JP2010212153A - Nonaqueous electrolyte secondary battery containing nonaqueous electrolyte, and the nonaqueous electrolyte - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte in which reduction of electric discharge capacity during a high temperature standing is suppressed, and discharge capacity during a low temperature charging is superior; and to provide a nonaqueous electrolyte secondary battery equipped with the nonaqueous electrolyte. <P>SOLUTION: The nonaqueous electrolyte contains at least one boronic acid compound or the like of a general formula (1), a boronic acid compound of a general formula (2), and a boronic acid compound having thiophene structure. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a nonaqueous electrolyte and a chargeable / dischargeable nonaqueous electrolyte secondary battery including the nonaqueous electrolyte and used mainly as a power source for portable electronic devices such as a video camera, a mobile computer, and a mobile phone.

A battery containing a non-aqueous electrolyte is widely used as a power source for consumer electronic devices because it has a high voltage, a high energy density, and high reliability such as storage stability.
As a typical example of a battery containing a nonaqueous electrolyte, a lithium battery and a lithium ion secondary battery can be given. These batteries include a negative electrode made of metal lithium or an active material capable of occluding and releasing lithium, a positive electrode made of transition metal oxide, fluorinated graphite, a composite oxide of lithium and transition metal, and the like. A water electrolyte.
The non-aqueous electrolyte is a solution obtained by mixing a Li electrolyte such as LiBF ₄ , LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , and Li ₂ SiF ₆ in an aprotic organic solvent.

In a non-aqueous electrolyte secondary battery (hereinafter referred to as a battery) such as a lithium ion secondary battery, the non-aqueous electrolyte transfers ions between a positive electrode and a negative electrode. In order to improve the charge / discharge characteristics of the battery, it is necessary to increase the ion transfer speed between the positive electrode and the negative electrode as much as possible, to increase the ionic conductivity of the nonaqueous electrolyte or to lower the viscosity of the nonaqueous electrolyte. For example, it is necessary to facilitate mass transfer by diffusion. In addition, non-aqueous electrolytes are used for positive and negative electrodes that are chemically and electrochemically highly reactive in order to improve the storage stability of batteries (such as storage characteristics) and cycle stability when charging and discharging are repeated. It needs to be stable.
Non-Patent Document 1 discloses a non-aqueous electrolyte that stabilizes an electrode, such as propylene carbonate, ethylene carbonate and other high dielectric constant carbonate solvents, diethyl carbonate, methyl ethyl carbonate, dimethyl carbonate and other low viscosity carbonate solvents such as LiPF _6. A lithium salt such as is dissolved.

However, the battery containing the nonaqueous electrolyte described above has a problem that the discharge capacity after the test is lowered when the battery is left in a charged state at a high temperature or repeatedly charged and discharged at a low temperature.
As described above, the discharge capacity of the battery decreases because the decomposition reaction of the nonaqueous electrolyte proceeds on the positive electrode and the negative electrode, the reaction resistance of the electrode increases, the electrical conductivity of the nonaqueous electrolyte decreases, This is because the internal resistance of the battery increases due to clogging or the like.
Moreover, patent documents 1-13 and nonpatent literature 1 are mentioned as a well-known example of boronic acid ester, borinic acid ester, boroxine, and other boric acid ester.

Japanese Patent Laid-Open No. 09-120825 JP-A-10-223258 Japanese Patent Application Laid-Open No. 11-054133 Japanese Patent Laid-Open No. 11-121033 Japanese Patent Laid-Open No. 11-100409 JP 2002-025609 A JP 2002-216844 A JP 2003-132946 A JP 2003-168476 A JP 2008-198542 A Japanese Patent No. 3439002 Japanese Patent No. 3462784 Japanese Patent No. 4092631

“Lithium Battery” edited by Jean-Paul Gabano, ACADMIC PRESS (1983)

In the above-mentioned patent documents, nonaqueous electrolytes in which various boronic acid esters, borinic acid esters, and boroxines are added to the nonaqueous electrolyte are proposed. However, it has been confirmed that when these nonaqueous electrolytes are used, the high-temperature storage characteristics are improved, but the discharge capacity decreases when charging and discharging are repeated at low temperatures. In the boronic acid ester, borinic acid ester, and boroxine in the above-mentioned patent documents, a film is formed on the negative electrode by reaction with the negative electrode, and when charged at a low temperature, it becomes a lithium ion migration resistance, and on the negative electrode. This is because metallic lithium is easily deposited.
In order to solve these problems, there is a demand for the development of an additive that satisfactorily suppresses the decomposition of the nonaqueous electrolyte on the surfaces of the positive electrode and the negative electrode and does not increase the lithium ion migration resistance of the film on the negative electrode. .

  The present invention has been made in view of such circumstances, and when a non-aqueous electrolyte secondary battery is produced by containing a specific boronic acid compound, borinic acid compound, or boroxine compound, the discharge after leaving at high temperature is performed. An object of the present invention is to provide a non-aqueous electrolyte that has a small decrease in capacity and a good discharge capacity during low-temperature charge / discharge.

  The present invention also provides a non-aqueous electrolyte secondary battery that includes the non-aqueous electrolyte and has a low decrease in discharge capacity after being left at high temperature and a good discharge capacity during low-temperature charge / discharge. Objective.

In order to solve the above-mentioned problems, the present inventor has paid attention to a boronic acid compound, a borinic acid compound, and a boroxine compound, and conducted intensive studies. As a result, it has been found that the above problem can be solved by configuring a nonaqueous electrolyte secondary battery using a nonaqueous electrolyte containing the following boronic acid compound, borinic acid compound, and boroxine compound, and to complete the present invention. It came.
That is, the nonaqueous electrolyte according to the first invention is a boronic acid compound or borinic acid compound represented by the following general formula (1), a boronic acid compound represented by the general formula (2), and the general formula (3). It contains at least one compound selected from the group consisting of boroxine compounds represented.

In the general formulas (1) to (2), X is an organic group having 20 or less carbon atoms including a sulfur atom, and may be the same or different. R < ₁ > -R < ₂ > is a C1-C20 organic group. n is an integer of 1-2. m is an integer of 1 to 3. In the general formula (3), Y is an organic group having 20 or less carbon atoms having a thiophene structure.

Here, the non-aqueous electrolyte means an electrolytic solution in which a supporting salt is dissolved in a non-aqueous solvent, or a solid electrolyte containing the electrolytic solution.
In the present invention, since the boronic acid compound, borinic acid compound, and boroxine compound are added to the nonaqueous electrolyte, when a nonaqueous electrolyte secondary battery is produced using this nonaqueous electrolyte, The decrease in discharge capacity is small, and the discharge capacity at low temperature charge / discharge is good.
Therefore, it is suppressed that the lifetime of a battery becomes short.

  Although the detailed reason why such an effect is obtained is unknown, the boronic acid compound, borinic acid compound, and boroxine compound act on the surface of the electrode (active material), and the interface between the electrode and the nonaqueous electrolyte is heated at a high temperature. However, since a stable protective film is formed and decomposition of the nonaqueous electrolyte (nonaqueous solvent) in the positive electrode and the negative electrode is suppressed, the capacity retention when charging / discharging is repeated at a high temperature is improved, and the negative electrode protective film This is probably because the lithium ion conductivity is good. This effect is first manifested by the addition of the boronic acid compound, borinic acid compound and boroxine compound.

  In addition, when X does not contain a sulfur atom or when Y does not have a thiophene structure, good film formation is insufficient, and as shown in a comparative example described later, when left at high temperature, As the electrolyte decomposes, the discharge capacity decreases. Moreover, the discharge capacity at the time of low temperature charge / discharge decreases.

  The nonaqueous electrolyte according to the second invention is characterized in that, in the first invention, X is an organic group having a thiophene structure.

  In the present invention, since X is an organic group containing a thiophene structure, when a non-aqueous electrolyte secondary battery is produced using this non-aqueous electrolyte, the decrease in discharge capacity after standing at high temperature is less, and Discharge capacity at low temperature charge / discharge is good.

The non-aqueous electrolyte according to the third invention is characterized by containing 0.01% by mass or more and 2% by mass or less of the boronic acid compound, borinic acid compound, or boroxine compound in the first or second invention.
When a non-aqueous electrolyte secondary battery is produced using this non-aqueous electrolyte, the decrease in discharge capacity after standing at high temperature is further reduced, and the discharge capacity during low-temperature charge / discharge is good.

  A non-aqueous electrolyte secondary battery according to a fourth invention includes the non-aqueous electrolyte according to the first to third inventions.

  In the present invention, since the nonaqueous electrolyte according to any one of the first to third inventions is included, the decrease in discharge capacity after leaving the battery at high temperature is further reduced, and the discharge capacity at low temperature charge / discharge is good, It can suppress that the lifetime of a battery becomes short.

  According to the non-aqueous electrolyte of the present invention, when a non-aqueous electrolyte secondary battery is produced using this non-aqueous electrolyte, a non-aqueous electrolyte secondary battery having good charge / discharge cycle life characteristics and high temperature storage characteristics is obtained. It is done.

  According to the nonaqueous electrolyte secondary battery of the present invention, high charge / discharge cycle life characteristics and high temperature storage characteristics are good.

It is sectional drawing which shows the nonaqueous electrolyte secondary battery which concerns on this invention.

Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof.
The battery (nonaqueous electrolyte secondary battery) of the present invention has a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte.

(1) Nonaqueous Electrolyte The nonaqueous electrolyte according to the present invention includes a boronic acid compound or a borinic acid compound represented by the general formula (1), a general formula (2), and a nonaqueous solvent and a lithium salt described later. It contains containing at least one compound selected from the group consisting of the boronic acid compound represented, the boroxine compound represented by the general formula (3).
In the general formulas (1) to (2), X is an organic group having 20 or less carbon atoms including a sulfur atom, and may be the same or different. R < ₁ > -R < ₂ > is a C1-C20 organic group. n is an integer of 1-2. m is an integer of 1 to 3. In the general formula (3), Y is an organic group having 20 or less carbon atoms having a thiophene structure.

  Specific examples of the organic group having 20 or less carbon atoms containing a sulfur atom represented by X include thiophene, dithiophene, methylthiophene, dimethylthiophene, ethylthiophene, diethylthiophene, fluorothiophene, difluorothiophene, chlorothiophene, dichlorothiophene , Cyanothiophene, phenylthiophene, hexyldithiophene, methylsulfonylbenzene, methylsulfonylaminobenzene, thioanisole, benzothiophene, cyanobenzothiophene, fluorobenzothiophene, difluorobenathiophene, thiazole, 2-methylthio-pyromidine, thieno [3 , 3-B] thiophene residues. The term “residue” refers to a structure obtained by removing a hydrogen atom from a bonding portion.

  From the viewpoint of good expression of the effects of the present invention, those having good lithium ion conductivity of the negative electrode film are preferred, and X preferably has a thiophene structure, specifically, thiophene, dithiophene, methylthiophene, dimethyl. Thiophene, ethylthiophene, diethylthiophene, fluorothiophene, difluorothiophene, chlorothiophene, dichlorothiophene, cyanothiophene, phenylthiophene, hexyldithiophene, benzothiophene, cyanobenzothiophene, fluorobenzothiophene, difluorobenathiophene, thiazole, thieno [ 3,3-B] thiophene residues.

  Specific examples of the organic group having 20 or less carbon atoms having a thiophene structure represented by Y include thiophene, dithiophene, methylthiophene, dimethylthiophene, ethylthiophene, diethylthiophene, fluorothiophene, difluorothiophene, chlorothiophene, dichlorothiophene , Cyanothiophene, phenylthiophene, hexyldithiophene, benzothiophene, cyanobenzothiophene, fluorobenzothiophene, difluorobenathiophene, thiazole, thieno [3,3-B] thiophene.

Specific examples of the organic group having 1 to 20 carbon atoms represented by R _{1 to} R ₂ include methanol, ethanol, propanol (including isomers), butanol (including isomers), and pentanol (including isomers). ), Hexanol (including isomers), heptanol (including isomers), octanol (including isomers), 2,2,2-trifluoroethanol, phenol, perfluorophenol, trimethylsilanol, triethylsilanol, tri OH group was removed from each structure of phenylsilanol, ethylene glycol, 1-methyl-ethylene glycol, 1,3-propanediol, pinacol, neopentyl glycol, 2-methyl-1,3-propanediol, catechol and oxalic acid Groups.

  Specific examples of the boronic acid compound or borinic acid compound represented by the general formula (1) include thiophene-2-boronic acid bistrimethylsilyl ester (the following compound 1), thiophene-2-boronic acid dimethyl ester (the following Compound 2), thiophene-2-boronic acid diethyl ester (the following compound 3), thiophene-2-boronic acid dibutyl ester (the following compound 4), thiophene-2-boronic acid butyl methyl ester (the following compound 5), thiophene-2 -Boronic acid bis (2,2,2-trifluoroethyl) ester (the following compound 6), thiophene-2-boronic acid diphenyl ester (the following compound 7), thioanisole-2-boronic acid dimethyl ester (the following compound 8) Dithiophene-2,2′-borinic acid methyl ester (compound 9 below), dithi Phen-2,2′-borinic acid butyl ester (compound 10 below), dithiophene-2,2′-borinic acid phenyl ester (compound 11 below), 2-thiophen-2-thioanisole-borinic acid methyl ester (compound below) 12).

  Specific examples of the boronic acid compound represented by the general formula (2) include thiophene-2-boronic acid pinacol ester (the following compound 13), 5-methyl-thiophene-2-boronic acid pinacol ester (the following compound 14). ), 2,2′-dithiophene-5-boronic acid pinacol ester (the following compound 15), 5′-hexyl-2,2′-dithiophene-5-boronic acid pinacol ester (the following compound 16), thiophene-3-boron Acid pinacol ester (the following compound 17), 5-chloro-thiophene-2-boronic acid pinacol ester (the following compound 18), 5-fluoro-thiophene-2-boronic acid pinacol ester (the following compound 19), thioanisole-2- Boronic acid pinacol ester (compound 20 below), thioanisole-4-boro Acid pinacol ester (the following compound 21), mesylaniline-4-boronic acid pinacol ester (the following compound 22), phenylthiomethylboronic acid pinacol ester (the following compound 23), benzothiophene-5-boronic acid pinacol ester (the following compound 24) ), Thiazole-2-boronic acid pinacol ester (the following compound 25), 2- (methylthio) pyrimidine-5-boronic acid pinacol ester (the following compound 26), thieno [3,2-B] thiophene-2-boronic acid pinacol Ester (the following compound 27), 5-cyano-benzonitrile-9-boronic acid pinacol ester (the following compound 28), (2-thienyl) benzene-4-boronic acid pinacol ester (the following compound 29), benzonitrile-9- Boronic acid neopentyl glycol ester The following compound 30), thiophene-2-boronic acid 1,3-propanediol ester (the following compound 31), thiophene-2-boronic acid ethylene glycol ester (the following compound 32), thiophene-2-boronic acid neopentyl glycol ester ( The following compound 33), thiophene-2-boronic acid oxalate (the following compound 34), thiophene-2,5-diboronic acid pinacol ester (the following compound 35), 2,2′-bithiophene-5,5′-diboronic acid Examples include pinacol ester (the following compound 36), thiophene-2,3,5-triboronic acid pinacol ester (the following compound 37), and thioanisole-1,3,5-triboronic acid pinacol ester (the following compound 38).

  Specific examples of the boroxine compound represented by the general formula (3) include tri (2-thiophene) boroxin (above compound 39), tri (5-methyl-2-thiophene) boroxin (above compound 40), And tri (5-boronic acid pinacol ester-2-thiophene) boroxine (compound 41 below).

  The boronic acid compound or borinic acid compound represented by the general formula (1), the boronic acid compound represented by the general formula (2), and the boroxine compound represented by the general formula (3) are selected. The content of at least one compound is 0.01% by mass or more and 2% by mass or less, and preferably 0.05% by mass or more and 1% by mass or less with respect to the total weight of the nonaqueous electrolyte. This range is preferable for achieving the object of the present invention.

The nonaqueous solvent used in the nonaqueous electrolyte of the present invention preferably contains at least a cyclic aprotic solvent and / or a chain aprotic solvent.
Examples of the cyclic aprotic solvent include cyclic carbonates such as ethylene carbonate, cyclic esters such as γ-butyrolactone, cyclic sulfones such as sulfolane, and cyclic ethers such as dioxolane.
Examples of the chain aprotic solvent include chain carbonates such as dimethyl carbonate, chain carboxylic acid esters such as methyl propionate, and chain ethers such as dimethoxyethane.

In particular, when the load characteristics and low temperature characteristics of the battery are intended to be improved, the non-aqueous solvent is preferably a mixture of a cyclic aprotic solvent and a chain aprotic solvent. Furthermore, when importance is attached to the electrochemical stability of the non-aqueous electrolyte, it is preferable to use a cyclic carbonate as the cyclic aprotic solvent and a chain carbonate as the chain aprotic solvent.
Specific examples of cyclic carbonates include ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, trans-2,3-butylene carbonate, cis-2,3-butylene carbonate, 1,2-pentylene carbonate, trans Examples include -2,3-pentylene carbonate, cis-2,3-pentylene carbonate, trifluoromethylethylene carbonate, fluoroethylene carbonate, 4,5-difluoroethylene carbonate, and the like.
Of these, ethylene carbonate and propylene carbonate having a high dielectric constant are preferable. When graphite is used for the negative electrode active material, it is more preferable to use ethylene carbonate. Moreover, you may use these cyclic carbonates in mixture of 2 or more types.

  Specific examples of the chain carbonate include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, methyl butyl carbonate, dibutyl carbonate, ethyl propyl carbonate, and methyl trifluoroethyl carbonate. Can be mentioned. Of these, dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate are preferred because of their low viscosity. These chain carbonates may be used in combination of two or more.

  As for the mixing ratio of the cyclic carbonate and the chain carbonate, the cyclic carbonate: chain carbonate (volume ratio) is preferably 5:95 to 70:30, more preferably 10:90 to 60:40. By setting such a ratio, the increase in the viscosity of the nonaqueous electrolyte can be suppressed and the dissociation degree of the nonaqueous electrolyte can be increased, so that the conductivity of the nonaqueous electrolyte contributing to the charge / discharge characteristics of the battery can be increased. it can.

  In the non-aqueous electrolyte according to the present invention, a compound other than the above-described boron-containing compound may be included as an additive in the non-aqueous solvent as long as the object of the present invention is not hindered. Specific examples of other compounds include carbonates having a carbon-carbon unsaturated bond such as vinylene carbonate, dimethyl vinylene carbonate, and divinyl carbonate; amides such as dimethylformamide; and chain-like compounds such as methyl-N, N-dimethylcarbamate. Carbamates; Cyclic amides such as N-methylpyrrolidone; Cyclic ureas such as N, N-dimethylimidazolidinone; Trimethyl borate, triethyl borate, tributyl borate, trioctyl borate, tri (trimethylsilyl) borate, etc. Boric acid esters; trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, tri (trimethylsilyl) phosphate, triphenyl phosphate, etc .; ethylene glycol dimethyl ether, diethylene glycol dimethyl ester Ethylene glycol derivatives such as tellurium and polyethylene glycol dimethyl ether; aromatic hydrocarbons such as biphenyl, fluorobiphenyl, o-terphenyl, toluene, ethylbenzene, fluorobenzene, cyclohexylbenzene, 2-fluoroanisole and 4-fluoroanisole; 3-propane sultone, 1,4-butane sultone, 1,3-prop-1-ene sultone, 1-methyl-1,3-prop-1-ene sultone, 2-methyl-1,3-prop-1-ene sultone, 3 -Sulfur systems such as methyl-1,3-prop-1-ene sultone, ethylene sulfite, propylene sulfite, ethylene sulfate, propylene sulfate, butene sulfate, hexene sulfate, vinylene sulfate, 3-sulfolene, divinyl sulfone, dimethyl sulfate, diethyl sulfate Compound; average It can be exemplified carboxylic acid anhydride having a maleic anhydride, a carbon-carbon unsaturated compounds such as norbornene dicarboxylic acid anhydride.

  These compounds may be added alone or in combination of two or more. Among these, use of vinylene carbonate, 1,3-prop-1-ene sultone, ethylene sulfate, propylene sulfate, butene sulfate, and hexene sulfate is preferable. The content of these compounds is preferably 0.1 to 10% by mass, and more preferably 0.5 to 5% by mass with respect to the total mass of the nonaqueous electrolyte.

As the lithium salt contained in the non-aqueous electrolyte of the present invention, any lithium salt that is used as a normal non-aqueous electrolyte can be used.
Specific examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ Li ₂ SiF ₆ , LiOSO ₂ C _k F _{(2k + 1)} (k = 1 to 8), LiN (SO ₂ C _{k F (2k + 1)) 2} (k = 1~8 _{integer), LiPF n (C k F} (2k + 1)) (6-n) (n = 1~5, k = 1~8 integer) LiBF _n C _k F _{(2k + 1)} (n = 1 to 3, k = 1 to 8), LiB (C ₂ O ₄ ) ₂ (lithium bisoxalyl borate), LiBF ₂ (C ₂ O ₄ ) (lithium difluorooxalyl borate), LiPF ₃ (C ₂ O ₄ ) (lithium trifluorooxalyl phosphate).
Moreover, the lithium salt shown by the following general formula can also be used.
LiC (SO ₂ R ¹¹ ) (SO ₂ R ¹² ) (SO ₂ R ¹³ )
LiN (SO ₂ OR ¹⁴ ) (SO ₂ OR ¹⁵ )
LiN (SO ₂ R ¹⁶ ) (SO ₂ OR ¹⁷ )
(In formula, R < ¹¹ > -R < ¹⁷ > may mutually be same or different, and is a C1-C8 perfluoroalkyl group).
These lithium salts may be used alone or in combination of two or more.
Of these, LiPF ₆ , LiBF ₄ , and LiN (SO ₂ C _k F _{(2k + 1)} ) ₂ (k = 1 to 8) are particularly preferable.
The above lithium salt is preferably contained in the non-aqueous electrolyte at a concentration of 0.1 to 3 mol / liter, more preferably 0.5 to 2 mol / liter.

(2) Positive electrode As the positive electrode active material used in the battery of the present invention, a composition formula Li _x MO ₂ , Li _y M ₂ O ₄ (where M is selected from transition metals), which is a compound capable of inserting and extracting lithium. Or a composite oxide represented by 0 ≦ x ≦ 1, 0 ≦ y ≦ 2), a metal chalcogenide or a metal oxide having a tunnel structure and a layered structure can be used. Specific examples thereof include LiCoO ₂ , LiCo _x Ni _1-x O ₂ , LiMn ₂ O ₄ , Li ₂ Mn ₂ O ₄ , MnO ₂ , FeO ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , TiS. ₂ etc. are mentioned.
Examples of the organic compound include conductive polymers such as polyaniline. Furthermore, the above-mentioned various active materials may be mixed and used regardless of an inorganic compound or an organic compound.
When a granular positive electrode active material is used, the positive electrode is produced, for example, by forming a mixture of positive electrode active material particles, a conductive additive and a binder on a metal current collector such as aluminum. .

(3) Negative electrode As the negative electrode active material of the present invention, all generally known materials such as metallic lithium, lithium alloys, and carbon materials capable of occluding and releasing lithium can be used. Examples of the negative electrode active material include alloys of lithium such as Al, Si, Pb, Sn, Zn, and Cd, metal oxides such as LiFe ₂ O ₃ , WO ₂ , MoO ₂ , SiO, and CuO, graphite, and carbon. A carbonaceous material, lithium nitride such as Li ₃ N, metallic lithium, or a mixture thereof can be used.

(4) Separator As the separator of the present invention, a woven fabric, a nonwoven fabric, a synthetic resin microporous membrane, or the like can be used, and a synthetic resin microporous membrane can be suitably used. Among these, a microporous membrane made of polyethylene and polypropylene, or a polyolefin microporous membrane such as a microporous membrane composed of these is preferably used in terms of thickness, membrane strength, membrane resistance, and the like.
Moreover, it can also serve as a separator by using solid electrolytes, such as a polymer solid electrolyte.
Further, a synthetic resin microporous membrane and a polymer solid electrolyte may be used in combination. In this case, a porous polymer solid electrolyte membrane may be used as the polymer solid electrolyte, and the polymer solid electrolyte may further contain a nonaqueous electrolyte.

  The shape of the battery of the present invention is not particularly limited, and can be applied to non-aqueous electrolyte secondary batteries having various shapes such as a square, a long cylinder, a coin, a button, a sheet, and a cylindrical battery. Although it is possible, the effect is satisfactorily exhibited in a battery in which the battery case is easily deformed, such as a rectangular shape, a long cylindrical shape, a coin shape, a button shape, and a sheet shape.

  Hereinafter, the present invention will be described with reference to preferred embodiments. However, the present invention is not limited to the embodiments in any way, and can be implemented with appropriate modifications within a range not changing the gist thereof. .

Example 1
FIG. 1 is a cross-sectional view showing a nonaqueous electrolyte secondary battery according to the present invention. In FIG. 1, 1 is a square nonaqueous electrolyte secondary battery (hereinafter referred to as a battery), 2 is an electrode group, 3 is a negative electrode, 4 is a positive electrode, 5 is a separator, 6 is a battery case, 7 is a case lid, 8 Is a safety valve, 9 is a negative electrode terminal, and 10 is a negative electrode lead. The electrode group 2 is obtained by winding the negative electrode 3 and the positive electrode 4 in a flat shape with the separator 5 interposed therebetween. The electrode group 2 and the nonaqueous electrolyte are housed in a battery case 6, and the opening of the battery case 6 is sealed by laser welding a case lid 7 provided with a safety valve 8. The negative electrode terminal 9 is connected to the negative electrode 3 via the negative electrode lead 10, and the positive electrode 4 is connected to the inner surface of the battery case 6.

The positive electrode 4 was produced as follows.
90% by mass of LiCoO ₂ as a positive electrode active material, 5% by mass of acetylene black as a conductive additive, and 5% by mass of polyvinylidene fluoride (PVDF) as a binder are mixed to form a positive electrode mixture. A paste was obtained by dispersing in N-methyl-2-pyrrolidone. This paste was uniformly applied to an aluminum current collector with a thickness of 20 μm and dried, and then compression molding was performed with a roll press to obtain the positive electrode 4.

The negative electrode 3 was produced as follows.
A slurry was prepared by mixing 97% by mass of graphite as a negative electrode active material, 1.5% by mass of carboxymethyl cellulose and 1.5% by mass of styrene butadiene rubber as a binder, and adding and dispersing distilled water as appropriate. . This slurry was uniformly applied to a 15 μm thick copper current collector, dried, dried at 100 ° C. for 5 hours, and then the density of the negative electrode active material layer composed of the binder and the active material was 1.40 g / cm ³ . Thus, the negative electrode 3 was obtained by compression molding with a roll press.

As the separator, a microporous polyethylene film having a thickness of 20 μm was used.
As a non-aqueous electrolyte, 1.1 mol / L of LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1: 1: 1 to obtain vinylene. 1% by mass of carbonate is added to the total mass of the non-aqueous electrolyte, and thiophene-2-boronic acid as a boronic acid ester represented by the general formula (1) with respect to the total mass of the non-aqueous electrolyte. What added 0.2 mass% of bis (trimethylsilyl) ester (the said compound 1) was used.

(Example 2)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of thiophene-2-boronic acid dimethyl ester (compound 2) was added to the total mass of the nonaqueous electrolyte.

Example 3
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of thioanisole-2-boronic acid dimethyl ester (Compound 8) was added to the total mass of the nonaqueous electrolyte.

Example 4
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of dithiophene-2,2′-borinic acid methyl ester (compound 9) was added to the total mass of the nonaqueous electrolyte.

(Example 5)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of thiophene-2-boronic acid pinacol ester (compound 13) was added to the total mass of the nonaqueous electrolyte.

(Example 6)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of 5-methyl-thiophene-2-boronic acid pinacol ester (compound 14) was added to the total mass of the nonaqueous electrolyte.

(Example 7)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of 2,2′-bithiophene-5-boronic acid pinacol ester (compound 15) was added to the total mass of the nonaqueous electrolyte. .

(Example 8)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of thiophene-2,5′-diboronic acid pinacol ester (compound 35) was added to the total mass of the nonaqueous electrolyte.

Example 9
A battery was prepared in the same manner as in Example 1 except that 0.2% by mass of 2,2′-bithiophene-5,5′-diboronic acid pinacol ester (compound 36) was added to the total mass of the nonaqueous electrolyte. Was made.

(Example 10)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of thiophene-2,3,5-triboronic acid pinacol ester (compound 37) was added to the total mass of the nonaqueous electrolyte.

(Example 11)
A battery was fabricated in the same manner as in Example 1 except that 0.01% by mass of tris (2-thiophene) boroxine (compound 39) was added to the total mass of the nonaqueous electrolyte.

Example 12
A battery was fabricated in the same manner as in Example 1 except that 0.05% by mass of tris (2-thiophene) boroxine (compound 39) was added to the total mass of the nonaqueous electrolyte.

(Example 13)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of tris (2-thiophene) boroxine (compound 39) was added to the total mass of the nonaqueous electrolyte.

(Example 14)
A battery was fabricated in the same manner as in Example 1 except that 1% by mass of tris (2-thiophene) boroxine (compound 39) was added to the total mass of the nonaqueous electrolyte.

(Example 15)
A battery was fabricated in the same manner as in Example 1 except that 2% by mass of tris (2-thiophene) boroxine (compound 39) was added to the total mass of the nonaqueous electrolyte.

(Example 16)
A battery was fabricated in the same manner as in Example 1 except that 3% by mass of tris (2-thiophene) boroxine (compound 39) was added to the total mass of the nonaqueous electrolyte.

(Comparative Example 1)
A battery was prepared in the same manner as in Example 1 except that the boronic acid compound, borinic acid compound and boroxine compound represented by the general formula (1), (2) or (3) were not added to the nonaqueous electrolyte. Produced.

(Comparative Example 2)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of diethylborinic acid methyl ester was added relative to the total mass of the nonaqueous electrolyte.

(Comparative Example 3)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of dimethylborinic acid methyl ester was added to the total mass of the nonaqueous electrolyte.

(Comparative Example 4)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of dimethylborinic acid ethyl ester was added to the total mass of the nonaqueous electrolyte.

(Comparative Example 5)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of trimethylboroxine was added to the total mass of the nonaqueous electrolyte.

(Comparative Example 6)
A battery was fabricated in the same manner as in Example 1 except that 0.2% by mass of triphenylboroxine was added to the total mass of the nonaqueous electrolyte.

  The following performance evaluation was performed on the batteries of the above-described examples and comparative examples.

[Initial capacity check test]
The initial capacity (mAh) was measured for the batteries of each Example and each Comparative Example. The batteries of each Example and each Comparative Example were prepared in 5 cells, and each battery was charged at a constant current and a constant voltage for 3 hours up to 4.2 V at a current of 800 mA, and then discharged to 3 V at a current of 800 mA. The discharge capacity (initial capacity) was measured, and the average value of 5 cells was determined.

[High temperature storage test]
The high temperature storage test was performed under the following conditions.
The battery after the initial capacity confirmation test was charged with constant current and constant voltage to 4.2 V at a current of 800 mA for 3 hours, and then left at 85 ° C. for 48 hours. The battery after standing was left at 25 ° C. for 5 hours, then discharged to 3 V at a current of 800 mA, and the discharge capacity (initial capacity) was measured. The capacity retention rate (%) after standing at high temperature was calculated from discharge capacity after standing / charged electric charge before standing at high temperature × 100.

[Low temperature cycle test]
The low temperature charge / discharge cycle test was performed under the following conditions.
The battery after the initial capacity confirmation test was charged in a constant temperature bath at −5 ° C. under the same conditions as the measurement of the initial capacity. The charged battery was left at 25 ° C. for 5 hours, and the discharge capacity was measured under the same conditions as the initial capacity measurement. The capacity retention ratio with respect to the initial capacity (= measured discharge capacity ÷ initial capacity × 100) was determined.

  Table 1 below shows the results of the initial capacity confirmation test, the high temperature storage test, and the low temperature charge / discharge cycle test.

〔Test results〕
Below, the result of an Example and a comparative example is considered.

As the results of Table 2 show, Examples 1 to 16 had larger initial capacities than Comparative Example 1 in which no addition was made. This is considered to be because these compounds suppressed electrolysis of the nonaqueous electrolyte on the negative electrode.
Moreover, Examples 1-16 also increased the capacity retention rate when left at a higher temperature than Comparative Example 1. This is considered to be because a stable protective film that suppresses the reaction with the nonaqueous electrolyte was formed on the positive electrode and the negative electrode.
Furthermore, Examples 1-16 had a larger capacity retention recovery effect after standing at a higher temperature than the compounds of Comparative Examples 2-6 that did not contain sulfur element. It is thought that this is because the presence of sulfur element in the compound made the film containing sulfur produced when decomposed at the positive electrode and the negative electrode more stable.
Elemental sulfur may be present as thiophene as in Example 2 or may be present as thioanisole as in Example 3. Thiophene having a large capacity retention after a high temperature storage test is more preferable.
Further, even when the methyl group was substituted for thiophene as in Example 6, there were two thiophenes in the structure according to Example 15 and Example 9, and the same effect was obtained.
As the types of boronic acid esters, trimethylsilyl ester as in Example 1, dimethyl ester as in Example 2, and pinacol ester as in Example 5 were all effective. In addition, by comparing Examples 2 and 4, n in the general formula (1) may be 1 or 2, and by comparing Examples, Example 5, Example 35, and Example 27. The same effect was obtained even when m in the compound general formula (2) was 1, 2, or 3.
A particularly preferred structure was the boroxine compound of Example 13, which was excellent in all the characteristics of initial discharge capacity, capacity retention after standing at high temperature, and capacity retention during low-temperature charging.
The reason is not clear, but it is thought that the reason is that the resistance increase between the positive electrode and the negative electrode can be suppressed most, and that the structure that contains the most boron and thiophene is considered to form a stable film. Yes.

In addition, in Examples 1 to 16, the capacity retention after low-temperature charging was higher than that of Comparative Example 1 without addition. On the other hand, Comparative Examples 2 to 6 to which a boron compound not containing sulfur was added had a small capacity retention after low-temperature charging.
The reason for this is that the resistance of the protective film formed on the electrode at a low temperature is increased in the film containing only boron, whereas the low-temperature lithium of the protective film formed on the electrode is included in the case of containing sulfur. It is thought that this is because the ion transfer resistance has decreased. In addition, it turned out that low-temperature discharge performance should just contain sulfur, and is not related in particular to a structure.

(Difference in amount added)
In Examples 11-16, the addition amount of the compound 39 was changed in 0.01 mass% or more and 3 mass% or less. In terms of initial discharge capacity and capacity retention after standing at high temperature, the addition amount is preferably 0.05% by mass or more, and the capacity retention during low-temperature charging is preferably 0.01% by mass or more and 2% by mass or less. The initial discharge capacity, the capacity retention after standing at high temperature, and the capacity retention during low-temperature charging, all of which are particularly preferably 0.05% by mass or more and 2% by mass or less.
From the above, the nonaqueous electrolyte is a boronic acid compound or a borinic acid compound represented by the general formula (1), a boronic acid compound represented by the general formula (2), and a boroxine compound represented by the general formula (3). By including, the discharge capacity when left at high temperature becomes good and the discharge capacity at low temperature charge can be improved.

DESCRIPTION OF SYMBOLS 1 Nonaqueous electrolyte secondary battery 2 Electrode group 3 Negative electrode plate 4 Positive electrode plate 5 Separator 6 Battery case 7 Case cover 8 Safety valve 9 Negative electrode terminal 10 Negative electrode lead

Claims (4)

Selected from the group consisting of boronic acid compounds or borinic acid compounds represented by the following general formula (1), boronic acid compounds represented by the general formula (2), and boroxine compounds represented by the general formula (3) A non-aqueous electrolyte containing at least one compound.

(In the general formulas (1) to (2), X is an organic group having 20 or less carbon atoms including a sulfur atom, and may be the same or different. R _{1 to} R ₂ are carbon atoms. An organic group having 1 to 20 atoms, n is an integer of 1 to 2, m is an integer of 1 to 3. In the general formula (3), Y is an organic group having 20 or less carbon atoms having a thiophene structure.   The nonaqueous electrolyte according to claim 1, wherein X is an organic group having a thiophene structure.   The boronic acid compound or borinic acid compound represented by the general formula (1), the boronic acid compound represented by the general formula (2), and the boroxine compound represented by the general formula (3) are selected. The nonaqueous electrolyte according to claim 1, wherein the nonaqueous electrolyte contains at least one compound in an amount of 0.01% by mass to 2% by mass.   A nonaqueous electrolyte secondary battery comprising the nonaqueous electrolyte according to claim 1.

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