JP2016178064A - Electrolytic solution for lithium ion secondary batteries and lithium ion secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an electrolytic solution for lithium ion secondary batteries which enables the formation of a lithium ion secondary battery superior in cycle characteristics; and a lithium ion secondary battery arranged by use of such an electrolytic solution.SOLUTION: The means for solving the problem is an electrolytic solution for lithium ion secondary batteries which comprises an ordinary temperature molten salt and an electrolyte. The ordinary temperature molten salt is a sulfonium salt. In the electrolytic solution, the content of halide ions except fluoride ions is less than 100 ppm in total.SELECTED DRAWING: None

Description

  The present invention relates to an electrolyte for a lithium ion secondary battery and a lithium ion secondary battery using the same.

  Lithium ion secondary batteries are widely applied as power sources for portable electronic devices because they are lighter and have a higher capacity than nickel cadmium batteries, nickel metal hydride batteries, and the like. It is also a promising candidate as a power source for use in hybrid vehicles and electric vehicles. With the recent miniaturization and higher functionality of portable electronic devices, further increase in capacity is expected for lithium ion secondary batteries that serve as these power sources.

  Examples of non-aqueous electrolytes in lithium ion secondary batteries include electrolyte solutions in which an electrolyte is dissolved in an organic solvent such as ethylene carbonate, propylene carbonate, dimethoxyethane, γ-butyrolactone, N, N-dimethylformamide, tetrahydrofuran, or acetonitrile. Has been used. However, since the organic solvent used in these electrolyte solutions is volatile and is itself a dangerous substance, it has problems in long-term reliability, durability, and safety.

  Therefore, it has been proposed to use a room temperature molten salt having high flame retardancy without using an electrolyte solution containing an organic solvent in the electrolyte (Patent Document 1). Among the room temperature molten salts, the sulfonium salt has low viscosity and high ionic conductivity (Patent Document 2), and therefore is considered promising as an electrolyte for a lithium ion secondary battery.

JP-A-4-349365 JP 2004-203763 A

  However, a lithium ion secondary battery using a sulfonium salt as an electrolytic solution is not always satisfactory in battery characteristics, and deterioration of cycle characteristics is particularly remarkable. As a result of extensive research on this matter, it was found that the halide ions remaining in the process of synthesizing the sulfonium salt promote the reductive decomposition of the sulfonium salt and cause deterioration of the characteristics.

  The present invention has been made in view of the above-described problems of the prior art, and provides an electrolyte for a lithium ion secondary battery that can constitute a lithium ion secondary battery having excellent cycle characteristics, and a lithium ion secondary battery using the same. The purpose is to provide.

  In order to solve the above problems, an electrolytic solution for a lithium ion secondary battery of the present invention has a room temperature molten salt and an electrolyte, the room temperature molten salt is a sulfonium salt, and a halogen other than fluoride ions in the electrolytic solution. The total content of chloride ions is less than 100 ppm.

  According to this, reductive decomposition of the sulfonium salt by the halide is suppressed, and the cycle characteristics when the battery is configured is improved.

  In the electrolytic solution for a lithium ion secondary battery of the present invention, the total content of halide ions is preferably less than 10 ppm.

  According to this, it is a value more suitable for suppressing reductive decomposition of the sulfonium salt by halide ions, and the cycle characteristics when the battery is constructed are further improved.

  Moreover, the lithium ion secondary battery of the present invention is characterized by using the above-described electrolyte for lithium ion secondary batteries.

  According to this, a lithium ion secondary battery excellent in cycle characteristics is provided.

  ADVANTAGE OF THE INVENTION According to this invention, the electrolyte solution for lithium ion secondary batteries which can comprise the lithium ion secondary battery excellent in cycling characteristics, and a lithium ion secondary battery using the same can be provided.

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

  Hereinafter, preferred embodiments of the present 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.

<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 negative electrode lead 62 whose other end protrudes outside the case, and a positive electrode 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)
Examples of the positive electrode active material include occlusion and release of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and counter anions (for example, PF ₆ ⁻ ) of the lithium ions. The electrode is not particularly limited as long as it can be reversibly advanced, and a 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 general 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) Oxide, 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 of lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 (0.9 <x + y + z <1.1) , etc.

(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, a negative electrode binder, and an amount of a negative electrode conductive additive as required.

(Negative electrode active material)
Also as the negative electrode active material, insertion and extraction of lithium ions, desorption and insertion (intercalation) of lithium ions, or doping and dedoping of lithium ions and a counter anion (for example, PF ₆ ⁻ ) of the lithium ions. The electrode is not particularly limited as long as it can be reversibly advanced, 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.

(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 for lithium ion secondary battery>
The electrolyte for a lithium ion secondary battery according to this embodiment includes a sulfonium salt that is a room temperature molten salt and an electrolyte, and the content of halide ions excluding fluoride ions in the electrolyte is less than 100 ppm. is there.

  The sulfonium salt has a tetrahedral structure similar to room temperature molten salts such as ammonium salts and phosphonium salts, but one of the orbitals is occupied by unshared electron pairs and has a three-dimensionally sparse structure. Therefore, the central sulfur cation is easily reduced. The electrolyte solution for a lithium ion secondary battery according to the present embodiment has a total content of halide ions excluding fluoride ions that act as a reducing agent, which is less than 100 ppm, and the reductive decomposition of the sulfonium salt is suppressed to constitute the battery. It is possible to improve the cycle characteristics at the time.

  Furthermore, from the viewpoint of inhibiting decomposition, the total content of halide ions excluding fluoride ions in the solution is more preferably less than 10 ppm.

  There is no limitation in particular as said sulfonium salt, For example, what is represented by following formula (1) can be used.

However, in the above formula (1), examples of R ₁ , R ₂ and R ₃ include linear or branched alkyl groups, phenyl groups, cycloalkyl groups, alkyloxy groups, alkylthia groups, dialkylaza groups, and the like. . Any hydrogen of these substituents may be substituted with any atom such as fluorine or chlorine, or any atomic group such as a perfluoroalkyl group or an alkyloxy group. In addition, a ring structure in which R ₁ and R ₂ are condensed with each other may be used. X ⁻ represents an anion.

When R ₁ , R ₂ and R ₃ are alkyl groups, for example, the number of carbon atoms can be 1 or more and 8 or less.

When R ₁ , R ₂ and R ₃ are cycloalkyl groups, for example, the number of carbon atoms can be 5 or more and 7 or less.

When R ₁ , R ₂ and R ₃ are alkyloxy groups or alkylthia groups, for example, the number of carbon atoms can be 1 or more and 4 or less.

When R ₁ , R ₂ and R ₃ are dialkylaza groups, for example, the number of carbon atoms can be 2 or more and 4 or less.

When R ₁ and R ₂ are ring structures condensed with each other, R ₁ and R ₂ can be combined to form an α, ω-alkylene group having 4 to 7 carbon atoms. R ₃ can be selected from the same structure as in the general formula (1).

In the formula (1), the anion represented by X ⁻ can include PF ₆ ⁻ , BF ₄ ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , and an imide anion. Further, X ^- a case of selecting the anion, _{^{e.g., - N (C n F 2n}} + 1 SO 2) (F m F 2m + 1 SO 2) (n, m is 1 or more independent 4 following a natural number), a compound represented by It can be.

There is no limitation in particular also as said electrolyte, The lithium salt used as an electrolyte of a lithium ion secondary battery can be used. For example, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ and LiBOB, organic acid anion salts such as LiCF ₃ SO ₃ , (CF ₃ SO ₂ ) ₂ NLi, and (FSO ₂ ) ₂ NLi can be used. .

  Representative examples of the compound of the above formula (1) are specifically exemplified by the formulas (2) to (6), but the sulfonium salts that can be used in the present embodiment are not limited to these.

（以下、本文中で化合物Ｎｏ．１と標記する。）

(Hereinafter referred to as Compound No. 1 in the text.)

（以下、本文中で化合物Ｎｏ．２と標記する。）

(Hereinafter referred to as Compound No. 2 in the text.)

（以下、本文中で化合物Ｎｏ．３と標記する。）

(Hereinafter referred to as Compound No. 3 in the text.)

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above 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]
(Synthesis of S, S, S-trimethylsulfonium nitrate (intermediate compound A))
Under Ar atmosphere, 10.7 g (0.110 mol) of silver was dispersed in 500 mL of dry ethanol, and then 81.3 mL (1.10 mol) of dimethyl sulfide was added dropwise over 30 minutes. This solution was stirred for 1 hour in an ice bath, and then 68.5 mL (1.10 mol) of iodomethane was added dropwise over 30 minutes. The mixture was stirred overnight, and the precipitate was filtered from the resulting solution, followed by liquid separation with water-diethyl ether. The aqueous phase was collected and dried to obtain 135 g (0.968 mol) of intermediate compound A.

(Synthesis of Compound No. 1 (Method A))
41.7 g (0.300 mol) of intermediate compound A was dissolved in 300 mL of water, and then 82.8 g (0.450 mol) of potassium hexafluorophosphate was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried, and 65.3 g (0.294 mol) of Compound No. 1 (Method A) was obtained.

[Synthesis Example 2]
(Synthesis of Compound No. 1 (Method B))
Synthesis was performed in the same manner as in Synthesis Example 1 except that the charge ratio was changed so that dimethyl sulfide: iodomethane: silver = 1: 1: 0.01 (molar ratio), and 32.2 g (0.145 mol) Compound No. 1 (Method B) was obtained.

[Synthesis Example 3]
(Synthesis of Compound No. 1 (Method C))
Synthesis was performed in the same manner as in Synthesis Example 1 except that the charge ratio was changed so that dimethyl sulfide: iodomethane: silver = 1: 1: 0.09 (molar ratio), and 31.3 g (0.141 mol) Compound No. 1 (Method C) was obtained.

[Synthesis Example 4]
(Synthesis of Compound No. 2)
41.7 g (0.300 mol) of intermediate compound A was dissolved in 300 mL of water, and then 129 g (0.450 mol) of lithium bis (trifluoromethanesulfonyl) imide was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried, and 89.9 g (0.288 mol) of Compound No. 2 was obtained.

[Synthesis Example 5]
(Synthesis of S, S-dimethyl-S-phenylsulfonium nitrate (intermediate compound B))
Under an Ar atmosphere, 10.7 g (0.110 mol) of silver was dispersed in 500 mL of dry ethanol, and then 129 mL (1.10 mol) of thioanisole was added dropwise over 30 minutes. This solution was stirred for 1 hour in an ice bath, and then 68.5 mL (1.10 mol) of iodomethane was added dropwise over 30 minutes. The mixture was stirred overnight, and the precipitate was filtered from the resulting solution, followed by liquid separation with water-diethyl ether. The aqueous phase was collected and dried to obtain 160 g (0.935 mol) of intermediate compound B.

(Synthesis of Compound No. 3 (Method A))
Intermediate compound B 51.3 g (0.300 mol) was dissolved in 300 mL of water, and then 82.8 g (0.450 mol) of potassium hexafluorophosphate was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried, and 72.4 g (0.285 mol) of Compound No. 3 (Method A) was obtained.

[Synthesis Example 6]
(Synthesis of Compound No. 1 (Method D))
The synthesis was performed in the same procedure as in Synthesis Example 1 except that bromomethane was used instead of iodomethane, and 48.9 g (0.220 mol) of Compound No. 1 (Method D) was obtained.

[Synthesis Example 7]
(Synthesis of Compound No. 1 (Method E))
The synthesis was performed in the same procedure as in Synthesis Example 1 except that chloromethane was used instead of iodomethane, and 51.1 g (0.230 mol) of Compound No. 1 (Method E) was obtained.

[Synthesis Comparative Example 1]
(Synthesis of S, S, S-trimethylsulfonium chloride (intermediate compound C))
200 mL of dry ethanol, 32.5 mL (0.440 mol) of dimethyl sulfide, and 9.9 mL (0.440 mol) of chloromethane were put into a PFA pressure vessel and sealed, 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 40.2 g (0.356 mol) of intermediate compound C.

(Synthesis of Compound No. 1 (Method F))
Intermediate compound C (33.9 g, 0.300 mol) was dissolved in water (300 mL), and then potassium hexafluorophosphate (82.8 g, 0.450 mol) was gradually added. The solution was stirred overnight, and the resulting solution was partitioned with water-dichloromethane. The organic phase was collected and dried, and 64.8 g (0.292 mol) of Compound No. 1 (Method F) was obtained.

[Synthesis Comparative Example 2]
(Synthesis of Compound No. 1 (Method G))
The synthesis was performed in the same procedure as in Synthesis Comparative Example 1 except that bromomethane was used instead of chloromethane, and 63.9 g (0.288 mol) of Compound No. 1 (Method G) was obtained.

[Synthesis Comparative Example 3]
(Synthesis of Compound No. 1 (Method H))
The synthesis was performed in the same procedure as in Synthesis Comparative Example 1 except that iodomethane was used instead of chloromethane, and 58.6 g (0.264 mol) of Compound No. 1 (Method H) was obtained.

[Synthesis Comparative Example 4]
(Synthesis of Compound No. 1 (Method I))
Synthesis was performed in the same procedure as in Synthesis Example 1 except that the charge ratio was changed so that dimethyl sulfide: iodomethane: silver = 1: 1: 0.005 (molar ratio), and 18.2 g (0.0820 mol) Compound No. 1 (Method I) was obtained.

[Synthesis Comparative Example 5]
(Synthesis of Compound No. 3 (Method B))
The synthesis was performed in the same procedure as in Synthesis Comparative Example 1 except that thioanisole was used instead of dimethyl sulfide, and 61.2 g (0.243 mol) of Compound No. 3 (Method B) was obtained.

[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. Similarly, the negative electrode sheet was obtained by apply | coating the slurry which mixed so that it might become a ratio of graphite: PVDF = 90: 10 (mass%), and was disperse | distributed in NMP on copper foil of thickness 16 micrometers.

(Preparation of electrolyte)
Compound No. LiPF ₆ was dissolved in 1 (Method A) so as to be 1 mol / L to prepare an electrolytic solution.

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

(Measurement of halide ion content excluding fluoride in electrolyte)
The electrolytic solution prepared above was subjected ThermoSCIENTFIC's ion chromatography apparatus DX-500, using an anion analytical column IonPacAS19, eluent 30mmolL ^-1 KOH, measured in terms of flow rate 1.0MLmin ^-1, Iodide ion (25.8 min) was detected at 9 ppm, and chloride ion (retention time 3.4 min) and bromide ion (15.2 min) were not detected.

(Measurement of cycle 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, charging / discharging at a current value of 0.5 C was defined as one cycle, charging / discharging for 300 cycles was performed, and a cycle capacity retention ratio (discharging capacity at 300 cycles / initial discharging capacity × 100) was obtained. It means that decomposition | disassembly of the electrolyte solution accompanying a cycle is suppressed, so that this value is high. The obtained results are shown in Table 1.

[Examples 2 to 7]
Lithium ion secondary batteries for evaluation of Examples 2 to 7 were produced in the same manner as in Example 1 except that the sulfonium salt used as the electrolytic solution was changed to that shown in Table 1.

  Table 1 shows the results of various measurements described in Example 1 performed on the evaluation lithium ion secondary batteries of Examples 2 to 7. In the table, “-” indicates that each ion was not detected. In Examples 4 to 7 in which the total content of halide ions excluding fluoride in the electrolytic solution was less than 10 ppm, the cycle capacity retention rate was excellent as in Example 1. Moreover, the cycle capacity maintenance factor which was good also in Example 4 and 5 in which iodide ion was measured 95 ppm and 13 ppm was shown.

[Comparative Examples 1-5]
Lithium ion secondary batteries for evaluation of Comparative Examples 1 to 5 were produced in the same manner as in Example 1 except that the sulfonium salt used as the electrolytic solution was changed to that shown in Table 1.

  Table 1 similarly shows the results of various measurements described in Example 1 performed on the evaluation lithium ion secondary batteries of Examples 1 to 5. In Comparative Examples 1 to 5 in which chloride ions, bromide ions, and iodide ions were measured at 100 ppm or more, the cycle capacity retention rate was worse than that in Example 1.

  By using the electrolytic solution for a lithium ion secondary battery of the present invention, a lithium ion secondary battery having improved cycle characteristics can be provided.

  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 (3)

  An electrolyte for a lithium ion secondary battery having an ambient temperature molten salt and an electrolyte, wherein the ambient temperature molten salt is a sulfonium salt, and the total content of halide ions excluding fluoride ions in the electrolyte is less than 100 ppm An electrolyte for a lithium ion secondary battery.   2. The electrolyte solution for a lithium ion secondary battery according to claim 1, wherein the total content of the halide ions is less than 10 ppm.   A lithium ion secondary battery comprising the electrolyte solution for a lithium ion secondary battery according to claim 1 or 2.

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