JP5622424B2 - Electrolyte for secondary battery - Google Patents

Description

  The present invention relates to an electrolyte for a non-aqueous secondary battery with improved safety and battery performance, for example, an electrolyte for a lithium ion secondary battery. More specifically, the present invention relates to an electrolytic solution that improves the cycle characteristics of a battery, which is deteriorated by the coexistence of a compound having flame retardancy, by coexisting a boron compound.

  Secondary batteries using non-aqueous electrolytes, especially lithium ion secondary batteries, are used as power storage devices for portable devices such as notebook computers and mobile phones due to their high drive voltage and high energy density. Technologies that can be applied to large-scale power storage applications such as electric vehicles and natural energy storage are drawing attention.

In these lithium-based batteries, due to the high operating voltage, an aqueous solution cannot be used basically, and a nonaqueous solvent having a wide electrochemically stable potential range (potential window) is used.
Volatile and flammable organic solvents are used for the electrolytes of these non-aqueous power storage devices, and there is a risk that they may ignite due to defects during production or excessive charge / discharge.

  As a method for improving the intrinsic safety of such a non-aqueous electrolyte solution, research on making the electrolyte solution flame-retardant and non-flammable has been actively conducted. For example, in a method in which a phosphate ester used in a flame retardant such as a resin material coexists in an electrolytic solution (see, for example, Patent Document 1 and Non-Patent Documents 1 and 2), it has no flash point and is high. Nonaqueous electrolytes having electrical conductivity have been proposed.

In addition, by adding flame retardant solvents such as phosphazene compounds having fluorine atoms in the side chain, fluorine-containing phosphate esters, fluorine-containing carbonates, and fluorine-containing ethers, high output density and excellent flame resistance performance of lithium secondary batteries (For example, refer to Patent Documents 2 and 3 and Non-Patent Document 3).
However, it is known that the non-aqueous electrolyte secondary battery containing a flame retardant having these fluorine atoms is deteriorated in battery performance such as cycle characteristics as compared with the case where the flame retardant is not added. It was sought after.

  On the other hand, proposals have been made to add boron-based compounds for the purpose of obtaining high battery performance rather than flame retardancy.

  For example, for the purpose of improving large current charge / discharge characteristics and large current cycle characteristics, tris (pentafluorophenyl) borane or tris borate (2H-hexafluoroisopropyl) is added together with fluoroethylene carbonate or difluoroethylene carbonate. Thus, a method for improving the large current charge / discharge characteristics of the lithium ion secondary battery has been proposed (see, for example, Patent Document 4).

  On the other hand, tris (pentafluorophenyl) borane is known to generate PF5 by capturing LiF in the electrolyte (LiPF6). It is also described that PF5 has a very high reactivity and thus reacts with the electrolyte solvent, and as a result, the electrolyte may be deteriorated (for example, Non-Patent Document 4).

JP-A-8-22839 International Publication No. 03/005479 Japanese Patent No. 3812495 JP 2009-43597 A

CMC Publishing, “Highly Safe Technology and Materials for Lithium Ion Batteries”, pp. 123-127 (issued February 13, 2009) Journal of the Electrochemical Society, 150, A161-A169, (2003). GS Yuasa Corporation, Technical Report, Vol. 5, No. 2, pp. 32-38, (issued on December 25, 2008). Journal of Power Sources, 162, 1379-1394 (2006).

Flame retardants with fluorine atoms in the side chain (hereinafter referred to as fluorinated flame retardants), which are proposed as electrolyte flame retardants, release fluorine during combustion and suppress oxygen by capturing oxygen. When a fluorine-based flame retardant is mixed in an electrolyte solution of a non-aqueous secondary battery, there is a problem that battery performance such as cycle characteristics deteriorates. Although this mechanism is not clear, it is thought that this mechanism is caused by the progress of the decomposition reaction of some of the fluorinated flame retardants on the electrode.
On the other hand, as an additive having an effect of improving battery performance, a number of compound groups such as vinylene carbonate, vinyl ethylene carbonate, propane sultone, and fluoroethylene carbonate have been proposed in addition to boron compounds.
However, it has been found that even when the above-mentioned additives excluding a boron compound are added together with a fluorine-based flame retardant, it is impossible to achieve both improvement in flame retardancy and battery performance.

As a result of intensive studies on improving battery performance, the present inventors have found that a non-aqueous electrolyte that satisfies both safety and battery characteristics can be obtained for boron compounds even in the presence of a fluorine-based flame retardant. confirmed.
The mechanism by which the battery performance such as cycle characteristics is improved by the addition of the boron compound is not clear, but it is thought that one or both of the two effects shown below are manifested and the cycle characteristics are improved. .
(1) The boron compound was preferentially decomposed on the electrode to form a good film, and the decomposition of the fluorine-based flame retardant was suppressed.
(2) The boron-based compound efficiently captures decomposition products by-produced when the fluorine-based flame retardant decomposes on the electrode.
In addition to boron compounds, many compound groups such as vinylene carbonate, vinyl ethylene carbonate, propane sultone, and fluoroethylene carbonate have been proposed, but only when boron compounds are used, the effect of coexistence with fluorine-based flame retardants ( The fact that both safety and battery performance have been confirmed is a surprising effect, and it is difficult to analogize easily.

  That is, the electrolyte solution for secondary batteries according to the present invention is a non-aqueous electrolyte solution in which a fluorine-based flame retardant and a boron compound are added together.

As the boron compound, one represented by the following general formula (1) can be considered.

(Wherein R ₁ , R ₂ and R ₃ are the same or different and each represents hydrogen, halogen, alkyl group, aryl group, alkoxy group, fluorine-containing alkyl group, fluorine-containing aryl group or fluorine-containing alkoxy group)

As the boron compound, one represented by the following general formula (2) can be considered.

(In the formula, R ₄ , R ₅ and R ₆ are the same or different C 1-10 linear or branched fluorine-containing alkyl groups)
Examples of the alkyl group include methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group, and decyl group.
As the fluorine-containing alkyl group, monofluoromethyl group, difluoromethyl group, trifluoromethyl group, monofluoroethyl group, difluoroethyl group, trifluoroethyl group, tetrafluoroethyl group, pentafluoroethyl group, octafluorobutyl group, Mention may be made of fluorine-containing alkyl groups having 1 to 10 carbon atoms such as nonafluorobutyl group, dodecafluorohexyl group, tridecafluorohexyl group, hexadecafluorooctyl group, heptadecafluorooctyl group, eicosafluorodecyl group, etc. .
Examples of such boric acid esters include trimethyl borate, triethyl borate, triisopropyl borate, tripropylpropyl borate, trinormal butyl borate, tripentyl borate, trihexyl borate, triheptyl borate, boric acid Examples include trioctyl, trinonyl borate, tridecyl borate and the like.
Examples of the fluorine-containing boric acid ester include tris borate (2-monofluoroethyl), tris borate (2,2-difluoroethyl), tris borate (2,2,2-trifluoroethyl), boron Acid tris (2,2,3,3-tetrafluoropropyl), boric acid tris (2,2,3,3,3-pentafluoropropyl), boric acid tris (hexafluoroisopropyl), boric acid tris (2, 2,3,3,4,4,5,5-octafluoropentyl), tris borate (2,2,2,3,3,4,4,5,5-nonafluoropentyl), tris borate ( 2,2,3,3,4,5,5,6,6,7,7-dodecafluoroheptyl), tris borate (2,2,3,3,4,4,5,5,6) , 6,7,7,8,8,9,9-hexadecafluorononyl) Tris borate (2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11-eicosadecafluoroundecyl And the like.

As said boron compound, what is shown by following General formula (3) can be considered.

(Wherein X ₁ to X ₁₅ are the same or different and each represents a hydrogen atom or a fluorine atom) Among these arylboron compounds, tris (pentafluorophenyl) borane, preferably all substituted with fluorine It is.

  In the present invention, in order to improve the battery performance of the non-aqueous electrolyte secondary battery, the concentration of the boron compound in the non-aqueous electrolyte is 0.1 to 50% by weight, preferably 10 to 30% by weight. The reason for this is that when an amount exceeding 50% by weight is added, the electrical conductivity decreases due to an increase in the viscosity of the electrolytic solution. Alternatively, battery performance may be reduced due to a decrease in electrolyte solubility. On the other hand, even when the content is less than 0.1%, the effect is recognized, but the effect is small.

  Examples of the flame retardant having a fluorine atom in the side chain used in the present invention include a fluorine-containing phosphate ester, a fluorine-containing phosphazene compound, a fluorine-containing ether, a fluorine-containing carbonate and a fluorine-containing ester.

As the fluorine-containing phosphate ester, for example, the following general formula (4)
_{(Wherein, R 7, R 8, R} 9 designate the same or non-identical straight or substituted branched alkyl group or a fluorine atom alkyl group having 1 to 10 carbon atoms having 1 to 10 carbon atoms, R ₇ ˜R ₉ is at least one alkyl group substituted with a fluorine atom).

As a fluorine-containing carbonate, for example, the general formula (5)

(In the formula, R ₁₀ and R ₁₁ represent the same or non-identical alkyl group having 1 to 10 carbon atoms or an alkyl group having 1 to 10 carbon atoms substituted with a fluorine atom, and at least of R ₁₀ and R ₁₁ , One is an alkyl group substituted with a fluorine atom.).

As fluorine-containing ether, for example, the general formula (6)

(Wherein R ₁₂ and R ₁₃ are the same or non-identical alkyl group having 2 to 6 carbon atoms, alkoxyalkyl group having 2 to 10 carbon atoms, alkyl group having 2 to 6 carbon atoms substituted with a fluorine atom, or fluorine. An alkoxyalkyl group having 2 to 10 carbon atoms substituted with an atom, and at least one of R ₁₂ and R ₁₃ is an alkyl group having a fluorine atom or an alkoxyalkyl group substituted with a fluorine atom. Mention may be made of the fluorinated ethers indicated.

As fluorine-containing ester, for example, the general formula (7)

(In the formula, R ₁₄ and R ₁₅ represent the same or non-identical alkyl group having 2 to 6 carbon atoms or an alkyl group having 2 to 6 carbon atoms substituted with a fluorine atom, and among R ₁₄ and R ₁₅ , at least One is an alkyl group substituted with a fluorine atom.).

As the fluorine-containing phosphazene, for example, the general formula (8)
(8)
(Wherein R _{16 to} R ₂₁ are the same or non-identical hydrogen atom, halogen atom, linear or branched alkyl group having 1 to 10 carbon atoms, alkyl group having 1 to 10 carbon atoms substituted with a fluorine atom, 1 represents an alkoxy group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms substituted with a fluorine atom, and at least one of R _{16 to} R ₂₁ is a fluorine atom, an alkyl group substituted with a fluorine atom, or fluorine And any one of alkoxy groups substituted with an atom).

  Among the fluorine-containing flame retardants described above, fluorine-containing phosphates and fluorine-containing phosphazene compounds are particularly preferable in terms of flame retardancy and battery performance.

  In the present invention, in order to improve the flame retardancy of the non-aqueous electrolyte secondary battery, the amount of the fluorine-containing flame retardant added in the non-aqueous electrolyte is preferably as much as possible, but when considering the balance with the battery performance Desirably, the content is 5% by weight or more and 50% by weight or less, and more preferably 10% by weight or more and 50% by weight or less. The reason is that sufficient flame retardancy cannot be exhibited at 5 wt% or less. On the other hand, if it is 50% by weight or more, the electrical conductivity decreases due to an increase in the viscosity of the electrolytic solution. Or the solubility of the electrolyte may be reduced.

Next, the non-aqueous electrolyte solution of the present invention will be described.
Representative examples of organic solvents that are usually used as non-aqueous electrolytes include, for example, cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, and chloroethylene carbonate, γ-butyrolactone, γ-valerolactone, and propiolactone. Cyclic esters, chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, diphenyl carbonate, chain esters such as methyl acetate, methyl butyrate, tetrahydrofuran, 1,3-dioxane, dimethoxyethane, diethoxyethane, methoxyethoxyethane , Ethers such as methyl diglyme, nitriles such as acetonitrile and benzonitrile, dioxolane or a derivative thereof alone or a mixture of two or more thereof. Yes.

As the electrolyte salt constituting the non-aqueous electrolyte solution, a lithium salt that is stable in a wide potential region used in a non-aqueous secondary battery can be used. Examples of such electrolyte salts include LiBF ₄ , LiPF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ). ₃ etc. are mentioned. These may be used alone or in combination of two or more. In order to improve the high rate charge / discharge characteristics of the battery, the concentration of the electrolyte salt in the non-aqueous electrolyte is 0.1 to 2.5 mol / L, particularly preferably 0.5 to 1.5 mol / L. A range is desirable.

  The electrolyte solution for secondary batteries according to the present invention satisfies both safety (flame retardancy) and battery characteristics (such as large current charge / discharge characteristics and large current cycle characteristics).

It is a figure which shows the battery of the lamination cell used for the Example and the comparative example.

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to these.

1. Preparation of electrolyte solution As a solvent for an electrolyte solution, a solvent in which ethylene carbonate (hereinafter abbreviated as EC) and dimethyl carbonate (hereinafter abbreviated as DMC) are mixed at a volume ratio of 1: 1 is used, and a predetermined amount of flame retardant and boron compound are added thereto. What mixed 1.0 mol / L of lithium hexafluorophosphate (LiPF6) as electrolyte was used as electrolyte solution in what was mixed.

2. Preparation of non-aqueous lithium ion secondary battery Lithium cobaltate (LiCoO ₂ ) was used as a positive electrode active material, carbon black as a conductive auxiliary agent, and polyvinylidene fluoride (PVDF) as a binder, LiCoO ₂ : carbon black: PVDF = 85 : 7: 8 The slurry was mixed with 1-methyl-2-pyrrolidone and applied to the aluminum current collector with a certain film thickness and dried to obtain a positive electrode.
A lithium metal foil was used as the negative electrode active material, and a negative electrode was obtained by pressure bonding to a copper current collector.
A glass filter was used as the separator.

  Using the above components, a lithium ion secondary battery using a laminate package having the structure shown in FIG. 1 was produced. In the lithium ion secondary battery, the positive electrode 3 and the negative electrode 5 are arranged opposite to each other with the separator 4 interposed therebetween, and a laminate composed of the positive electrode 3, the separator 4 and the negative electrode 5 is accommodated in the laminate package 6. This is carried out via the positive electrode lead 1 and the negative electrode lead 2 provided in FIG.

3. Charging / discharging test The battery thus prepared was charged at a constant temperature of 25 ° C. with a charging current of 0.1 C and an upper limit voltage of 4.2 V, and then discharged to 3.0 V with a discharging current of 0.1 C. did. After this operation, a constant current / constant voltage charge of 4.2 V was performed with a charging current of 1 C under a constant temperature of 55 ° C., and a constant current discharge was performed up to a final voltage of 3.0 V with a discharging current of 1 C. The discharge capacity at this time was defined as the initial discharge capacity, the discharge capacity when this operation was repeated 50 times was measured, and the comparison was performed using the discharge capacity / initial discharge capacity ratio after 50 cycles as the deterioration rate.

4). Flame retardancy (self-extinguishing) test 0.5 g of electrolyte solution was dropped into 0.25 g of glass wool in a circular shape of about 2 cm. The glass wool in which this electrolyte solution was immersed was exposed to flame and ignited, and the presence or absence of ignition and the time until extinguishing when ignited were measured. This test was measured 5 times, and the average value of 3 points, excluding the upper and lower 2 points out of 5 times, was compared as the fire extinguishing time per 1 g (SET, sec / g).

Example 1
As a flame retardant, 15% by weight of tris phosphate (2,2,2-trifluoroethyl) (hereinafter abbreviated as TFEP) is mixed and tris (2,2,2-trifluoroethyl borate) (hereinafter referred to as TFEB). And an electrolyte solution in which 5 wt% was mixed. Using this electrolytic solution, the above-mentioned non-aqueous lithium ion secondary battery 2 was prepared, and 0.1 C charge / discharge was repeated 4 times according to the theoretical capacity of the battery, followed by 1 C charge / discharge. At that time, the discharge capacity of the battery was 131 mAh / g.

Furthermore, using this battery, a charge / discharge test was performed 50 times at 1C under a temperature condition of 55 ° C. As a result, the discharge capacity of the battery was 118 mAh / g, and the cycle deterioration rate after 50 cycles was 90%. In addition, the flame retardancy test of 4 above was performed using this electrolytic solution. As a result, the self-extinguishing time (SET) per 1 g was 10 seconds.
The results are shown in Table 1.

Comparative Example 1
An electrolyte solution containing no boron compound and flame retardant was prepared. Using this electrolytic solution, a battery was prepared in the same manner as in Example 1, and a charge / discharge test and a flame retardancy test were performed. As a result, the initial discharge capacity was 135 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 120 mAh / g. The deterioration rate of this battery was 89%. Moreover, the flame retardance test result using this electrolyte solution was 128 seconds / g.

Comparative Example 2
An electrolytic solution in which 15% by weight of TFEP was mixed as a flame retardant without using a boron compound was prepared. Using this electrolytic solution, a battery was prepared in the same manner as in Example 1, and a charge / discharge test was performed.
As a result, the initial discharge capacity was 132 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 66 mAh / g. The deterioration rate of this battery was 50%. Moreover, the flame retardance test result using this electrolyte solution was 32 seconds / g.

Example 2
As a flame retardant, 15% by weight of tris (2,2,2-trifluoroethyl) phosphate (abbreviated as TFEP) was mixed, and tris (pentafluorophenyl) borane (hereinafter abbreviated as FAB) as a boron compound as a boron compound. An electrolyte solution mixed with 5% by weight was prepared. A battery was prepared using this electrolytic solution in the same manner as in Example 1, and a charge / discharge test was performed. As a result, the initial discharge capacity was 126 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 115 mAh / g. This battery was hardly deteriorated, and the deterioration rate was 91%. Moreover, the flame retardance test result using this electrolyte solution was 12 seconds / g.

Example 3
As a flame retardant, tris (2,2,3,3-tetrafluoropropyl) phosphate (abbreviated as TFPP) is mixed in an amount of 15% by weight. As a boron compound, tris (2H-hexafluoroisopropyl) borate (hereinafter abbreviated as HFPB) is mixed. ) Was mixed with 5 wt% to prepare an electrolyte solution. Using this electrolytic solution, a battery was prepared according to Example 1, and a charge / discharge test was performed. As a result, the initial discharge capacity was 130 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 117 mAh / g. The deterioration rate of this battery was 90%. Moreover, the flame retardance test result using this electrolyte solution was 9 seconds / g.

Example 4
As a flame retardant, 15% by weight of bis (2,2,2-trifluoroethyl) phosphate (2,2,3,3-tetrafluoropropyl) (abbreviated as E2PP) was mixed, and triethyl borate ( Hereinafter, an electrolytic solution in which 5% by weight of TEB) was mixed was prepared. Using this electrolytic solution, a battery was prepared according to Example 1, and a charge / discharge test was performed. As a result, the initial discharge capacity was 132 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 106 mAh / g. The deterioration rate of this battery was 80%. Moreover, the flame retardance test result using this electrolyte solution was 32 seconds / g.

Example 5
An electrolyte solution was prepared by mixing 15% by weight of TFEP as a flame retardant and 1% by weight of TFEB as a boron compound. A battery was prepared using this electrolytic solution in the same manner as in Example 1, and a charge / discharge test was performed. As a result, the initial discharge capacity was 132 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 104 mAh / g. The deterioration rate of this battery was 79%. Moreover, the flame retardance test result using this electrolyte solution was 18 seconds / g.

Example 6
An electrolyte solution was prepared by mixing 30% by weight of TFEP as a flame retardant and 11% by weight of TFEB as a boron compound. A battery was prepared using this electrolytic solution in the same manner as in Example 1, and a charge / discharge test was performed. As a result, the initial discharge capacity was 121 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 115 mAh / g. The deterioration rate of this battery was 95%. Moreover, in the flame retardancy test using this electrolyte solution, it did not ignite in all the tests.

Example 7
An electrolyte solution was prepared by mixing 40% by weight of TFEP as a flame retardant and 15% by weight of TFEB as a boron compound. A battery was prepared using this electrolytic solution in the same manner as in Example 1, and a charge / discharge test was performed. As a result, the initial discharge capacity was 115 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 113 mAh / g. The deterioration rate of this battery was 98%. Moreover, in the flame retardancy test using this electrolyte solution, it did not ignite in all the tests.

Example 8
An electrolyte solution was prepared by mixing 10% by weight of TFPP as a flame retardant and 30% by weight of TFEB as a boron compound. A battery was prepared using this electrolytic solution in the same manner as in Example 1, and a charge / discharge test was performed. As a result, the initial discharge capacity was 129 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 125 mAh / g. The deterioration rate of this battery was 97%. Moreover, the flame retardance test result using this electrolyte solution was 13 seconds / g.

Example 9
As a flame retardant, 10% by weight of 2-ethoxy-2,4,4,6,6-pentafluoro-1,3,5,2λ ⁵ , 4λ ⁵ , 6λ ⁵ -triazatriphosphine (hereinafter referred to as PFPN) An electrolytic solution in which 5% by weight of TFEB was mixed as a boron compound was prepared. A battery was prepared using this electrolytic solution in the same manner as in Example 1, and a charge / discharge test was performed. As a result, the initial discharge capacity was 120 mAh / g, and the discharge capacity after 50 cycles at 55 ° C. was 110 mAh / g. The deterioration rate of this battery was 92%. Moreover, the flame retardance test result using this electrolyte solution was 35 seconds / g.

DESCRIPTION OF SYMBOLS 1 Positive electrode lead 2 Negative electrode lead 3 Positive electrode 4 Separator 5 Negative electrode 6 Laminated package

Claims (5)

Nonaqueous secondary battery electrolyte that uses a combination of positive and negative electrodes capable of occluding and releasing ions, including fluorine-containing phosphate ester, fluorine-containing phosphazene, fluorine-containing ether, fluorine-containing chain carbonate and An electrolyte solution for a secondary battery, wherein at least one fluorine-based flame retardant selected from the group consisting of fluorine carboxylates and a boron compound represented by the following general formula (1) are coexistingly added.

Wherein R ₁ , R ₂ and R ₃ are the same or different and each represents hydrogen, halogen, alkyl group, aryl group, alkoxy group, fluorine-containing alkyl group, fluorine-containing aryl group or fluorine-containing alkoxy group. The electrolyte solution for a secondary battery according to claim 1, which is a boron compound. Nonaqueous secondary battery electrolyte that uses a combination of positive and negative electrodes capable of occluding and releasing ions, including fluorine-containing phosphate ester, fluorine-containing phosphazene, fluorine-containing ether, fluorine-containing chain carbonate and An electrolytic solution for a secondary battery, wherein at least one fluorine-based flame retardant selected from the group consisting of fluorine carboxylates and a boron compound represented by the following general formula (2) are coexistingly added.

The fluorinated boric acid ester represented by the formula (wherein R ₄ , R ₅ and R ₆ represent the same or different linear or branched fluorine-containing alkyl group having 1 to 10 carbon atoms): Secondary battery electrolyte. Nonaqueous secondary battery electrolyte that uses a combination of positive and negative electrodes capable of occluding and releasing ions, including fluorine-containing phosphate ester, fluorine-containing phosphazene, fluorine-containing ether, fluorine-containing chain carbonate and An electrolytic solution for a secondary battery, wherein at least one fluorine-based flame retardant selected from the group consisting of fluorine carboxylates and a boron compound represented by the following general formula (3) are coexistingly added.








The electrolyte solution for a secondary battery according to claim 1, which is an arylborane represented by the formula (wherein X _{1 to} X ₁₅ each represent the same or different hydrogen atom or fluorine atom). The electrolytic solution for a secondary battery according to claim 1 or 2 , wherein the boron compound is tris (trifluoroethyl) borate.   The secondary battery electrolyte according to any one of claims 1 to 4, wherein the secondary battery is a lithium ion secondary battery.