JP2012009158A - Electrolyte and lithium secondary battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte having excellent performance, e.g. a high conductivity or a wide potential window, exhibiting excellent safety, and useful in a lithium secondary battery.SOLUTION: The electrolyte contains constituents (A1), (A2), and (B). (A1) lithium cation, (A2) organic cation, (B) cyanofluorophosphate based anion represented by general formula (1),P(CN)F...(1)(where, n is an integer of 1-5).

Description

  The present invention relates to an electrolyte useful for a lithium secondary battery, more specifically, an electrolyte useful for a lithium secondary battery, which has excellent performance as an electrolyte such as conductivity and potential window, and is excellent in safety, and The present invention relates to a lithium secondary battery using the same.

  In recent years, lithium secondary batteries have been widely used in information electronic devices such as notebook computers, mobile phones, and PDAs, and the demand for more comfortable portability has led to rapid reductions in size, thickness, weight, and performance of batteries. Is going on. In addition, the application of lithium secondary batteries is also being considered for electric vehicles expected as next-generation vehicles, and further miniaturization, weight reduction, and higher performance are required.

On the other hand, the safety of current lithium secondary batteries has become a problem, such as the firing and explosion of information electronic equipment. The current lithium secondary battery uses an electrolyte in which a lithium salt such as LiPF ₆ or LiBF ₄ is dissolved in a flammable and volatile organic solvent such as propylene carbonate or diethyl carbonate. I am involved. These risks increased as the capacity of the battery increased, and hindered the spread of electric vehicles.

  In view of this, a lithium secondary battery using such an electrolyte has been studied in which a flame-retardant and low-volatile ionic liquid is used as an electrolyte instead of an organic solvent and a lithium salt is dissolved in the electrolyte. The ionic liquid referred to herein is a salt containing a cation and an anion, and is a compound having a melting point below room temperature.

  As an electrolyte using an ionic liquid, for example, an electrolyte containing a specific organic cation, a lithium cation, and a specific nitrogen-containing organic anion has been proposed (see, for example, Patent Document 1). Further, it has been proposed that an ionic liquid containing a specific nitrogen-containing anion can be applied as an ion conductor material (see, for example, Patent Document 2). An ion conductive material containing a lithium cation and a dicyanoamide anion has been proposed (see, for example, Patent Document 3).

JP 2004-303642 A Special table 2001-527505 gazette JP 2004-6240 A

However, in the technique disclosed in Patent Document 1, as an anion composing the ^{_{electrolyte, - N (SO 2 CF 3}} ) 2, - N (SO 2 C 2 F 5) 2, - N (SO 2 CF 3) (SO ₂ C ₄ F ₉ ) or the like is used, but an electrolyte using such an anion has a problem that the conductivity is inferior. In particular, the electrical conductivity at a low temperature (for example, −30 ° C.) necessary for an electrolyte for an automobile battery cannot be secured.

Further, in the technique disclosed in Patent Document 2, as the anion of the ionic liquid, ^- but N (SO ₂ F) ₂ or the like is used, an ionic liquid having such anions are large heating value in the case of degradation, organic solvents Is more dangerous. For example, the temperature inside the battery sometimes reaches several hundred degrees due to a short circuit or the like, and the battery will explode or ignite due to such thermal runaway, but when an ionic liquid with a large calorific value is used as the electrolyte material, This thermal runaway will be accelerated.

  In the disclosed technique of Patent Document 3, a dicyanoamide anion or the like is used as an anion. An ionic liquid having such an anion has a narrow potential window of 4 V, and it is difficult to increase the battery output.

  Furthermore, in the above-described electrolyte, since a decomposition reaction by oxidation and / or reduction proceeds on the positive electrode surface and / or the negative electrode surface of the battery, even if a positive electrode material and a negative electrode material having a wide potential window are used, the wide potential window is reduced. It cannot be used effectively. For example, the potential window of an ionic liquid electrolyte composed of an imidazolium cation and a nitrogen-containing organic anion is generally about 3 to 5 V, whereas an electrolyte used in an automobile battery requires 5 V or more. is there. In particular, in order to ensure the safety of the battery even in an overcharged state, an electrolyte having a wide potential window even at 0.1 V is required.

  In view of this, the present invention provides an electrolyte, particularly an electrolyte for a lithium secondary battery, and further a lithium secondary battery, which is excellent in performance as an electrolyte such as conductivity and potential window and excellent in safety under such a background. It is intended to do.

  However, as a result of intensive studies in view of such circumstances, the present inventors have found that an electrolyte containing a phosphate anion having a cyano group and a fluorine atom has high conductivity, a wide potential window, and is excellent in safety. The present invention has been completed.

That is, the gist of the present invention relates to an electrolyte comprising components (A1), (A2), and (B).
(A1) Lithium cation (A2) Organic cation (B) Cyanofluorophosphate anion represented by the following general formula (1)

[Chemical 1]
^{_{- P (CN) n F 6}} -n ··· (1)
(Here, n is an integer of 1 to 5.)
Furthermore, the present invention relates to a lithium secondary battery in which the electrolyte is sandwiched between a positive electrode and a negative electrode.

  The electrolyte of the present invention can provide a lithium secondary battery that is excellent in performance as an electrolyte having a high conductivity and a wide potential window, in particular, as an electrolyte for a lithium secondary battery and excellent in safety.

The present invention is described in detail below.
The electrolyte in the present invention represents a conductive substance containing a lithium cation as an essential component and containing other cations and anions. Moreover, the ionic liquid in this invention represents the ionic substance containing the cation and anion which hold | maintain a liquid at the fixed temperature of room temperature vicinity. In the present invention, room temperature usually means a temperature of 25 ° C.

The electrolyte of the present invention contains the following components (A1), (A2), and (B).
(A1) Lithium cation (A2) Organic cation (B) Cyanofluorophosphate anion represented by the following general formula (1)

[Chemical 2]
^{_{- P (CN) n F 6}} -n ··· (1)
(Here, n is an integer of 1 to 5.)

  n is an integer of 1 to 5, preferably 2 to 4 from the viewpoint of melting point, and particularly preferably 3 from the viewpoint of conductivity.

  The main effect of the electrolyte of the present invention is to have a low viscosity and a high conductivity. In particular, when n is 3, the anion has an asymmetric chemical structure, thereby further reducing the viscosity. Furthermore, since the anion has geometric isomers (facial and meridional), it is presumed that the mixture of these anions contributes to further lowering the viscosity.

  In addition, the remarkable effect of the electrolyte of the present invention is that when a battery is charged / discharged, a minute amount of anion decomposition products coat the surface of the positive electrode material, and express the function of forming an electrode protective film. It has a function of stabilizing the potential window.

  In general, when an ionic liquid is used as the electrolytic solution, anions are decomposed on the positive electrode side to lower the oxidation resistance potential of the battery, and cations are decomposed on the negative electrode side to lower the reduction resistance potential of the battery. In either case, the potential window becomes unstable and the charge / discharge characteristics are deteriorated.

  On the other hand, when an organic solvent is used as the electrolytic solution, an electrode protective film forming agent such as vinylene carbonate is generally added. It is known that these additives decompose during use of the battery to form a stable SEI (Solid Electrolyte Interface) on the surface of the electrode material. Such SEI protects the electrode, suppresses oxidation and reduction of the electrolytic solution, and can utilize a wide potential window inherently possessed by the positive and negative electrodes.

  The electrolyte of the present invention has the same effect as the electrode protective film forming agent described above. That is, a part of the anion is decomposed on the positive electrode side to protect the surface of the positive electrode material and prevent further decomposition of the electrolyte. As a result, the oxidation-reduction potential of the battery is stabilized, and a wide potential window inherently possessed by the positive and negative electrodes can be utilized.

  The mechanism by which the anion of the present invention forms SEI is not clear, but it is presumed that the phosphorus atom, cyano group, and / or fluorine atom of the anion react with the electrode surface to form a stable electrode protective film. Is done. The above-described electrode protective film forming agent such as vinylene carbonate generally forms a protective film on the negative electrode to stabilize the reduction resistance potential, but the electrolyte of the present invention forms a protective film on the positive electrode. The oxidation resistance potential can be stabilized.

  Examples of the organic cation (A2) of the electrolyte of the present invention include nitrogen-containing organic cations such as imidazolium cation, pyrrolidinium cation, piperidinium cation, aliphatic quaternary ammonium cation, tetramethylphosphonium cation, and tetraethylphosphonium. Cations, tetrapropylphosphonium cations, tetrabutylphosphonium cations, tetraoctylphosphonium cations, trimethylethylphosphonium cations, triethylmethylphosphonium cations, hexyltrimethylphosphonium cations, trimethyloctylphosphonium, triethyl (methoxymethyl) phosphonium, triethyl (methoxymethyl) phosphonium, etc. Phosphorus-containing organic cations such as phosphonium cations, trimethylsulfonium Examples include thione, triethylsulfonium cation, tributylsulfonium cation, diethylmethylsulfonium cation, dimethylpropylsulfonium, dimethylhexylsulfonium and other sulfur-containing organic cations, oxonium cation, etc. In this respect, nitrogen-containing organic cations are preferable.

  Examples of the nitrogen-containing organic cation include an imidazolium cation, a pyrrolidinium cation, a piperidinium cation, and an aliphatic quaternary ammonium cation.

  Examples of the imidazolium-based cation include 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-methyl-3-propylimidazolium, 1-butyl-3-methylimidazolium, and 1-methyl. -3-pentylimidazolium, 1-hexyl-3-methylimidazolium, 1-heptyl-3-methylimidazolium, 1-methyl-3-octylimidazolium, 1-decyl-3-methylimidazolium, 1-dodecyl -3-methylimidazolium, 1-ethyl-3-propylimidazolium, 1-butyl-3-ethylimidazolium, 1-methoxyethyl-3-methylimidazolium, 1-cyanomethyl-3-methylimidazolium, 1- Cyanoethyl-3-methylimidazolium, 1- (3-cyanopropyl) 3-methylimidazolium, 1-methyl-3-trifluoromethylimidazolium, 1-methyl-3-perfluoroethylimidazolium, 1-methyl-3-perfluoropropylimidazolium, 1-methyl-3-perfluoro Butylimidazolium, 1-ethyl-3-trifluoromethylimidazolium, 1-ethyl-3-perfluoroethylimidazolium, 1-ethyl-3-perfluoropropylimidazolium, 1-ethyl-3-perfluorobutylimidazole 1-perfluoroethyl-3-propylimidazolium, 1-perfluoropropyl-3-propylimidazolium, 1-perfluorobutyl-3-propylimidazolium, 1-perfluorobutyl-3-propylimidazolium, 1-butyl-3-trifluorome Louis imidazolium, 1-butyl-3-perfluoroethylimidazolium, 1-butyl-3-perfluoropropylimidazolium, 1-butyl-3-perfluorobutylimidazolium, 1,3-bis (trifluoromethyl) Imidazolium, 1-perfluoroethyl-3-trifluoromethylimidazolium, 1-perfluoropropyl-3-trifluoromethylimidazolium, 1-perfluorobutyl-3-trifluoromethylimidazolium, 1,3-bis (Perfluoroethyl) imidazolium, 1-perfluoroethyl-3-perfluoropropylimidazolium, 1-perfluorobutyl-3-perfluoroethylimidazolium, 1,3-bis (perfluoropropyl) imidazolium, 1 -Perfluorobutyl-3-pa Disubstituted imidazolium cations such as fluoropropylimidazolium, 1,3-bis (perfluorobutyl) imidazolium, 1,2,3-trimethylimidazolium, 1,3,5-trimethylimidazolium, 3-ethyl- 1,2-dimethylimidazolium, 2-ethyl-1,3-dimethylimidazolium, 3-ethyl-1,5-dimethylimidazolium, 1,2-dimethyl-3-propylimidazolium, 1,3-dimethyl- 2-propylimidazolium, 1,5-dimethyl-3-propylimidazolim, 1-butyl-2,3-dimethylimidazolium, 2-butyl-1,3-dimethylimidazolium, 3-butyl-1,5- Dimethylimidazolium, 1,2-dimethyl-3-hexylimidazolium, 1,2-dimethyl-3-octi Louis imidazolium, 1-ethyl-3,4-dimethylimidazolium, 1-isopropyl-2,3-dimethylimidazolium, 3-trifluoromethyl-1,2 dimethylimidazolium, 3-perfluoroethyl-1,2 -Dimethylimidazolium, 3-perfluoropropyl-1,2-dimethylimidazolim, 3-perfluorobutyl-1,2-dimethylimidazolium, 1,3-bis (trifluoromethyl) -2-methylimidazolium, 1-perfluoroethyl-2-methyl-3-trifluoromethylimidazolim, 2-cyano-1,3-dimethylimidazolium, 2-cyano-1-ethyl-4-methylimidazolium, 2-cyano-1- Propyl-4-methylimidazolium, 1-butyl-2-cyano-4-methylimidazolium, etc. And a substituted imidazolium cation and the like.

  Examples of the pyrrolidinium cation include N, N-dimethylpyrrolidinium, N-ethyl-N-methylpyrrolidinium, N-methyl-N-propylpyrrolidinium, N-butyl-N-methylpyrrolidinium, N-methyl-N-pentylpyrrolidinium, N-hexyl-N-methylpyrrolidinium, N-methyl-N-octylpyrrolidinium, N-decyl-N-methylpyrrolidinium, N-dodecyl-N- Methylpyrrolidinium, N- (2-methoxyethyl) -N-methylpyrrolidinium, N- (2-ethoxyethyl) -N-methylpyrrolidinium, N- (2-propoxyethyl) -N-methylpyrrole Examples thereof include dinium and N- (2-isopropoxyethyl) -N-methylpyrrolidinium.

  Examples of piperidinium-based cations include N, N-dimethylpiperidinium, N-ethyl-N-methylpiperidinium ion, N-methyl-N-propylpiperidinium, N-butyl-N-methylpiperidinium, N-methyl-N-pentylpiperidinium, N-hexyl-N-methylpiperidinium, N-methyl-N-octylpiperidinium, N-decyl-N-methylpiperidinium, N-dodecyl-N- Methylpiperidinium, N- (2-methoxyethyl) -N-methylpiperidinium, N- (2-methoxyethyl) -N-ethylpiperidinium, N- (2-ethoxyethyl) -N-methylpi Peridinium, N-methyl-N- (2-methoxyphenyl) piperidinium, N-methyl-N- (4-methoxyphenyl) piperidinium Ethyl -N- N-(2-methoxyphenyl) piperidinium, N- ethyl -N- (4-methoxyphenyl) piperidinium and the like.

  Examples of the aliphatic quaternary ammonium cation include N, N, N, N-tetramethylammonium, N, N, N-trimethylethylammonium, N, N, N-trimethylpropylammonium, N, N, N- Trimethylbutylammonium, N, N, N-trimethylpentylammonium, N, N, N-trimethylhexylammonium, N, N, N-trimethylheptylammonium, N, N, N-trimethyloctylammonium, N, N, N- Trimethyldecylammonium, N, N, N-trimethyldodecylammonium, N-ethyl-N, N-dimethylpropylammonium, N-ethyl-N, N-dimethylbutylammonium, N-ethyl-N, N-dimethylhexylammonium, 2-methoxy-N, N, N-trimethyl Tillammonium, 2-ethoxy-N, N, N-trimethylethylammonium, 2-propoxy-N, N, N-trimethylethylammonium, N- (2-methoxyethyl) -N, N-dimethylpropylammonium, N- (2-methoxyethyl) -N, N-dimethylbutylammonium and the like can be mentioned.

  Among the above nitrogen-containing organic cations, imidazolium-based cations are particularly preferable from the viewpoint of electrical conductivity, and 1-ethyl-3-methylimidazolium and 1-propyl-3-methylimidazolium are further preferable from the viewpoint of potential window. Dialkylimidazolium cations such as 1-butyl-3-methylimidazolium and trialkylimidazolium cations such as 1-butyl-2,3-dimethylimidazolium are preferred.

In the present invention, an electrolyte containing a lithium cation is provided. As described above, for example, an electrolyte for a lithium secondary battery is manufactured by dissolving a lithium salt (solid electrolyte), an electrode protective film forming agent, or the like in an electrolytic solution. Therefore, these dissolving steps eliminate moisture as much as possible. Although it is performed in a dry environment and in a clean environment, it requires a large amount of equipment and labor. Furthermore, in the electrolyte for a lithium secondary battery, a lithium salt counter anion must be present where a lithium cation is sufficient. In general, LiPF ₆ or LiBF ₄ is used as a lithium salt, is these counter anions ^- PF ₆ or ^- BF ₄ is easily decomposed by water, reducing the conductivity to increase the viscosity of the electrolyte There are harmful effects such as.

In the electrolyte of the present invention, it is not necessary to newly add the lithium salt as described above, and unnecessary counter anions can be excluded. Moreover, even if the lithium salt as described above is newly added, the addition amount can be made small, and accordingly, the content of unnecessary counter anions can be reduced.

Moles b moles a _1, the number of moles a _2, and cyano fluoro phosphate based anionic organic cation (A2) of lithium cation (A1) (B) in the present invention, from the viewpoint of conductivity, the following formula It is preferable to satisfy (1-1) and (2-1).
Formula (1-1) 0.05 ≦ a ₁ / (a ₁ + a ₂ ) ≦ 0.4
Formula (2-1) 1 ≦ (a ₁ + a ₂ ) /b≦1.3

Regarding the formula (1-1), from the point of the potential window, more preferably,
Formula (1-2) 0.08 ≦ a ₁ / (a ₁ + a ₂ ) ≦ 0.3
From the viewpoint of charge / discharge characteristics, particularly preferably,
Formula (1-3) 0.1 ≦ a ₁ / (a ₁ + a ₂ ) ≦ 0.25
It is. If it is less than the above lower limit value, the lithium cation tends to be insufficient, and there is a tendency that the performance as a lithium secondary battery cannot be ensured, and if it exceeds the upper limit value, the conductivity tends to decrease.

Regarding the formula (2-1), from the point of the potential window, more preferably,
Formula (2-2) 1 ≦ (a ₁ + a ₂ ) /b≦1.2
From the viewpoint of charge / discharge characteristics, particularly preferably,
Formula (2-3) 1 ≦ (a ₁ + a ₂ ) /b≦1.1
It is. Even if it is less than the above lower limit value or exceeds the upper limit value, the conductivity tends to decrease. In this case, less than 1 means that an inorganic cation other than the lithium cation exists, and exceeding 1 means that an anion other than the cyanofluorophosphate anion exists.

  The electrolyte of the present invention preferably has a conductivity of 3 mS / cm or more at 25 ° C., more preferably 5 mS / cm or more, still more preferably 7 mS / cm or more, and particularly preferably 8 mS / cm or more. The upper limit of the conductivity at 25 ° C. is usually 100 mS / cm. If the electrical conductivity at 25 ° C. is too small, high-speed charge / discharge of the battery becomes difficult.

  Furthermore, in the present invention, the conductivity at low temperature is important. For example, the conductivity of the electrolyte at −30 ° C. is preferably 0.01 mS / cm or more, more preferably 0.1 mS / cm or more. More preferably, it is 0.2 mS / cm or more, and particularly preferably 0.3 mS / cm or more. In addition, the upper limit of the conductivity at −30 ° C. is usually 10 mS / cm. If the conductivity at −30 ° C. is too small, the operation of the battery in a cold region becomes difficult.

  Furthermore, the potential window of the electrolyte of the present invention is preferably 5 V or more, more preferably 6 V or more, and particularly preferably 7 V or more. If the potential window is too narrow, there is a tendency that it cannot cope with the high output of the battery. The upper limit value of the potential window is usually 10V.

  Furthermore, the melting point of the electrolyte of the present invention is preferably −30 ° C. or lower, more preferably −40 ° C. or lower, and particularly preferably −50 ° C. or lower. If the melting point is too high, the electrolyte tends to freeze. In addition, as a lower limit of melting | fusing point, it is -250 degreeC normally.

  Furthermore, the flash point of the electrolyte is preferably 200 ° C. or higher, more preferably 300 ° C. or higher, and particularly preferably 350 ° C. or higher. If the flash point is too low, the safety of the battery tends to decrease. The upper limit of the flash point is usually 600 ° C. The flash point is a value measured according to JIS K 2265 (-1, 2, 3, 4).

Next, the manufacturing method of the electrolyte of this invention is demonstrated.
The electrolyte of the present invention can be obtained, for example, by cation exchange of an ionic liquid (I) comprising an organic cation (A2) and a cyanofluorophosphate anion (B). That is, it can be obtained by exchanging a part of the organic cation (A2) in the ionic liquid with the lithium cation (A1).

  First, a method for producing an ionic liquid (I) comprising an organic cation (A2) and a cyanofluorophosphate anion (B) will be described.

  Taking an alkylimidazolium cation as an example of the organic cation (A2), a chlorinated product of such a cation and phosphorus pentachloride are reacted, and the resulting hexachlorophosphate anion is cyanated and then fluorinated to form a cyanofluorophosphate anion. To do. However, the present invention is not limited to this, and it can be produced according to the following description even if the raw materials are different.

  The chlorinated alkyl imidazolium used as a cation source can be obtained by reacting imidazole and chloroalkyl in equimolar amounts and quaternizing ammonium chloride.

  Next, the chlorinated product of alkylimidazolium and an equimolar amount of phosphorus pentachloride are stirred in a reaction solvent, usually at room temperature to 100 ° C., preferably at room temperature to 50 ° C., usually for several minutes to several hours, preferably 10 The reaction is carried out for from 1 to 1 hour to obtain an intermediate containing an alkylimidazolium cation and a hexachlorophosphate anion. The reaction is preferably performed in an inert gas atmosphere, and particularly preferably performed in a dry atmosphere. As the reaction solvent, polar solvents such as acetone, methyl ethyl ketone, acetonitrile, dichloromethane, dichloroethylene, chloroform, tetrahydrofuran (THF), dimethylformamide (DMF), dimethyl sulfoxide (DMSO) are preferable, and dehydrated dichloromethane and dehydrated acetonitrile are particularly preferable.

  Further, 1 to 10 equivalents of a cyano compound is added to and stirred with respect to 1 mol of an intermediate containing a cyano compound, for example, an alkylimidazolium cation and a hexachlorophosphate anion, in a reaction system. The cyanochlorophosphate anion is preferably reacted at 0 to 50 ° C., particularly preferably 10 to 30 ° C., usually for several minutes to several tens of hours, preferably 10 minutes to 50 hours, particularly preferably 1 to 24 hours. The ionic liquid which has can be obtained. The reaction is preferably performed in an inert gas atmosphere, and particularly preferably performed in a dry atmosphere. Examples of the cyano compound include hydrogen cyanide, trimethylsilyl cyanide, potassium cyanide, sodium cyanide, silver cyanide and copper cyanide. Among these, silver cyanide and copper cyanide are preferable.

  The obtained ionic liquid comprising an alkylimidazolium cation and a cyanochlorophosphate anion is preferably purified in order to remove the generated metal halide and impurities. Examples of the purification technique include techniques such as filtration, extraction, washing, column chromatography, reprecipitation, and adsorption. Among these, column chromatograph is preferable from the viewpoint of improving the electrochemical characteristics of the ionic liquid. Examples of the column include alumina, silica gel, diatomaceous earth, activated carbon, and the like. Alumina is particularly preferable because impurity ions can be efficiently removed.

Thus, an ionic liquid having an alkyl imidazolium cation and a cyanochlorophosphate anion can be obtained from the alkyl imidazolium halide and phosphorus pentachloride as raw materials. Cyano chloro phosphate based anionic formula of ^- n in _{P (CN) n X 6-} n (X = Cl) can be controlled amount of cyano compounds, reaction time, and the like purification conditions.

Next, an ionic liquid having a cyanofluorophosphate anion is obtained by fluorinating the ionic liquid having a cyanochlorophosphate anion with a fluorinating agent. More specifically, 1 to 10 equivalents of a fluorinating agent is added to 1 equivalent of an ionic liquid having a cyanochlorophosphate anion, and usually 0 to 100 ° C, preferably 20 to 80 ° C, particularly preferably 25. An ionic liquid having a desired cyanofluorophosphate anion can be obtained by reacting at -60 ° C for usually several minutes to several tens of hours, preferably 1 to 24 hours, particularly preferably 3 to 8 hours. Examples of the fluorinating agent include HF, LiF, NaF, KF, AgF, LiBF ₄ , NaBF ₄ , KBF ₄ , AgBF _4, and among these, AgBF ₄ and AgF are preferable.

  When using a metal salt as the fluorinating agent, it is preferable to add a metal cation scavenger to the obtained ionic liquid and perform post-treatment in order to remove by-product metal cations. As the metal cation scavenger, a compound comprising a target ionic liquid cation and a halogen anion is preferable. For example, when producing an ionic liquid having a 1-ethyl-3-methylimidazolium cation and a cyanofluorophosphate anion, a halide of 1-ethyl-3-methylimidazolium cation is used. Chloride is preferred as the halide. The metal cation scavenger is preferably 0.1 to 1 equivalent, particularly preferably 0.3 to 0.8 equivalent, relative to 1 equivalent of the fluorinating agent used. As conditions for the post-treatment, a metal cation scavenger is added to the ionic liquid, and the reaction is usually carried out at 0 to 50 ° C., preferably 10 to 40 ° C., usually for several minutes to several hours, preferably 10 minutes to 1 hour. Thereafter, the precipitated metal halide is removed by filtration, and the filtrate is washed with water and dried to obtain an ionic liquid with high purity.

  The obtained ionic liquid composed of alkylimidazolium cation and cyanofluorophosphate anion is preferably purified in order to remove further impurities. Examples of the purification technique include techniques such as filtration, extraction, washing, column chromatography, reprecipitation, and adsorption. Among these, column chromatograph is preferable from the viewpoint of improving the electrochemical characteristics of the ionic liquid. Examples of the column include alumina, silica gel, diatomaceous earth, activated carbon, and the like. Alumina is particularly preferable because impurity ions can be efficiently removed.

  The electrolyte of the present invention is produced using the ionic liquid (I) comprising the organic cation (A2) and the cyanofluorophosphate anion (B) thus obtained. The electrolyte of the present invention can be obtained by substituting a part of the organic cation (A2) with the lithium cation (A1) by an existing cation exchange method using an ion exchange resin, but is more preferable from the viewpoint of productivity. For example, by reacting the ionic liquid (I) with a lithium salt (II) composed of a lithium cation (A1) and a counter anion, the counter anion derived from the lithium salt is removed by a technique such as solvent washing or decomposition. can get.

  As a cation exchange method using an ion exchange resin, any known method may be used. For example, after an aqueous solution of lithium hydroxide is allowed to act on an ion exchange resin such as a styrene type, the resin is loaded with lithium cations. Then, the ionic liquid (I) is allowed to act on this resin to replace a part of the organic cation (A2) with the lithium cation (A1).

Examples of the lithium salt (II) include LiBF ₄ , LiF, LiCl, LiBr, LiI, LiPF ₆ , LiOH, LiCO ₂ H, LiCO ₂ CH ₃ , LiCO ₂ CF ₃ , LiSO ₂ CH ₃ , LiSO ₂ CF ₃ , Examples include LiCN, LiN (CN) ₂ , LiC (CN) ₃ , LiSCN, LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ F) _{2 and the} like. Among these, from the viewpoint of solubility in the ionic liquid (I), LiBF ₄ , LiPF ₆ , LiCO ₂ H, LiCO ₂ CH ₃ , LiCO ₂ CF ₃ , LiSO ₂ CH ₃ , LiSO ₂ CF ₃ , LiN (SO ₂ CF ₃ ) ₂ is preferable, and LiBF ₄ , LiPF ₆ , LiCO ₂ H, LiCO ₂ CH ₃ , LiCO ₂ CF ₃ , LiSO ₂ CH ₃ , and LiSO ₂ CF ₃ are more preferable because of the solubility of the counter anion in water. Although a reaction solvent is not particularly required, when a reaction solvent is used, for example, methylene chloride, chloroform, and toluene are preferable from the viewpoint of cation exchange efficiency, and methylene chloride is preferable from the viewpoint of efficiency of solvent washing.

As a specific method of the reaction, for example, 10 to 90 mol% of the ionic liquid (I) and 90 to 10 mol% of the lithium salt (II) are stirred at 10 to 60 ° C for 1 minute to 24 hours, A composition comprising a lithium cation, an organic cation, a counter anion, and a cyanofluorophosphate anion is obtained. A preferable range of the blending ratio is 20 to 80 mol% of ionic liquid (I) and 80 to 20 mol% of lithium salt (II), more preferably 30 to 75 mol% of ionic liquid (I) and 70 of lithium salt (II). ~ 25 mol%. If the amount of lithium salt is small, the lithium cation in the electrolyte tends to be insufficient, and the performance as a lithium secondary battery tends not to be ensured. Conversely, if the amount of lithium salt is large, the electrolyte becomes highly viscous. There is a tendency for the conductivity to decrease. A preferable range of the reaction temperature is 20 to 50 ° C, more preferably 25 to 40 ° C. If the reaction temperature is too low, the reaction tends to take time, and if it is too high, the counter anion tends to decompose. A preferable range of the reaction time is 30 minutes to 12 hours, more preferably 1 to 6 hours. If the reaction time is too short, the lithium salt (II) is not sufficiently dissolved in the ionic liquid (I), and the cation exchange tends not to proceed sufficiently. If it is too long, the productivity tends to be poor.

As a specific method of solvent washing, 10 to 1000 parts by weight of a washing solvent such as alcohol, ether, and a mixed solvent of these and water are added to 100 parts by weight of the composition described above, and 0 to 60 parts are added. The mixture was stirred at 1 ° C. for 1 to 5 hours, and the layer in which the counter anion was dissolved was removed, followed by filtration and then drying. The target lithium cation (A1), organic cation (A2), and cyanofluorophosphate anion ( An electrolyte containing B) is obtained. The blending amount of the cleaning solvent is preferably 20 to 700 parts by weight, and more preferably 30 to 500 parts by weight with respect to 100 parts by weight of the composition. When the washing solvent is small, the counter anion removal rate tends to decrease, and conversely, when it is too large, the productivity tends to decrease. Such solvent cleaning may be repeated twice or more.

The amount of lithium cation in the electrolyte, that is, a ₁ / (a ₁ + a ₂ ) can be controlled by the composition of the ionic liquid, the type and amount of the lithium salt, and the type and amount of the cleaning solvent.

  As a specific method for removing the counter anion by decomposition, 60 to 99 mol% of ionic liquid (I) and 1 to 40 mol% of a lithium salt (II) of a carboxylic acid such as lithium formate are usually 10 to 60 ° C. Then, after stirring and mixing for 1 minute to 24 hours, an anion can be removed by adding a proton source such as water and heating at 100 to 500 ° C., more preferably 200 to 300 ° C. If the temperature is too low, the reaction tends not to proceed, and if it is too high, the organic cation tends to decompose. A preferable range of the reaction time is 30 minutes to 12 hours, more preferably 1 to 6 hours. If the time is too short, the decomposition of the anion becomes insufficient, and if it is too long, the organic cation tends to be decomposed.

Thus, although the electrolyte of the present invention is obtained, the electrolyte of the present invention is preferably used as an electrolyte for a lithium secondary battery.

  When the electrolyte of the present invention is used as an electrolyte for a lithium secondary battery, such electrolytes may be used alone or in combination of two or more. For example, those having different organic cations (A2) or those represented by the general formula (1) Or a combination of two or more of the different cyanofluorophosphate anions (B) represented by formula (B).

  In the electrolyte of the present invention, an additive such as a lithium salt, an ionic liquid, an organic solvent, and an electrode protective film forming agent is added to the electrolyte of the present invention as necessary within a range satisfying the formula (1-1) and the formula (2-1). You may mix | blend.

Examples of the lithium salt include LiBF ₄ , LiBR ₄ (R represents a phenyl group or an alkyl group), LiBF _m R _4-m (R is an alkyl group that may contain a fluorine atom, and m is an integer of 1 to 3). _{, LiPF 6, LiSbF 6, LiAsF} 6, LiCIO 4, LiSO 3 CF 3, LiN (SO 2 CF 3) 2, LiN (SO 2 F) 2, LiN (SO 2 CF 3) (SO 2 F), LiN ( CN) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiSO ₃ C ₆ F ₁₃ , LiSO ₃ C ₈ F ₁₇ , LiAlCl ₄ , lithium tetrakis [3,5-bis (trifluoromethyl) phenyl] borate and the like. . Of these, lithium salts such as LiBF ₄ , LiPF ₆ , LiN (SO ₂ CF ₃ ) ₂ , and LiN (SO ₂ F) ₂ are preferably used from the viewpoint of solubility in the electrolyte of the present invention.

Examples of the ionic liquid include, as anions, chlorine anions, bromine anions, iodine anions, BF ₄ ⁻ , BF ₃ CF ₃ ⁻ , BF ₃ C ₂ F ₅ ⁻ , PF ₆ ⁻ , NO ₃ ⁻ , CF ₃ CO ₂ ^−. , CF ₃ SO ₃ ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ , (FSO ₂ ) ₂ N ⁻ , (CF ₃ SO ₂ ) (FSO ₂ ) N ⁻ , (CN) ₂ N ⁻ , (CN) ₃ C ^{_{-, (CF 3 SO 2)}} 3 C -, (C 2 F 5 SO 2) 2 N -, AlCl 4 -, Al 2 Cl 7 - and ion liquid containing such. Corresponding cations include 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-octyl- And alkyl imidazolium cations such as 3-methylimidazolium, 1-butyl-2,3-dimethylimidazolium, 1,2,3-trimethylimidazolium, 1,2-dimethyl-3-ethylimidazolium, and the like. Other than imidazolium-based cations, ionic liquids containing quaternary ammonium-based cations, pyridinium-based cations, quaternary phosphonium-based cations, and the like can be given. These ionic liquids can be used alone or in combination of two or more.

  Examples of the electrode protective film forming agent include vinylene carbonate, 1,3-propane sultone, ethylene sulfite, triethyl borate, butyl methyl sulfonate, and the like. Among these, vinylene carbonate is particularly preferable from the viewpoint of forming stable SEI on the negative electrode side.

  The content of the electrode protective film forming agent is preferably 0.1 to 5 parts by weight, more preferably 0.2 to 3 parts by weight, particularly preferably 100 parts by weight of the entire electrolyte. 0.3 to 1 part by weight. If the content is too large, the safety of the battery tends to decrease, and if it is too small, the effect of stabilizing the potential window of the lithium secondary battery tends not to be obtained.

  In the present invention, it is also possible to add a small amount of an organic solvent to improve the electrical conductivity. Examples of the organic solvent include carbonate solvents (propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, etc.), amide solvents (N-methylformamide, N-ethylformamide, N, N-dimethylformamide, N-methylacetamide, N-ethylacetamide, N-methylpyrrolidinone, etc.), lactone solvents (γ-butyllactone, γ-valerolactone, δ-valerolactone, 3-methyl-1,3-oxazolidine-2- ON), alcohol solvents (ethylene glycol, propylene glycol, glycerin, methyl cellosolve, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, diglycerin, polyoxyalkylene glycol Chlorohexanediol, xylene glycol, etc.), ether solvents (methylal, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-2-methoxyethane, alkoxypolyalkylene ether, etc.), nitrile solvents ( Benzonitrile, acetonitrile, 3-methoxypropionitrile, etc.), phosphoric acids and phosphate ester solvents (eg orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, polyphosphoric acid, phosphorous acid, trimethyl phosphate), 2-imidazolidinones (1, 3-dimethyl-2-imidazolidinone, etc.), pyrrolidones, sulfolane solvents (sulfolane, tetramethylene sulfolane, etc.), furan solvents (tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, etc.), dioxolane, dioxane Etc. These alone or a mixed solvent can be used. Among these, carbonate-based solvents, ether-based solvents, furan-based solvents, and sulfolane-based solvents are preferable from the viewpoint of conductivity, and sulfolane-based solvents are particularly preferably used from the viewpoint of battery safety.

  The content in the case of using an organic solvent is preferably 30% by volume or less, particularly preferably 5 to 20% by volume when the entire electrolyte is 100% by volume. When there is too much content, it will become the tendency for the flame retardance of electrolyte to fall.

Next, a lithium secondary battery obtained using the electrolyte of the present invention will be described.
In the present invention, the lithium secondary battery is produced by sandwiching the electrolyte of the present invention obtained above between a positive electrode and a negative electrode.

  Such a positive electrode is preferably a composite positive electrode. A composite positive electrode refers to a composition in which a positive electrode active material is mixed with a conductive additive such as ketjen black or acetylene black, a binder such as polyvinylidene fluoride, and an ion conductive polymer as required. It is applied to a conductive metal plate.

  Examples of the positive electrode active material include an inorganic active material, an organic active material, and a composite thereof. However, an inorganic active material or a composite of an inorganic active material and an organic active material has a large energy density. This is preferable.

As the inorganic active material, Li _0.3 MnO ₂ , Li ₄ Mn ₅ O ₁₂ , V ₂ O ₅ , LiFePO ₄ , LiMnO _3, etc. in the 3V system, LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _{1 / 3} Mn _1/3 Co _1/3 O ₂ , LiNi _1/2 Mn _1/2 O ₂ , LiNi _0.8 Co _0.2 O ₂ , LiMnPO ₄ , Li ₂ MnO _3, etc. In the 5V system, Li ₂ MnO ₃ And metal oxides such as LiNi _0.5 Mn _1.5 O ₂ , metal sulfides such as TiS ₂ , MoS ₂ and FeS, and composite oxides of these compounds and lithium. Examples of the organic active material include conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, and polyparaphenylene, organic disulfide compounds, carbon disulfide, and active sulfur.

  With regard to the negative electrode, for example, a metal-based negative electrode in which a negative electrode active material is directly applied to a current collector, an active material such as a conductive polymer, a carbon body, and an oxide with a binder such as polyvinylidene fluoride on an alloy-based current collector. Examples thereof include a negative electrode coated with a substance.

Examples of the negative electrode active material include lithium metal and silicon metal, alloys of metals such as lithium, silicon, aluminum, lead, tin, silicon, and magnesium, metal oxides such as SnO ₂ and TiO ₂ , polypyridine, polyacetylene, polythiophene, or Examples thereof include cation-doped conductive polymers composed of these derivatives, carbon bodies capable of occluding lithium, and in particular, when the electrolyte of the present invention is used, lithium metal and silicon metal having high energy density are preferable. .

  In the present invention, when such lithium metal is used, the thickness of the lithium metal is preferably 1 to 100 μm, more preferably 3 to 50 μm, and particularly preferably 5 to 20 μm. It is economical and preferable to use a thin lithium foil as the lithium metal.

  The lithium secondary battery of the present invention is manufactured by sandwiching an electrolyte between the positive electrode and the negative electrode, but it is preferable to use a separator from the viewpoint of preventing a short circuit. Specifically, a lithium secondary battery is obtained by impregnating a separator with an electrolyte and sandwiching the separator between a positive electrode and a negative electrode.

  As the separator, those having low resistance to ion migration of lithium ions are used, and for example, selected from polypropylene, polyethylene, polyester, polytetrafluoroethylene, polyvinyl alcohol, and saponified ethylene-vinyl acetate copolymer. Examples thereof include a microporous film made of one or more materials, an organic or inorganic nonwoven fabric, or a woven fabric. In these, the microporous film and glass nonwoven fabric which consist of polypropylene or polyethylene are preferable at the point of short circuit prevention and economical efficiency.

  Although it does not specifically limit as a form of the lithium secondary battery of this invention, The battery cell of various forms, such as a coin, a sheet | seat, a cylinder, is mentioned.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to a following example, unless the summary is exceeded.
In the examples, “parts” and “%” mean weight basis.
The measurement conditions for each characteristic are as follows.

(Flash point)
The measurement was performed according to JIS K 2265 (-1, 2, 3, 4).

(conductivity)
Using a CG-511B cell made by Toa DKK as a measurement cell, immersed in an electrolyte for 5 hours, and then measured by an alternating current impedance method using an electrochemical measurement system “Solartron 1280Z” (manufactured by Solartron, UK). . The AC amplitude was measured at 5 mV, and the frequency range was 20 k to 0.1 Hz.

(Potential window)
As a measurement cell, a V-4C voltammetry cell manufactured by BAES Co., Ltd. was used, and an electrode manufactured by BAS Co., Ltd. was used. Glassy carbon (diameter 1 mm) was used for the working electrode, platinum was used for the counter electrode, and lithium was used for the reference electrode. The potential sweep rate was 5 mV / sec, and the temperature was 25 ° C. As a measuring apparatus, an electrochemical measuring system “Solartron 1280Z” (manufactured by Solartron, UK) was used. The limiting current density was ^{± 1mA / cm 2, + 1mA} / cm 2 oxidation potential the potential to reach the potential to reach -1 mA / cm ² and reduction source potential, the potential difference of the oxidation potential and the reduction based on potential The window (V) was used.

(Analysis equipment)
Mass spectrometry uses “JMS-T100LP AccuTOF LC-plus” manufactured by JEOL Ltd., IR spectrum uses “Avatar 360” manufactured by Nicolet, and NMR uses “Unity-300” (solvent: heavy acetonitrile) manufactured by Varian. Measured.

Example 1
[Production of electrolyte]
Under a stream of argon, 41.7 g of phosphorus pentachloride, 140 mL of dehydrated acetonitrile, and 29.5 g of 1-ethyl-3-methylimidazolium chloride were placed in a 300 mL round bottom flask and stirred for 1 hour. The obtained solution was added to a 500 mL two-necked round bottom flask containing 81.7 g of silver cyanide and 150 mL of dehydrated acetonitrile, stirred for 1 day, filtered, and the filtrate was concentrated. 100 mL of dichloromethane was added to the concentrated solution, and washed with 50 mL of water three times. Thereafter, the dichloromethane layer was separated and concentrated to obtain 41.4 g of an ionic liquid composed of 1-ethyl-3-methylimidazolium / trichlorotricyanophosphate.
Next, 38.8 g of 1-ethyl-3-methylimidazolium trichlorotricyanophosphate prepared above and 70 mL of dehydrated acetonitrile were dissolved in a 500 mL Erlenmeyer under an argon stream. The solution was added to 1 L Erlenmeyer containing 146 g of AgBF ₄ as a fluorinating agent and 500 mL of dehydrated acetonitrile, stirred at 60 ° C. for 5 hours, and further 1-ethyl-3-methyl as a metal cation scavenger. A solution prepared by dissolving 56 g of imidazolium chloride in 40 mL of dehydrated acetonitrile was added, and the mixture was stirred at 25 ° C. for 30 minutes. After the reaction, the reaction solution was filtered, and the filtrate was concentrated. The concentrated solution was dissolved in 200 mL of dichloromethane and washed four times with 100 mL of water. The dichloromethane layer was extracted and then vacuum dried to obtain 16.6 g of an ionic liquid composed of 1-ethyl-3-methylimidazolium trifluorotricyanophosphate.

  The ionic liquid was identified and confirmed to be 1-ethyl-3-methylimidazolium trifluorotricyanophosphate. The main chart attributions are shown below.

MS m / z = 165.99717
IR 2200cm ^-1 [CN]
³¹ P-NMR -227.06 ppm [q, Hz = 739.6 Hz, P]
¹ H-NMR 8.35 ppm [s, ¹ H]
7.34 ppm [s, 1H]
7.29 ppm [s, 1H]
4.15 ppm [q, Hz = 7.2 Hz, 2H]
3.78 ppm [s, 3H]
1.42 ppm [t, Hz = 7.2, 3H]

To 12.8 g of the obtained ionic liquid, 1.7 g of LiBF ₄ was added and stirred for 2 hours at room temperature to dissolve the lithium salt in the ionic liquid. Thereafter, 13 mL of a mixed solvent of propylene glycol monomethyl ether acetate and water was added, and the ionic liquid layer was washed twice. After separating and filtering the ionic liquid layer, the electrolyte containing lithium cation, 1-ethyl-3-methylimidazolium cation, and trifluorotricyanophosphate anion is obtained by vacuum drying at 60 ° C. for 3 hours. 5 g was obtained.

  The analysis results of the obtained electrolyte are as follows, and a liquid electrolyte composed of the target lithium cation, 1-ethyl-3-methylimidazolium cation, and trifluorotricyanophosphate anion was obtained.

Elemental analysis Lithium 0.72%
Mass spectrometry m / z = 165.97917
IR 2200cm ^-1 [CN]
³¹ P-NMR -227.06 ppm [q, Hz = 739.6 Hz, P]
¹ H-NMR 8.35 ppm [s, ¹ H]
7.34 ppm [s, 1H]
7.29 ppm [s, 1H]
4.15 ppm [q, Hz = 7.2 Hz, 2H]
3.78 ppm [s, 3H]
1.42 ppm [t, Hz = 7.2, 3H]

  The component ratio of the electrolyte is 13 mol% lithium cation, 37 mol% 1-ethyl-3-methylimidazolium cation, and 50 mol% trifluorotricyanophosphate anion.

Various characteristics of the electrolyte are as shown in Table 1. It was confirmed that the electrochemical characteristics were excellent by having high conductivity and a wide potential window even at low temperatures.
Moreover, the obtained electrolyte can manufacture a lithium secondary battery as follows, for example, and is useful as an electrolyte for lithium secondary batteries.

[Manufacture of lithium secondary batteries]
(1) Preparation of positive electrode 9.0 g of LiCoO ₂ powder, 0.5 g of ketjen black, and 0.5 g of polyvinylidene fluoride were mixed, and 7.0 g of 1-methyl-2-pyrrolidone was further added and mixed well in a mortar. A positive electrode slurry is obtained. The obtained positive electrode slurry was applied on a 20 μm thick aluminum foil using a wire bar in the air, dried at 100 ° C. for 15 minutes, and further dried at 130 ° C. for 1 hour under reduced pressure to obtain a film thickness of 30 μm. A composite positive electrode is produced.

(2) Battery assembly The above electrolyte is impregnated into a glass nonwoven fabric (GF / A manufactured by Whatman Japan, thickness 260 μm) and a composite positive electrode as a separator, and a separator and a lithium foil (thickness) as a negative electrode are formed on the composite positive electrode. Are stacked in the order of 500 μm), inserted into a 2032 type coin cell and sealed to obtain a lithium secondary battery.

Comparative Example 1
The electrolyte was adjusted by adding 15.1 g of LiPF6 to 120 g of ethylene carbonate (50 vol%) / dimethyl carbonate (50 vol%), which is a general organic electrolyte, and evaluated in the same manner as in Example 1.
The results are as shown in Table 1.

  As is clear from the evaluation results of the above examples and comparative examples, the electrolytes of the examples are superior in conductivity at room temperature and low temperature, and have a wider potential window, compared to the electrolytes of the comparative examples. Therefore, it is very effective as an electrolyte for a lithium secondary battery.

  The electrolyte of the present invention has excellent performance as an electrolyte having a high conductivity and a wide potential window, and is excellent in safety, and is very useful as an electrolyte for a lithium secondary battery. It is also useful as an electrolyte for other secondary batteries, primary batteries, capacitors, capacitors, actuators, electrochromic elements, various sensors, dye-sensitized solar cells, fuel cells, and further, antistatic agents, polymerization initiators. It is also useful as an ion exchange membrane material or an ion glass material.

Claims (9)

An electrolyte comprising components (A1), (A2), and (B).
(A1) Lithium cation (A2) Organic cation (B) Cyanofluorophosphate anion represented by the following general formula (1)
^{_{- P (CN) n F 6}} -n ··· (1)
(Here, n is an integer of 1 to 5.) Moles _{a 1} of lithium cation (A1), moles b moles _{a 2} organic cation (A2), and cyano-fluoro phosphate based anionic (B) is a compound represented by the following formula (1-1) and (2-1 The electrolyte according to claim 1, wherein:
Formula (1-1) 0.05 ≦ a ₁ / (a ₁ + a ₂ ) ≦ 0.4
Formula (2-1) 1 ≦ (a ₁ + a ₂ ) /b≦1.3 3. The electrolyte according to claim 1, wherein n in the general formula (1) is 3.   The electrolyte according to any one of claims 1 to 3, wherein the organic cation (A2) is a nitrogen-containing organic cation.   The electrolyte according to claim 4, wherein the nitrogen-containing organic cation is an imidazolium cation. The electrolyte according to any one of claims 1 to 5, wherein the conductivity at 25 ° C is 3 mS / cm or more.   The electrolyte according to any one of claims 1 to 6, wherein the conductivity at -30 ° C is 0.01 mS / cm or more. The electrolyte according to any one of claims 1 to 7, wherein the potential window is 5 V or more. A lithium secondary battery comprising the electrolyte according to claim 1 sandwiched between a positive electrode and a negative electrode.