JP2017004603A - Non-aqueous electrolyte and non-aqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-aqueous electrolyte which achieves both safety and high ion conductivity and, furthermore, to provide a non-aqueous secondary battery superior in safety and output characteristics.SOLUTION: As a non-aqueous solvent, a non-aqueous electrolyte contains fluorine-containing phosphate ester having a 1-6C linear or branched alkyl group, a 6-10C aryl group or 1-6 linear, or a branched fluorine-containing alkyl group. Also, as electrolyte salt, the non-aqueous electrolyte contains fluorosulfonyl imide lithium salt having a fluorine atom or 1-4C linear, a branched alkyl group or 1-4C linear, or a branched fluorine-containing alkyl group.SELECTED DRAWING: None

Description

  The present invention relates to a non-aqueous electrolyte excellent in safety and ion conductivity used for a non-aqueous secondary battery. More specifically, a nonaqueous electrolytic solution having safety and good ionic conductivity by containing a fluorine-containing phosphate ester in a solvent of the nonaqueous electrolytic solution and using a fluorosulfonylimide lithium salt as an electrolyte salt, The present invention also relates to a non-aqueous secondary battery having excellent safety and output characteristics.

  Non-aqueous secondary batteries represented by lithium ion secondary batteries have high output density and high energy density, and are widely used as power sources for mobile phones, notebook computers, tablet computers, and the like. In recent years, it has been put into practical use as clean energy with low carbon dioxide emissions and as a large capacity power source for electric power storage and electric vehicles. Such a large storage battery, particularly an electric vehicle or a hybrid vehicle, needs to have a performance for taking out a large current value instantaneously, that is, an excellent output performance. In response to such demands, high ion conductivity is required for non-aqueous electrolytes (Non-Patent Document 1).

On the other hand, the non-aqueous electrolyte used in this lithium ion secondary battery is a flammable electrolyte obtained by dissolving lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte in a flammable solvent such as ethylene carbonate or dimethyl carbonate. Therefore, improvement of the safety of these combustible non-aqueous electrolytes is required with the increase in size of batteries (Non-Patent Document 2).

  For improving the safety of such non-aqueous electrolytes, fluorine-containing phosphate esters (Patent Documents 1 to 4), fluorine-containing ethers (Patent Documents 5 and 6), and fluorine-containing esters (patents) Documents 7 and 8) and fluorine-containing solvents such as fluorine-containing carbonates (Patent Documents 9 and 10) have been proposed. Among these, non-aqueous electrolytes containing fluorine-containing phosphates are particularly excellent in flame retardancy, and it is known that an electrolyte with high safety that does not have a flash point can be constructed (Patent Document 4). .

  This fluorine-containing phosphate ester is derived from the decomposition of the electrolytic solution even for a lithium ion secondary battery including a positive electrode active material that operates at a potential of 4.5 V or more with respect to lithium for the purpose of increasing energy density. It has been proposed as a non-aqueous electrolyte solvent that contributes to excellent cycle characteristics with less gas generation (Patent Document 11). In addition, as a film forming agent for further suppressing the decomposition of the non-aqueous electrolyte for a lithium ion secondary battery operating at such a high potential, the addition of a fluorosulfonyl anion to the non-aqueous electrolyte has also been proposed. (Patent Document 12).

JP-A-8-88023 JP 2007-141760 A JP 2007-258067 A JP 2013-20713 A Japanese Patent Laid-Open No. 9-097627 JP-A-11-26015 JP-A-6-20719 JP-A-10-116627 Japanese Patent Laid-Open No. 7-6786 JP 2007-305352 A International Publication 2012/0777712 International Publication No. 2014/080870

"Automotive Lithium Ion Battery" edited by Seiji Kanamura, published by Nikkan Kogyo Shimbun, December 20, 2010 "High-safety technology and materials for lithium-ion batteries", CMC Publishing Co., Ltd., supervised by Noboru Sato and Akira Yoshino, published February 13, 2009

  Thus, the fluorine-containing phosphate ester is known to be useful as an electrolyte solution for non-aqueous secondary batteries, particularly lithium secondary batteries. For example, as described in Patent Document 3, It is known that the addition of phosphoric acid esters causes an increase in the viscosity of the nonaqueous electrolyte and a decrease in the degree of ionic dissociation, resulting in a decrease in ionic conductivity and a decrease in the output characteristics of the nonaqueous secondary battery. It was done.

  The present invention has been made in view of these problems. That is, the present invention provides a highly safe non-aqueous electrolyte containing a fluorinated phosphate ester as a non-aqueous electrolyte solvent, and provides a non-aqueous electrolyte having excellent ion conductivity, which has been difficult in the past. An object of the present invention is to provide a non-aqueous secondary battery having excellent output characteristics by using this non-aqueous electrolyte.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have included a fluorine-containing phosphate ester in the solvent of the non-aqueous electrolyte solution, and by using a specific imidolithium salt as an electrolyte salt, safety can be achieved. It was found that a non-aqueous electrolyte having excellent ion conductivity and excellent ion conductivity can be obtained. Furthermore, the nonaqueous electrolytic solution of the present invention has also been found to have an unexpected effect that the freezing point is low and can be used at a lower temperature than when conventional LiPF6 is used as an electrolyte salt. The invention has been completed.

  That is, the present invention relates to the following gist.

(1) As a nonaqueous solvent, the following general formula (1)

(In the formula, Rf ¹ , Rf ² and Rf ³ are each independently a linear or branched alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 10 carbon atoms, or a linear chain having 1 to 6 carbon atoms. Or a branched fluorine-containing alkyl group, and at least one of Rf ^{1 to} Rf ³ is a fluorine-containing alkyl group), and an electrolyte salt represented by the following general formula (2)

(In the formula, Rf ⁴ represents a fluorine atom, a linear or branched alkyl group having 1 to 4 carbon atoms, or a linear or branched fluorine-containing alkyl group having 1 to 4 carbon atoms). Non-aqueous electrolyte containing.

(2) The nonaqueous electrolytic solution according to (1), wherein at least one selected from the group consisting of a cyclic carbonate, a chain carbonate, a cyclic ester, and a phosphate ester is present as a nonaqueous solvent.

(3) The fluorine-containing phosphate represented by the general formula (1) is tris (2,2-difluoroethyl) phosphate, tris (2,2,2-trifluoroethyl) phosphate, trisphosphate. (2,2,3,3-tetrafluoropropyl), bis (2,2,2-trifluoroethyl) methyl phosphate, bis (2,2,2-trifluoroethyl) ethyl phosphate, bis ( 2,2,2-trifluoroethyl) phenyl, bis (2,2,2-trifluoroethyl) phosphate 2,2-difluoroethyl, bis (2,2,2-trifluoroethyl) phosphate 2,2 , 3,3-tetrafluoropropyl, bis (2,2-difluoroethyl) phosphate 2,2,2-trifluoroethyl, bis (2,2,3,3-tetrafluoropropyl) 2,2, 2-Trifluoroe Or (2), which is at least one selected from the group consisting of (2,2,2-trifluoroethyl) (2,2,3,3-tetrafluoropropyl) methyl phosphate and phosphoric acid The non-aqueous electrolyte described.

(4) The nonaqueous electrolytic solution according to any one of (1) to (3), wherein the electrolyte salt is difluorosulfonylimide lithium.

(5) further comprises LiPF ₆ or LiBF ₄ as an electrolyte salt, the amount of LiPF ₆ or LiBF ₄ relative fluorosulfonyl imide lithium salt is 0.01 to 2 molar ratio (1) to (4) of section The nonaqueous electrolytic solution according to any one of the above.

(6) The nonaqueous solvent to be further present is at least one cyclic carbonate selected from the group consisting of ethylene carbonate, propylene carbonate, vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate and difluoroethylene carbonate. Non-aqueous electrolyte.

(7) The nonaqueous electrolytic solution according to (2), wherein the nonaqueous solvent to be further present is at least one chain carbonate selected from the group consisting of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

(8) The nonaqueous solvent to be further present is at least one cyclic ester selected from the group consisting of γ-butyrolactone, γ-valerolactone, δ-valerolactone, ε-caprolactone and γ-hexanolactone (2) The nonaqueous electrolytic solution according to Item.

(9) Further, the non-aqueous solvent to be present comprises trimethyl phosphate, triethyl phosphate, tri-n-propyl phosphate, triisopropyl phosphate, tri-n-butyl phosphate, triisobutyl phosphate, and triphenyl phosphate. The nonaqueous electrolytic solution according to item (2), which is at least one phosphate ester selected from the group.

(10) A nonaqueous secondary battery comprising the nonaqueous electrolyte solution according to any one of items (1) to (9).

  According to the present invention, in an electrolytic solution containing a fluorine-containing phosphate ester in a non-aqueous solvent, by using a specific fluorosulfonylimide lithium salt as an electrolyte salt, safety and high performance that have been difficult with conventional electrolytic solutions are improved. A nonaqueous electrolytic solution having both ionic conductivity can be provided. Furthermore, by using the nonaqueous electrolytic solution of the present invention, a nonaqueous secondary battery excellent in safety and output characteristics can be provided.

It is explanatory drawing which shows the electrochemical cell made from glass which combined the platinum electrode used by the measurement of the measurement example 1 [ion conductivity] facing each other. It is a schematic cross section which shows the structure of the coin cell type non-aqueous secondary battery used in Example 6-1 and Comparative Example 6-1.

The nonaqueous electrolytic solution of the present invention contains a fluorine-containing phosphate represented by the general formula (1) as a first component. In the general formula (1), Rf ¹ , Rf ² and Rf ³ are each independently a linear or branched alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 10 carbon atoms, or 1 to 6 carbon atoms. A linear or branched fluorine-containing alkyl group, and at least one of Rf ^{1 to} Rf ³ is a fluorine-containing alkyl group.

  Examples of the linear or branched alkyl group having 1 to 6 carbon atoms include methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-pentyl group, and n -Hexyl group and the like, and examples of the linear or branched fluorine-containing alkyl group having 1 to 6 carbon atoms include trifluoromethyl group, 2,2-difluoroethyl group, 2,2,2-trifluoroethyl group, 2,2,3,3-tetrafluoropropyl group, 2,2,3,3,3-pentafluoropropyl group, hexafluoroisopropyl group, 2,2,3,3,4,4,5,5-octa Fluoropentyl group, 2,2,3,3,4,4,5,5,5-nonafluoropentyl group, 3,3,4,4,5,5,6,6,6-nonafluorohexyl group, etc. Can be mentioned. Examples of such a fluorine-containing phosphate ester include tris phosphate (trifluoromethyl), tris phosphate (2,2-difluoroethyl), tris phosphate (2,2,2-trifluoroethyl), and phosphoric acid. Tris (2,2,3,3-tetrafluoropropyl), Tris phosphate (2,2,3,3,3-pentafluoropropyl), Tris phosphate (hexafluoroisopropyl), Tris phosphate (2,2 , 3,3,4,4,5,5-octafluoropentyl), tris phosphate (2,2,3,3,4,4,5,5,5-nonafluoropentyl), tris phosphate (3 , 3,4,4,5,5,6,6,6-nonafluorohexyl), bis (2,2,2-trifluoroethyl) methyl phosphate, bis (2,2,2-trifluorophosphate) Ethyl) ethyl, biphosphate (2,2,2-trifluoroethyl) 2,2-difluoroethyl, bis (2,2,2-trifluoroethyl) phosphate 2,2,3,3-tetrafluoropropyl, bis (2, 2,3,3-tetrafluoropropyl) 2,2,2-trifluoroethyl and bis (2,2,2-trifluoroethyl) phosphate (2,2,3,3-tetrafluoropropyl) methyl, etc. Can be mentioned.

  Among these fluorine-containing phosphate esters, in particular, tris phosphate (2,2-difluoroethyl) having two or more fluorine-containing alkyl groups in the structure, tris phosphate (2,2,2-trifluoroethyl), Tris (2,2,3,3-tetrafluoropropyl) phosphate, bis (2,2,2-trifluoroethyl) methyl phosphate, bis (2,2,2-trifluoroethyl) ethyl phosphate, phosphorus Bis (2,2,2-trifluoroethyl) phenyl phosphate, bis (2,2,2-trifluoroethyl) phosphate, 2,2-difluoroethyl phosphate, bis (2,2,2-trifluoroethyl phosphate) 2,2,3,3-tetrafluoropropyl, bis (2,2,3,3-tetrafluoropropyl) 2,2,2-trifluoroethyl phosphate, bis (2,2,2-trifluorophosphate) Echi ) (2,2,3,3-tetrafluoro propyl) methyl and the like are themselves more preferable because it exhibits a non-flammable.

  These fluorine-containing phosphate esters are superior in solubility to electrolyte salts compared to fluorine-containing ethers and fluorine-containing carbonates. Therefore, fluorine-containing phosphate esters can be used alone as an electrolyte solvent. In order to improve further, it is preferable to coexist other non-aqueous solvent. Nonaqueous solvents include ethylene carbonate, propylene carbonate, vinylene carbonate, vinyl ethylene carbonate, cyclic carbonates such as fluoroethylene carbonate and difluoroethylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, γ-butyrolactone, Cyclic esters such as γ-valerolactone, δ-valerolactone, ε-caprolactone, γ-hexanolactone, trimethyl phosphate, triethyl phosphate, tri-n-propyl phosphate, triisopropyl phosphate, tri-n phosphate -Phosphate esters such as butyl, triisobutyl phosphate, triphenyl phosphate, chain esters such as methyl acetate, ethyl acetate, methyl butyrate, tetrahydrofuran, 1,3-dioxane, di- Ethers such as methoxyethane, diethoxyethane, methoxyethoxyethane, triglyme and tetraglyme, nitriles such as acetonitrile, benzonitrile, adiponitrile and glutaronitrile, dioxolane or derivatives thereof, and chain sulfones such as dimethylsulfone and diethylsulfone And cyclic sulfones such as sulfolane, propane sultone, butane sultone cyclic sulfonate, and the like. Among these non-aqueous solvents, at least one kind or a mixture of two or more kinds selected from the group consisting of cyclic carbonate, cyclic ester, chain carbonate, and phosphate ester is preferable from the viewpoint of ion conductivity.

  The nonaqueous electrolytic solution of the present invention uses, as a second essential component, a fluorosulfonylimide lithium salt represented by the general formula (2) as an electrolyte salt. Examples of typical fluorosulfonylimide lithium salts include bis (fluorosulfonyl) imide lithium, fluorosulfonyl (trifluoromethylsulfonyl) imide lithium, and fluorosulfonyl (pentafluoroethylsulfonyl) imide lithium. Bis (fluorosulfonyl) imidolithium is preferred because it can further improve ionic conductivity.

In the non-aqueous electrolyte of the present invention, other lithium salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ are used as the electrolyte salt. You may use together. In particular, when LiPF ₆ or LiBF ₄ is used in combination, effects such as improvement of the lifetime of the non-aqueous secondary battery may be obtained while maintaining high ionic conductivity. Under the present circumstances, the usage-amount with the other lithium salt with respect to the fluoro sulfonyl imide lithium salt is 0.01 to 2 times by mole ratio, Preferably it is 0.1 to 1 time. When the amount used is less than 0.01 times, the combined effect cannot be obtained, and when it exceeds 2 times, the ion conductivity is not sufficient. Further, the total concentration of the electrolyte salt in the non-aqueous electrolyte is preferably in the range of 0.2 to 3.0 mol / L, particularly when dissolved at a high concentration of 1.0 to 3.0 mol / L. It is preferable because a non-aqueous secondary battery having high ion conductivity and a long life can be easily obtained. When the concentration is less than 0.2 mol / L, the ion conductivity is not sufficient, and when it exceeds 3.0 mol / L, there is a problem that the electrolyte salt is likely to precipitate.

The non-aqueous secondary battery using the non-aqueous electrolyte of the present invention comprises at least a positive electrode, a negative electrode, and a separator. As the negative electrode material, metallic lithium, a lithium alloy, a carbon material that can be doped / undoped with lithium ions, a silicon material, a composite oxide of tin, titanium, and lithium, or the like can be used. Examples of the positive electrode material include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _1/4 Mn _3/4 O ₄ , LiFeO ₂ , LiFePO _{4, and the} like. A composite oxide of lithium and transition metal is used. The separator can be used as long as it is ion-permeable and electrically insulating. For example, a polyolefin resin such as polyethylene or polypropylene, or a microporous film of a fluorine resin such as polyvinylidene fluoride, cellulose or nonwoven fabric. Fibrous substances such as can be used. The shape and form of the nonaqueous secondary battery are not particularly limited, and for example, a cylindrical shape, a square shape, a coin shape, a card shape, and the like are appropriately selected.

  Hereinafter, the present invention will be described in detail using examples, but the present invention is not limited to the examples.

Measurement example 1
[Ionic conductivity]
The ion conductivity (unit: mS / cm) of the non-aqueous electrolyte was measured using the method described in “Electrochemical Measurement Manual, Fundamentals, 2002, 45, Electrochemical Society, Maruzen Co., Ltd.”. . That is, as shown in FIG. 1, cell constants were calculated in advance using a standard solution having a known electrical conductivity in a glass electrochemical cell 1 in which platinum electrodes 2 were combined facing each other. The prepared electrolyte was poured into this electrochemical cell and sealed. The obtained electrochemical cell was allowed to stand in a constant temperature bath at 20 ° C. for 1 hour, and then the solution resistance was measured by a complex impedance method using a potentiostat / galvanostat (manufactured by Toyo Technica, VersaSTAT4-400). The ionic conductivity of each non-aqueous electrolyte was calculated from the obtained solution resistance value.

  Calculation formula: ion conductivity (mS / cm) = solution resistance value (Ω) / cell constant

Measurement example 2
[viscosity]
The viscosity (unit: mPa · sec) of the non-aqueous electrolyte was measured using a cone plate type rotational viscometer (manufactured by BrookField, DV-I PRIME). That is, the prepared electrolytic solution was introduced into a cup of a rotational viscometer connected with a fluid thermostat, and measured until the temperature became constant at 20 ° C.

Measurement example 3
[Freezing point]
Measurement of the freezing point (unit: ° C.) of the non-aqueous electrolyte was performed according to JIS K0065 “Method for measuring freezing point of chemical products”. That is, the prepared electrolyte solution is introduced into a glass double tube container and indirectly cooled by an ice bath (or cryogen). Stir at a constant speed with a glass stirring rod that shakes up and down, and gradually cool the electrolyte. The freezing point was measured by reading the static temperature when there was no supercooling, and by reading the maximum temperature of the temperature rise when it showed a certain freezing point again after the temperature once decreased below the freezing point due to supercooling. .

Example 1-1
Lithium bis (fluorosulfonyl) imide (hereinafter abbreviated as LiFSI) as an electrolyte is adjusted to a concentration of 1.0 mol / L in tris (2,2,2-trifluoroethyl) phosphate (hereinafter abbreviated as TFEP). In addition, the mixture was sufficiently stirred at 20 ° C. and completely dissolved to prepare an electrolyte solution (LiFSI concentration = 11.5% by mass). Using this electrolytic solution, the ionic conductivity was measured by the method of Measurement Example 1. Moreover, the viscosity was measured by the method of Measurement Example 2 using the same electrolytic solution. The results are shown in Table 1.

Comparative Example 1-1
To TFEP, lithium hexafluorophosphate (hereinafter abbreviated as LiPF ₆ ) was added as an electrolyte so as to have a concentration of 0.5 mol / L, and the mixture was thoroughly stirred at 20 ° C. to completely dissolve the electrolyte. It was prepared (LiPF ₆ concentration = 4.5% by mass). Using this electrolytic solution, the ionic conductivity was measured by the method of Measurement Example 1. Moreover, the viscosity was measured by the method of Measurement Example 2 using the same electrolytic solution. The results are shown in Table 1.

Comparative Example 1-2
An electrolyte solution was prepared in the same manner as in Example 1-1 except that lithium bis (trifluoromethylsulfonyl) imide (hereinafter abbreviated as LiTFSI) was used as the electrolyte, and the ionic conductivity and viscosity were measured (LiTFSI concentration). = 17.2 mass%). The results are shown in Table 1.

Example 1-2
The same as Example 1-1 except that LiFSI was added to bis (2,2,2-trifluoroethyl) methyl phosphate (hereinafter abbreviated as BTFEMP) as an electrolyte so as to have a concentration of 1.0 mol / L. An electrolytic solution was prepared by the method, and ionic conductivity and viscosity were measured (LiFSI concentration = 12.0% by mass). The results are shown in Table 1.

Comparative Example 1-3
A 1.0 mol / L electrolytic solution was prepared in the same manner as in Example 1-2 except that LiPF ₆ was used as the electrolyte, and the ionic conductivity and viscosity were measured (LiPF ₆ concentration = 9.7% by mass). . The results are shown in Table 1.

Comparative Example 1-4
A 1.0 mol / L electrolytic solution was prepared in the same manner as in Example 1-2 except that LiTFSI was used as the electrolyte, and the ionic conductivity and viscosity were measured (LiTFSI concentration = 18.1% by mass). The results are shown in Table 1.

Example 1-3
The same as Example 1-1 except that LiFSI was added to bis (2,2,2-trifluoroethyl) ethyl phosphate (hereinafter abbreviated as BTFEEP) as an electrolyte so as to have a concentration of 1.0 mol / L. An electrolytic solution was prepared by the method, and ionic conductivity and viscosity were measured (LiFSI concentration = 12.4% by mass). The results are shown in Table 1.

Comparative Example 1-5
A 1.0 mol / L electrolytic solution was prepared in the same manner as in Example 1-3 except that LiPF ₆ was used as the electrolyte, and the ionic conductivity and viscosity were measured (LiPF ₆ concentration = 10.2% by mass). . The results are shown in Table 1.

Comparative Example 1-6
A 1.0 mol / L electrolytic solution was prepared in the same manner as in Example 1-3 except that LiTFSI was used as the electrolyte, and the ionic conductivity and viscosity were measured (LiTFSI concentration = 19.1% by mass). The results are shown in Table 1.

LiFSI: Lithium bis (fluorosulfonyl) imide LiPF ₆ : Lithium hexafluorophosphate LiTFSI: Lithium bis (trifluoromethylsulfonyl) imide TFEP: Tris (2,2,2-trifluoroethyl phosphate)
BTFEMP: bis (2,2,2-trifluoroethyl) methyl phosphate BTFEEP: bis (2,2,2-trifluoroethyl) ethyl phosphate

From the results of Examples and Comparative Examples shown in Table 1, when LiFSI is used as an electrolyte in a solvent system of a fluorine-based phosphate ester alone, the ionic conductivity is greatly improved compared to LiPF ₆ and LiTFSI. I know that. The reason why the ionic conductivity is improved in this way is not clear, but is related to the dissociation state of LiFSI due to the interaction between LiFSI and the fluorine-containing phosphate ester and the coordination state of the fluorine-containing phosphate ester to the Li cation. It is thought that there is.

Example 1-4
The saturation solubility of LiFSI in TFEP was measured. That is, an excess amount of LiFSI was mixed with TFEP and dissolved under constant temperature conditions of 20 ° C. under stirring conditions. After stirring for 10 hours or more and confirming that it was sufficiently dissolved, the mixture was allowed to stand for 2 hours, and the LiFSI concentration in the supernatant was determined by ¹⁹ F-NMR (AVANCE II 400 manufactured by BRUKER) to determine the LiFSI concentration in TFEP. Was calculated. As a result, the LiFSI saturated solubility in TFEP at 20 ° C. was 22.9% by mass (1.27 mol / L).

Comparative Example 1-7
Saturation solubility was measured in the same manner as in Example 1-4, except that the electrolyte was changed to LiPF ₆ . As a result, the LiPF ₆ saturated solubility in TFEP at 20 ° C. was 5.6% by mass (0.58 mol / L).

Example 2-1
Ethylene carbonate (hereinafter abbreviated as EC), dimethyl carbonate (hereinafter abbreviated as DMC) and TFEP were mixed at a volume ratio of 35/35/30, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L ( LiFSI concentration = 12.5% by mass). The electrolyte solution was measured for ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 2.

Comparative Example 2-1
Ion conductivity and viscosity were measured in the same manner as in Example 2-1, except that LiPF ₆ was used as the electrolyte (LiPF ₆ concentration = 10.1% by mass). The results are shown in Table 2.

EC: ethylene carbonate, DMC: dimethyl carbonate

  Example 2-1 and Comparative Example 2-1 are measurement examples of ion conductivity and viscosity when a cyclic carbonate and a chain carbonate are mixed with a fluorine-based phosphate ester. As is apparent from this result, an improvement in ionic conductivity is observed when combined with the LiFSI of the present invention.

Example 2-2
EC and TFEP were mixed at a volume ratio of 7/3, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L (LiFSI concentration = 12.7% by mass). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 3.

Example 2-3
An electrolytic solution was prepared in the same manner as in Example 2-2 except that the EC / TFEP mixing ratio was 5/5 by volume, and the ionic conductivity and viscosity were measured (LiFSI concentration = 12). .3% by mass). The results are shown in Table 3.

Furthermore, as a result of measuring the freezing point by the method of Measurement Example 3 using this electrolytic solution, the freezing point was -5.1 ° C. If the LiFSI when compared with the results of Comparative Example 2-3 was used as the electrolyte, the freezing point as compared with LiPF ₆ was found to have a surprising effect decreases.

Example 2-4
An electrolytic solution was prepared in the same manner as in Example 2-2 except that the EC / TFEP mixing ratio was 3/7 by volume, and the ionic conductivity and viscosity were measured (LiFSI concentration = 12). 0.0 mass%). The results are shown in Table 3.

Comparative Example 2-2
Except using LiPF6 as the electrolyte, an electrolyte solution was prepared in the same manner as in Example 2-2, the ion conductivity, the measurement of the viscosity was performed (LiPF ₆ concentration = 10.1 mass%). The results are shown in Table 3.

Comparative Example 2-3
An electrolyte solution was prepared in the same manner as in Example 2-3, except that LiPF ₆ was used as the electrolyte, and ion conductivity and viscosity were measured (LiPF ₆ concentration = 9.9% by mass). The results are shown in Table 3.

  Furthermore, as a result of measuring the freezing point by the method of Measurement Example 3 using this electrolytic solution, the freezing point was 4.1 ° C.

Comparative Example 2-4
An electrolyte solution was prepared in the same manner as in Example 2-4 except that LiPF ₆ was used as the electrolyte, and the ionic conductivity and viscosity were measured (LiPF ₆ concentration = 9.8% by mass). The results are shown in Table 3.

Examples 2-2 to 2-4 and Comparative Examples 2-2 to 2-4 are examples examined by changing the ratio of the fluorine-based phosphate ester and the nonaqueous solvent. In all proportions, who were mixed LiFSI indicates ionic conductivity of greater than when mixed with LiPF _6.

Example 2-5
Propylene carbonate (hereinafter abbreviated as PC) and TFEP were mixed at a volume ratio of 5/5, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L (LiFSI concentration = 12.8 mass%). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 4.

Example 2-6
Fluoroethylene carbonate (hereinafter abbreviated as FEC) and TFEP were mixed at a volume ratio of 5/5, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L (LiFSI concentration = 11.8% by mass). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 4.

Example 2-7
γ-Butyrolactone (hereinafter abbreviated as GBL) and TFEP were mixed at a volume ratio of 5/5, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L (LiFSI concentration = 13.4% by mass). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 4.

Example 2-8
PC and BTFEMP were mixed at a volume ratio of 5/5, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L (LiFSI concentration = 13.3 mass%). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 4.

Example 2-9
GBL and BTFEMP were mixed at a volume ratio of 5/5, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L (LiFSI concentration = 13.3 mass%). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 4.

Comparative Example 2-5
An electrolyte solution was prepared in the same manner as in Example 2-5, except that LiPF ₆ was used as the electrolyte, and ionic conductivity and viscosity were measured (LiPF ₆ concentration = 10.3 mass%). The results are shown in Table 4.

Comparative Example 2-6
An electrolyte solution was prepared in the same manner as in Example 2-6, except that LiPF ₆ was used as the electrolyte, and ionic conductivity and viscosity were measured (LiPF ₆ concentration = 9.5 mass%). The results are shown in Table 4.

Comparative Example 2-7
An electrolyte solution was prepared in the same manner as in Example 2-7, except that LiPF ₆ was used as the electrolyte, and ionic conductivity and viscosity were measured (LiPF ₆ concentration = 10.7% by mass). The results are shown in Table 4.

Comparative Example 2-8
Except for using LiPF ₆ as the electrolyte, an electrolytic solution was prepared in the same manner as in Example 2-8, and the ionic conductivity and viscosity were measured (LiPF ₆ concentration = 10.7% by mass). The results are shown in Table 4.

Comparative Example 2-9
Except for using LiPF ₆ as the electrolyte, an electrolytic solution was prepared in the same manner as in Example 2-9, and the ionic conductivity and viscosity were measured (LiPF ₆ concentration = 11.0% by mass). The results are shown in Table 4.

PC: Propylene carbonate FEC: Fluoroethylene carbonate GBL: γ-butyrolactone Examples 2-5 to 2-9 and Comparative examples 2-5 to 2-9 show examples using cyclic carbonates and cyclic esters other than EC. Thus, it turns out that ion conductivity improves not only by EC but in combination with various solvents.

Example 3-1
Triethyl phosphate (hereinafter abbreviated as TEP) and TFEP were mixed at a volume ratio of 7/3, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L (LiFSI concentration = 14.5% by mass). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 5.

Example 3-2
An electrolyte solution was prepared in the same manner as in Example 3-1, except that the mixing ratio of TEP / TFEP was 5/5 by volume ratio, and ionic conductivity and viscosity were measured (LiFSI concentration = 14). 0.0 mass%). The results are shown in Table 5.

Example 3-3
An electrolyte solution was prepared in the same manner as in Example 3-1, except that the TEP / TFEP mixing ratio was 3/7 by volume, and the ionic conductivity and viscosity were measured (LiFSI concentration = 13). .1% by mass). The results are shown in Table 5.

Example 3-4
Trimethyl phosphate (hereinafter abbreviated as TMP) and TFEP were mixed at a volume ratio of 7/3, and LiFSI as an electrolyte was dissolved at 1 mol / L. The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2 (LiFSI concentration = 13.5% by mass). The results are shown in Table 5.

Example 3-5
An electrolyte solution was prepared in the same manner as in Example 3-4, except that the mixing ratio of TMP / TFEP was 5/5 by volume ratio, and ionic conductivity and viscosity were measured (LiFSI concentration = 13). .2% by mass). The results are shown in Table 5.

Example 3-6
An electrolyte solution was prepared in the same manner as in Example 3-4, except that the TMP / TFEP mixing ratio was 3/7 by volume, and the ionic conductivity and viscosity were measured (LiFSI concentration = 12). .2% by mass). The results are shown in Table 5.

Comparative Example 3-1
An electrolyte solution was prepared in the same manner as in Example 3-1, except that lithium bis (trifluoromethylsulfonyl) imide (hereinafter abbreviated as LiTFSI) was used as the electrolyte, and ionic conductivity and viscosity were measured. (LiTFSI concentration = 22.3 mass%). The results are shown in Table 5.

Comparative Example 3-2
Except that LiTFSI was used as the electrolyte, an electrolytic solution was prepared in the same manner as in Example 3-2, and ionic conductivity and viscosity were measured (LiTFSI concentration = 21.2 mass%). The results are shown in Table 5.

  Furthermore, as a result of measuring the freezing point by the method of Measurement Example 3 using this electrolytic solution, the freezing point was 4.1 ° C.

Comparative Example 3-3
An electrolyte solution was prepared in the same manner as in Example 3-3 except that LiTFSI was used as the electrolyte, and ionic conductivity and viscosity were measured (LiTFSI concentration = 19.5 mass%). The results are shown in Table 5.

Comparative Example 3-4
An electrolyte solution was prepared in the same manner as in Example 3-4 except that LiTFSI was used as the electrolyte, and ionic conductivity and viscosity were measured (LiTFSI concentration = 20.9 mass%). The results are shown in Table 5.

Comparative Example 3-5
Except having used LiTFSI for the electrolyte, an electrolytic solution was prepared in the same manner as in Example 3-5, and ionic conductivity and viscosity were measured (LiTFSI concentration = 20.0 mass%). The results are shown in Table 5.

Comparative Example 3-6
Except having used LiTFSI for the electrolyte, an electrolytic solution was prepared in the same manner as in Example 3-5, and ionic conductivity and viscosity were measured (LiTFSI concentration = 19.0 mass%). The results are shown in Table 5.

TEP: Triethyl phosphate TMP: Trimethyl phosphate LiTFSI: Lithium bis (trifluoromethylsulfonyl) imide

  Examples 3-1 to 3-6 and Comparative Examples 3-1 to 3-6 show examples in which a non-fluorine phosphate ester is mixed with a fluorine phosphate ester and LiTFSI is used as a comparative electrolyte. As is clear from this result, even when combined with a non-fluorine phosphate ester, it is understood that the ionic conductivity is improved by using the LiFSI electrolyte.

Example 4-1
EC and BTFEMP were mixed at a volume ratio of 7/3, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L (LiFSI concentration = 12.8% by mass). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 6.

Example 4-2
An electrolytic solution was prepared in the same manner as in Example 4-1, except that the EC / BTFEMP mixing ratio was 5/5 by volume ratio, and the ionic conductivity and viscosity were measured (LiFSI concentration = 12). .5% by mass). The results are shown in Table 6.

Example 4-3
An electrolyte solution was prepared in the same manner as in Example 4-1, except that the EC / BTFEMP mixing ratio was 3/7 by volume, and the ionic conductivity and viscosity were measured (LiFSI concentration = 12). .4 mass%). The results are shown in Table 6.

Comparative Example 4-1
Except using LiPF6 as the electrolyte, an electrolyte solution was prepared in the same manner as in Example 4-1, the ion conductivity, the measurement of the viscosity was performed (LiPF ₆ concentration = 10.4 mass%). The results are shown in Table 6.

Comparative Example 4-2
Except using LiPF6 as the electrolyte, an electrolyte solution was prepared in the same manner as in Example 4-1, the ion conductivity, the measurement of the viscosity was performed (LiPF ₆ concentration = 10.2 mass%). The results are shown in Table 6.

Comparative Example 4-3
Except using LiPF6 as the electrolyte, an electrolyte solution was prepared in the same manner as in Example 4-1, the ion conductivity, the measurement of the viscosity was performed (LiPF ₆ concentration = 9.9 wt%). The results are shown in Table 6.

Example 5-1
EC and TFEP were mixed at a volume ratio of 5/5, and LiFSI as an electrolyte was dissolved at 0.5 mol / L and LiPF6 was dissolved at 0.5 mol / L (LiFSI concentration = 6.1 mass%, LiPF ₆ concentration = 5.0 mass%). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 7.

Comparative Example 5-1
An electrolyte solution was prepared in the same manner as in Example 5-1, except that 1 mol / L of LiPF6 was dissolved in the electrolyte, and 1 wt% LiFSI was added to the electrolyte solution, and ionic conductivity and viscosity were measured. (LiPF ₆ concentration = 9.9% by mass). The results are shown in Table 7.

Example 5-2
EC, TFEP, and 1,3-propane sultone were mixed at a volume ratio of 47.5 / 47.5 / 5, and LiFSI as an electrolyte was dissolved at 1 mol / L (LiFSI concentration = 12.3 mass). %). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 8.

Comparative Example 5-2
Except that LiPF6 in the electrolyte was dissolved 1 mol / L, the electrolytic solution was prepared in the same manner as in Example 5-2, the ion conductivity, the measurement of the viscosity was performed (LiPF ₆ concentration = 9.9 wt%). The results are shown in Table 8.

Example 5-3
EC and TFEP were mixed at a volume ratio of 5/5, and LiFSI as an electrolyte was dissolved so as to be 0.5 mol / L (LiFSI concentration = 6.2% by mass). The electrolyte solution thus prepared was subjected to measurement of ion conductivity and viscosity by the methods of Measurement Example 1 and Measurement Example 2. The results are shown in Table 9.

Example 5-4
Except that the LiFSI concentration was adjusted to 2 mol / L, an electrolyte solution was prepared in the same manner as in Example 5-3, and ionic conductivity and viscosity were measured (LiFSI concentration = 23.7 mass). %). The results are shown in Table 9.

Example 5-5
Except that the LiFSI concentration was adjusted to 3 mol / L, an electrolyte solution was prepared in the same manner as in Example 5-3, and ion conductivity and viscosity were measured (LiFSI concentration = 34.3 mass). %). The results are shown in Table 9.

Comparative Example 5-3
Except that LiPF6 in the electrolyte was dissolved at a concentration of 0.5 mol / L, the electrolytic solution was prepared in the same manner as in Example 5-3, the ion conductivity was measured for viscosity (LiPF ₆ concentration = 5. 1% by mass). The results are shown in Table 9.

Comparative Example 5-4
An electrolytic solution was prepared in the same manner as in Example 5-4 except that LiPF6 was dissolved in the electrolyte at a concentration of 2 mol / L. However, the electrolyte was not completely dissolved, and the electrolytic solution itself solidified, and the electrolytic solution could not be prepared.

Comparative Example 5-5
An electrolytic solution was prepared in the same manner as in Example 5-5 except that LiPF6 was dissolved in the electrolyte at a concentration of 3 mol / L. However, the electrolyte was not completely dissolved, and the electrolytic solution itself solidified, and the electrolytic solution could not be prepared.

  For reference, the results of Example 2-3 and Comparative Example 2-3 are also shown.

  The reason for the difference in solubility as in Examples 5-4 and 5-5 and Comparative Examples 5-4 and 5-5 is not clear, but the interaction between the fluorine-containing phosphate ester and LiFSI or LiPF6 is not clear. It is thought that such a difference in solubility was shown due to the difference.

Creation example 1
[Creation of coin cell type lithium ion secondary battery]
In Example 6-1 and Comparative Example 6-1, a coin cell type lithium ion secondary battery was prepared as a nonaqueous electrolyte secondary battery by the following method, and a charge / discharge cycle test was performed.

That is, LiCoO ₂ is used as the positive electrode active material, carbon black is used as the conductive auxiliary agent, and polyvinylidene fluoride (PVDF) is used as the binder, so that LiCoO ₂ : carbon black: PVDF = 90: 5: 5. What was blended and slurried using 1-methyl-2-pyrrolidone was applied on an aluminum current collector with a constant film thickness and dried to obtain a positive electrode.

  Natural spherical graphite is used as the negative electrode active material, PVDF is blended as a binder in a weight ratio of graphite: PVDF = 9: 1, and a slurry obtained using 1-methyl-2-pyrrolidone is made of copper. The negative electrode was obtained by applying a constant film thickness on the electric conductor and drying it.

  As the separator, an inorganic filler-containing polyolefin porous membrane was used.

  The non-aqueous electrolyte of the present invention was added to the above components to produce a lithium secondary battery using a coin-type cell having the structure shown in FIG. In the lithium secondary battery, the positive electrode 3 and the negative electrode 7 were disposed opposite to each other with the separator 10 interposed therebetween, and a laminate including the positive electrode 3, the separator 10, and the negative electrode 7 was fitted into the gasket 11. A positive electrode stainless steel cap 5 and a negative electrode stainless steel cap 6 are attached to the gasket 11, and the positive electrode 3 constituting the laminate is pressed against the inside of the positive electrode stainless steel cap 5 by a stainless spring 9 provided inside the negative electrode stainless steel cap 6. A lithium ion secondary battery was prepared.

Example 6-1
EC and TFEP were mixed at a volume ratio of 5/5, and LiFSI as an electrolyte was dissolved so as to be 1 mol / L. Using this electrolytic solution, a coin cell type lithium secondary battery was prepared by the method of Preparation Example 4. Under a constant temperature condition of 25 ° C., the battery was charged with an upper limit voltage of 4.2 V with a charging current of 0.1 C, and subsequently discharged to 3.0 V with a discharging current of 0.1 C. This battery was subjected to constant current-constant voltage charging at a constant current of 25 ° C. with a charging current of 1 C and a constant current discharging to a final voltage of 3.0 V with a discharging current of 1 C. The results are shown in Table 10.

Comparative Example 6-1
An electrolytic solution was prepared in the same manner as in Example 6-1 except that LiPF6 was used as the electrolyte, and a coin cell type lithium secondary battery was prepared. Under a constant temperature condition of 25 ° C., the battery was charged with an upper limit voltage of 4.2 V with a charging current of 0.1 C, and subsequently discharged to 3.0 V with a discharging current of 0.1 C. This battery was subjected to constant current-constant voltage charging at a constant current of 25 ° C. with a charging current of 1 C and a constant current discharging to a final voltage of 3.0 V with a discharging current of 1 C. The results are shown in Table 10.

  In Example 6-1 and Comparative Example 6-1, the charge / discharge performance of the batteries was compared. As a result, it has been clarified that the composition of the present invention makes it possible to charge and discharge the battery, and that the difference becomes remarkable particularly in a test at a high rate. Although the details of this reason are not clear, it is considered that the ionic conductivity of the nonaqueous electrolytic solution is increased by the interaction between the fluorine-containing phosphate ester and LiFSI, and as a result, the battery capacity at a particularly high rate is improved.

  INDUSTRIAL APPLICABILITY The present invention is useful as a non-aqueous electrolyte that achieves both safety and high ionic conductivity, which was difficult with conventional electrolytes, and a non-aqueous secondary battery excellent in safety and output characteristics.

DESCRIPTION OF SYMBOLS 1 Electrochemical cell made of glass 2 Electrode made of platinum 3 Positive electrode active material 4 Positive electrode current collector 5 Positive electrode stainless steel cap 6 Negative electrode stainless steel cap 7 Negative electrode material 8 Negative electrode current collector 9 Stainless steel leaf spring 10 Inorganic filler-containing polyolefin porous separator 11 Gasket

Claims (10)

The following general formula (1) as a non-aqueous solvent
(In the formula, Rf ¹ , Rf ² and Rf ³ are each independently a linear or branched alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 10 carbon atoms, or a linear chain having 1 to 6 carbon atoms. Or a branched fluorine-containing alkyl group, and at least one of Rf ^{1 to} Rf ³ is a fluorine-containing alkyl group), and an electrolyte salt represented by the following general formula (2)
(In the formula, Rf ⁴ represents a fluorine atom, a linear or branched alkyl group having 1 to 4 carbon atoms, or a linear or branched fluorine-containing alkyl group having 1 to 4 carbon atoms). Non-aqueous electrolyte containing.   The nonaqueous electrolytic solution according to claim 1, wherein at least one selected from the group consisting of a cyclic carbonate, a chain carbonate, a cyclic ester, and a phosphate ester is further present as the nonaqueous solvent.   The fluorine-containing phosphoric acid ester represented by the general formula (1) is tris (2,2-difluoroethyl) phosphate, tris (2,2,2-trifluoroethyl) phosphate, tris (2,2). , 3,3-tetrafluoropropyl), bis (2,2,2-trifluoroethyl) methyl phosphate, bis (2,2,2-trifluoroethyl) ethyl phosphate, bis (2,2,2) phosphate 2-trifluoroethyl) phenyl, bis (2,2,2-trifluoroethyl) phosphate 2,2-difluoroethyl, bis (2,2,2-trifluoroethyl) phosphate 2,2,3,3 -Tetrafluoropropyl, bis (2,2-difluoroethyl) phosphate 2,2,2-trifluoroethyl, bis (2,2,3,3-tetrafluoropropyl) phosphate 2,2,2-trifluoro Ethyl and phosphorus The nonaqueous electrolytic solution according to claim 1 or 2, which is at least one selected from the group consisting of (2,2,2-trifluoroethyl) (2,2,3,3-tetrafluoropropyl) methyl. .   The nonaqueous electrolytic solution according to any one of claims 1 to 3, wherein the fluorosulfonylimide lithium salt is bis (fluorosulfonyl) imide lithium. Further comprising a LiPF ₆ or LiBF ₄ as an electrolyte salt, any one of claims 1 to 4 usage LiPF ₆ or LiBF ₄ relative fluorosulfonyl imide lithium salt is 0.01 to 2 molar ratio The non-aqueous electrolyte described.   The nonaqueous electrolysis according to claim 2, wherein the nonaqueous solvent to be further present is at least one cyclic carbonate selected from the group consisting of ethylene carbonate, propylene carbonate, vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate and difluoroethylene carbonate. liquid.   The nonaqueous electrolytic solution according to claim 2, wherein the nonaqueous solvent to be further present is at least one chain carbonate selected from the group consisting of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.   The nonaqueous solvent to be further present is at least one cyclic ester selected from the group consisting of γ-butyrolactone, γ-valerolactone, δ-valerolactone, ε-caprolactam and γ-hexanolactone. Non-aqueous electrolyte.   Further, the non-aqueous solvent to be present is selected from the group consisting of trimethyl phosphate, triethyl phosphate, tri-n-propyl phosphate, triisopropyl phosphate, tri-n-butyl phosphate, triisobutyl phosphate and triphenyl phosphate. The nonaqueous electrolytic solution according to claim 2, which is at least one phosphate ester.   A nonaqueous secondary battery comprising the nonaqueous electrolyte solution according to any one of claims 1 to 9.