, and normalized with Comparative Example 1 as 100. The results are shown in Table 2.

From the results of Table 2, it can be seen that the output at 0 ° C. in the examples is highest when the amount of LiPO ₂ F ₂ is around 0.5 mass%. Also, the output at 0 ℃ in the comparative example, it can be seen that the concentration of LiPF ₆ is the highest in the vicinity of 1.2 mol / L. From these results, it is presumed that the output at 0 ° C. does not depend on the type of the lithium salt and is influenced by the lithium ion concentration.

From the results of Evaluation Example 1 and Evaluation Example 2, it can be said that the lithium ion secondary battery of the present invention can smoothly release lithium ions from the silicon material even under low temperature conditions. In particular, it can be said that the output improvement effect specific to LiPO ₂ F ₂ at −20 ° C. shown in Evaluation Example 1 is remarkable. It can be said that the lithium ion secondary battery of the present invention is excellent in output even when used under a low temperature condition such as in a cold district or in a freezer. Therefore, it can be said that the lithium ion secondary battery of this invention is suitable as a battery for low temperature environments.

Claims (6)

A positive electrode;
A negative electrode comprising a silicon material having a structure in which a plurality of plate-like silicon bodies are laminated in the thickness direction;
An electrolyte containing LiPO ₂ F ₂ ;
A lithium ion secondary battery comprising: The lithium ion secondary battery according to claim 1, wherein the electrolytic solution contains LiPO ₂ F ₂ and other lithium salts. The lithium ion secondary battery according to claim 1 or 2, wherein a ratio of LiPO ₂ F _{2 to} the whole electrolyte solution is 0.1 to 5% by mass.   The lithium ion secondary battery according to any one of claims 1 to 3, wherein the electrolytic solution contains a fluorine-containing cyclic carbonate.   The lithium ion secondary battery according to any one of claims 1 to 4, wherein the negative electrode includes a negative electrode active material layer containing a crosslinked polymer obtained by crosslinking a carboxyl group-containing polymer with a diamine and the silicon material. The positive electrode has the general formula of the layered rock salt _{structure: Li a Ni b Co c Mn} d D e O f (0.2 ≦ a ≦ 2, b + c + d + e = 1,0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Al, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, At least one element selected from Sn, In, Y, Bi, S, Si, Na, K, P, and V, 1.7 ≦ f ≦ 3), lithium composite metal oxide represented by LiMPO ₄ , LiMVO ₄ or a Li ₂ MSiO ₄ (wherein M is selected from at least one of Co, Ni, Mn, and Fe) and a positive electrode active material layer. The lithium ion secondary of any one of -5 battery.