JP2017178791A - Sulfonium salt, electrolytic solution for lithium secondary battery, and lithium secondary battery therewith - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sulfonium salt having low viscosity, electrolytic solution for a lithium secondary battery and a lithium secondary battery.SOLUTION: There is provided a sulfonium salt shown by a chemical formula (1).SELECTED DRAWING: None

Description

The present invention relates to a sulfonium salt, an electrolytic solution for a lithium secondary battery, and a lithium secondary battery using the same.

  Salts that take a liquid state at room temperature are called ionic liquids, and many salts having a structure with a nitrogen atom as a cation as typified by ammonium salts have been reported. However, ammonium salts have a relatively high melting point and high viscosity, and only a part of the structure becomes a low viscosity liquid around room temperature. In addition, ionic liquids with nitrogen atoms as cations have low reduction stability and are electrolytes, electrolytes or additives for lithium secondary batteries, electric double layer capacitors, fuel cells or dye-sensitized solar cells, or power storage devices. It is a major obstacle for its application as a drug.

  As an ionic liquid in a liquid state over a wide temperature range, a sulfonium salt having a sulfur atom as a cation is known. (See Patent Document 1)

Japanese Patent Laid-Open No. 2008-233103

  However, the characteristics of conventional sulfonium salts are not satisfactory in consideration of application to various electric devices, and there is a problem that the viscosity is particularly high.

  This invention is made | formed in view of the subject which the said prior art has, and aims at providing the sulfonium salt with low viscosity, the electrolyte solution for lithium secondary batteries, and a lithium secondary battery using the same.

  In order to solve the above problems, the sulfonium salt according to the present invention is represented by the following chemical formula (1).

［式中の置換基Ｒ_１〜Ｒ_５は一価の置換基、Ｘ⁻は一価のアニオンを示す。（ただし、Ｒ_１≠Ｒ_２≠Ｒ_３、かつＲ_Ａ≠Ｒ_４≠Ｒ_５）］

[In the formula, substituents R _{1 to} R ₅ represent a monovalent substituent, and X ⁻ represents a monovalent anion. (However, R ₁ ≠ R ₂ ≠ R ₃ and R _A ≠ R ₄ ≠ R ₅ )]

  According to this, in the sulfonium salt according to the present invention, the adjacent carbon of the sulfur atom which is the cation center is an asymmetric carbon, and forms a diastereomer. The valence of the cation center of the sulfonium salt is 3, which is sterically sparse compared to the ammonium salt and phosphonium salt. For this reason, a cation and an anion are easy to approach, the whole system forms a long-period order, and a viscosity increases. However, by forming a diastereomer, the steric repulsion between cations is increased, the long-period order is disturbed, and the viscosity is lowered.

  The sulfonium salt according to the present invention is further preferably represented by the following chemical formula (2) or chemical formula (3).

［式中の置換基Ｒ_６およびＲ_７は一価の置換基、Ｘ⁻は一価のアニオンを示す］

[In the formula, substituents R ₆ and R ₇ represent a monovalent substituent, and X ⁻ represents a monovalent anion.]

  According to this, it is more preferable as a cation structure, and the viscosity is further reduced.

  The electrolyte solution for a lithium secondary battery according to the present invention includes the sulfonium salt.

  The lithium secondary battery according to the present invention includes a positive electrode, a negative electrode, a separator positioned between the positive electrode and the negative electrode, and the electrolyte solution for a lithium secondary battery.

  According to this, even if a sulfonium salt is used as the electrolytic solution, a lithium secondary battery in which deterioration of rate characteristics is suppressed is provided.

  ADVANTAGE OF THE INVENTION According to this invention, the sulfonium salt with low viscosity, the electrolyte solution for lithium secondary batteries, and a lithium secondary battery using the same are provided.

It is a schematic cross section of the lithium ion secondary battery of this embodiment.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments according to the invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

<Sulphonium salt>
The sulfonium salt according to this embodiment is represented by the chemical formula (1).

  In the sulfonium salt according to this embodiment, the adjacent carbon of the sulfur atom that is the cation center is an asymmetric carbon, and forms a diastereomer. The valence of the cation center of the sulfonium salt is 3, which is sterically sparse compared to the ammonium salt and phosphonium salt. For this reason, a cation and an anion are easy to approach, the whole system forms a long-period order, and a viscosity increases. However, by forming a diastereomer, the steric repulsion between cations is increased, the long-period order is disturbed, and the viscosity is lowered.

R _{1 to} R ₅ are not particularly limited, but are preferably an alkyl group or a fluoroalkyl group, and more preferably a linear alkyl group or a fluoroalkyl group. In this case, the viscosity is further reduced.

R _{4 to} R ₅ are not particularly limited, but are preferably an alkyl group or a fluoroalkyl group, and more preferably a linear alkyl group or a fluoroalkyl group. In this case, the number of carbon atoms is preferably 1 to 3, and the viscosity is further reduced.

The monovalent anion is not particularly limited, and a known anion such as BF ₄ ⁻ , PF ₆ ⁻ , CClO ₄ ⁻ , N (SO ₂ CF ₃ ) ₂ ⁻ or N (SO ₂ F) ₂ ⁻ may be used. it can.

  The ammonium salt according to this embodiment is preferably represented by chemical formula (2) or (3).

  According to this, it is more preferable as a cation structure, and the viscosity is further reduced.

R ₆ and R ₇ are not particularly limited, but are preferably an alkyl group or a fluoroalkyl group, and more preferably a linear alkyl group or a fluoroalkyl group. In this case, the viscosity is further reduced.

The number of carbon atoms of R ₆ and _{R 7} are 1 to 3 is preferable, 1 to 2 is more preferred. In this case, the viscosity is further reduced.

(Synthesis method)
A general synthesis method of the sulfonium salt according to the present embodiment is as follows.

  First, an appropriate sulfide and an alkyl halide are mixed and heated as necessary to obtain a tertiary sulfonium halide. In addition, you may make it react under pressure using an autoclave etc. The obtained ammonium halide is dissolved in a polar solvent such as water and reacted with a reagent that generates the required anion species such as lithium bis (trifluorosulfonyl) imide, and an anion exchange reaction is performed to obtain a desired sulfonium salt. Can be obtained.

<Lithium ion secondary battery>
As shown in FIG. 1, a lithium ion secondary battery 100 according to the present embodiment is disposed adjacent to each other between a plate-like negative electrode 20 and a plate-like positive electrode 10 facing each other, and the negative electrode 20 and the positive electrode 10. A plate-like separator 18, an electrolyte solution containing lithium ions, a case 50 containing these in a sealed state, and one end of the negative electrode 20 being electrically connected. A lead 62 whose other end protrudes outside the case and a lead 60 whose one end is electrically connected to the positive electrode 10 and whose other end protrudes outside the case are provided.

  The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. The separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14.

<Positive electrode>
(Positive electrode current collector)
The positive electrode current collector 12 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as aluminum, an alloy thereof, or stainless steel can be used.

(Positive electrode active material layer)
The positive electrode active material layer 14 is mainly composed of a positive electrode active material, a positive electrode binder, and a necessary amount of positive electrode conductive additive.

(Positive electrode active material)
As the positive electrode active material, lithium ion occlusion and release, lithium ion desorption and insertion (intercalation), or doping and dedoping of a counter anion (for example, PF ₆ ⁻ ) of the lithium ion are reversibly performed. If it can be made to advance, it will not specifically limit, A well-known electrode active material can be used. For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and the chemical formula: LiNi _x Co _y Mn _z MaO ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1) , 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, and M is one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, and Cr) Product, lithium vanadium compound (LiV ₂ O ₅ ), olivine-type LiMPO ₄ (where M is one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al, Zr or VO) shown), and composite metal oxides such as lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1)

(Binder for positive electrode)
The positive electrode binder bonds the positive electrode active materials to each other and bonds the positive electrode active material layer 14 to the positive electrode current collector 12. The binder is not particularly limited as long as it can be bonded as described above. For example, fluorine resin such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), cellulose, styrene / butadiene rubber, ethylene / propylene rubber, polyimide A resin, a polyamideimide resin, or the like may be used. Alternatively, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene, polythiophene, and polyaniline. Examples of the ion conductive conductive polymer include those obtained by combining a polyether polymer compound such as polyethylene oxide and polypropylene oxide and a lithium salt such as LiClO ₄ , LiBF ₄ , and LiPF _6. It is done.

  Although content of the binder in the positive electrode active material layer 14 is not specifically limited, When adding, it is preferable that it is 0.5-5 mass parts with respect to the mass of a positive electrode active material.

(Conductive aid for positive electrode)
The conductive aid for positive electrode is not particularly limited as long as it improves the conductivity of the positive electrode active material layer 14, and a known conductive aid can be used. Examples thereof include carbon-based materials such as graphite and carbon black, metal fine powders such as copper, nickel, stainless steel, and iron, and conductive oxides such as ITO.

<Negative electrode>
(Negative electrode current collector)
The negative electrode current collector 22 may be a conductive plate material, and for example, a metal thin plate (metal foil) such as copper can be used.

(Negative electrode active material layer)
The negative electrode active material layer 24 is mainly composed of a negative electrode active material.

(Negative electrode active material)
The negative electrode active material is not particularly limited as long as it can reversibly advance occlusion and release of lithium ions and desorption and insertion (intercalation) of lithium ions, and a known electrode active material can be used. . For example, carbon materials such as graphite and hard carbon, silicon materials such as silicon oxide (SiO _x ) metal silicon (Si), metal oxides such as lithium titanate (LTO), metal materials such as lithium, tin, and zinc Is mentioned.

  Among these, in particular, lithium is preferably used as the negative electrode active material, and a negative electrode in which the formation of dendrites accompanying charge / discharge is suppressed can be obtained.

  When a metal material is not used as the negative electrode active material, the negative electrode active material layer 24 may further include a negative electrode binder and a negative electrode conductive additive.

(Binder for negative electrode)
There is no limitation in particular as a binder for negative electrodes, The thing similar to the binder for positive electrodes described above can be used.

(Conductive aid for negative electrode)
There is no limitation in particular as a conductive support agent for negative electrodes, The thing similar to the conductive support agent for positive electrodes described above can be used.

<Electrolyte>
The electrolytic solution according to the present embodiment includes a sulfonium salt represented by the chemical formula (1).

The electrolyte is not particularly limited as long as it is a lithium salt used as an electrolyte of a lithium ion secondary battery. For example, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , lithium bisoxalate borate, LiCF ₃ SO ₃ , An organic acid anion salt such as (CF ₃ SO ₂ ) ₂ NLi, (FSO ₂ ) ₂ NLi, or the like can be used.

  As mentioned above, although preferred embodiment which concerns on this invention was described, this invention is not limited to the said embodiment.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

[Synthesis Example 1]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of ethyl methyl sulfide, and 1.00 mol of 2-iodobutane were put into a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. After concentrating the solution after the reaction, recrystallization was performed with an ethanol-diethyl ether mixed solvent to obtain 0.900 mol of S- (sec-butyl) -S-propyl-S-methylsulfonium iodide.

(Synthesis of sulfonium salt)
0.500 mol of S- (sec-butyl) -S-propyl-S-methylsulfonium iodide was dissolved in 300 mL of water, and then 0.750 mol of lithium bis (trifluoromethylsulfonyl) imide (LiTFSI) was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S- (sec-butyl) -S-propyl-S-methylsulfonium bis (trifluoromethylsulfonyl) imide.
The compound was identified with a nuclear magnetic resonance analyzer (JEOL ECA-500) in CDCl3. The spectral data is shown in Table 1.

(Measurement of viscosity of sulfonium salt)
The viscosity of the synthesized sulfonium salt was measured using an EMS viscometer (Kyoto Electronics Industry EMS-1000). The results are shown in Table 1.

[Synthesis Example 2]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of tert-butylmethylsulfide and 1.00 mol of 2-iodobutane were sealed in a pressure-resistant container made of PFA, and then reacted at 75 ° C. for 24 hours. After concentrating the solution after the reaction, recrystallization was performed with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S- (sec-butyl) -S- (tert-butyl) -S-methylsulfonium iodide. .

  S- (sec-butyl) -S- (tert-butyl) -S-methylsulfonium iodide 0.500 mol was dissolved in 300 mL of water, and then lithium bis (trifluoromethylsulfonyl) imide (LiTFSI) 0.750 mol was gradually added. Added to. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S- (sec-butyl) -S- (tert-butyl) -S-methylsulfonium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 3]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methyltetrahydrothiophene, and 1.00 mol of iodomethane were put into a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-methyl-2-methyltetrahydrothiophenium iodide.

  0.500 mol of S-methyl-2-methyltetrahydrothiophenium iodide was dissolved in 300 mL of water, and then 0.750 mol of LiTFSI was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-methyl-2-methyltetrahydrothiophenium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 4]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methyltetrahydrothiophene, and 1.00 mol of iodoethane were sealed in a pressure-resistant container made of PFA, and then reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-ethyl-2-methyltetrahydrothiophenium iodide.

  0.500 mol of S-ethyl-2-methyltetrahydrothiophenium iodide was dissolved in 300 mL of water, and then 0.750 mol of LiTFSI was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-ethyl-2-methyltetrahydrothiophenium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 5]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methyltetrahydrothiophene, and 1.00 mol of 1-iodopropane were put into a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-propyl-2-methyltetrahydrothiophenium iodide.

  0.500 mol of S-propyl-2-methyltetrahydrothiophenium iodide was dissolved in 300 mL of water, and then 0.750 mol of LiTFSI was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-propyl-2-methyltetrahydrothiophenium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 6]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methyltetrahydrothiophene, and 1.00 mol of 1-iodobutane were put into a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-butyl-2-methyltetrahydrothiophenium iodide.

  0.500 mol of S-butyl-2-methyltetrahydrothiophenium iodide was dissolved in 300 mL of water, and then 0.750 mol of LiTFSI was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-butyl-2-methyltetrahydrothiophenium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 7]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methyltetrahydrothiophene, and 1.00 mol of fluoroiodomethane were put in a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-fluoromethyl-2-methyltetrahydrothiophenium iodide.

  0.500 mol of S-fluoromethyl-2-methyltetrahydrothiophenium iodide was dissolved in 300 mL of water, and then 0.750 mol of LiTFSI was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-fluoromethyl-2-methyltetrahydrothiophenium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 8]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methyltetrahydrothiophene, and 1.00 mol of iodomethane were put into a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-methyl-2-methyltetrahydrothiophenium iodide.

  0.500 mol of S-methyl-2-methyltetrahydrothiophenium iodide was dissolved in 300 mL of water, and then 0.750 mol of lithium bis (fluorosulfonyl) imide (LiFSI) was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-methyl-2-methyltetrahydrothiophenium bis (fluorosulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 9]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methyltetrahydrothiophene, and 1.00 mol of iodomethane were put into a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-methyl-2-methyltetrahydrothiophenium iodide.

  0.500 mol of S-methyl-2-methyltetrahydrothiophenium iodide was dissolved in 300 mL of water, and then 0.750 mol of lithium perchlorate was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-methyl-2-methyltetrahydrothiophenium perchloride. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 10]
(Synthesis of alkyltian)
Starting from 1,5-hexadiene, Chem. Zentralbl. , 1923, vol. 94, #I p. According to 1504, bromination of the terminal diene and cyclization with NaS were performed to obtain 2-methylthiane.

  Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methylthian, and 1.00 mol of iodomethane were put into a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-methyl-2-methylthianium iodide.

  0.500 mol of S-methyl-2-methylthianium iodide was dissolved in 300 mL of water, and then 0.750 mol of LiTFSI was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-methyl-2-methylthianium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Example 11]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of 2-methylthian, and 1.00 mol of 1-iodobutane were put in a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and then recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-butyl-2-methylthianium iodide.

  0.500 mol of S-butyl-2-methylthianium iodide was dissolved in 300 mL of water, and then 0.750 mol of LiTFSI was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-butyl-2-methylthianium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

[Synthesis Comparative Example 1]
Under an Ar atmosphere, 200 mL of dry ethanol, 1.00 mol of tetrahydrothiophene, and 1.00 mol of iodomethane were placed in a pressure-resistant container made of PFA, sealed, and reacted at 75 ° C. for 24 hours. The solution after the reaction was concentrated and recrystallized with a mixed solvent of ethanol-diethyl ether to obtain 0.900 mol of S-methyltetrahydrothiophenium iodide.

  0.500 mol of S-methyltetrahydrothiophenium iodide was dissolved in 300 mL of water, and then 0.750 mol of LiTFSI was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried to obtain 0.480 mol of S-methyltetrahydrothiophenium bis (trifluoromethylsulfonyl) imide. The spectral data and viscosities of the compounds are shown in Table 1.

  The sulfonium salt synthesized in Synthesis Examples 1 to 11 showed a lower viscosity than the sulfonium salt of Comparative Example 1. It is thought that the formation of diastereomers disturbed long-period order and reduced the viscosity.

[Example 1]
(Production of electrodes)
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM): carbon black: PVDF = 80: 10: 10 (mass%) The mixture was mixed to obtain N-methyl-2-pyrrolidone (NMP ) Was applied to an aluminum metal foil having a thickness of 20 μm, and NMP was evaporated to obtain a positive electrode sheet. A negative electrode sheet was obtained by affixing a 100 μm lithium foil on a 16 μm thick copper foil.

(Preparation of electrolyte)
LiFSI was dissolved in the sulfonium salt produced in Synthesis Example 1 to 1 mol / L to prepare an electrolyte solution.

(Production of evaluation lithium-ion secondary battery)
The positive electrode and negative electrode produced above, and a separator made of a polyethylene microporous film sandwiched between them and put in an aluminum laminate pack, into this aluminum laminate pack, after injecting the electrolyte prepared above, vacuum sealed, A lithium ion secondary battery for evaluation was produced.

(Measurement of rate capacity maintenance rate)
About the lithium ion secondary battery for evaluation produced above, using a secondary battery charge / discharge test apparatus (manufactured by Hokuto Denko Co., Ltd.), the voltage range was 2.5 V to 4.2 V, and 1 C = 170 mAh / g. The battery was charged and discharged at a current value of 0.05 C, and the initial discharge capacity was determined. Subsequently, charge and discharge were performed at a current value of 0.5 C, and a rate capacity retention rate (0.5 C discharge capacity / initial discharge capacity × 100) was obtained. The obtained results are shown in Table 1.

[Examples 2 to 11]
The lithium ion secondary batteries for evaluation of Examples 2 to 11 were produced in the same manner as in Example 1 except that the sulfonium salt used in the production of the electrolytic solution was the sulfonium salt produced in Synthesis Examples 2 to 11. .

[Comparative Example 1]
A lithium ion secondary battery for evaluation of Comparative Example 1 was produced in the same manner as Example 1 except that the ammonium salt used in the production of the electrolytic solution was the sulfonium salt produced in Synthesis Comparative Example 1.

Table 1 shows the results of measurement of the rate capacity retention rate described in Example 1 for the lithium ion secondary batteries for evaluation of Examples 2 to 11. Examples 1 to 11 all showed a favorable rate capacity retention rate relative to Comparative Example 1. It is presumed that rate characteristics were improved by using a sulfonium salt having a low viscosity.

  The present invention provides a sulfonium salt having a low viscosity.

  DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 20 ... Negative electrode, 22 ... Negative electrode collector, 24 ... Negative electrode active material layer, 30 ... Laminate, 50 ... Case, 60 62 ... Lead, 100 ... Lithium ion secondary battery.

Claims (4)

A sulfonium salt represented by the following chemical formula (1).

[In the formula, substituents R _{1 to} R ₅ represent a monovalent substituent, and X ⁻ represents a monovalent anion. (However, R ₁ ≠ R ₂ ≠ R ₃ and R _A ≠ R ₄ ≠ R ₅ )] A sulfonium salt represented by the following chemical formula (2) or (3).

[In the formula, substituents R ₆ and R ₇ represent a monovalent substituent, and X ⁻ represents a monovalent anion. ]   The electrolyte solution for lithium secondary batteries containing the sulfonium salt in any one of Claim 1 or 2. The lithium secondary battery provided with the positive electrode, the negative electrode, the separator located between the said positive electrode and the said negative electrode, and the electrolyte solution for lithium secondary batteries of Claim 3.

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