JP2016018732A - Nonaqueous electrolyte and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte which makes possible to obtain a secondary battery superior in capacity-keeping rate.SOLUTION: According to an embodiment of the present invention, a nonaqueous electrolyte comprises an azetidine-based compound expressed by a predetermined formula. It is inferred that the addition of the azetidine-based compound working as a coat-forming agent to the nonaqueous electrolyte allows a stable coat film to be formed on a negative electrode in initial charge and discharge. The coat film enables the suppression of the reductive decomposition of a component (especially a nonaqueous electrolytic solvent) in the nonaqueous electrolyte and the increase in capacity-keeping rate.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a non-aqueous electrolyte and a lithium ion secondary battery.

  Lithium ion secondary batteries are attracting attention as power sources for mobile phones and notebook computers because they can achieve high energy density. Recently, lithium ion secondary batteries are also attracting attention as motor-driven power sources for hybrid vehicles (HEV) due to improved output characteristics and long-term reliability such as storage characteristics.

  In a lithium ion secondary battery, a film (SEI film) is formed on the surface of the negative electrode in the non-aqueous electrolyte for the purpose of suppressing the decomposition reaction of components (eg, non-aqueous electrolytic solvent) contained in the non-aqueous electrolyte on the negative electrode surface. ) Is known to add additives. The film forming agent uses an electrochemical reaction in the initial charge / discharge process to form a film on the surface of the negative electrode, and this film can improve the basic characteristics and reliability of the secondary battery. However, since the film forming agent is not consumed at all in the initial charge / discharge process, it remains in the non-aqueous electrolyte even after the film is formed. Therefore, a new film may be formed by repeated charge and discharge during use, and the internal resistance of the negative electrode may increase.

  As a solution to such a problem, Patent Document 1 discloses a method of adding vinylene carbonate to a nonaqueous electrolytic solution. Patent Document 2 discloses a method of adding a sultone-based compound to a non-aqueous electrolyte.

Japanese Patent Laid-Open No. 5-13088 JP 2000-3724 A

  However, in recent years, demands for improving the cycle characteristics of lithium ion secondary batteries are increasing.

  Accordingly, an object of the present invention is to provide a non-aqueous electrolyte that can provide a secondary battery having an excellent capacity retention rate.

  One embodiment of the present invention is a nonaqueous electrolytic solution characterized by including an azetidine compound represented by the following formula (1).

[In Formula (1), X represents a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group, a substituted or unsubstituted alkenyl group, or a substituted or unsubstituted alkynyl group. R ¹ and R ² are each independently a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted cycloalkyl group, or substituted or unsubstituted An unsubstituted aryl group. R ¹ and R ² may be bonded via a single bond or an ether bond. ].

  One embodiment of the present invention is a lithium ion secondary battery including the nonaqueous electrolyte solution.

  According to one embodiment of the present invention, it is possible to provide a nonaqueous electrolytic solution that can provide a secondary battery having an excellent capacity retention rate. According to one embodiment of the present invention, a lithium ion secondary battery having an excellent capacity maintenance rate can be provided.

It is typical sectional drawing which shows the structural example of a lithium ion secondary battery.

  Hereinafter, examples of the electrode of the present invention and a secondary battery in which this electrode can be used will be described for each component.

[1] Negative electrode <Negative electrode active material layer>
The negative electrode is formed, for example, by binding a negative electrode active material to a negative electrode current collector with a negative electrode binder. As long as the negative electrode active material in this embodiment can occlude and release lithium, any material can be used as long as the effects of the present invention are not significantly impaired.

  The negative electrode active material is not particularly limited as long as it is a material capable of occluding and releasing lithium ions. For example, a known negative electrode active material can be arbitrarily used. For example, carbonaceous materials such as coke, acetylene black, mesophase micro beads, and graphite; lithium metal; lithium alloys such as lithium-silicon and lithium-tin, lithium titanate, and the like can be used. Among these, it is preferable to use a carbonaceous material in that the cycle characteristics and safety are good and the continuous charge characteristics are also excellent. In addition, a negative electrode active material may be used individually by 1 type, and may be used in combination of 2 or more type.

Furthermore, the average particle diameter (D ₅₀ ) of the negative electrode active material is not particularly limited, but is preferably 1 μm or more from the viewpoint of battery characteristics such as initial efficiency, rate characteristics, cycle characteristics, More preferably, it is 15 μm or more, preferably 50 μm or less, and more preferably 30 μm or less. In addition, for example, those obtained by coating the above carbonaceous material with an organic substance such as pitch and then firing, those obtained by forming amorphous carbon on the surface using the CVD method, etc. It can be suitably used as a quality material. Here, organic substances used for coating include coal tar pitch from soft pitch to hard pitch; coal heavy oil such as dry distillation liquefied oil; straight heavy oil such as atmospheric residual oil and vacuum residual oil; crude oil And petroleum heavy oils such as cracked heavy oil (for example, ethylene heavy end) produced as a by-product during thermal decomposition of naphtha and the like. Moreover, what grind | pulverized the solid residue obtained by distilling these heavy oils at 200-400 degreeC to 1-100 micrometers can also be used. Furthermore, a vinyl chloride resin, a phenol resin, an imide resin, etc. can also be used. For example, the negative electrode active material layer can be formed into a sheet electrode by roll molding the negative electrode active material described above, or a pellet electrode by compression molding. Usually, as in the case of the positive electrode active material layer, The negative electrode active material, the binder, and, if necessary, various auxiliary agents and the like can be produced by applying a coating solution obtained by slurrying with a solvent onto a current collector and drying it. it can.

Examples of the negative electrode active material containing silicon include silicon and silicon compounds. Examples of silicon include simple silicon. Examples of the silicon compound include silicon oxide, silicate, a compound of transition metal such as nickel silicide and cobalt silicide and silicon, and the like. The silicon compound has a role of relaxing expansion and contraction due to repeated charge / discharge of the negative electrode active material itself, and is preferably used from the viewpoint of charge / discharge cycle characteristics. Furthermore, depending on the type of silicon compound, it also has a role of ensuring conduction between silicons. From this point of view, silicon oxide is preferably used as the silicon compound. The silicon oxide is not particularly limited, but is represented by, for example, SiO _x (0 <x <2). The silicon oxide may contain Li, and the silicon oxide containing Li is represented by, for example, SiLi _y O _z (y> 0, 2>z> 0). Further, the silicon oxide may contain a trace amount of a metal element or a nonmetal element. The silicon oxide can contain, for example, 0.1 to 5% by mass of one or more elements selected from nitrogen, boron and sulfur. By containing a trace amount of a metal element or a nonmetal element, the electrical conductivity of the silicon oxide can be improved. Further, the silicon oxide may be crystalline or amorphous. The negative electrode active material preferably contains a carbon material capable of inserting and extracting lithium ions in addition to silicon or silicon oxide. The carbon material can also be contained in a composite state with silicon or silicon oxide. Similar to silicon oxide, the carbon material has the role of relaxing expansion and contraction due to repeated charge and discharge of the negative electrode active material itself and ensuring conduction between silicon as the negative electrode active material. Therefore, better cycle characteristics can be obtained by the coexistence of silicon, silicon oxide, and carbon material.

  As the carbon material, for example, graphite, amorphous carbon, diamond-like carbon, carbon nanotube, or a composite thereof can be used. Here, graphite with high crystallinity has high electrical conductivity, and is excellent in adhesiveness and voltage flatness with a positive electrode current collector made of a metal such as copper. On the other hand, since amorphous carbon having low crystallinity has a relatively small volume expansion, it has a high effect of relaxing the volume expansion of the entire negative electrode, and deterioration due to non-uniformity such as crystal grain boundaries and defects hardly occurs. The content of the carbon material in the negative electrode active material is preferably 2% by mass or more and 50% by mass or less, and more preferably 2% by mass or more and 30% by mass or less.

  As a method for producing a negative electrode active material containing silicon and a silicon compound, when silicon oxide is used as the silicon compound, for example, a method of mixing simple silicon and silicon oxide and sintering under high temperature and reduced pressure Is mentioned. Further, when a compound of transition metal and silicon is used as the silicon compound, for example, a method of mixing and melting simple silicon and the transition metal, and a method of coating the transition metal on the surface of the simple silicon by vapor deposition or the like can be mentioned. .

  In addition to the manufacturing method described above, the compounding with carbon can be combined. For example, a method of introducing a mixed sintered product of simple silicon and silicon compound into a gas atmosphere of an organic compound in a high temperature non-oxygen atmosphere, or a mixed sintered product of single silicon and silicon oxide and carbon in a high temperature non-oxygen atmosphere. By the method of mixing the precursor resins, a coating layer made of carbon can be formed around the cores of simple silicon and silicon oxide. Thereby, the suppression of volume expansion with respect to charging / discharging and the further improvement effect of cycling characteristics are acquired.

  When silicon is used as the negative electrode active material in the present embodiment, the negative electrode active material is preferably composed of a composite containing silicon, silicon oxide, and a carbon material (hereinafter also referred to as Si / SiO / C composite). Furthermore, it is preferable that all or part of the silicon oxide has an amorphous structure. The silicon oxide having an amorphous structure can suppress the volume expansion of a carbon material or silicon which is another negative electrode active material. Although this mechanism is not clear, it is presumed that the formation of a film on the interface between the carbon material and the electrolytic solution has some influence due to the amorphous structure of silicon oxide. The amorphous structure is considered to have relatively few elements due to non-uniformity such as crystal grain boundaries and defects. Note that it can be confirmed by X-ray diffraction measurement (general XRD measurement) that all or part of silicon oxide has an amorphous structure. Specifically, when silicon oxide does not have an amorphous structure, a peak peculiar to silicon oxide is observed, but when all or part of silicon oxide has an amorphous structure, silicon oxide is observed. A unique peak is observed as a broad peak.

  In the Si / SiO / C composite, it is preferable that all or part of silicon is dispersed in silicon oxide. By dispersing at least a part of silicon in silicon oxide, volume expansion as a whole of the negative electrode can be further suppressed, and decomposition of the electrolytic solution can also be suppressed. Note that all or part of silicon is dispersed in the silicon oxide because transmission electron microscope observation (general TEM observation) and energy dispersive X-ray spectroscopy measurement (general EDX measurement). It can confirm by using together. Specifically, the cross section of the sample is observed, the oxygen concentration of the silicon portion dispersed in the silicon oxide is measured, and it can be confirmed that the sample is not an oxide.

  In the Si / SiO / C composite, for example, all or part of silicon oxide has an amorphous structure, and all or part of silicon is dispersed in silicon oxide. Such a Si / SiO / C composite can be produced, for example, by a method disclosed in Patent Document 3 (Japanese Patent Laid-Open No. 2004-47404). That is, the Si / SiO / C composite can be obtained, for example, by performing a CVD process on silicon oxide in an atmosphere containing an organic gas such as methane gas. The Si / SiO / C composite obtained by such a method has a form in which the surface of particles made of silicon oxide containing silicon is coated with carbon. Silicon is nanoclustered in silicon oxide.

  In the Si / SiO / C composite, the ratio of silicon, silicon oxide and carbon material is not particularly limited. Silicon is preferably 5% by mass or more and 90% by mass or less, and more preferably 20% by mass or more and 50% by mass or less with respect to the Si / SiO / C composite. The silicon oxide is preferably 5% by mass or more and 90% by mass or less, and more preferably 40% by mass or more and 70% by mass or less with respect to the Si / SiO / C composite. The carbon material is preferably 2% by mass or more and 50% by mass or less, and more preferably 2% by mass or more and 30% by mass or less with respect to the Si / SiO / C composite.

  Further, the Si / SiO / C composite can be composed of a mixture of simple silicon, silicon oxide and carbon material, and can also be produced by mixing simple silicon, silicon oxide and carbon material by mechanical milling. it can. For example, the Si / SiO / C composite can be obtained by mixing particulate silicon, silicon oxide and carbon materials. For example, the average particle diameter of simple silicon can be made smaller than the average particle diameter of the carbon material and the average particle diameter of the silicon oxide. In this way, simple silicon with a small volume change during charge / discharge has a relatively small particle size, and carbon materials and silicon oxides with a large volume change have a relatively large particle size. Is more effectively suppressed.

  Moreover, the average particle diameter of single-piece | unit silicon can be 20 micrometers or less, for example, and it is preferable to set it as 15 micrometers or less. The average particle diameter of silicon oxide is preferably 1/2 or less of the average particle diameter of the carbon material, and the average particle diameter of simple silicon is 1/2 or less of the average particle diameter of silicon oxide. preferable. Furthermore, it is more preferable that the average particle diameter of the silicon oxide is 1/2 or less of the average particle diameter of the carbon material, and the average particle diameter of the simple silicon is 1/2 or less of the average particle diameter of the silicon oxide. . By controlling the average particle diameter in such a range, the effect of relaxing the volume expansion can be obtained more effectively, and a secondary battery excellent in the balance of energy density, cycle life and efficiency can be obtained. More specifically, it is preferable that the average particle diameter of silicon oxide is ½ or less of the average particle diameter of graphite, and the average particle diameter of simple silicon is ½ or less of the average particle diameter of silicon oxide. . More specifically, the average particle diameter of the single silicon can be, for example, 20 μm or less, and is preferably 15 μm or less. Moreover, you may use what processed the surface of the above-mentioned Si / SiO / C composite_body | complex with a silane coupling agent as a negative electrode active material.

<Binder for negative electrode>
The binder for the negative electrode is not particularly limited. For example, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, Polypropylene, polyethylene, polyimide, polyamideimide, various polyurethanes and the like can be used. Among these, polyimide and polyamideimide are preferable because of their high binding properties. An aqueous binder can also be used. The aqueous binder is not particularly limited, but usually, a water-dispersible polymer is used in the form of a latex or an emulsion. For example, an acrylic resin emulsion, a styrene resin emulsion, a vinyl acetate polymer emulsion, a urethane resin emulsion, or the like can be used. Among these, a water-dispersible synthetic rubber latex or emulsion is preferable from the viewpoint of viscoelastic properties. Examples of the water-dispersible synthetic rubber latex (emulsion) include polybutadiene rubber latex, styrene-butadiene rubber latex, acrylonitrile-butadiene rubber latex, (meth) acrylic acid ester-butadiene rubber latex, chloroprene rubber latex, and the like. Styrene-butadiene rubber latex (SBR latex) is preferable from the viewpoint of resistance to the electrolytic solution. These binders can be used alone or in combination of two or more. The amount of the binder for the negative electrode to be used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoints of “sufficient binding force” and “high energy” which are in a trade-off relationship. .

<Thickener for negative electrode>
In order to facilitate the production of the negative electrode slurry, a thickener can also be used. Such thickeners include carboxymethylcellulose (including alkali neutralized lithium, sodium and potassium salts), polyethylene oxide, polypropylene oxide, hydroxyethylcellulose, hydroxypropylcellulose, carboxymethylhydroxyethylcellulose, polyvinyl alcohol. , Polyacrylamide, hydroxyethyl polyacrylate, ammonium polyacrylate, polyacrylic acid (including lithium salt, sodium salt and potassium salt neutralized with alkali). These thickeners can be used alone or in combination of two or more. As a ratio of the thickener in a negative electrode active material layer, 0.1-5 mass% is preferable.

<Surfactant for negative electrode>
When water is used as the dispersion solvent, a nonionic surfactant can also be used for the purpose of improving the dispersibility of the carbon particles in the slurry. Although it does not specifically limit as a nonionic surfactant, Polyoxyalkylene alkyl ether can be used preferably. The polyoxyalkylene alkyl ether is represented by the general formula: R—O— (AO) _n H (wherein R represents an alkyl group, A represents an alkylene group, and n represents a natural number). Here, the carbon number of the alkyl group represented by the symbol R, the carbon number of the alkylene group represented by the symbol A, and the degree of polymerization of the alkyleneoxy group (AO) represented by n are not particularly limited. The polyoxyalkylene alkyl ether is different in at least one of the number of carbon atoms of the alkyl group represented by the symbol R, the number of carbon atoms of the alkylene group represented by the symbol A, and the degree of polymerization of the alkyleneoxy group (AO) represented by the symbol n. A mixture of polyoxyalkylene alkyl ethers may be used.

<Negative electrode current collector>
As a material of the current collector for the negative electrode, a known material can be arbitrarily used. For example, a metal material such as copper, nickel, or SUS is used. Among these, copper is particularly preferable from the viewpoint of ease of processing and cost. The current collector is preferably subjected to a roughening treatment in advance. Furthermore, the shape of the current collector is also arbitrary, and examples thereof include a foil shape, a flat plate shape, and a mesh shape. Also, a perforated current collector such as expanded metal or punching metal can be used. Moreover, the preferable thickness and shape when using a thin film as the current collector are also arbitrary.

<Method for producing negative electrode>
For example, the negative electrode is produced by forming a negative electrode active material layer containing a negative electrode active material, a negative electrode binder, a negative electrode thickener, a surfactant, and the like on a negative electrode current collector. Can do. Examples of the method for forming the negative electrode active material layer include a doctor blade method, a die coater method, a CVD method, and a sputtering method. After forming a negative electrode active material layer in advance, a thin film of aluminum, nickel, or an alloy thereof may be formed by a method such as vapor deposition or sputtering to form a negative electrode current collector. In particular, a method of preparing a slurry by mixing a negative electrode active material, a negative electrode binder, a negative electrode thickener, a surfactant and the like in a dispersion solvent, applying the slurry onto a current collector, and then drying by heating is inexpensive. It is preferable because it can be manufactured. The heating and drying temperature is preferably 50 ° C. or higher and 140 ° C. or lower, more preferably 80 ° C. or higher and 120 ° C. or lower. The dispersion solvent is preferably NMP or water.

[2] Positive electrode <Positive electrode active material layer>
The positive electrode active material layer includes a positive electrode active material, and has a structure in which the positive electrode active material is bound on the positive electrode current collector by a positive electrode binder. The positive electrode active material releases lithium ions into the electrolyte during charging and occludes lithium from the electrolyte during discharging, and has a layered structure such as LiMnO ₂ and LixMn ₂ O ₄ (0 <x <2). Lithium manganate having a spinel structure; LiCoO ₂ , LiNiO ₂ , or a part of these transition metals replaced with another metal; LiNi _1/3 Co _1/3 Mn _1/3 O _And lithium transition metal oxides in which the number of specific transition metals such as ₂ does not exceed half; those lithium transition metal oxides in which Li is excessive in comparison with the stoichiometric composition. In particular, Li _α Ni _β Co _γ Al _δ O ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.7, γ ≦ 0.2) or Li _α Ni _β Co _γ Mn _δ O ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.6, γ ≦ 0.2) are preferable. A positive electrode active material can be used individually by 1 type or in combination of 2 or more types.

  A conductive auxiliary material may be added to the positive electrode active material layer for the purpose of reducing the impedance of the positive electrode active material. As the conductive auxiliary material, carbonaceous fine particles such as graphite, carbon black, and acetylene black can be used.

<Binder for positive electrode>
The binder for the positive electrode is not particularly limited. For example, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer. Rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide and the like can be used. Among these, polyimide, polyamideimide, polyacrylic acid (including lithium salt, sodium salt and potassium salt neutralized with alkali), carboxymethylcellulose (lithium salt neutralized with alkali) due to its strong binding properties , Sodium salts and potassium salts) are preferred. The amount of the binder for the positive electrode to be used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoints of “sufficient binding force” and “high energy” which are in a trade-off relationship. .

<Current collector for positive electrode>
The positive electrode current collector may be any material that supports the positive electrode active material layer including the positive electrode active material integrated by the binder and has conductivity that enables conduction with the external terminal. The same negative electrode current collector as described above can be used.

<Method for producing positive electrode>
The method for producing the positive electrode is not particularly limited. For example, only the surface-treated Mn-based positive electrode powder, or the surface-treated Mn-based positive electrode powder and the lithium nickel composite oxide powder, After mixing with a binder and an appropriate dispersion medium that can dissolve the binder (slurry method), apply it on a current collector such as an aluminum foil, dry the solvent, and then compress it with a press or the like. Form a film. In addition, there is no restriction | limiting in particular as a conductive support material, Usually used things, such as carbon black, acetylene black, natural graphite, artificial graphite, carbon fiber, can be used.

[3] Electrolyte Solution The nonaqueous electrolyte solution of the present embodiment contains an azetidine compound represented by the following formula (1). The azetidine-based compound is presumed to function as a film-forming additive that forms a film on the electrode. More specifically, by adding an azetidine-based compound that functions as a film forming agent to the non-aqueous electrolyte, a stable film is formed on the negative electrode during the first charge / discharge. The film suppresses reductive decomposition of components (particularly nonaqueous electrolytic solvents) in the nonaqueous electrolytic solution, thereby improving the capacity retention rate.

[In Formula (1), X represents a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group, a substituted or unsubstituted alkenyl group, or a substituted or unsubstituted alkynyl group. R ¹ and R ² are each independently a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted cycloalkyl group, or substituted or unsubstituted An unsubstituted aryl group. R ¹ and R ² may be bonded via a single bond or an ether bond. ].

  In X of Formula (1), the aryl group preferably has 6 to 18 carbon atoms, more preferably 6 to 12 carbon atoms, and still more preferably 6 to 10 carbon atoms. The number of carbon atoms of the heterocyclic group is preferably 2 to 12, more preferably 2 to 10, and further preferably 2 to 8. The hetero atom of the heterocyclic group is preferably a nitrogen atom, an oxygen atom or a sulfur atom. The alkenyl group preferably has 2 to 6 carbon atoms, more preferably 2 to 5 carbon atoms, and still more preferably 2 to 4 carbon atoms. The alkynyl group preferably has 2 to 6 carbon atoms, more preferably 2 to 5 carbon atoms, and still more preferably 2 to 4 carbon atoms. The alkenyl group and alkynyl group may be linear or branched.

In R ¹ and R ² of formula (1), the alkyl group preferably has 1 to 6 carbon atoms, more preferably 1 to 4 carbon atoms, and still more preferably 1 to 3 carbon atoms. The alkenyl group preferably has 2 to 6 carbon atoms, more preferably 2 to 5 carbon atoms, and still more preferably 2 to 4 carbon atoms. The alkynyl group preferably has 2 to 6 carbon atoms, more preferably 2 to 5 carbon atoms, and still more preferably 2 to 4 carbon atoms. The alkyl group, alkenyl group and alkynyl group may be linear or branched. The cycloalkyl group preferably has 4 to 7 carbon atoms, and more preferably 5 to 6 carbon atoms. The aryl group preferably has 6 to 18 carbon atoms, more preferably 6 to 12 carbon atoms, and still more preferably 6 to 10 carbon atoms.

R ¹ and R ² may be bonded via a single bond or an ether bond to form a ring structure. As the ring structure, for example, both R ¹ and R ² are substituted or unsubstituted alkyl groups, and the alkyl groups are bonded via a single bond to form a 5-membered or 6-membered ring. The structure can be mentioned.

In the formula (1), examples of the substituent for X, R ¹ and R ² are each independently an optionally branched alkyl group having 1 to 4 carbon atoms (for example, methyl group, ethyl group, propyl group). Group, isopropyl group, butyl group), optionally substituted halogen-substituted alkyl group having 1 to 4 carbon atoms (for example, perfluoromethyl group, perfluoroethyl group), optionally branched carbon number of 1 to 4 Alkoxy groups (for example, methoxy group, ethoxy group, n-propoxy group, iso-propoxy group, n-butoxy group, tert-butoxy group), aryl groups having 6 to 10 carbon atoms (for example, phenyl group, naphthyl group) , Amino group, hydroxy group, or halogen atom (for example, fluorine atom, chlorine atom, bromine atom) and the like. When there are a plurality of substituents in one group, they may be independent from each other.

  The halogen-substituted alkyl group represents a substituted alkyl group having a structure in which at least one hydrogen atom in the unsubstituted alkyl group is substituted with a halogen atom (for example, a fluorine atom, a chlorine atom, or a bromine atom). The halogen-substituted alkyl group is preferably a fluorine-substituted alkyl group. The fluorine-substituted alkyl group represents a substituted alkyl group having a structure in which at least one hydrogen atom in the unsubstituted alkyl group is substituted with a fluorine atom.

  The heterocyclic group is preferably a thiophene group from the viewpoint of oxidation resistance.

  An azetidine compound can be synthesized by a general chemical reaction. For example, an azetidine compound can be synthesized by dehydration condensation or cyclic amidation using acid chloride and primary amine or isocyanate as raw materials.

  An azetidine type compound may be used individually by 1 type, and may be used in combination of 2 or more type.

  The content of the azetidine-based compound in the nonaqueous electrolytic solution is preferably 0.001% by mass or more and 5.0% by mass or less, and preferably 0.01% by mass or more and 2.0% by mass or less. More preferably, it is 0.01 mass% or more and 0.3 mass% or less. When the content of the azetidine-based compound is in the above range, it is easy to form a film only on the surface, and it is possible to suppress an excessive increase in the thickness of the film. As a result, an increase in internal resistance of the electrode is suppressed, ion conductivity and electron conductivity in the electrode are improved, and battery characteristics are improved.

  The azetidine-based compound is preferably a compound represented by the following formula (Az-1) or (Az-2).

  Moreover, as an azetidine type compound, the following compounds (Az-3)-(Az-9) are mentioned preferably other than a compound (Az-1) and (Az-2).

  These compounds (Az-3) to (Az-9) can be prepared by those skilled in the art by referring to the production methods and examples described below without undue trial and error, complicated experiments, and the like. It can be easily produced according to the compound (Az-1).

  Examples of the nonaqueous electrolytic solvent contained in the nonaqueous electrolytic solution include cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, γ-lactones, cyclic ethers, chain ethers, and fluorine derivatives thereof. One or more solvents selected from the group consisting of: Specific examples of the nonaqueous electrolytic solvent include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl. Chain carbonates such as methyl carbonate (EMC) and dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate, γ-lactones such as γ-butyrolactone, 1, 2 -Chain ethers such as diethoxyethane (DEE) and ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylform Amide, acetonitrile, propyl nitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl Examples include ether and N-methylpyrrolidone. These can be used individually by 1 type or in mixture of 2 or more types.

The nonaqueous electrolytic solution can contain a lithium salt as an electrolyte salt. Examples of the lithium salt include lithium imide salt, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiClO ₄ , LiAlCl ₄ , LiN (CnF _{2n + 1} SO ₂ ) (CmF _{2m + 1} SO ₂ ) (n and m are natural numbers), etc. Is mentioned. These can be used individually by 1 type or in mixture of 2 or more types. In particular, it is preferable to use LiPF ₆ or LiBF ₄ . By using these, the electrical conductivity of the lithium salt can be increased, and the cycle characteristics of the secondary battery can be further improved.

[4] Separator The separator is not particularly limited, and a porous film such as polypropylene or polyethylene or a nonwoven fabric can be used. Moreover, what laminated | stacked them can also be used as a separator.

[5] Exterior Body The exterior body is not particularly limited, and for example, a laminate film can be used. The laminate film can be appropriately selected as long as it is stable to the electrolytic solution and has a sufficient water vapor barrier property. As the laminate film, for example, a laminate film made of polypropylene, polyethylene or the like coated with aluminum, silica, or alumina can be used as the outer package. In particular, an aluminum laminate film is preferable from the viewpoint of suppressing volume expansion.

  In the case of a secondary battery using a laminate film as an exterior body, the distortion of the electrode element is greatly increased when gas is generated, compared to a secondary battery using a metal can as the exterior body. This is because the laminate film is more easily deformed by the internal pressure of the secondary battery than the metal can. Furthermore, when sealing a secondary battery using a laminate film as an exterior body, the internal pressure of the battery is usually lower than the atmospheric pressure, so there is no extra space inside, and if gas is generated, it is immediately It may lead to battery volume changes and electrode element deformation.

  The secondary battery according to the present embodiment can overcome the above problem. As a result, it is possible to provide a laminate-type lithium ion secondary battery that is inexpensive and has excellent flexibility in designing the cell capacity by changing the number of layers. A typical layer configuration of the laminate film includes a configuration in which a metal thin film layer and a heat-fusible resin layer are laminated. In addition, as a typical layer structure of a laminate film, a protective layer made of a film of polyester such as polyethylene terephthalate or nylon is further laminated on the surface of the metal thin film layer opposite to the heat fusion resin layer. The structure which was made is mentioned. When sealing the battery element, the battery element is surrounded with the heat-fusible resin layer facing each other. As the metal thin film layer, for example, a foil of Al, Ti, Ti alloy, Fe, stainless steel, Mg alloy or the like having a thickness of 10 to 100 μm is used. The resin used for the heat-fusible resin layer is not particularly limited as long as it can be heat-sealed. For example, polypropylene, polyethylene, these acid modified products, polyphenylene sulfide, polyesters such as polyethylene terephthalate, polyamide, ethylene-vinyl acetate copolymer, ethylene-methacrylic acid copolymer and ethylene-acrylic acid copolymer with metal ions. An ionomer resin bonded between molecules is used as the heat-fusible resin layer. The thickness of the heat-fusible resin layer is preferably 10 to 200 μm, more preferably 30 to 100 μm.

  In addition, examples of the exterior body include various shapes such as a square shape, a paper shape, a laminated shape, a cylindrical shape, and a coin shape. The material of the exterior body is not particularly limited, and may be selected according to the battery shape.

[6] Battery configuration The configuration of the secondary battery is not particularly limited. For example, a laminated laminate in which an electrode assembly in which a positive electrode and a negative electrode are arranged to face each other and an electrolytic solution are included in an outer package. Can be a mold. FIG. 1 is a schematic cross-sectional view showing a structure of an electrode element included in a laminated laminate type secondary battery. This electrode element is formed by alternately stacking a plurality of positive electrodes 1 and a plurality of negative electrodes 3 having a planar structure with a separator 2 interposed therebetween. The positive electrode current collector 1b of each positive electrode 1 is welded and electrically connected to each other at an end portion not covered with the positive electrode active material layer 1a, and the positive electrode terminal 4 is welded to the welded portion. The negative electrode current collector 3b included in each negative electrode 3 is welded and electrically connected to each other at an end portion not covered with the negative electrode active material layer 3a, and the negative electrode terminal 6 is welded to the welded portion. Further, the positive electrode terminal 4 is welded to the positive electrode tab 5, and the negative electrode terminal 6 is welded to the negative electrode tab 7.

  The secondary battery of the present embodiment is a laminated laminate type having an electrode assembly in which a positive electrode and a negative electrode are arranged to face each other, a non-aqueous electrolyte, and an exterior body containing the electrode assembly and the non-aqueous electrolyte. The present invention relates to a secondary battery. Further, the negative electrode includes a negative electrode active material including at least one of a metal (a) capable of being alloyed with lithium and a metal oxide (b) capable of occluding and releasing lithium ions, and a negative electrode active material including a binder for the negative electrode. It has a material layer, and a film made of the film forming agent is formed on the negative electrode active material layer.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited to these.

(Production Example 1)
[Synthesis of Azetidine Compound Az-1]
Under an argon atmosphere, 2.7 mL of isobutyryl chloride and 5.2 mL of phenyl isocyanate were dissolved in 40 mL of toluene and stirred for 5 minutes. To this mixed solution, a mixed solution of 5 mL of toluene and 4.2 mL of triethylamine was dropped, and subsequently the reaction was performed by refluxing at 135 ° C. for 8 hours under an argon atmosphere. After allowing to cool, the hydrochloride was removed with filter paper, and the resulting filtrate was distilled off under reduced pressure. The residue was separated on a silica gel column (developing solvent: chloroform) to obtain 920.4 mg of the target compound (Compound Az-1) (yield 19%).

Example 1
[Production of positive electrode]
92 parts by mass of lithium nickel manganese cobalt composite oxide, 0.03 part of oxalic acid, 4 parts by mass of carbon-based conductive agent, and 3 parts by mass of polyvinylidene fluoride were mixed, and the mixture was mixed with N-methylpyrrolidone (NMP). To prepare a positive electrode slurry. Thereafter, a positive electrode slurry was applied to an aluminum foil as a positive electrode current collector so that the thickness after drying was 80 μm and dried to prepare a positive electrode.

[Production of negative electrode]
96.7 parts by mass of graphite, 2 parts by mass of styrene butadiene rubber, 1 part by mass of sodium carboxymethylcellulose (CMC), and 0.3 parts by mass of carbon-based conductive additive are mixed, and the mixture is dispersed in water. A negative electrode slurry was prepared. Thereafter, a negative electrode slurry was applied to a copper foil as a negative electrode current collector so that the thickness after drying was 50 μm and dried to prepare a negative electrode.

[Preparation of non-aqueous electrolyte]
The content of the compound (Az-1) is 0.1% by mass in an ethylene carbonate / diethyl carbonate mixed solution (mixing ratio (volume basis) 3: 7) containing 1.0 mol / L LiPF ₆ as an electrolyte salt. Thus, a non-aqueous electrolyte was prepared.

[Production of small cells]
A negative electrode obtained by punching to a size of 23 mm × 25 mm, a polypropylene porous film separator, and a positive electrode obtained by punching to a size of 22 mm × 24 mm were laminated in this order to obtain an electrode assembly. The electrode assembly was wrapped with an aluminum laminate film, and the nonaqueous electrolyte was injected into the film, followed by vacuum sealing to produce a sheet-like lithium ion secondary battery.

[Evaluation]
<Cycle test>
About the produced lithium ion secondary battery, charging / discharging was repeated 120 times in the voltage range of 2.5V to 4.15V in a 20 degreeC thermostat, and discharge capacity (the first time, after 120 cycles) was measured. The capacity retention rate was the ratio (%) of the discharge capacity after 120 cycles to the initial discharge capacity. The discharge capacity is a value per unit mass of the positive electrode active material. The results of the measured capacity retention ratio are shown in Table 1.

(Comparative Example 1)
A lithium ion secondary battery was prepared and evaluated in the same manner as in Example 1 except that the non-aqueous electrolyte was prepared without adding the compound (Az-1).

  As shown in Table 1, it was confirmed that the cycle characteristics of the lithium ion secondary battery can be improved by adding the azetidine-based compound.

  The present embodiment can be used in, for example, all industrial fields that require a power source and industrial fields related to transportation, storage, and supply of electrical energy. Specifically, power supplies for mobile devices such as mobile phones and notebook computers; power supplies for transportation and transportation media such as trains, satellites, and submarines, including electric vehicles such as electric cars, hybrid cars, electric bikes, and electric assist bicycles A backup power source such as a UPS; a power storage facility for storing power generated by solar power generation, wind power generation, etc .;

1a positive electrode active material layer 1b positive electrode current collector 2 separator 3a negative electrode active material layer 3b negative electrode current collector 4 positive electrode terminal 5 positive electrode tab 6 negative electrode terminal 7 negative electrode tab

Claims (8)

A nonaqueous electrolytic solution comprising an azetidine compound represented by the following formula (1);

[In Formula (1), X represents a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group, a substituted or unsubstituted alkenyl group, or a substituted or unsubstituted alkynyl group. R ¹ and R ² are each independently a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted cycloalkyl group, or substituted or unsubstituted An unsubstituted aryl group. R ¹ and R ² may be bonded via a single bond or an ether bond. ]. The non-aqueous electrolyte according to claim 1, wherein the azetidine compound is at least one selected from the following compounds (Az-1) to (Az-9);

.   The azetidine-based compound is 1-phenyl-3,3-dimethyl-2,4-azetidinedione, or 1-allyl-3,3-dimethyl-2,4-azetidinedione. Non-aqueous electrolyte.   4. The non-aqueous electrolyte solution according to claim 1, wherein a content of the azetidine compound in the non-aqueous electrolyte solution is 0.001% by mass or more and 5.0% by mass or less.   The nonaqueous electrolytic solution according to any one of claims 1 to 4, further comprising an electrolyte salt and a nonaqueous electrolytic solvent.   A lithium ion secondary battery comprising the nonaqueous electrolytic solution according to any one of claims 1 to 5.   The lithium ion secondary battery according to claim 6, comprising: an electrode assembly in which a positive electrode and a negative electrode are arranged to face each other via a separator; and an outer package that encloses the electrode assembly and the non-aqueous electrolyte.   The lithium ion secondary battery according to claim 6 or 7, wherein the outer package is a laminate film.

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