JP7004201B2 - Electrolyte for lithium-ion secondary batteries - Google Patents

Description

The present invention relates to an electrolytic solution for a lithium ion secondary battery.

Japanese Unexamined Patent Publication No. 2017-134986 discloses an electrolytic solution for a lithium ion secondary battery using a fluorinated solvent. In the same publication, in particular, it is considered to use a fluorination solvent in which a fluorinated cyclic carbonate and a fluorinated chain carbonate are mixed.
Japanese Unexamined Patent Publication No. 2010-73489 describes an electrolytic solution having a solvated ionic liquid in which only glyme is used as a solvent, an alkali metal salt is used as a solute, and at least a part of glyme and the alkali metal salt form a complex. Has been done.
Japanese Unexamined Patent Publication No. 2013-225496 discloses an electrolytic solution used for an alkali metal-sulfur-based secondary battery. The electrolytic solution disclosed here contains an ether compound such as THF and grime, and a solvent. Further, at least a part of the ether compound and the alkali metal salt forms a complex. The solvent is a fluorine-based solvent that has hydrophobicity, is completely mixed with the complex, and does not chemically react with alkali metals and polysulfide alkali metals ( _M2 Sn: 1 ≦ _n ≦ 8) (however, however). F ₃ CH ₂ CO-CF ₂ CF ₂ H), ionic liquids, and solvents represented by one or more selected from the group of hydrocarbons are used.

Japanese Unexamined Patent Publication No. 2017-134986 Japanese Unexamined Patent Publication No. 2010-73489 Japanese Unexamined Patent Publication No. 2013-225496

By the way, among the fluorine-based solvents, the fluorinated ether solvent has a wide potential window and easily has a low viscosity structure, so that improvement in diffusibility can be expected. Further, when the fluorinated ether solvent and the solvated ionic liquid are mixed to form an electrolytic solution, it can contribute to lowering the resistance in charging and discharging at a high rate. However, according to the new findings obtained by the present inventor, even when an electrolytic solution obtained by mixing a fluorinated ether and a solvated ionic liquid is used, the capacity retention rate of the lithium ion secondary battery after the charge / discharge cycle is sufficient. Sometimes it wasn't.

硫黄含有化合物Ｂは、下記の構造式（Ｂ）で示された化合物である。

構造式（Ａ）と構造式（Ｂ）において、Ｘは、Ｃ，Ｓｉ，Ｂ及びＰの中から選択される元素である。Ｒ^１およびＲ^２は、それぞれ独立した水素、ハロゲン原子、ハロゲン原子で置換されていてもよいアルキル基、ハロゲン原子で置換されていてもよいフェニル基である。ｎは、構造式（Ａ）および（Ｂ）における括弧内のメチレン基（ＣＨ_２）の数を示しており、ｎ＝０～２である。 The electrolytic solution for a lithium ion secondary battery proposed here includes a fluorinated monoether, a solvated ionic liquid, and an additive containing at least one sulfur-containing compound among sulfur-containing compound A and sulfur-containing compound B. Includes.
The sulfur-containing compound A is a compound represented by the following structural formula (A).

The sulfur-containing compound B is a compound represented by the following structural formula (B).

In the structural formula (A) and the structural formula (B), X is an element selected from C, Si, B and P. R ¹ and R ² are independent hydrogen, halogen atom, alkyl group which may be substituted with a halogen atom, and phenyl group which may be substituted with a halogen atom, respectively. n indicates the number of methylene groups (CH ₂ ) in parentheses in the structural formulas (A) and (B), and n = 0 to 2.

According to the electrolytic solution for the lithium ion secondary battery, it can be expected that the cycle capacity retention rate [%], which is the capacity retention rate after the charge / discharge cycle, is improved.

Hereinafter, an embodiment of the electrolytic solution for the lithium ion secondary battery proposed here will be described. The embodiments described herein are, of course, not intended to specifically limit the invention. The invention is not limited to the embodiments described herein, unless otherwise noted.

The present inventor is studying the use of an electrolytic solution in which a solvated ion liquid is mixed with a fluorinated solvent as an electrolytic solution for a lithium ion secondary battery.
Here, the solvated ionic liquid is a melt of a complex composed of a glyme or a glyme derivative and a lithium salt. Grime is a general term for linear glycol diethers. Examples of the grime used here include triglyme and tetraglyme.

In the study of the electrolytic solution for the lithium ion secondary battery, when the mixed molar ratio of the Li salt to the glyme in the solvated ionic liquid is around 1, the glyme molecule coordinates with Li (in other words, forms a complex). I think I'm doing it. Then, the electrochemical stability (in other words, oxidation resistance) is improved as compared with the case where the grime molecule is present alone. However, it has been newly found that the solvated ionic liquid has lower potential stability (oxidation resistance) than the generally used carbonate-based solvent. For example, when the battery is repeatedly charged to a high potential in a high temperature environment, a part of the solvated ionic liquid may be decomposed, and the capacity retention rate of the lithium ion secondary battery is high and difficult to maintain.

The electrolytic solution for a lithium ion secondary battery proposed here includes a fluorinated monoether, a solvated ionic liquid, and an additive containing at least one sulfur-containing compound among sulfur-containing compound A and sulfur-containing compound B. Includes.

[Sulfur-containing compound A]
The sulfur-containing compound A is a compound represented by the following structural formula (A).

[Sulfur-containing compound B]
The sulfur-containing compound B is a compound represented by the following structural formula (B).

In the structural formula (A) and the structural formula (B),
X is an element selected from C, Si, B and P.
R ¹ and R ² are independent hydrogen, halogen atom, alkyl group which may be substituted with a halogen atom, and phenyl group which may be substituted with a halogen atom, respectively.
n indicates the number of methylene groups (CH ₂ ) in parentheses in the structural formulas (A) and (B). In the structural formula (A) and the structural formula (B), n is preferably n = 0 to 2.

[Example of sulfur-containing compound]
Examples of the sulfur-containing compound may be 1,3-propanesulton in which the compound of the following structural formula (C) is exemplified. 1,3-Propane sulton is appropriately referred to herein as "PS". In PS, in the above-mentioned structural formula (A), X is C, R ¹ and R ² are independent hydrogens, and n = 0.

As another example of the sulfur-containing compound, structural formula (D) is 1-propene 1,3-sultone. 1-Propene 1,3-sultone is appropriately referred to herein as "PRS". In PRS, in the above-mentioned structural formula (B), X is C, R ¹ and R ² are independent hydrogens, and n = 0.

As another example of the sulfur-containing compound, structural formula (E) is 3,3-dimethyl-3-sila-1,3 propanesulton. 3,3-dimethyl-3-sila-1,3 propanesulton is appropriately referred to herein as "DMSPS". In DMSPS, in the above-mentioned structural formula (B), X is Si, R ¹ and R ² are independent hydrogens, and n = 0.

Here, among the sulfur-containing compounds as the exemplified additives, 3,3-dimethyl-3 is particularly preferable from the viewpoint of improving the cycle capacity retention rate [%] for the lithium ion secondary battery. -Sila-1,3 Propane Sulfur (DMSPS).

[Solvate ionic liquid]
The solvated ionic liquid may be, for example, a melt of a complex composed of an ether compound represented by the following structural formula (F) and a lithium salt. Here, the ether compound represented by the structural formula (F) is a material for forming a solvated ionic liquid with a lithium salt.

R ^1- (OCHR ³ CH ₂ ) _X- OR ² ... (F)

In the structural formula (F), R ¹ and R ² are independent of each other.
R ¹ and R ² can be a functional group selected from the group consisting of an alkyl group having 1 to 9 carbon atoms, a phenyl group and a cyclohexyl group. Here, a part of the alkyl group may be substituted with fluorine. Further, the phenyl group may be substituted with a halogen atom. The cyclohexyl group may be substituted with a halogen atom.
R ¹ and R ² may each form a ring.
R ³ is H or CH ₃ .
X can be any number selected from 1-10.

The ether compound represented by the structural formula (F) may include, for example, triglyme or tetraglyme.
Here, triethylene glycol means an ether compound represented by the structural formula of (Triethylene glycol dimethyl ether), CH ₃ (OCH ₂ CH ₂ ) ₃ OCH ₃ .
Further, tetraglyme means an ether compound represented by the structural formula of (Tetraethylene glycol dimethyl ether), CH ₃ (OCH ₂ CH ₂ ) ₄ OCH ₃ .

[Lithium salt]
Further, as the lithium salt, for example, LiFSI (lithium bis (fluorosulfonyl) imide) can be used. As used herein, "LiFSI" means lithium bis (fluorosulfonyl) imide, unless otherwise specified. Lithium salts are not limited to LiFSI. Lithium salts include, for example, LiPF ₆ (lithium hexafluorophosphate), LIBF ₄ (lithium tetrafluoroborate), LiClO ₄ (lithium perchlorate), LiTFSI (LiN (SO ₂ CF ₃ ) ₂ : lithium bis ( Trifluoromethanesulfonyl) imide: Lithium bis (trifluoromethanesulfonyl) imide), LiBETI (LiN (SO ₂ C ₂ F ₅ ) ₂ : Lithium bis (pentafluoroethanesulfonyl) imide) can be used.

In the solvated ionic liquid, the mixing ratio of the ether compound with the lithium salt is, for example, a molar ratio of ether compound: lithium salt = 1: 0.9 to 1.1 (that is, 0.9 or more and 1.1 or less). It would be nice to have it.

[Fluorinated monoether]
The fluorinated monoether is used as a solvent. The fluorinated monoether can be, for example, the following structural formula (G).

HCFY ^1- (CFZ ¹ ) _m1 -O-CH _2- (CFZ ² ) _m2 -CFY ² H ... (G)

In the structural formula (G), Y ¹ , Y ² , Z ¹ , and Z ² independently represent H or F, respectively. m1 and m2 can be any number selected from 1 to 5.
Such fluorinated monoether is used as a solvent. It is preferable that the fluorinated monoether is contained at least one kind. That is, the solvent may be one kind of fluorinated monoether or a mixed solvent in which two or more kinds of fluorinated monoether are mixed.

[Example of fluorinated monoether]
Examples of the fluorinated monoether used as a solvent include FCH ₂ OCH ₂ CF ₂ CF ₂ H, HCF ₂ OCH ₂ CF ₂ CF ₂ H (FME1), HCF ₂ CF ₂ OCH ₂ CF ₂ CF ₂ H (FME2). Can be done. HCF ₂ OCH ₂ CF ₂ CF ₂ H is appropriately referred to herein as "FME1". HCF ₂ CF ₂ OCH ₂ CF ₂ CF ₂ H is appropriately referred to herein as "FME2".

As described above, the electrolytic solution for the lithium ion secondary battery is an electrolytic solution containing a solvated ion liquid with fluorinated monoether as a solvent, and is at least one of the above-mentioned sulfur-containing compound A and sulfur-containing compound B. It is preferable that the sulfur-containing compound of the above is contained as an additive.
Here, the sulfur-containing compound A may be, for example, a compound represented by the above-mentioned structural formula (A) such as PS and DMSPS.
Here, the sulfur-containing compound B may be, for example, a compound represented by the above-mentioned structural formula (B), such as PRS.

[Evaluation test]
The present inventor prepared an evaluation battery using the above-mentioned electrolytic solution, and evaluated the cycle capacity retention rate [%]. Table 1 is a table showing a sample of the evaluation battery and its cycle capacity retention rate [%]. The evaluation batteries prepared here differ only in the prepared electrolytic solutions, and have the same configuration in other respects.

To prepare the positive electrode of the evaluation battery, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is used as the positive electrode active material, acetylene black (AB) is used as the conductive auxiliary agent, and polyvinylidene fluoride (PVDF) is used as the binder. Were used respectively. The positive electrode active material, the conductive auxiliary agent, and the binder are mixed at a ratio (% by weight) of positive electrode active material: conductive auxiliary agent (AB): binder (PVDF) = 87: 10: 3 to form a slurry. Made. N-Methyl-2-pyrrolidone (NMP) was used as the dispersion solvent for the slurry. The obtained slurry was applied onto an aluminum foil as a positive electrode current collector foil and dried. As a result, a positive electrode active material layer was formed on the positive electrode current collector foil. Then, the obtained sheet was pressed by a roll press until the density of the formed positive electrode active material layer reached 2.3 g / cm ³ , to prepare a positive electrode sheet.

For the production of the negative electrode, a natural graphite-based material having an average particle diameter of 20 μm (D50) was used as the negative electrode active material. Further, a styrene-butadiene copolymer (SBR) was used as a binder, and carboxymethyl cellulose (CMC) was used as a thickener. The negative electrode active material, the binder, and the thickener were mixed at a ratio of negative electrode active material: binder (SBR): thickener (CMC) = 98: 1: 1 to prepare a slurry. Water was used as the dispersion solvent of the slurry. The obtained mixed slurry was applied onto the negative electrode current collector foil and dried. As a result, a negative electrode active material layer was formed on the negative electrode current collector foil. Then, the obtained sheet was pressed by a roll press until the density of the formed negative electrode active material layer reached 1.2 g / cm ³ , and a negative electrode sheet was produced.

Here, the positive electrode current collector foil has a weight ratio of 2: 1 between the positive electrode active material contained in the positive electrode active material layer per unit area and the negative electrode active material contained in the negative electrode active material layer per unit area. And the amount of the mixed slurry applied to the negative electrode current collector foil was adjusted respectively.

The electrolyte of each sample of the evaluation battery is different. The differences in the materials contained in the electrolytic solution of each sample are as shown in Table 1, and the electrolytic solution of each sample is substantially the same unless otherwise specified.

Here, in the evaluation battery samples 1 to 14 prepared here, the composition of the solvent of the electrolytic solution, the mixing ratio, the presence / absence of the additive, the type, and the amount of the additive are different as shown in Table 1. .. In Table 1 and herein, "G3" means triglyme. Further, "G4" means tetraglyme. Further, LiFSI is used as the lithium salt. In the column of mixed molar ratio [LiFSI / grime] in Table 1, the molar ratio of LiFSI to the ether compound is shown. Further, as shown in the additive column of Table 1, the additives include lithium bisoxalate (LiBOB) as a boron-based additive and vinylene as a carbonate-based additive, in addition to the above-mentioned PS, PRS and DMSPS. Carbonate (VC) is used. In Table 1 and herein, "LiBOB" means lithium bisoxalate. Further, "VC" means vinylene carbonate. In the column of the additive amount (wt%), the weight ratio of the additive in the electrolytic solution is shown. In the same column, "-" indicates that no additive has been added.

The manufactured evaluation battery is a so-called laminated type evaluation battery. Specifically, an electrode body is produced in which the positive electrode active material layer of the positive electrode sheet and the negative electrode active material layer of the negative electrode sheet prepared as described above are opposed to each other via a separator. The produced electrode body is housed in a bag-shaped laminate together with the electrolytic solution. Then, by sealing the laminate, a laminate type evaluation battery was produced. A conducting wire is wired to the positive electrode current collector foil of the positive electrode sheet and the negative electrode current collector foil of the negative electrode sheet so that they can be charged and discharged.

In the initial charging of the evaluation battery produced as described above, a constant current method was used in a constant temperature bath set at 25 ° C., and the battery was charged to 4.10 V at a current value of 0.3 C. After that, it was discharged to 3.00 V at a current value of 0.3 C by a constant current method. Further, in order to inspect the initial characteristics, the battery was charged to 4.10 V by a constant current-constant voltage method. Here, the battery was charged to 4.10 V with a constant current value of 0.2 C, and then charged at a constant voltage until the current value at the time of constant voltage charging became 1/50 C, and the battery was fully charged. The capacity when discharged to 3.00 V at a constant current of 0.2 C by the constant current method from such a fully charged state was defined as the initial capacity.

A cycle test was carried out on the evaluation battery whose initial capacity was measured as described above. The cycle test was carried out in a constant temperature bath set at 60 ° C. Here, in the cycle test, 200 cycles of charging up to 4.10 V and discharging up to 3.00 V were repeated as one cycle. Here, charging and discharging were performed by a constant current method, respectively. The current rate was 2C. After 200 cycles, the post-cycle capacity was obtained in the same manner as the initial capacity described above. That is, after 200 cycles, the battery was charged to 4.10 V with a constant current value of 0.2 C, and then charged at a constant voltage until the current value at the time of constant voltage charging became 1/50 C to complete the charge. The capacity when discharged to 3.00 V at a constant current of 0.2 C by the constant current method from such a fully charged state was defined as the post-cycle capacity. The cycle capacity retention rate [%] shown in Table 1 was obtained by dividing the obtained capacity after the cycle by the initial capacity.

Here, in the above-mentioned sample 1, LiFSI, which is an electrolyte, and triglyme (G3) are mixed at a molar ratio of 1: 1 to form a solvated ionic liquid. A solvated ionic liquid applied to the fluorinated monoether as a solvent was added to adjust the concentration of LiFSI to 1.5 mol / L. Further, the amount of the additive PS added was 1 wt%. Here, HCF ₂ CF ₂ OCH ₂ CF ₂ CF ₂ H (FME2) was used as the fluorinated monoether. The cycle capacity retention rate of such sample 1 was 89%.

In sample 2, tetraglyme (G4) is used instead of triglyme (G3) for sample 1. Sample 2 is the same as Sample 1 in other respects. The cycle capacity retention rate of sample 2 was 85%.

In Sample 3, PRS is used as an additive. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 3 was 84%.

In Sample 4, DMSPS is used as an additive. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 4 was 88%.

In Sample 5, the amount of the additive PS added is 0.6 wt%. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 5 was 83%.

In Sample 6, the mixed molar ratio [LiFSI / Glyme] of the ether compound G4 of the solvated ionic liquid is 0.9. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 6 was 85%.

In Sample 7, the mixed molar ratio [LiFSI / grime] of the ether compound G4 of the solvated ionic liquid is 1.1. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 7 was 84%.

In Sample 8, G3 is used as the ether compound of the solvated ionic liquid, but no additive is added. In other respects, it is the same as sample 1. The cycle capacity retention rate of sample 8 was 75%. The cycle capacity retention rate of the sample 1 to which the additive is added is 89%, which is much higher than that of the sample 8. It can be considered that this is because the additive is added.

In Sample 9, G4 is used as the ether compound of the solvated ionic liquid, but no additive is added. In other respects, it is the same as Samples 2-4. The cycle capacity retention rate of sample 9 was 70%. The cycle capacity retention rate of the samples 2 to 4 to which the additive is added is 84 to 88%, which is much higher than that of the sample 9. It can be considered that this is because the additive is added. Further, considering the results of Samples 1 and 8 together, the cycle capacity retention rate [%] when the additives PS, PRS or DMSPS are sufficiently added regardless of the ether compound of the solvated ionic liquid. ] Is improving.

In Sample 10, LiBOB, which is a boron-based additive, is used as an additive. The amount of LiBOB added was 0.3 wt%. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 10 was 72%. Here, in order to suppress the reductive decomposition of a general carbonate solvent, a sufficient effect of LiBOB can be obtained even with an addition amount of about 0.3 wt%. However, the oxidative decomposition of the solvated ionic liquid (grime) cannot be suppressed. From these facts, it is considered that the cycle capacity retention rate of the sample 10 remains at about 72%. In order to suppress the oxidative decomposition of the solvated ionic liquid (grime), it is preferable to use an additive such as PS that also acts well on the positive electrode.

In Sample 11, VC, which is a carbonate-based additive, is used as an additive. The amount of VC added was 1.0 wt%. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 11 was 69%. That is, when VC is used as an additive, the cycle capacity retention rate does not improve.

In sample 12, the amount of the additive PS added was 0.3 wt%. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 12 was 72%. In Sample 2, the amount of PS added was small, and the effect of adding PS was not sufficiently obtained.

In sample 13, the mixed molar ratio of LiFSI to the ether compound G4 is 0.8 in the solvated ionic liquid. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 13 was 74%. In sample 13, the molar ratio of lithium salt to grime is low. In this case, excess grime that does not form a complex with the lithium salt increases. Therefore, it is presumed that grime decomposes during the cycle and the capacity retention rate decreases.

In sample 14, the mixed molar ratio of LiFSI to the ether compound G4 is 1.2 in the solvated ionic liquid. In other respects, it is the same as sample 2. In sample 14, the molar ratio of lithium salt to grime is high. In the step of preparing the electrolytic solution, excess salt was precipitated when mixed with the fluorinated monoether. Therefore, in the sample 14, the cycle capacity retention rate has not been measured.

In Sample 15, instead of PS, 1,5-pentane sultone having n = 2 in the above-mentioned structural formula (A) was used as an additive. In other respects, it is the same as sample 2. In sample 15, the cycle capacity retention rate is 81%, and the cycle capacity retention rate is maintained as high as 80% or more. Here, an additive having n = 2 in the structural formula (A) is used, but the present invention is not limited to this. As described above, the additive for the sulfur-containing compound is not limited to PS, PRS, and DMSPS. Although not all of them are exemplified as samples, according to the findings of the present inventor, additives having n = 0 to 2 in the structural formulas (A) and (B) are adopted as additives for sulfur-containing compounds. Can be done.

In sample 16, PS was used as an additive for the sulfur-containing compound, and the amount added was 1.5 wt%. In other respects, it is the same as sample 2. The cycle capacity retention rate of sample 16 was 83%. As described above, the amount of the sulfur-containing compound additive added may be about 1.5 wt%.

In sample 17, HCF ₂ OCH ₂ CF ₂ CF ₂ H (FME1) is used as the fluorinated monoether. In other respects, it is the same as sample 9. The cycle capacity retention rate of sample 17 was 68%.
In sample 18, PS was used as an additive for the sulfur-containing compound, and the amount added was 1.0 wt%. In other respects, it is the same as sample 17. In another view, in sample 18, HCF ₂ OCH ₂ CF ₂ CF ₂ H (FME1) is used as the fluorinated monoether. In other respects, it is the same as sample 2. The cycle capacity retention rate of such sample 18 was 82%.

As described above, even when HCF ₂ OCH ₂ CF ₂ CF ₂ H (FME1) is used as the fluorinated monoether, the cycle capacity retention rate is maintained high when the additive of the sulfur-containing compound is not added. Not done (Sample 17). On the other hand, the cycle capacity retention rate is maintained high by adding an appropriate amount of the sulfur-containing compound additive (Sample 18). The fluorinated monoether is not limited to HCF ₂ OCH ₂ CF ₂ CF ₂ H (FME1) or HCF ₂ CF ₂ OCH ₂ CF ₂ CF ₂ H (FME2). According to the findings of the present inventor, various fluorinated monoethers can be adopted as exemplified above.

As described above, in the samples 1 to 7, 15, 16 and 18, the cycle capacity retention rate [%] of the lithium ion secondary battery is maintained high. Samples 1 to 7, 15, 16 and 18 are examples, but the electrolytic solution for the lithium ion secondary battery is an electrolytic solution containing a solvated ionic liquid with fluorinated monoether as a solvent, and the above-mentioned sulfur. It is preferable to contain an appropriate amount as an additive containing at least one of the contained compound A and the sulfur-containing compound B.
Here, the sulfur-containing compound A may be, for example, a compound represented by the above-mentioned structural formula (A) such as PS and DMSPS.
Here, the sulfur-containing compound B may be, for example, a compound represented by the above-mentioned structural formula (B), such as PRS.

According to the electrolytic solution for the lithium ion secondary battery, it can be expected that the oxidation resistance of the electrolytic solution is improved and the cycle capacity retention rate [%] is improved. Specifically, in an electrolytic solution for a lithium ion secondary battery in which a fluorinated monoether containing a solvated ionic liquid is used as a solvent and the solubility of the lithium salt is enhanced, the cycle capacity retention rate [%] is further improved. .. This makes it possible to provide a lithium ion secondary battery having high solubility of a lithium salt, low resistance, and good cycle characteristics in an electrolytic solution using a fluorinated monoether as a solvent.

From this point of view, the electrolytic solution for the lithium ion secondary battery is an additive containing a fluorinated monoether, a solvated ionic liquid, and at least one sulfur-containing compound among the above-mentioned sulfur-containing compound A and sulfur-containing compound B. And should be included. The amount of the sulfur-containing compound added to the electrolytic solution is, for example, 0.5 wt% or more, preferably 0.6 wt% or more, from the viewpoint of improving the cycle capacity retention rate [%] described above.
The amount of the sulfur-containing compound added to the electrolytic solution is, for example, 1.5 wt% or less, preferably 1.0 wt% or less from the viewpoint of suppressing an increase in resistance.
There are various known proposals for additives for electrolytic solutions for lithium ion secondary batteries. The electrolytic solution for a lithium ion secondary battery may further contain other additives in addition to the sulfur-containing compound.

In the above, various electrolytic solutions for lithium ion secondary batteries proposed here have been described. Unless otherwise specified, the embodiments and samples of the electrolytic solution for the lithium ion secondary battery mentioned here are not limited to the present invention.

Claims (1)

Fluorinated monoether and
Solvation ionic liquid and
It contains an additive containing at least one sulfur-containing compound among the sulfur-containing compound A and the sulfur-containing compound B.
The solvated ionic liquid is a melt of a complex composed of an ether compound represented by the following structural formula (F) and a lithium salt.
R ^1- (OCHR ³ CH ₂ ) _X- OR ² ... (F)
Here, the ether compound represented by the structural formula (F) is a material for forming a solvated ionic liquid with a lithium salt.
R ¹ and R ² are functional groups selected from the group consisting of an alkyl group having 1 to 9 carbon atoms, a phenyl group and a cyclohexyl group.
Here, some of the alkyl groups include those substituted with fluorine.
Phenyl groups include those substituted with halogen atoms and
Cyclohexyl groups include those substituted with halogen atoms.
R ¹ and R ² include those forming a ring, respectively.
R ³ is H or CH ₃
X in the structural formula (F) is an arbitrary number selected from 1 to 10.
The lithium salt is composed of one of lithium bis (fluorosulfonyl) imide, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ and LiN (SO ₂ C ₂ F ₅ ) ₂ .
In the solvated ionic liquid, the mixing ratio of the ether compound with the lithium salt is, in terms of molar ratio, the ether compound: lithium salt = 1: 0.9 to 1.1.
The sulfur-containing compound A is a compound represented by the following structural formula (A).

The sulfur-containing compound B is a compound represented by the following structural formula (B), and is a compound represented by the following structural formula (B).

In the structural formula (A) and the structural formula (B),
X is an element selected from C, Si, B and P.
R ¹ and R ² are independent hydrogen, halogen atom, alkyl group which may be substituted with a halogen atom, and phenyl group which may be substituted with a halogen atom, respectively.
n indicates the number of methylene groups (CH ₂ ) in parentheses in the structural formulas (A) and (B), and n = 0 to 2.
Electrolyte for lithium-ion secondary batteries.