JP2019220450A - Electrolytic solution for rocking chair type secondary battery, and rocking chair type secondary battery - Google Patents

Abstract

To raise the charge/discharge performance of a rocking chair type secondary battery which is superior in oxidation stability and thermal stability to a level which has not been achieved in the past, and/or to suppress the corrosion of an Al current collector of a rocking chair type secondary battery having the Al current collector when used under a high-positive electrode electric potential environment.SOLUTION: An electrolytic solution for a rocking chair type secondary battery comprises: an alkali metal salt; and an oxygen-containing solvent including an ester bond. In the electrolytic solution, the alkali metal salt of a quantity which is 2.0 mol or more to 1 kg of the oxygen-containing solvent is included within a range of a concentration of saturation or below.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an electrolyte for a rocking chair type secondary battery and a rocking chair type secondary battery.

  2. Description of the Related Art Conventionally, the development of a so-called rocking chair type secondary battery, in which ions are charged and discharged by reciprocating ions in an electrolytic solution disposed between electrodes, has been advanced. In recent years, for next-generation rocking chair type secondary batteries, electrolytes having excellent oxidation stability and thermal stability have been demanded. For example, while improving the charge and discharge performance by increasing the concentration of the alkali metal salt as an electrolyte, focused on improving the oxidation stability and thermal stability by solvating the alkali metal salt and a solvent, Electrolytes with high alkali metal salt concentrations (ie, rich electrolytes) have received attention. In addition, for the current collector for the positive electrode of the rocking chair type secondary battery, a metal (for example, Al (aluminum) or an Al alloy) that forms a stable passivation film on the surface is used in order to withstand corrosion by the electrolytic solution. However, in the next-generation rocking chair type secondary battery, a higher positive electrode potential (for example, around 4.0 V or higher) than in the past is assumed. However, in an environment with a high positive electrode potential (for example, exceeding 3.5 V), even if the current collector has a passivation film formed on its surface, corrosion gradually progresses, and Al is removed from the current collector. There is a concern that battery performance will be reduced due to elution. Therefore, there is a demand for an electrolyte solution that does not easily corrode the positive electrode current collector even under a high positive electrode potential environment.

  Regarding the improvement of the oxidative stability and thermal stability of the electrolytic solution, as the solvent for the concentrated electrolytic solution, a glyme-based solvent (an alkyl-terminated ether compound) or a carbonate-based solvent (an alkyl-terminated carbonate ester) is used. Compounds) are the mainstream and have been widely studied. For example, Patent Literature 1 discloses that a rechargeable lithium battery manufactured using 1,2-dimethoxyethane, ethylene carbonate, ethyl methyl carbonate, or the like as a solvent for a concentrated electrolyte has achieved high energy density and improved cycle life. Is described.

  In the lithium battery described in Patent Literature 1, although the charge / discharge performance is improved due to the high alkali metal salt concentration, the viscosity of the electrolytic solution tends to increase. In general, the movement of alkali metal ions derived from an alkali metal salt is restricted in a high-viscosity electrolytic solution. Therefore, when the salt concentration is the same, a lower electrolytic solution viscosity increases the mobility of the alkali metal ion. Thereby, the charge / discharge performance is improved.

Patent Literature 2 discloses that a solvent for an electrolytic solution composed of a chain ester represented by the following general formula is used for the purpose of broadening the operating temperature range of a non-aqueous electrolyte secondary battery or the like. .
R ¹ -OR ² -COO-R ³
In the above formula, R ¹ and R ³ represent a linear, branched or cyclic alkyl group or fluoroalkyl group having 1 to 20 carbon atoms, and R ² represents a linear chain having 1 to 20 carbon atoms. , A branched or cyclic alkylene group or a fluoroalkylene group. Patent Document 2 teaches that the concentration of the electrolyte in the electrolytic solution is such that if the electrolyte is present at a ratio of 50% by mass or more with respect to the mass of the solvent, the viscosity of the electrolytic solution increases, and ion migration is greatly inhibited. I have. That is, it is suggested that the chain ester described in Patent Document 2 should be avoided from being used as a solvent in a concentrated electrolytic solution having an electrolyte concentration of more than 50% by mass.

  On the other hand, as a method of suppressing corrosion of the current collector, a method of selecting an electrolyte salt (for example, a lithium salt in a lithium ion battery) and a solvent in an electrolytic solution, and a method of decomposing on a current collector surface to form a protective film There are known a method of incorporating such a compound into an electrolytic solution as an additive, alloying of a current collector, various processes (coating of an inorganic layer or an organic layer, etc.), and the like.

  Patent Document 3 relates to an electrolytic solution that focuses on corrosion inhibition of a current collector. Patent Document 3 discloses a nonaqueous electrolytic solution used for a power storage device including a current collector containing aluminum or an aluminum alloy, and a polyethylene oxide derivative having a specific structure. And a non-aqueous solvent containing an aprotic organic solvent, wherein lithium bis (trifluoromethanesulfonyl) amide is dissolved as a supporting electrolyte.

Patent Literature 4 discloses a lithium ion secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte. The non-aqueous electrolyte includes LiPF ₆ and an imide salt having a specific structure. A lithium ion secondary battery, wherein the body is an aluminum foil having a purity of 97.0% or more and 99.0% or less is described.

  Patent Document 5 is a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and an electrolytic solution, wherein the positive electrode has a positive electrode current collector made of aluminum or an aluminum alloy, An alkali metal, an alkaline earth metal or a salt having aluminum as a cation, and an organic solvent having a hetero element, the peak intensity derived from the organic solvent in the vibration spectrum of the electrolytic solution, the original peak of the organic solvent A non-aqueous electrolyte secondary battery characterized by Is> Io where Io is the intensity and Is is the intensity of the peak shifted from the peak.

  Patent Document 6 describes an electrolyte solution containing an electrolyte salt and an electrolyte solvent, wherein the electrolyte solvent contains a specific ratio of fluorinated acyclic dialkyl carbonate with respect to the total amount of the electrolyte solvent of 100% by weight. .

JP-T-2018-505538 JP-A-10-92222 JP 2014-157672 A JP 2017-84739 A JP-A-2006-189340 JP 2006-532237 A

  As described above, as a measure for improving the charge / discharge performance, it is conceivable to increase the charge amount, that is, to increase the salt concentration of the electrolytic solution, and to increase the mobility of alkali metal ions, that is, to decrease the viscosity of the electrolytic solution. However, as suggested in Patent Literature 2, generally, increasing the concentration of the electrolyte and lowering the viscosity of the electrolyte have contradictory properties. One embodiment of the present invention is to improve the charge / discharge performance of a rocking chair type secondary battery having excellent oxidation stability and heat stability by achieving the contradictory properties of increasing the concentration of the electrolyte and decreasing the viscosity. The task is to raise the level to a level that has never been seen before.

In addition, as described above, as a measure for suppressing corrosion of a current collector containing Al or an Al alloy (also referred to as “Al current collector” in the present disclosure), a specific method is used in which the molecular structure of a solvent or an electrolyte salt is specified. In this case, a passivation film (for example, Al ₂ O ₃ , and further, when the electrolyte salt is, for example, fluoride, LiF, AlF _3, etc.) is provided on the surface of the Al current collector. Can be formed. However, in the conventional technique, even under an environment of a high positive electrode potential (for example, 4.0 V or more) as expected in a next-generation rocking chair type secondary battery, the effect of suppressing corrosion of the current collector has not been sufficiently obtained. . An object of one embodiment of the present invention is to avoid corrosion of an Al current collector even when a rocking chair secondary battery is used in an environment with a high positive electrode potential (for example, 4.0 V or more). .

The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, the use of a specific oxygen-containing solvent has suppressed the increase in the viscosity of the concentrated electrolytic solution, and thus has been confirmed to be a concentrated electrolytic solution having excellent oxidation stability and thermal stability. It has been found that the charge and discharge performance of a rocking chair type secondary battery using a liquid is significantly improved as compared with the conventional one. That is, the present invention includes the following embodiments.
[1] An alkali metal salt and an oxygen-containing solvent containing an ester bond are contained, and the alkali metal salt is contained in an amount of 2.0 mol or more and a saturation concentration or less with respect to 1 kg of the oxygen-containing solvent. An electrolyte solution for a rocking chair type secondary battery, wherein the electrolyte is used.
[2] The electrolytic solution according to aspect 1, wherein the alkali metal salt is contained in an amount within a range from 4.0 mol to 12.0 mol per 1 kg of the oxygen-containing solvent.
[3] The electrolytic solution according to aspect 1 or 2, wherein the oxygen-containing solvent is represented by the following general formula (1).
R ¹ -O- (A-O) _n -CH ₂ CH (R ³ ) -COO-R ² (1)
(Wherein, R ¹ and R ² each independently represent a linear or branched alkyl group having 1 to 4 carbon atoms, R ³ represents a methyl group or H 2, and A each independently represents 2 to 4 carbon atoms. 4 represents an alkylene group, and n represents an integer of 0 to 5.)
[4] The electrolytic solution according to aspect 3, wherein in the general formula (1), R ¹ and R ² are both methyl groups, R ³ is H, and n is 0.
[5] The alkali metal salt is selected from lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), lithium bis (fluorosulfonyl) imide (LiFSI), lithium tetrafluoroborate (LiBF ₄ ), and lithium hexafluorophosphate (LiPF _6). ), Lithium perchlorate (LiClO ₄ ) and lithium bis (pentafluoroethanesulfonyl) imide (LiBETI). The electrolytic solution according to any one of aspects 1 to 4, wherein the electrolytic solution is selected from the group consisting of:
[6] The electrolytic solution according to any one of aspects 1 to 5, wherein no solvent is present other than the oxygen-containing solvent.
[7] The electrolytic solution according to any one of aspects 1 to 6, further comprising a metal ester compound represented by the following general formula (2).
M [-O- (A ¹ -O) _m -R ⁴ ] ₃ (2)
(Wherein, M represents a trivalent element selected from the group consisting of boron and aluminum, A ¹ each independently represents an alkylene group having 2 to 4 carbon atoms, and m each independently represents an integer of 0 to 50. And each R ⁴ independently represents an aliphatic hydrocarbon group having 1 to 13 carbon atoms.)
[8] The electrolytic solution according to any one of aspects 1 to 7, which comprises a complex of the alkali metal derived from the alkali metal salt and the oxygen-containing solvent.
[9] A rocking chair secondary battery including a positive electrode, a negative electrode, and the electrolytic solution according to any one of aspects 1 to 8.

  According to one aspect of the present invention, by realizing suppression of the increase in viscosity of a concentrated electrolytic solution having excellent oxidation resistance and thermal stability, an electrolytic solution for a rocking chair type secondary battery having further improved charge / discharge performance than before, And a rocking chair type secondary battery using the same. Further, according to one embodiment of the present invention, even when a rocking chair secondary battery including an Al current collector is used in a high positive electrode potential (eg, 4.0 V or higher) environment, corrosion of the Al current collector is avoided. Can be done.

5 is a graph showing the relationship between electrolyte concentration and electrolyte viscosity. 5 is a graph showing a relationship between a discharge capacity and an electrolyte concentration. 6 is a graph showing the results of a charge / discharge test for an embodiment in which a metal ester compound was added.

Hereinafter, exemplary embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.
The present invention comprises an alkali metal salt and an oxygen-containing solvent containing an ester bond, wherein the alkali metal salt is contained in an amount within a range of 2.0 mol or more to 1 kg of the oxygen-containing solvent and a saturation concentration or less. It is intended to provide an electrolyte solution for a rocking chair type secondary battery characterized by being included.

(Alkali metal salt)
The alkali metal salt used in the electrolyte according to the present invention may be any alkali metal salt understood by those skilled in the art to be useful as an electrolyte for a rocking chair secondary battery. The alkali metal constituting the alkali metal salt can be selected from lithium, sodium, potassium, rubidium and cesium, but is preferably lithium, sodium or potassium from the viewpoint of easy availability or production. Further, lithium is more preferable from the viewpoint of easy complex formation with an oxygen-containing solvent containing an ester bond. That is, the electrolyte in a more preferred embodiment is an electrolyte for a lithium ion secondary battery.

As counter anions constituting the alkali metal salt, Cl, Br, I, BF ₄ , PF ₆ , CF ₃ SO ₃ , SCN, ClO ₄ , CF ₃ CO ₂ , AsF ₆ , SbF ₆ , AlCl ₄ , N (CF ₃ SO ₂ ) ₂ , PF ₃ (C ₂ F ₅ ) ₃ , N (FSO ₂ ) ₂ , N (FSO ₂ ) (CF ₃ SO ₂ ), N (C ₂ F ₅ SO ₂ ) ₂ , N (C _{2 F 4 S 2 O 4),} N (C 3 F 6 S 2 O 4), N (CN) 2, N (CF 3 SO 2) (CF 3 CO), illustrate C (CF ₃ SO _{2) 3,} etc. it can.

Lithium salt, which is a preferred example of an alkali metal salt, may be any lithium salt understood by those skilled in the art to be useful as an electrolyte for a rocking chair type secondary battery, for example, LiTFSI, LiFSI, LiBF ₄ , LiPF ₆ , LiClO ₄ , LiBETI, LiAsF ₆ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₃ C, LiI, LiSCN, and the like. Among them, preferred lithium salts are LiTFSI (lithium bis (trifluoromethanesulfonyl) imide), LiFSI (lithium bis (fluorosulfonyl) imide), LiBF ₄ (lithium tetrafluoroborate), LiPF ₆ (lithium hexafluorophosphate), It is selected from the group consisting of lithium perchlorate (LiClO ₄ ) and LiBETI (lithium bis (pentafluoroethanesulfonyl) imide). These lithium salts are commercially available.

When the alkali metal salt is LiPF ₆ , since LiPF ₆ can form a passivation film of a fluorine compound (LiF, AlF _3, etc.) on the Al current collector, for example, when the alkali metal salt is an imide-based lithium salt or the like This is preferable because the corrosion effect of the current collector is even better.

(Oxygen-containing solvent containing ester bond)
The oxygen-containing solvent containing an ester bond used in the electrolytic solution according to the present invention is a compound capable of forming a complex with the above alkali metal. One embodiment of such a compound includes a compound represented by the following general formula (1).
R ¹ -O- (A-O) _n -CH ₂ CH (R ³ ) -COO-R ² (1)
In the formula (1), R ¹ and R ² each independently represent a linear or branched alkyl group having 1 to 4 carbon atoms, R ³ represents a methyl group or H 2, and A represents each independently a carbon number. Represents an alkylene group of 2 to 4, and n represents an integer of 0 to 5.

R ¹ and R ² in the above general formula (1) are linear or branched alkyl groups having 1 to 4 carbon atoms from the viewpoint of further enhancing the complexability with an alkali metal. Specific examples of such an alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a t-butyl group, and the like. Among them, a methyl group, an ethyl group, and a propyl group are preferable. Particularly, a methyl group is preferable. Also, R ³ is preferably H 2.

The total number of carbon atoms of R ¹ , R ² and R ³ in the general formula (1) is preferably 5 or less, more preferably 3 or less, from the viewpoint of increasing the electrolyte salt concentration while suppressing the increase in the viscosity of the electrolyte. is there. Here, R ¹ and R ² may have a structure having a substituent, and in this case, each carbon number of R ¹ and R ² is a number including the substituent. Examples of the substituent that R ¹ and R ² may have include an alkoxy group having 1 to 3 carbon atoms, a fluoro group, a chloro group, a bromo group, a cyano group, a nitro group, a hydroxyl group, an aldehyde group, and a carboxyl group. Can be

  A in the general formula (1) is preferably an ethylene group having 2 carbon atoms or a propylene group having 3 carbon atoms, and particularly preferably an ethylene group, from the viewpoint of further increasing the complexability with an alkali metal. . Further, n in the above general formula (1) is preferably 2 or less, more preferably 1 or less, and most preferably 0 from the viewpoint of further enhancing the complexability with an alkali metal.

  Particularly preferred compounds as the oxygen-containing solvent containing an ester bond according to the present invention include methyl 3-methoxypropionate, ethyl 3-methoxypropionate, methyl 3-ethoxypropionate, methyl 3-methoxy-2-methylpropionate, and the like. Is mentioned. These compounds can be used by appropriately obtaining commercially available products.

  In the electrolytic solution according to the present invention, the alkali metal salt is contained in an amount of 2.0 mol or more and a saturation concentration or less with respect to 1 kg of the oxygen-containing solvent containing an ester bond. When the molar concentration of the alkali metal salt with respect to 1 kg of the oxygen-containing solvent is within the above range, the increase in the viscosity of the concentrated electrolytic solution having excellent oxidation resistance and thermal stability is suppressed, and the charge and discharge performance of the rocking chair type secondary battery is reduced. It has been found that it is even better. The molar concentration at which the discharge capacity is particularly improved among the charge and discharge performances is 3.0 mol or more, preferably 4.0 mol or more, more preferably 5.0 mol or more. On the other hand, the upper limit of the molar concentration is 12.0 mol or less, preferably 10.0 mol or less, and more preferably 8.0 mol or less. When the molar concentration is less than 2.0 mol, the concentration of the alkali metal salt decreases, and the concentration falls outside the range of the concentrated electrolytic solution contemplated by the present invention.

  In the electrolytic solution according to the present invention, at least a part, preferably substantially all, of the alkali metal derived from the alkali metal salt forms a complex with the oxygen-containing solvent containing an ester bond. The formation of such a complex improves the oxidation stability and thermal stability of the electrolyte of the secondary battery, and suppresses corrosion of the Al current collector when the Al current collector is used as the current collector. You. The presence of the complex can be confirmed by thermogravimetry. The electrolytic solution according to the present invention favorably suppresses corrosion of the Al current collector even in a secondary battery used under a high positive electrode potential environment of, for example, 4.0 V or more by including an oxygen-containing solvent containing an ester bond. This contributes to higher performance of next-generation batteries. Without being bound by theory, the oxygen-containing solvent containing an ester bond forms a complex with an alkali metal salt in the electrolytic solution, and the complex has a stable coordination structure even under an environment of a high positive electrode potential. It is considered that by holding, the corrosion of the current collector can be favorably suppressed. Such an effect cannot be obtained with a glyme-based solvent and a carbonate-based solvent conventionally used for a concentrated electrolyte. In particular, the electrolytic solution according to the present invention can suppress the corrosion of the current collector without requiring the processing of the current collector, and thus has an advantage of simplifying the battery manufacturing process and thereby reducing the cost.

  As long as the intended effect is not impaired, the electrolytic solution according to the present invention contains a small amount of another oxygen-containing solvent capable of forming a complex with an alkali metal (50% of the total amount of the solvent) in addition to the above-described oxygen-containing solvent containing an ester bond. % By mass). Such other oxygen-containing solvents include conventional grime solvents (alkyl-terminated ether compounds) and carbonate solvents (alkyl-terminated carbonate compounds). However, it is preferable that the solvent of the electrolytic solution according to the present invention is substantially free of a conventional oxygen-containing solvent and is composed of the above-described oxygen-containing solvent containing an ester bond.

(Metal ester compound)
The electrolytic solution according to the present invention may further contain a metal ester compound as long as the intended effect is not impaired. Examples of such a metal ester compound include those represented by the following general formula (2).
M [-O- (A ¹ -O) _m -R ⁴ ] ₃ (2)
In the above formula, M represents a trivalent element selected from the group consisting of boron and aluminum, A ¹ each independently represents an alkylene group having 2 to 4 carbon atoms, and m is each independently an integer of 0 to 50. And each R ⁴ independently represents an aliphatic hydrocarbon group having 1 to 13 carbon atoms.

  Without being bound by any particular theory, the Lewis acidity of boron or aluminum of the metal ester compound attracts the oxygen of the oxygen-containing solvent in the complex composed of the alkali metal and the oxygen-containing solvent. It is conceivable to lower the activation energy of desolvation. For this reason, the coexistence of such a metal ester compound maintains good oxidative stability and thermal stability due to the complex formation of the alkali metal and the oxygen-containing solvent, and at the same time, the alkali metal ion in the electrolyte solution. , High ionic conductivity and low charge transfer resistance due to good mobility can be realized. In particular, in the metal ester compound having an alkylene oxide chain, the ether oxygen in the alkylene oxide chain bridges the electrode together with the desolvated alkali metal ion, and the charge transfer resistance in the vicinity of the electrode-electrolyte interface. Is considered to be reduced.

  Further, the metal ester compound represented by the general formula (2) contributes to suppressing corrosion of the Al current collector. Without being bound by theory, current collector corrosion is caused by solvent molecules and electrolyte components in the electrolytic solution. It is effective to suppress direct contact with the body. The metal ester compound represented by the general formula (2) is arranged on the current collector surface due to the high affinity of the group 13 element (that is, boron or aluminum) with the Al current collector, and It is believed that a protective layer is formed thereon. Thus, it is considered that the solvent molecules and the electrolyte component in the electrolytic solution are less likely to approach the surface of the Al current collector, and a corrosion inhibiting effect is obtained.

In the general formula (2), M is a trivalent element selected from the group consisting of boron and aluminum from the viewpoint of promoting the desolvation of alkali metals by the contribution of Lewis acidity. M is preferably aluminum from the viewpoint of good function as Lewis acidity and good affinity with the Al current collector. A ¹ is independently an alkylene group having 2 to 4 carbon atoms, preferably an ethylene group, from the viewpoint of obtaining a good effect of reducing charge transfer resistance. m is independently an integer of 0 to 50, but from the viewpoint of obtaining a good effect of reducing the charge transfer resistance, the lower limit is preferably 3 or more. On the other hand, the upper limit of m is such that (1) the metal part of the metal ester compound becomes bulky, so that the oxygen in the oxygen-containing solvent and the metal part hardly interact with each other. If the alkylene oxide chain is too long, it will be difficult to transport the desolvated alkali metal ion. (3) If the alkylene oxide chain of the metal ester compound is too long, the alkylene oxide chain will be oxidatively decomposed and the oxidation stability will decrease. From the viewpoints of performing, etc., it is 50 or less, preferably 20 or less, and more preferably 9 or less. m is 3 from the viewpoint of a favorable corrosion inhibiting effect of the Al current collector.

In the general formula (2), R ⁴ is each independently an aliphatic hydrocarbon group having 1 to 13 carbon atoms, from the viewpoint of obtaining a good effect of reducing the charge transfer resistance and a good effect of suppressing corrosion of the Al current collector. . An aliphatic hydrocarbon group can be straight, branched, or cycloaliphatic, and can be saturated or unsaturated. Examples of R ⁴ include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a t-butyl group, a pentyl group, an isopentyl group, a hexyl group, a 2-ethylhexyl group, a lauryl group, and a tridecyl group. And preferably a methyl group, an ethyl group, and a propyl group, and particularly preferably a methyl group. Examples of the alkenyl group having 3 to 6 carbon atoms for R ⁴ include an allyl group, a 2-methylallyl group, a 3,3-dimethylallyl group, and a 2,3,3-trimethylallyl group. , 2-methylallyl group, particularly preferably allyl group. As the acryloyl group or methacryloyl group for R ⁴ , a methacryloyl group is particularly preferred. In the formula, the total number of carbon atoms of a plurality of R ⁴ satisfies the viewpoints of good affinity with oxygen in the oxygen-containing solvent, and desolvated alkali metal ions, and good corrosion inhibiting effect of the Al current collector. Therefore, it is preferably 9 or less, more preferably 5 or less. R ⁴ may be a structure having a substituent, and in this case, the carbon number of R ^{4 is} a number including the substituent. As the substituent, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, isopentyl, hexyl, 2-ethylhexyl, lauryl, tridecyl, carbon number Examples include 1 to 5 alkoxy groups, fluoro groups, chloro groups, bromo groups, cyano groups, nitro groups, hydroxyl groups, aldehyde groups, carboxyl groups, and the like. In one embodiment, R ⁴ has no substituent. R ⁴ is preferably a methyl group, an ethyl group, or a propyl group, and more preferably a methyl group. In a particularly preferred embodiment, in the general formula (2), M is aluminum, A ¹ is an ethylene group, m is 3 to 9, and R ⁴ is a methyl group.

  The method for producing the metal ester compound represented by the general formula (2) is not particularly limited, but can be produced, for example, by the following method. In a typical embodiment, the metal ester compound represented by the general formula (2) can be obtained by a transesterification reaction between a trialkyl metal ester and a polyalkylene glycol derivative. The metal in the trialkyl metal ester is a metal contained in the target metal ester compound, that is, boron or aluminum. As the alkyl group of the trialkyl metal ester, a linear or branched lower alkyl group having 1 to 8 carbon atoms, for example, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a t- Butyl group, sec-butyl group, pentyl group, isopentyl group, hexyl group, octyl group, and 2-ethylhexyl group, preferably methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group , A t-butyl group and a sec-butyl group, particularly preferably an isopropyl group and a sec-butyl group.

The polyalkylene glycol derivative is represented by the following general formula (2a).
HO- (A ¹ -O) _m -R ⁴ (2a)
In the formula, A ¹ , m and R ⁴ are the same as defined in the general formula (2).

  The transesterification is usually performed under heating. Since the transesterification reaction is an equilibrium reaction, it is preferable to carry out the reaction while removing the alcohol generated as a result of the reaction from the reaction system in order to make the reaction proceed quickly. As the removal method, a known method is used, and examples thereof include a method of removing by distillation, a method of removing the compound by adsorption to an adsorbent such as molecular sieve, and the like. The reaction temperature and distillation pressure vary depending on the types of the raw material polyalkylene glycol derivative and trialkyl metal ester, but the reaction temperature is usually room temperature to 250 ° C, preferably 50 to 200 ° C, and particularly preferably. 50 to 150 ° C. The distillation pressure is from normal pressure to 0.5 mmHg, preferably from normal pressure to 5 mmHg, and particularly preferably from normal pressure to 10 mmHg. The reaction solvent may not be used, but when used, there is no particular limitation as long as it does not react with the produced metal ester compound, and aromatic hydrocarbons such as toluene, xylene and benzene, hexane, heptane and octane are used. Aliphatic hydrocarbons such as cyclohexane and cycloheptane, cyclic ethers such as dioxane and tetrahydrofuran, ethers such as diethyl ether and isopropyl ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and triethylene glycol Glymes such as dimethyl ether; preferably, aromatic hydrocarbons such as toluene, xylene and benzene; aliphatic hydrocarbons such as hexane, heptane and octane; alicyclic hydrocarbons such as cyclohexane and cycloheptane. , Cyclic ethers such as oxane and tetrahydrofuran; ethers such as diethyl ether and isopropyl ether; glymes such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and triethylene glycol dimethyl ether; and particularly preferably, aromatics such as toluene, xylene and benzene. Hydrocarbons, aliphatic hydrocarbons such as hexane, heptane and octane; and alicyclic hydrocarbons such as cyclohexane and cycloheptane.

In the general formula (2), when R ⁴ particularly has a polymerizable group among the unsaturated hydrocarbon groups, a polymerization inhibitor may be added to prevent polymerization of the metal ester compound which is a reaction product. Good. Examples of the polymerization inhibitor include hydroquinone compounds, sterically hindered phenols, sterically hindered amines, and hydroxyamine derivatives. Examples of hydroquinone compounds include hydroquinone and hydroquinone monomethyl ether. Examples of sterically hindered phenols include sterically hindered amines such as 4-methyl 2,6-di-t-butylphenol and 2,4-dimethyl-6-t-butylphenol. Are hydroxyamine derivatives such as 2,2,6,6-tetramethylpiperidine-N-oxyl and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl; -Diethylhydroxylamine and the like, and two or more polymerization inhibitors may be used in combination. The reaction time varies depending on the reaction temperature and the starting compound, but is usually 1 to 30 hours, preferably 2 to 20 hours, and particularly preferably 2 to 10 hours.

  The produced metal ester compound represented by the general formula (2) is isolated by a known method. For example, when the reaction yield is high, and particularly when the purification operation and the decolorization operation do not need to be performed, only filtration is performed. It is enough. When a solvent is used, after filtration, the solvent can be distilled off and isolated. Further, a purification operation and a decolorization operation can be performed as necessary. As a purification operation, a known method is used, and examples include column chromatography. As the decolorizing operation, a known method is used, and examples thereof include activated carbon treatment, silica treatment, and adsorbent treatment such as aluminum hydroxide treatment.

[Other ingredients]
The electrolyte solution according to the present invention may further comprise a viscosity modifier (for example, hydrofluoroether), an antioxidant, a film-forming agent, a preservative, a surfactant, a stabilizer, an ultraviolet absorber, an inorganic or organic material, in addition to the components described above. It may further contain additives such as fillers and flame retardants.

  The electrolyte solution according to the present invention is obtained by mixing the oxygen-containing solvent containing an ester bond and the alkali metal salt with the metal ester compound and other additives as necessary under a dry atmosphere such as in a normal dry room. Can be manufactured.

  In a rocking chair type secondary battery, stability against electrode potential (that is, potential window) is important. The potential (oxidation or reduction decomposition potential) at which the decomposition current (obtained as a function of the potential) is measured when the oxidative or reductive decomposition of the electrolytic solution is tested with the electrode whose potential is controlled is determined as a potential window. It is an indicator of the oxidative stability of the liquid. The potential window of the electrolyte is preferably 3 V or more, more preferably 4 V or more, and still more preferably 5 V or more. The potential window is a value measured by a linear sweep voltammetry method.

  The current value measured for the Al / Li electrode by the linear sweep voltammetry method can be an indicator of the corrosion of the Al current collector by the electrolytic solution. In particular, in the measurement by the linear sweep voltammetry method, the fact that the current value does not remarkably increase even at a high potential of, for example, 4.0 V or more is based on a rocking chair type secondary using an Al current collector at a high positive electrode potential of, for example, 4.0 V. This indicates that in the battery, the corrosion of the Al current collector is favorably suppressed by the electrolytic solution. In the linear sweep voltammetry method, the current value measured under the conditions described in [Example] of the present specification or equivalent thereto is, in a preferred embodiment, a current value at a potential of 4.0 V and a current value at a potential of 3.5 V. Is 5.0 μA or less, and in a more preferred embodiment, the difference from the current value at a potential of 3.5 V at a potential of 4.5 V is 15.0 μA or less.

<Rocking chair type secondary battery>
The present invention further provides a rocking chair secondary battery having a positive electrode, a negative electrode, and the electrolyte solution according to the present invention. In a preferred embodiment, the rocking chair secondary battery is a lithium ion secondary battery. The positive electrode and the negative electrode are not particularly limited, and the active material for the positive electrode includes metal oxides such as lithium cobalt oxide, lithium manganate, lithium iron phosphate, nickel manganese cobalt oxide (NMC), and nickel cobalt lithium aluminate (NCA). Substance, elemental sulfur, lithium polysulfide (Li ₂ S _x (0 <x ≦ 8)), sulfur-based metal polysulfide (MS _x (M = Ni, Cu, Fe, 0 <x ≦ 2)), organic Sulfur compounds such as disulfide compounds and carbon sulfide compounds can be used as appropriate. Examples of the negative electrode active material include graphite, hard carbon, soft carbon and other carbon-based materials, crystalline or amorphous silicon alone, silicon oxide, silicone resins, silicon-containing compounds such as silicon-containing polymer compounds, and titanium. Lithium oxide, lithium metal simple substance, LiC, LiSi, LiGr, LiAl, LiIn and the like can be used. In one aspect, a rocking chair secondary battery has a separator disposed between a positive electrode and a negative electrode. As the separator, various sheet materials capable of holding an electrolytic solution, for example, a porous film, a nonwoven fabric, or the like formed from a material such as polyolefin, polyimide, cellulose, or glass can be used.

  Hereinafter, the present invention will be described specifically with reference to Examples, but the scope of the present invention is not limited to the following Examples.

Example I:
<Preparation of oxygen-containing solvent>
The following compounds were prepared as oxygen-containing solvents in Examples and Comparative Examples.
-MMP: methyl 3-methoxypropionate (Example)
G3: triethylene glycol dimethyl ether (comparative example)
-EMC: ethyl methyl carbonate (comparative example)
・ PC: Propylene carbonate (comparative example)
MMP used M3MP manufactured by Nippon Emulsifier Co., Ltd., G3 used DMTG manufactured by Nippon Emulsifier Co., Ltd., EMC used ethyl methyl carbonate manufactured by Ube Industries, Ltd., and PC used propylene carbonate manufactured by Mitsui Chemicals, Inc.

<Preparation of electrolyte>
Each of the above oxygen-containing solvents, lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) dried and dehydrated (lithium bis (trifluoromethanesulfonyl) imide lithium manufactured by Tokyo Chemical Industry Co., Ltd.), lithium bis (fluorosulfonyl) imide (LiFSI), Or, lithium hexafluorophosphate (LiPF ₆ ) was mixed at a predetermined ratio shown in Tables 1 and 2 with a stirrer at room temperature overnight to prepare each electrolyte solution. Note that for each electrolyte using LiTFSI, the formation of a complex of lithium and a solvent was confirmed by thermogravimetry (TG-DTA 2000S, Mac Science).

<Performance evaluation>
[Shear viscosity]
Each of the prepared electrolyte solutions was set in a viscoelasticity measuring device MCR101 (manufactured by Anton Paar), and the shear viscosity was measured at 30 ° C. at a shear rate of 100 sec ⁻¹ .
[Charge and discharge test]
Each of the prepared electrolytes was impregnated with cellulose (25 μm in thickness), and incorporated into a coin battery (20 mm in diameter, 3.2 mm in thickness).
A charge / discharge test was performed at 30 ° C. under the following charge / discharge conditions using a charge / discharge test apparatus (HJD1010mSM8 manufactured by Hokuto Denko Corporation) using the following combinations of electrodes.
Electrode combination: LFP (positive electrode) / LTO (negative electrode)
Charge / discharge conditions: 4.0 V or 4.5 V, 1.0 C (0.1 C every 50 cycles)
(Description of abbreviations)
LFP: lithium iron phosphate LTO: lithium titanate The results are shown in Table 1 and FIGS.

  Regarding the relationship between the lithium salt concentration and the viscosity of the electrolytic solution, as is clear from FIG. 1, the oxygenated solvent (MMP) according to the present invention was compared with the oxygenated solvent (EMC, PC, G3) of the comparative example at the same concentration. Therefore, there was a tendency to suppress an increase in the viscosity of the electrolytic solution. Here, the electrolyte concentration of 4.7 mol / kg in the LiTFSI / MMP-based electrolyte is 135% by mass in terms of mass, but the concentration of the electrolyte exceeds 50% by mass. In light of the teachings of Patent Document 2 that it was greatly inhibited, the viscosity behavior in the high concentration region when using the oxygen-containing solvent according to the present invention was surprising.

  Regarding the relationship between the lithium salt concentration of the electrolytic solution and the discharge capacity, as is clear from FIG. 2, the oxygen-containing solvent (MMP) according to the present invention is different from the oxygen-containing solvent (EMC, PC, G3) of the comparative example at the same concentration. In comparison, a significantly higher discharge capacity was exhibited particularly in a high concentration region of an electrolyte concentration of 4.0 mol / kg or more. It is considered that the discharge capacity increased due to the synergistic effect of the viscosity increase suppression effect and the high concentration electrolyte by MMP.

  Further, regarding the relationship between the type of Li salt and the discharge capacity, as is clear from Table 1, in all three types of different Li salts, the oxygenated solvent according to the present invention was compared with the comparative oxygenated solvent at the same concentration. The discharge capacity was significantly higher.

Example II:
<Preparation of electrolyte>
An electrolyte was prepared using the same MMP and G3 as described in Example I as the solvent and the same LiTFSI as described in Example I as the alkali metal salt.
MMP and LiTFSI were mixed overnight at room temperature with a stirrer at a molar ratio of 1: 1 to obtain an electrolyte solution of the present invention.
G3 and LiTFSI were mixed overnight in a stirrer at room temperature at a molar ratio of 1: 1 to obtain a comparative electrolyte.

<Performance evaluation>
[Aluminum corrosion evaluation]
The current value (first cycle) at a potential of 4.0 V and 4.5 V was measured by the linear sweep voltammetry (LSV) method under the following conditions. An increase in the current value at the Al / Li electrode is an indicator of aluminum corrosion due to the electrolytic solution.
Measurement cell: Bipolar PP electrochemical measurement cell Test temperature: 30 ° C
Potential sweep speed: 20 mV / sec Counter electrode / reference electrode: Li metal working electrode: Al foil The results are shown in Table 2.

  As is clear from Table 2, the electrolytic solution of the present disclosure has a higher positive electrode potential of 4.0 V, and further has an extremely high potential of 4.5 V, as compared with the comparative electrolytic solution having the same solvent: alkali metal salt molar ratio. It can be seen that at a high positive electrode potential, the aluminum corrosion inhibiting effect is excellent.

Example III: (an embodiment in which a metal ester compound is further added)
<Metal ester compound>
(Metal ester compound 1 (ALsec))
Aluminum-trisec butoxide ("Aluminum sec-butoxide A-0244" manufactured by Tokyo Chemical Industry Co., Ltd.) was used.

(Synthesis of Metal Ester Compound 2 (ALE n = 3))
In a dry room having a dew point of −40 ° C. or lower, 20.20 g (0.082 mol) of aluminum-trisec butoxide (manufactured by Tokyo Chemical Industry Co., Ltd.) and triethylene glycol monomethyl ether (molecular weight: 164.19: Japanese emulsifier) (MTG) 40.39 g (0.246 mol) was mixed to form a transesterification reaction system. The reaction system was heated to 60 to 65 ° C. by a water bath while stirring under nitrogen bubbling. After dissolving the aluminum-trisec butoxide, the transesterification reaction was allowed to proceed while maintaining the temperature at 60 to 65 ° C. for about 2 hours. Thereafter, while the pressure of the reaction system was gradually reduced to 10 mmHg (1.3 kPa) over about 3 hours, by-product free isobutyl alcohol was removed. Then, the reaction system was cooled to room temperature and filtered to obtain 42.3 g (yield 99.9%) of metal ester compound 2 (Al [O (CH ₂ CH ₂ O) ₃ CH ₃ ] ₃ ). Was.

(Synthesis of Metal Ester Compound 3 (ALE n = 9))
In a dry room having a dew point of −40 ° C. or lower, 20.20 g (0.082 mol) of aluminum-trisec butoxide (manufactured by Tokyo Chemical Industry Co., Ltd.) and polyethylene glycol monomethyl ether (molecular weight 427.83: Japanese emulsifier ( (MPG-130) (105.2 g, 0.246 mol) was mixed to form a transesterification reaction system. The reaction system was heated to 60 to 65 ° C. by a water bath while stirring under nitrogen bubbling. After dissolving the aluminum-trisec butoxide, the transesterification reaction was allowed to proceed while maintaining the temperature at 60 to 65 ° C for about 2 hours. Thereafter, while the pressure of the reaction system was gradually reduced to 10 mmHg (1.3 kPa) over about 3 hours, by-product free isobutyl alcohol was removed. Thereafter, the reaction system was cooled to room temperature and filtered to thereby obtain 106.7 g of a metal ester compound of the chemical formula: Al [O (CH ₂ CH ₂ O) ₉ CH ₃ ] ₃ (ALE n = 9) (yield: 99). 0.5%).

(Metal ester compound 4 (BBO))
Tributyl borate (Tributyl borate B-0518 manufactured by Tokyo Chemical Industry Co., Ltd.) was used.

(Synthesis of Metal Ester Compound 5 (BE n = 3))
Metal ester compound 5 was prepared in the same manner as in the synthesis of metal ester compound 2 except that 18.87 g (0.082 mol) of metal ester compound 4 was used instead of 20.20 g (0.082 mol) of metal ester compound 1. 40.98 g (99.9% yield) of (B [O (CH ₂ CH ₂ O) ₃ CH ₃ ] ₃ ) was obtained.

(Synthesis of Metal Ester Compound 6 (BE n = 9))
Metal ester compound 6 was prepared in the same manner as in the synthesis of metal ester compound 3, except that 18.87 g (0.082 mol) of metal ester compound 4 was used instead of 20.20 g (0.082 mol) of metal ester compound 1. _{(B [O (CH 2 CH} 2 O) 9 CH 3] 3) to afford 105.6g of (99.7% yield).

<Preparation of electrolyte solution and performance evaluation-1>
The above-mentioned MMP and LiTFSI were mixed overnight at room temperature with a stirrer at a molar ratio of 1.0: 1.0 to prepare a mixed solution (electrolyte concentration of 8.5 mol / kg). With respect to 100 parts by mass of the mixed solution, 5.0% by mass of the above-mentioned metal ester compound 3 (ALE n = 9) (1.6% by mol based on 100% by mol of alkali metal salt) was added; 5.8% by mass (1.8% by mol based on 100% by mol of the alkali metal salt) was prepared, and mixed with a stirrer at room temperature overnight to prepare each electrolytic solution.

  The charge / discharge test described above was repeated for each prepared electrolyte solution, and the results are shown in FIG. First, with respect to the electrolyte before adding the metal ester compound, the electrolyte concentration of 8.5 mol / kg is 243% by mass in terms of mass, and the discharge capacity is constant at about 50% up to 25 cycles in such a concentrated electrolyte. Was surprising in light of the teaching of Patent Document 2. Further, in the electrolytic solution to which the metal ester compound was added, the discharge capacity maintained in accordance with the amount added increased. It is presumed that even in such a concentrated electrolyte, the activation energy of desolvation of lithium was effectively reduced due to the Lewis acidity of aluminum of the metal ester compound.

<Preparation of electrolytic solution and performance evaluation-2>
The above-mentioned MMP and LiTFSI were mixed overnight at room temperature with a stirrer at a molar ratio of 1.0: 1.0 to prepare a mixed solution (electrolyte concentration of 8.5 mol / kg). To the mixture, each of the above-mentioned metal ester compounds 1 to 6 was added so as to be 1.8 mol% as an outer ratio with respect to 100 mol% of the alkali metal salt. And mixed overnight to prepare each electrolyte solution.

[Ionic conductivity]
Each of the prepared electrolytes was impregnated with cellulose (thickness: 25 μm) and incorporated into a UFO-type two-electrode cell (electrode: SUS-SUS).
The impedance was measured at 30 ° C. using a VSP potentiogalvanostat (manufactured by BioLogic).

[Shear viscosity] [Charge / discharge test] and [Aluminum corrosion test]
For each of the prepared electrolytes, a shear viscosity measurement, a charge / discharge test, and an aluminum corrosion test were performed in the same procedure as in Examples I and II except that the charge / discharge condition in the charge / discharge test was 0.5 C (15 cycles). .
Table 3 shows the results.

  As is clear from Table 3, when the electrolytic solution of the present disclosure further contains a metal ester compound having a specific structure, the discharge capacity is improved and / or the voltage is as high as 4.0 V or more as compared with the case where the metal ester compound is not contained. It can be seen that the effect of suppressing aluminum corrosion under a positive electrode potential environment is particularly remarkable.

  The electrolyte solution according to the present invention is suitably applied to various rocking chair type secondary batteries, particularly lithium ion secondary batteries.

Claims (9)

   An alkali metal salt and an oxygen-containing solvent containing an ester bond, wherein the alkali metal salt is contained in an amount of 2.0 mol or more per 1 kg of the oxygen-containing solvent and not more than the saturation concentration. Characteristic electrolyte for rocking chair type secondary battery.    The electrolytic solution according to claim 1, wherein the alkali metal salt is contained in an amount within a range of 4.0 mol or more and 12.0 mol or less based on 1 kg of the oxygen-containing solvent. The electrolytic solution according to claim 1, wherein the oxygen-containing solvent is represented by the following general formula (1).
R ¹ -O- (A-O) _n -CH ₂ CH (R ³ ) -COO-R ² (1)
(Wherein, R ¹ and R ² each independently represent a linear or branched alkyl group having 1 to 4 carbon atoms, R ³ represents a methyl group or H 2, and A each independently represents 2 to 4 carbon atoms. 4 represents an alkylene group, and n represents an integer of 0 to 5.) 4. The electrolytic solution according to claim 3, wherein in the general formula (1), R ¹ and R ² are both methyl groups, R ³ is H, and n is 0. 5. The alkali metal salt is lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), lithium bis (fluorosulfonyl) imide (LiFSI), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), It is selected from the group consisting of lithium chlorate (LiClO ₄₎ and lithium bis (pentafluoroethane sulfonyl) imide (LiBETI), electrolyte solution according to any one of claims 1-4.    The electrolytic solution according to any one of claims 1 to 5, wherein no solvent is present other than the oxygen-containing solvent. The electrolytic solution according to any one of claims 1 to 6, further comprising a metal ester compound represented by the following general formula (2).
M [-O- (A ¹ -O) _m -R ⁴ ] ₃ (2)
(Wherein, M represents a trivalent element selected from the group consisting of boron and aluminum, A ¹ each independently represents an alkylene group having 2 to 4 carbon atoms, and m each independently represents an integer of 0 to 50. And each R ⁴ independently represents an aliphatic hydrocarbon group having 1 to 13 carbon atoms.)    The electrolytic solution according to any one of claims 1 to 7, comprising a complex of the alkali metal derived from the alkali metal salt and the oxygen-containing solvent.    A rocking chair type secondary battery comprising a positive electrode, a negative electrode, and the electrolytic solution according to claim 1.