WO2013122146A1

WO2013122146A1 - Lithium secondary battery and electrolyte for lithium secondary battery

Info

Publication number: WO2013122146A1
Application number: PCT/JP2013/053537
Authority: WO
Inventors: 洋子橋詰; 井上　和彦; 信也須藤; 須黒　雅博; 緑志村
Original assignee: 日本電気株式会社
Priority date: 2012-02-17
Filing date: 2013-02-14
Publication date: 2013-08-22

Abstract

Provided are: a lithium secondary battery wherein the deterioration of a negative electrode active material is suppressed, having excellent cycle characteristics, in particular, having a long life relative to use in high-temperature environments; and an electrolyte for lithium secondary batteries used in same. The lithium secondary battery has an electrolyte that permeates a positive electrode and a negative electrode that absorb/release lithium during charging/discharging; and the negative electrode therein includes a silicon negative electrode active material. The electrolyte includes an unsaturated compound indicated by formula (1). (In formula: R₁-R₃ independently indicate a direct bond or a C1-6 alkylene group; the alkylene group may have an unsaturated double bond, an unsaturated triple bond, an oxy group, or a substituent group; and R₄ indicates a vinyl group.)

Description

Lithium secondary battery and electrolyte for lithium secondary battery

The present invention relates to a lithium secondary battery having a high capacity, particularly excellent in cycle characteristics for use in a high temperature environment, and having a long life, and an electrolyte for a lithium secondary battery used therefor.

Lithium secondary batteries are widely used in portable electronic devices and personal computers, and are required to be smaller and lighter. On the other hand, lithium secondary batteries have a high energy density that can be used in high-performance electronic devices and electric vehicles, and are charged and discharged. It is required to suppress deterioration, have excellent cycle characteristics, and have a long life. A lithium secondary battery includes a positive electrode active material layer containing a positive electrode active material formed on a positive electrode current collector and a negative electrode active material layer containing a negative electrode active material formed on a negative electrode current collector. The electrode active material reversibly stores and releases lithium ions, so that the charge / discharge cycle is performed. Is called.

As this type of negative electrode active material, from the viewpoint of high energy density, low cost, and safety, metals such as silicon and silicon oxide, tin that forms an alloy with lithium, and metal oxides are used instead of carbon-based materials. It has been. However, the negative electrode active material containing silicon has a large volume expansion / contraction due to charging / discharging, and falls off as a fine powder from the negative electrode active material layer with repeated charging / discharging, resulting in a decrease in battery capacity. In particular, when used in a high temperature environment of 45 ° C. or higher, the capacity of the battery is greatly reduced and the deterioration tends to be remarkable.

In order to suppress deterioration due to charging / discharging of the silicon-based negative electrode active material having a large volume expansion / contraction rate due to such occlusion / release of lithium, a film is formed on the negative electrode active material layer, and the negative electrode active material from the negative electrode active material layer is formed. It has been done to suppress the loss of material. However, it is difficult to form a stable coating having a uniform thickness on the silicon-based negative electrode active material that can sufficiently suppress deterioration of cycle characteristics due to use.

On the other hand, in order to improve the charge / discharge cycle characteristics of the lithium secondary battery, the cycle characteristics are improved by adding a specific substance to the electrolytic solution to be used. Specifically, in a non-aqueous electrolyte secondary battery using a negative electrode manufactured using a crystalline carbon material with high crystallinity such as graphite as an active material and a polymer carboxylic oxide as a binder, an organic solvent, In a non-aqueous electrolyte secondary battery including a negative electrode containing an amorphous oxide, an electrolyte solution containing an electrolyte salt and a specific unsaturated phosphate ester is used (Patent Documents 1 and 2). A lithium secondary battery such as a patent document 3 containing an aromatic compound such as a specific condensed ring has been reported. In addition, as a method applicable to a silicon-based negative electrode active material having a large volume expansion / shrinkage ratio due to insertion and extraction of lithium as described above, a lithium secondary having a negative electrode formed by depositing an active material thin film on a current collector A battery including a non-electrolyte containing at least one of a phosphate ester compound, a phosphite ester, and a borate ester (Patent Document 4) has been reported.

However, in order to realize a battery with a high energy density, it is necessary to increase the thickness of the electrode containing the silicon-based negative electrode active material. Even when such an electrode is used, it follows the volume change accompanying charging and discharging. Forms a uniform and stable film with flexibility, suppresses the deterioration of the negative electrode active material due to charge and discharge, and improves cycle characteristics and extends the life especially when used under high temperature environments There is a need for lithium secondary batteries that can be used.

JP2011-124039 JP2011-77029A JP-A-9-330739 JP 2002-319431 A

An object of the present invention is to provide a silicon-based negative electrode active material having a large volume expansion / shrinkage ratio associated with occlusion / release of lithium, having a flexibility capable of following a volume change accompanying charge / discharge, and having a uniform thickness and stable coating. To provide a lithium secondary battery that can suppress the deterioration of the negative electrode active material caused by charging and discharging, improve cycle characteristics, and extend the life, particularly for use in a high-temperature environment. It is in. In addition, the problem of the present invention is to form a flexible and stable coating with a uniform thickness on the negative electrode active material, to suppress the deterioration of the negative electrode active material due to charge / discharge, especially for use in a high temperature environment. An object of the present invention is to provide an electrolytic solution for a lithium secondary battery that can improve cycle characteristics and extend the life.

As a substance capable of forming a flexible and stable film capable of following a volume change accompanying charge / discharge on a silicon-based negative electrode active substance, the present inventors have added three hydroxyl groups of phosphoric acid at the end with unsaturated dioxygen. The knowledge that by adding unsaturated compounds substituted with hydrocarbon groups having a heavy bond to the electrolyte, they can be polymerized with charge and discharge to form a uniform thickness coating on the silicon-based negative electrode active material. Based on this finding, the present invention has been completed.

That is, the present invention is a lithium secondary battery having an electrolyte solution that immerses a positive electrode and a negative electrode that occlude and release lithium during charge and discharge, and the negative electrode includes a silicon-based negative electrode active material,
The electrolyte is the formula (1)

(Wherein R ₁ to R ₃ independently represent a direct bond or an alkylene group having 1 to 6 carbon atoms, and the alkylene group represents an unsaturated double bond, an unsaturated triple bond, an oxy group or a substituent. The present invention relates to a lithium secondary battery characterized by containing an unsaturated compound represented by the following formula: R ₄ may be a vinyl group.

The present invention also relates to an electrolyte for a lithium secondary battery in which a positive electrode and a negative electrode that absorb and release lithium in accordance with charge and discharge are immersed,

(Wherein R ₁ to R ₃ independently represent a direct bond or an alkylene group having 1 to 6 carbon atoms, and the alkylene group represents an unsaturated double bond, an unsaturated triple bond, an oxy group or a substituent. And R ₄ represents a vinyl group). The present invention relates to an electrolytic solution for a lithium secondary battery, characterized by comprising an unsaturated compound represented by:

The lithium secondary battery of the present invention is to form a flexible and uniform thickness coating film capable of following a volume change associated with charge / discharge on a silicon-based negative electrode active material having a large volume expansion / contraction rate due to insertion and extraction of lithium. It is possible to suppress the deterioration of the negative electrode active material due to charge / discharge, and particularly to improve the cycle characteristics and extend the life for use in a high temperature environment. In addition, the electrolyte solution for a lithium secondary battery of the present invention forms a stable film having a flexible and uniform thickness on the negative electrode active material, suppresses deterioration of the negative electrode active material due to charge / discharge, and particularly in a high temperature environment. With respect to the use of the battery below, the cycle characteristics can be improved and the life can be extended.

It is a block diagram which shows the structure of an example of the lithium secondary battery of this invention.

DESCRIPTION OF SYMBOLS 1 Negative electrode active material layer 2 Negative electrode collector 3 Negative electrode 4 Positive electrode active material layer 5 Positive electrode collector 6 Positive electrode 7 Separator 8 Exterior body 11 Lithium secondary battery

The lithium secondary battery of the present invention has a positive electrode and a negative electrode, and an electrolytic solution for immersing them. *

[Negative electrode]
The negative electrode includes a silicon-based negative electrode active material capable of reversibly occluding and releasing lithium ions during charge and discharge, and is laminated on the current collector as a negative electrode active material layer integrated with a negative electrode binder. Has a structured.

The negative electrode active material may be any material as long as it contains a silicon-based negative electrode active material. Examples of the silicon-based negative electrode active material include silicon and silicon oxide (SiOx: 0 <x ≦ 2). . Any one of these may be used, but it is preferable to include both of them. These have different lithium ion charge / discharge potentials as a negative electrode active material. Specifically, silicon has a lower lithium ion charge / discharge potential than silicon oxide. Lithium ions can be gradually released as the voltage changes, and rapid volume shrinkage of the negative electrode active material layer due to lithium ions being released at a specific potential at a time can be suppressed. Silicon oxide hardly reacts with the electrolyte and can exist stably. Specific examples include SiO and SiO ₂ .

The content of silicon in the negative electrode active material may be 100% by mass, and may be 0% by mass when silicon oxide is contained in the negative electrode active material, but is preferably 5% by mass or more and 95% by mass or less. More preferably, they are 10 mass% or more and 90 mass% or less, More preferably, they are 20 mass% or more and 50 mass% or less. In addition, the content of silicon oxide in the negative electrode active material may be 100% by mass, and may be 0% by mass when silicon is contained in the negative electrode active material, but may be 5% by mass or more and 90% by mass or less. More preferably, it is 40 mass% or more and 80 mass% or less, More preferably, it is 50 mass% or more and 70 mass% or less.

Further, the negative electrode active material may contain a metal other than silicon or a metal oxide. Examples of the metal other than silicon include metals capable of forming an alloy with lithium, and capable of releasing lithium ions from the lithium alloy during discharging and forming the lithium alloy during charging. Specific examples of the negative electrode active material include aluminum, lead, tin, indium, bismuth, silver, barium, calcium, mercury, palladium, platinum, tellurium, zinc, and lanthanum. These can select 1 type (s) or 2 or more types. Of these, tin is preferred.

Specific examples of the metal oxide as the negative electrode active material include aluminum oxide, tin oxide, indium oxide, zinc oxide, and lithium oxide, and these can be used alone or in combination of two or more. . These metal oxides are preferably used together with the above metals, and in particular, when used together with the same metal as the metal contained in the metal oxide, occlusion / release of lithium ions is performed at different potentials during charging and discharging, It is preferable to use a tin oxide together with the tin because a rapid volume change of the negative electrode active material layer can be suppressed.

These silicon oxides and metal oxides are preferably at least partially amorphous. When the silicon oxide or the metal oxide is amorphous, it is possible to suppress pulverization of the negative electrode active material layer and to suppress reaction with the electrolytic solution. In the negative electrode active material layer having amorphous silicon oxide or metal oxide, elements due to non-uniformity such as defects and crystal grain boundaries included in the crystal structure are reduced, and non-uniform volume change is suppressed. it is conceivable that.

It can be confirmed from the X-ray diffraction measurement that the silicon oxide and the metal oxide are amorphous, since the peak specific to the crystal structure observed when having a crystal structure is broad.

As the negative electrode active material, it is preferable that silicon or the above metal is dispersed in such amorphous silicon oxide or metal oxide. In particular, silicon has a large volume change with charge and discharge, and is dispersed and contained in amorphous silicon oxide, so that non-uniform volume change with charge and discharge is suppressed, and the negative electrode active material layer and the electrolyte solution are suppressed. Can be prevented. The dispersion state of silicon and metal in silicon oxide and metal oxide is determined by observing the cross section of the sample by TEM observation with a transmission electron microscope, and by dispersing the dispersed particles in the matrix by energy dispersive X-ray spectroscopy (EDX measurement). This can be confirmed by obtaining a measurement result of not containing oxygen.

Examples of the average particle diameter of silicon dispersed in amorphous silicon oxide include several nanometers to several hundred nanometers.

For the particle diameter, an average value of 100 particles measured by observation with a transmission electron microscope (TEM) can be adopted. The same method can be employed for the average particle size of the following particles.

Further, it is preferable that a carbon material is included as the negative electrode active material. Examples of the carbon material include graphite, amorphous carbon, diamond-like carbon, and carbon nanotube. Graphite with high crystallinity has high electrical conductivity and can improve the current collecting performance of the negative electrode active material layer. Amorphous carbon with low crystallinity suppresses deterioration of the negative electrode active material layer due to charge / discharge. can do. 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.

It is preferable that the silicon, silicon oxide, metal, metal oxide, and carbon material are in a particulate form because deterioration due to charging / discharging of the negative electrode active material can be suppressed. As the particulate negative electrode active material, the larger the volume change associated with charging / discharging, the smaller the diameter is preferable because the volume change of the negative electrode active material layer due to the volume change of these particles can be suppressed. Specifically, the average particle diameter of silicon oxide is smaller than the average particle diameter of the carbon material. For example, the average particle diameter of silicon oxide is preferably ½ or less of the average particle diameter of the carbon material. The average particle diameter of silicon is smaller than the average particle diameter of silicon oxide. For example, the average particle diameter of silicon is preferably ½ or less of the average particle diameter of silicon oxide. If the average particle size is controlled in such a range, particles with large volume change due to charge / discharge become small diameter, the effect of relaxing the volume change of the negative electrode active material layer is large, and the balance between energy density, cycle life and efficiency is excellent A secondary battery can be obtained. Specifically, the average particle diameter of silicon is preferably 20 μm or less, for example, because it can ensure contact with the current collector, and more preferably 15 μm or less.

In addition, from the viewpoint of suppressing the decrease in conductivity and suppressing the deterioration of the negative electrode active material due to the charge / discharge cycle, amorphous silicon oxide exists around the silicon cluster, and its surface is in the form of particles coated with carbon. May be. The thickness of the carbon film covering the surface of the silicon-based material particles is preferably 0.1 to 5 μm because it is possible to suppress deterioration of the negative electrode active material due to charge and discharge and increase conductivity. . The thickness of the carbon coating can be measured by observation with a transmission electron microscope (TEM), and an average value of measured values for 100 particles can be adopted.

As a method for producing a negative electrode active material having a carbon film in which silicon or metal is dispersed in the amorphous silicon oxide, a method described in JP-A-2004-47404 can be exemplified. Specifically, silicon oxide or metal oxide is formed by CVD in the atmosphere of organic gas such as methane gas to form amorphous silicon oxide or metal oxide around the silicon or metal nanocluster. A carbon coating can be formed around. Moreover, the method of mixing a silicon oxide, a metal oxide, silicon, a metal, and a carbon material by mechanical milling can be mentioned. The average particle diameter of the negative electrode active material having such a carbon film can be about 1 to 20 μm.

Examples of the negative electrode binder that binds the negative electrode active material include polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and styrene-butadiene copolymer. Examples thereof include polymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide and the like. These can be used alone or in combination of two or more. Among these, it is preferable that polyimide and polyamideimide are included from the viewpoint of binding force. The amount of the binder for the negative electrode used is 5 to 25 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoints of “sufficient binding force” and “higher energy” which are in a trade-off relationship It is preferable.

The current collector that supports the negative electrode active material layer in which the negative electrode active material is integrated with the negative electrode binder may be any material as long as it has conductivity that enables conduction between the negative electrode active material layer and the external terminal. In view of chemical stability, aluminum, nickel, copper, silver, or an alloy thereof is preferable. Examples of the shape include foil, flat plate, and mesh. The thickness of the current collector can be about 5 to 30 μm.

The negative electrode can be manufactured using a negative electrode active material layer material including a negative electrode active material and a negative electrode binder on a current collector. As a method for producing the negative electrode active material layer, a doctor blade method, a coating method such as a die coater method, a CVD method, a sputtering method, or the like can be employed. 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. The thickness of the negative electrode active material layer can be about 10 to 200 μm.

[Positive electrode]
The positive electrode includes a positive electrode active material capable of reversibly occluding and releasing lithium ions during charge and discharge, and the positive electrode active material is laminated on the current collector as a positive electrode active material layer integrated with a positive electrode binder. It has a structure.

The positive electrode active material releases lithium ions into the electrolytic solution during charging and occludes lithium from the electrolytic solution during discharging, and is layered such as LiMnO ₂ and Li _x Mn ₂ O ₄ (0 <x <2). Lithium manganate having a structure, or 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 /} Examples thereof include lithium transition metal oxides in which a specific transition metal such as ₃ O ₂ does not exceed half; those lithium transition metal oxides in which Li is more excessive than the stoichiometric composition. In particular, LiαNiβCoγAlδO ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.7, γ ≦ 0.2) or LiαNiβCoγMnδO ₂ (1 ≦ α ≦ 1.2, β + γ + δ = 1, β ≧ 0.6). Γ ≦ 0.2) is preferable. A positive electrode active material can be used individually by 1 type or in combination of 2 or more types.

As the positive electrode binder that binds and integrates the positive electrode active material, specifically, the same negative electrode binder as that described above can be used. As the positive electrode binder, polyvinylidene fluoride is preferable from the viewpoint of versatility and low cost. The amount of the positive electrode binder used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material. When the content of the positive electrode binder is 2 parts by mass or more, the adhesion between the active materials or between the active material and the current collector is improved, and the cycle characteristics are improved. The substance ratio is improved and the positive electrode capacity can be improved.

In the positive electrode active material layer, a conductive auxiliary material may be added 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.

The current collector that supports the positive electrode active material layer in which the positive electrode active material is integrated with the positive electrode binder may be any material that has conductivity that enables conduction between the positive electrode active material layer and the external terminal. Specifically, the same thing as the electrical power collector used for the said negative electrode can be mentioned.

The positive electrode can be produced on a current collector using a positive electrode active material layer material including a positive electrode active material and a positive electrode binder. As a method for manufacturing the positive electrode active material layer, a method similar to the method for manufacturing the negative electrode active material layer can be used.

[Electrolyte]
The electrolytic solution is obtained by dissolving the electrolyte in a non-aqueous organic solvent capable of immersing the positive electrode and the negative electrode and dissolving lithium ions so that lithium can be absorbed and released in the positive electrode and the negative electrode during charging and discharging.

It is preferable that the solvent of the electrolytic solution is stable at the operating potential of the battery and has a low viscosity so that the electrode can be immersed in the usage environment of the battery. Specific examples of the solvent include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and vinylene carbonate (VC); dimethyl carbonate (DMC), diethyl carbonate (DEC). ), Acyclic carbonic acid esters such as ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC); propylene carbonate derivatives; aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate; Can be mentioned. These can be used alone or in combination of two or more. Among these, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), dipropyl carbonate A cyclic or chain carbonate such as (DPC) is preferred.

It is preferable that the solvent further contains a fluorinated ether compound. The fluorinated ether compound has high affinity with silicon and improves cycle characteristics, particularly capacity retention. The fluorinated ether compound may be a fluorinated chain ether compound or a fluorinated cyclic ether compound. These can be obtained by substituting a part of hydrogen of a chain ether compound or a cyclic ether compound with fluorine.

Examples of the chain ether compounds include dimethyl ether, methyl ethyl ether, diethyl ether, methyl propyl ether, ethyl propyl ether, dipropyl ether, methyl butyl ether, ethyl butyl ether, propyl butyl ether, dibutyl ether, methyl pentyl ether, ethyl pentyl ether, propyl Chain monoether compounds such as pentyl ether, butyl pentyl ether, dipentyl ether; 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), ethoxymethoxyethane (EME), 1,2-di Propoxyethane, propoxyethoxyethane, propoxymethoxyethane, 1,2-dibutoxyethane, butoxypropoxyethane, butoxyethoxyethane, butoxymethoxy Ethane, 1,2-pentoxy ethane, can be cited pentoxy butoxy ethane, pent propoxy ethane, pentoxy ethoxy ethane, a chain diether compounds such as pentoxy methoxyethane.

Examples of cyclic ether compounds include cyclic monoethers such as ethylene oxide, propylene oxide, oxetane, tetrahydrofuran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran, tetrahydropyran, 2-methyltetrahydropyran, 3-methyltetrahydropyran, 4-methyltetrahydropyran, etc. Compound; 1,3-dioxolane, 2-methyl-1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,4-dioxane, 2-methyl-1,4-dioxane, 1,3-dioxane, 2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 5-methyl-1,3-dioxane, 2,4-dimethyl-1,3-dioxane, 4-ethyl-1,3- And cyclic diether compounds such as dioxane.

Of these, fluorinated chain ether compounds having good stability are more preferred. As the fluorinated chain ether compound,
H- (CX ¹ X ² -CX ³ X ⁴ ) _n -CH ₂ O-CX ⁵ X ⁶ -CX ⁷ X ⁸ -H
The thing represented by these is preferable. In the formula, n represents 1, 2, 3 or 4, and X ¹ to X ⁸ independently represent a fluorine atom or a hydrogen atom. However, at least one of X ¹ to X ⁴ represents a fluorine atom, and at least one of X ⁵ to X ⁸ represents a fluorine atom.

Further, a compound having an atomic ratio (total number of fluorine atoms) / (total number of hydrogen atoms) of fluorine atoms and hydrogen atoms bonded to the compound of 1 or more is preferable,
H— (CF ₂ —CF ₂ ) _n —CH ₂ O—CF ₂ —CF ₂ —H
Is more preferable. In the formula, n is more preferably 1 or 2.

As an electrolyte contained in the electrolytic solution, a lithium salt is preferable. Specific examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , Li (CF ₃ SO ₂ ) ₃ , LiN ( CF ₃ SO ₂ ) _{2 and the} like.

The concentration of the electrolyte in the electrolytic solution is preferably 0.01 mol / L or more and 3 mol / L or less, more preferably 0.5 mol / L or more and 1.5 mol / L or less. When the electrolyte concentration is within this range, safety can be improved, and a battery having high reliability and contributing to reduction of environmental load can be obtained.

The electrolyte contains a terminal unsaturated compound whose terminal represented by the formula (1) is a hydrocarbon group having an unsaturated double bond.

The terminal unsaturated compound represented by the formula (1) is a polymer produced by the polymerization reaction proceeding with the unsaturated double bond becoming a radical on the negative electrode active material surface as the battery is charged and discharged. The negative electrode active material is coated to form a uniform film made of a polymer. Since this polymer coating allows lithium ions to permeate and inhibits permeation of the electrolytic solution, it suppresses the reaction between the negative electrode active material and the electrolytic solution and suppresses a decrease in battery capacity due to repeated charge and discharge. Can do.

In formula (1), R ₁ to R ₃ independently represent a direct bond or an alkylene group having 1 to 6 carbon atoms, and the alkylene group is an unsaturated double bond, an unsaturated triple bond, an oxy group or a substituted group. It may have a group. R ₄ represents a vinyl group. Among these, the phosphoric acid triester represented by the formula (2) in which R ₁ to R ₃ are bonded to a phosphorus atom through an oxy group is a uniform thickness on the surface of the negative electrode active material by charge / discharge. It is preferable because a coating film can be formed.

In formula (2), R ₇ to R ₉ independently represent a direct bond or an alkylene group having 1 to 6 carbon atoms, and the alkylene group is an unsaturated double bond, an unsaturated triple bond, an oxy group or a substituted group. May have a group, and R ₄ represents a vinyl group. R ₇ to R ₉ are preferably the same group because a uniform thickness film can be formed on the surface of the negative electrode active material.

Specific examples of the terminal unsaturated compound include trivinyl phosphate, triallyl phosphate, tri (3-butenyl) phosphate, tri (4-pentenyl) phosphate, diallyl monovinyl phosphate, monohexyl dibutenyl phosphate. And dihexyl monobutenyl phosphate.

The content of the terminal unsaturated compound in the electrolytic solution is preferably selected as appropriate so that a film having an appropriate thickness is formed on the negative electrode active material. The terminal unsaturated compound represented by the formula (1) contained in the electrolytic solution is polymerized or decomposed during the initial charge / discharge of the battery and the subsequent relatively early charge / discharge. For this reason, when the amount of the terminal unsaturated compound represented by the formula (1) contained in the electrolytic solution is excessive, the terminal unsaturation represented by the formula (1) may be obtained at an early stage of the charge / discharge cycle. The compound is polymerized or decomposed, and excessive decomposition products may adhere to the electrode, etc., inhibiting the occlusion / release of lithium ions in the subsequent charge / discharge cycle, and reducing the discharge capacity of the battery, or the rate The characteristics may be deteriorated. The content of the terminal unsaturated compound represented by the formula (1) contained in the electrolytic solution is generally about 0.05 to 5.0% by mass, for example, preferably 0.5 to It is 3.0% by mass, more preferably 0.5 to 2.0% by mass.

Further, the upper limit of the content of the terminal unsaturated compound represented by the formula (1) in the electrolytic solution can be defined by the impedance (charge transfer resistance) between the electrodes at the end of charging. Specifically, the content of the terminal unsaturated compound represented by formula (1) in the electrolytic solution is between the electrodes at the end of charging when the terminal unsaturated compound represented by formula (1) is added. It is preferable that the impedance is less than about 10 times that in the case of no addition since the rate characteristic or charge / discharge characteristic is not deteriorated.

When the terminal unsaturated compound represented by the above formula (1) is contained in the electrolytic solution in the above range, the thickness of the coating formed on the negative electrode active material particles can be set to 1 to 100 nm. A more preferable range of the thickness of the coating is about 5 to 50 nm. The thickness of the coating film formed on the negative electrode active material can be measured by observation with an electron microscope.

[separator]
Any separator may be used as long as it suppresses the conduction between the positive electrode and the negative electrode, does not inhibit the permeation of the charged body, and has durability with respect to the electrolytic solution. Specific examples of the separator material include polyolefin microporous membranes such as polypropylene and polyethylene, cellulose, polyethylene terephthalate, polyimide, and polyvinylidene fluoride. These can be used as porous films, woven fabrics, non-woven fabrics and the like.

[Cell exterior body]
As the outer package, those having a strength capable of stably holding the positive electrode, the negative electrode, the separator, and the electrolytic solution, electrochemically stable with respect to these substances, and watertight are preferable. Specifically, for example, a laminate film coated with stainless steel, nickel-plated iron, aluminum, silica, and alumina can be used as the material of the exterior body. As the resin used for the laminate film, polyethylene, polypropylene, and the like can be used. Polyethylene terephthalate or the like can be used. These may be a structure of one layer or two or more layers.

[Secondary battery]
The shape of the secondary battery may be any of the cylindrical type, flat wound square type, laminated square type, coin type, flat wound laminated type, or laminated laminated type.

As an example of the secondary battery, a laminated laminate type secondary battery 11 shown in FIG. 1 can be given. The laminated laminate type secondary battery includes a negative electrode 3 having a negative electrode active material layer 1 provided on a negative electrode current collector 2 made of metal such as copper foil, and a positive electrode current collector 5 made of metal such as aluminum foil. The positive electrode 6 having the positive electrode active material layer 4 provided on the substrate is disposed so as to face each other through a separator 7 made of a polypropylene microporous film that avoids these contacts, and these are accommodated in a laminate outer package 8. . The laminate outer package is filled with an electrolytic solution, and the negative electrode active material layer 1 and the positive electrode active material layer 4 are immersed in the electrolytic solution and electrically connected to a portion of the negative electrode current collector where the active material layer is not formed. The negative electrode terminal 9 and the positive electrode terminal 10 electrically connected to the positive electrode current collector of the portion where the active material layer is not formed are drawn out of the laminate outer package, and at the time of charge / discharge, an external power source, It is designed to be connected to the device used.

Hereinafter, the lithium secondary battery of the present invention will be described in detail.
[Example 1]
[Production of lithium secondary battery]
As the negative electrode active material, a silicon-based negative electrode active material in which a carbon coating was formed on the surface of silicon-based particles in which silicon was dispersed in amorphous silicon oxide (SiOx, 0 <x ≦ 2) was obtained. The mass ratio of silicon, amorphous silicon oxide, and carbon in the silicon-based negative electrode active material was 29:61:10. This negative electrode active material and polyamic acid, which is a precursor of polyimide as a negative electrode binder, were weighed at a mass ratio of 90:10 and mixed with n-methylpyrrolidone to obtain a negative electrode slurry. The negative electrode slurry was applied to a copper foil having a thickness of 10 μm, dried, and then subjected to a heat treatment in a nitrogen atmosphere at 300 ° C. to produce a negative electrode.

Lithium nickelate (LiNi0.80Co0.15Al0.05O2) as a positive electrode active material, carbon black as a conductive auxiliary, and polyvinylidene fluoride as a positive electrode binder in a mass ratio of 90: 5: 5 They were weighed and mixed with n-methylpyrrolidone to form a positive electrode slurry. The positive electrode slurry was applied to an aluminum foil having a thickness of 20 μm, dried, and then pressed to prepare a positive electrode.

3 layers of the positive electrode and 4 layers of the negative electrode obtained were alternately stacked while sandwiching a polypropylene porous film as a separator. The end of each positive electrode current collector not covered with the positive electrode active material is welded, the end of each negative electrode current collector not covered with the negative electrode active material is welded, and aluminum is further applied to the welded portion of the positive electrode current collector. A positive electrode terminal made of nickel was welded, and a negative electrode terminal made of nickel was welded to the welded portion of the negative electrode current collector to obtain an electrode element having a planar laminated structure.

99 parts by mass of a carbonate-based non-aqueous electrolyte composed of EC / DEC = 30/70 (volume ratio) in which LiPF ₆ is dissolved at a concentration of 1 mol / l is represented by the formula (3) as a terminal unsaturated compound. Triaryl phosphate (compound (A)) was mixed with 1 part by mass (content in the electrolyte: 1% by mass) to obtain an electrolyte.

The obtained electrode element was wrapped with an aluminum laminate film as an outer package, and an electrolyte solution was injected therein, and then sealed while reducing the pressure to 0.1 atm to prepare a secondary battery.

[Evaluation of charge / discharge cycle characteristics]
The obtained lithium secondary battery was repeatedly charged and discharged in a constant temperature bath at 60 ° C. in a voltage range of 2.5 V to 4.2 V. The discharge capacities at the 100th and 250th charge / discharge cycles were measured. The discharge capacity ratio after 100 cycles and the discharge capacity ratio after 250 cycles with respect to the initial discharge amount were calculated to obtain a capacity retention rate. The results are shown in Table 1.

[Comparative Example 1]
A lithium secondary battery was produced in the same manner as in Example 1 except that the terminal unsaturated compound was not used, and the charge / discharge cycle characteristics were evaluated. The results are shown in Table 1.

[Comparative Example 2]
A lithium secondary battery was produced in the same manner as in Example 1 except that triethyl phosphate represented by formula (4) (compound (B)) was used as the terminal unsaturated compound instead of triallyl phosphate. The charge / discharge cycle characteristics were evaluated. The results are shown in Table 1.

From the results, the charge / discharge capacity retention rate at 60 ° C. of the lithium secondary battery of the example is higher than that of the lithium secondary battery of the comparative example, and the lithium of the present invention using the electrolytic solution containing the terminal unsaturated compound. It can be seen that the secondary battery is excellent in charge / discharge cycle characteristics.

This application includes all matters described in Japanese Patent Application No. 2012-33128 filed on February 17, 2012 as its contents.

The present invention can be used in all industrial fields that require a power source and industrial fields related to the transport, storage and supply of electrical energy. Specifically, it can be used as a power source for mobile devices such as mobile phones and notebook computers.

Claims

A lithium secondary battery having a positive electrode that occludes and releases lithium with charge and discharge, and an electrolyte that immerses the negative electrode, the negative electrode including a silicon-based negative electrode active material,
The electrolyte is the formula (1)

(Wherein R 1 to R 3 independently represent a direct bond or an alkylene group having 1 to 6 carbon atoms, and the alkylene group represents an unsaturated double bond, an unsaturated triple bond, an oxy group or a substituent. And a lithium secondary battery characterized by comprising an unsaturated compound represented by R 4 represents a vinyl group.
The lithium secondary battery according to claim 1 , wherein R 1 to R 3 are the same in formula (1).
The unsaturated compound represented by the formula (1) is represented by the formula (2)

(Wherein R 7 to R 9 independently represent a direct bond or an alkylene group having 1 to 6 carbon atoms, and the alkylene group represents an unsaturated double bond, an unsaturated triple bond, an oxy group, or a substituent. may have, lithium secondary battery according to claim 1, wherein the R 4 is an unsaturated compound represented by showing a vinyl group.).
The lithium secondary battery according to claim 3, wherein R 7 to R 9 are the same in formula (1).
The lithium secondary battery according to any one of claims 1 to 4, wherein the electrolytic solution contains at least one selected from a carbonate ester and an ether compound.
The negative electrode has a negative electrode active material layer containing a particulate silicon-based negative electrode active material, and a film containing the polymer of the unsaturated compound is formed on the particulate silicon-based negative electrode active material with charge / discharge. The lithium secondary battery according to any one of claims 1 to 5, characterized in that:
An electrolyte for a lithium secondary battery that immerses a positive electrode and a negative electrode that occlude and release lithium in accordance with charge and discharge, wherein the formula (1)

(Wherein R 1 to R 3 independently represent a direct bond or an alkylene group having 1 to 6 carbon atoms, and the alkylene group represents an unsaturated double bond, an unsaturated triple bond, an oxy group or a substituent. An electrolyte for a lithium secondary battery, comprising an unsaturated compound which may be present and R 4 represents a vinyl group.
The electrolyte solution for a lithium secondary battery according to claim 7, wherein R 1 to R 3 in formula (1) are the same.
The unsaturated compound represented by the formula (1) is represented by the formula (2)

(Wherein R 7 to R 9 independently represent a direct bond or an alkylene group having 1 to 6 carbon atoms, and the alkylene group represents an unsaturated double bond, an unsaturated triple bond, an oxy group, or a substituent. The electrolyte solution for a lithium secondary battery according to claim 7, wherein the electrolyte solution may be an unsaturated compound represented by: R 4 may be a vinyl group.
10. The electrolyte for a lithium secondary battery according to claim 9 , wherein R 7 to R 9 are the same in formula (1).
The electrolyte solution for a lithium secondary battery according to any one of claims 7 to 10, comprising any one or more selected from a carbonate ester and an ether compound.