Electrolytic solution

Abstract

To provide a novel electrolytic solution.SOLUTION: An electrolytic solution according to the present disclosure includes an organic solvent and a lithium salt dissolved in the organic solvent, and the organic solvent has a sulfone structure, an asymmetric structure, and a cyclic structure having 10 or less carbon atoms, the lithium salt is lithium 1,1,2,2,3,3-hexafluoropropene-1,3-disulfonimide, and the molar ratio of the organic solvent to the lithium salt is 3.0 or less. Preferably, the cyclic structure has 3 or more carbon atoms.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to electrolytes. [Background technology]

[0002] Patent Document 1 discloses an electrolyte solution for a lithium ion secondary battery, which contains a lithium imide salt, at least one solvent selected from the group consisting of carbonates, esters, ethers, and room-temperature molten salts, and at least one element selected from Group 1 and Group 2 elements, and the molar ratio of the lithium imide salt to the solvent is in the range of 1:0.8 to 1:2.0. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-96463 Summary of the Invention [Problem to be solved by the invention]

[0004] There is a need for new electrolyte solutions, for example, new electrolyte solutions for lithium ion secondary batteries. That is, an object of the present disclosure is to provide a new electrolyte solution. [Means for solving the problem]

[0005] The present inventors have found that the above object can be achieved by the following means: <<Aspect 1>> An electrolyte solution comprising an organic solvent and a lithium salt dissolved in the organic solvent, the organic solvent has a sulfone structure, an asymmetric structure, and a cyclic structure having 10 or less carbon atoms; the lithium salt is lithium 1,1,2,2,3,3-hexafluoropropene-1,3-disulfonimide; the molar ratio of the organic solvent to the lithium salt is 3.0 or less; Electrolyte. <<Aspect 2>> 2. The electrolyte solution according to aspect 1, wherein the cyclic structure has 3 or more carbon atoms. Aspect 3 3. The electrolyte solution according to aspect 2, wherein the cyclic structure has 4 or more and 8 or less carbon atoms. Aspect 4 4. The electrolytic solution according to any one of aspects 1 to 3, wherein the sulfone structure is contained within the skeleton of the cyclic structure. Aspect 5 5. The electrolyte solution according to any one of aspects 1 to 4, having a viscosity at 25° C. of 1100 mPas or less. [Effects of the Invention]

[0006] According to the present disclosure, a novel electrolyte solution can be provided. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of a secondary battery. [Figure 2] 1 is a graph showing the relationship between the molar ratio of solvent to LiCFSA (solvent / LiCFSA=x / 1) and ionic conductivity (ionic conductivity / Scm −1 ) in Examples 1 to 4 and Comparative Examples 1 to 7. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0009] 1. Electrolyte The electrolyte solution of the present disclosure contains an organic solvent and a lithium salt dissolved in the organic solvent. The organic solvent has a sulfone structure, an asymmetric structure, and a cyclic structure containing 10 or less carbon atoms. The lithium salt is lithium 1,1,2,2,3,3-hexafluoropropene-1,3-disulfonimide (LiCFSA), and has a structure represented by the following formula 1:

[0010] [ka]

[0011] Here, the molar ratio of the organic solvent to the lithium salt is 3.0 or less.

[0012] We investigated electrolytes using organic solvents with sulfone structures and lithium 1,1,2,2,3,3-hexafluoropropene-1,3-disulfonimide (LiCFSA) as a lithium salt, and found that, depending on the type of organic solvent, increasing the concentration of the lithium salt can actually decrease the ionic conductivity. This is thought to be because a high concentration of lithium salt increases the viscosity of the electrolyte, and, depending on the type of organic solvent, components in the electrolyte may solidify or form molecular crystals under the temperature conditions when the battery is used.

[0013] The organic solvent contained in the electrolyte solution of the present disclosure has a sulfone structure, an asymmetric structure, and a cyclic structure having 10 or less carbon atoms. Because the organic solvent in the electrolyte solution of the present disclosure has an asymmetric structure, the organic solvent is less likely to solidify or form molecular crystals. Furthermore, because the electrolyte solution of the present disclosure has a cyclic structure having 10 or less carbon atoms, the viscosity of the electrolyte solution is relatively low, and the viscosity of the electrolyte solution does not become too high even when the concentration of the lithium salt is increased.

[0014] By employing such an organic solvent, the electrolyte solution of the present disclosure can maintain a liquid state while containing a high concentration of lithium salt, and can achieve significantly higher ionic conductivity than when, for example, sulfolane is used as the organic solvent.

[0015] The electrolyte solution of the present disclosure preferably has a viscosity of 1100 mPas or less at 25° C. When the viscosity at 25° C. is in this range, the ionic conductivity of the electrolyte solution can be maintained at a significantly high value. The viscosity of the electrolyte solution at 25° C. may be 1100 mPas or less, 1050 mPas or less, or 1000 mPas or less, or may be 800 mPas or more, 900 mPas or more, or 950 mPas or more.

[0016] 1-1. Organic Solvents The organic solvent contained in the electrolyte solution of the present disclosure has a sulfone structure, an asymmetric structure, and a cyclic structure with 10 or less carbon atoms. The sulfone structure is a structure having a sulfonyl group, which is a functional group represented by -S(=O)2-. The organic solvent may have the sulfone structure in the backbone of a cyclic structure.

[0017] The organic solvent having an asymmetric structure means that the organic solvent has an asymmetric structure in its structural formula. When the organic solvent has an asymmetric structure, the organic solvent is less likely to crystallize.

[0018] The organic solvent has a carbon number of 10 or less. The organic solvent may have a carbon number of 10 or less, 9 or less, or 8 or less, or may have a carbon number of 3 or more, 4 or more, or 5 or more. The organic solvent preferably has a cyclic structure with a carbon number of 3 or more, and more preferably has a cyclic structure with a carbon number of 4 to 8.

[0019] Specific examples of the organic solvent include, but are not limited to, 1,4-butanesultone and 3-methylsulfolane. The structural formulas of 1,4-butane sultone and 3-methylsulfolane are as follows:

[0020] [ka] and

[0021] [ka] It can be expressed as:

[0022] In the electrolyte solution of the present disclosure, the molar ratio of the organic solvent to the lithium salt is 3.0 or less. The molar ratio of the organic solvent to the lithium salt may be 3.0 or less, 2.9 or less, 2.8 or less, 2.7 or less, 2.6 or less, or 2.5 or less, or may be 1.5 or more, 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2.0 or more. The molar ratio of the organic solvent to the lithium salt is preferably 2.0 to 3.0. Such a molar ratio provides particularly good ionic conductivity.

[0023] The electrolyte solution of the present disclosure may contain another solvent (sub-solvent) in addition to the organic solvent, but in the electrolyte solution of the present disclosure, higher performance is likely to be ensured when the amount of the sub-solvent is small. In the electrolyte solution of the present disclosure, the molar ratio of the sub-solvent to the organic solvent ([sub-solvent (mol)] / [the organic solvent (mol)]) may be 0 or more and 0.10 or less, 0.05 or less, or 0.03 or less.

[0024] 1-2.Optional ingredients The electrolyte solution of the present disclosure may contain other components in addition to the organic solvent and lithium salt described above. Examples of other components include the auxiliary solvents described above and other lithium salts. The electrolyte solution of the present disclosure may also be used in combination with a solid material (e.g., a solid electrolyte). Furthermore, the electrolyte solution of the present disclosure may contain various additives in addition to the above. The type of additive may be selected depending on the application of the electrolyte solution.

[0025] 2. Lithium-ion secondary battery The electrolyte solution of the present disclosure is used, for example, as an electrolyte material for lithium-ion secondary batteries. A lithium-ion secondary battery having the electrolyte solution of the present disclosure will be described below. As shown in FIG. 1 , a lithium-ion secondary battery 100 according to one embodiment has a positive electrode 10, an electrolyte layer 20, and a negative electrode 30. Here, at least one of the positive electrode 10, the electrolyte layer 20, and the negative electrode 30 contains the electrolyte solution of the present disclosure. The positive electrode 10 is composed of a positive electrode active material layer 11 and a positive electrode current collector 12. The negative electrode 30 is composed of a negative electrode active material layer 31 and a negative electrode current collector 32.

[0026] As described above, the electrolyte solution of the present disclosure has high ionic conductivity. In this regard, the performance of the secondary battery 100 is likely to be improved by including the electrolyte solution of the present disclosure in at least one of the positive electrode 10, the electrolyte layer 20, and the negative electrode 30 of the secondary battery 100. For example, by using an electrolyte solution with high ionic conductivity (high lithium ion activity) such as the electrolyte solution of the present disclosure in a secondary battery, the lithium ions are more likely to react selectively at the interface between the electrode and the electrolyte solution, and the resistance at the interface between the electrode and the electrolyte solution is more likely to be reduced.

[0027] In the secondary battery 100, the above-described electrolytic solution of the present disclosure may be used alone as the electrolyte, or the electrolytic solution may be used in combination with a solid electrolyte (particularly a sulfide solid electrolyte).

[0028] The lithium-ion secondary battery 100 may have the above components housed inside an exterior body. Any known exterior body for a battery can be used as the exterior body. Furthermore, a plurality of secondary batteries 100 may be electrically connected and stacked in any manner to form an assembled battery. In this case, the assembled battery may be housed inside a known battery case. The secondary battery 100 may also have other obvious components such as necessary terminals. The shape of the secondary battery 100 may be, for example, a coin type, a laminate type, a cylindrical type, a prismatic type, or the like.

[0029] The lithium-ion secondary battery 100 can be manufactured by applying a known method. For example, it can be manufactured as follows. However, the manufacturing method of the secondary battery 100 is not limited to the following method, and each layer may be formed by, for example, dry molding or the like.

[0030] (1) A positive electrode layer slurry is obtained by dispersing the positive electrode active material and other components that constitute the positive electrode active material layer in a solvent. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The positive electrode layer slurry is applied to the surface of a positive electrode current collector using a doctor blade or the like, and then dried to form a positive electrode active material layer on the surface of the positive electrode current collector, resulting in a positive electrode.

[0031] (2) The negative electrode active material and other components that constitute the negative electrode active material layer are dispersed in a solvent to obtain a negative electrode layer slurry. The solvent used in this case is not particularly limited, and water or various organic solvents can be used. The negative electrode layer slurry is applied to the surface of the negative electrode current collector using a doctor blade or the like, and then dried to form a negative electrode active material layer on the surface of the negative electrode current collector, resulting in a negative electrode.

[0032] (3) The layers are stacked so that the electrolyte layer (solid electrolyte layer or separator) is sandwiched between the negative electrode and the positive electrode to obtain a laminate having, in this order, the negative electrode current collector, the negative electrode active material layer, the electrolyte layer, the positive electrode active material layer, and the positive electrode current collector. Other members such as terminals are attached to the laminate as necessary.

[0033] (4) The laminate is housed in a battery case, and the battery case is filled with an electrolyte solution, and the laminate is immersed in the electrolyte solution and sealed in the battery case to form a secondary battery. Note that the electrolyte solution may be impregnated into the negative electrode active material layer, the solid electrolyte layer or the separator, or the positive electrode active material layer in the step (3) above. [Example]

[0034] 1. Electrolyte Preparation 1-1. Example 1 1,4-butane sultone (melting point 14.5°C) as an organic solvent and lithium 1,1,2,2,3,3-hexafluoropropene-1,3-disulfonimide (LiCFSA) as a lithium salt were weighed out so that the molar ratio of organic solvent:lithium salt was 3.0:1.0, and the mixture was mixed and stirred to prepare the electrolyte of Example 1.

[0035] 1-2. Example 2 An electrolyte of Example 2 was prepared in the same manner as in Example 1, except that the molar ratio of organic solvent:lithium salt was 2.0:1.0.

[0036] 1-3. Example 3 3-Methylsulfolane (melting point 1°C) as an organic solvent and LiCFSA as a lithium salt were weighed out so that the molar ratio of organic solvent:lithium salt was 3.0:1.0, and the mixture was stirred to prepare the electrolyte of Example 3.

[0037] 1-4. Example 4 An electrolyte of Example 4 was prepared in the same manner as in Example 3, except that the molar ratio of organic solvent:lithium salt was 2.0:1.0.

[0038] 1-5. Comparative Example 1 An electrolyte of Comparative Example 1 was prepared by weighing sulfolane (melting point 27.8°C) as an organic solvent and LiCFSA as a lithium salt so that the molar ratio of organic solvent:lithium salt was 3.0:1.0, mixing and stirring the mixture.

[0039] 1-6. Comparative Examples 2 to 7 The electrolytes of Comparative Examples 2 to 7 were prepared in the same manner as in Comparative Example 1, except that the molar ratios of organic solvent:lithium salt were 2.9:1.0, 2.8:1.0, 2.7:1.0, 2.6:1.0, 2.5:1.0, and 2.0:1.0, respectively.

[0040] 2. Thermal Characterization DSC measurements were carried out from -50°C to 200°C at a temperature rise rate of 5°C / min, and the melting point of each electrolyte was obtained from the rising temperature of the melting peak.

[0041] 3.Viscosity measurement A rheometer (manufactured by Thermo Scientific) was used to measure the viscosity of the electrolytes of Examples 1 to 4 at 25° C. The viscosity of the electrolytes of Comparative Examples 1 to 7 was not measured because they were solid at 25° C.

[0042] 4. Ionic Conductivity Evaluation Two-electrode symmetrical cells were fabricated using Li metal for the electrodes and the various electrolyte solutions listed above, with fixed electrode area and inter-electrode distance, and the resistance was measured at 25°C using an AC impedance method. The ionic conductivity was calculated from the obtained resistance and the cell shape (electrode area, inter-electrode distance). The impedance measurement conditions were as follows:

[0043] Impedance measurement conditions: temperature 25°C, amplitude 10mV, frequency 1M~10mHz

[0044] 5.Results The preparation conditions and measurement / evaluation results for each example are shown in FIG. 2 and Table 1 below.

[0045] [Table 1]

[0046] 1, the electrolytes of Examples 1 to 4, which used 1,4-butanesultone or 3-methylsulfolane as the organic solvent and LiCFSA as the lithium salt, were liquid at room temperature and had high lithium ion conductivity. In contrast, the electrolytes of Comparative Examples 1 to 7, which used sulfolane as the organic solvent and LiCFSA as the lithium salt, were solid at room temperature and had lower lithium ion conductivity than the respective Examples. [Explanation of symbols]

[0047] 10 positive electrode 11 Cathode active material layer 12 Positive electrode current collector 20 Electrolyte layer 30 negative electrode 31 Negative electrode active material layer 32 Negative electrode current collector 100 Lithium-ion secondary battery

Claims

1. An electrolyte solution comprising an organic solvent and a lithium salt dissolved in the organic solvent, the organic solvent has a sulfone structure, an asymmetric structure, and a cyclic structure having 10 or less carbon atoms; the lithium salt is lithium 1,1,2,2,3,3-hexafluoropropene-1,3-disulfonimide; the molar ratio of the organic solvent to the lithium salt is 3.0 or less; Electrolyte.

2. The electrolyte solution according to claim 1 , wherein the cyclic structure has 3 or more carbon atoms.

3. The electrolyte solution according to claim 2 , wherein the cyclic structure has 4 or more and 8 or less carbon atoms.

4. The electrolyte solution according to claim 1 or 2, wherein the sulfone structure is contained in the skeleton of the cyclic structure.

5. 3. The electrolyte solution according to claim 1, having a viscosity of 1100 mPas or less at 25°C.

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