Solid electrolyte

Abstract

To provide a solid electrolyte that exhibits excellent ion conductivity.SOLUTION: The solid electrolyte of the present disclosure comprises sulfolane, a specified cyclic lithium salt, and a linear lithium salt, wherein the molar ratio of the sulfolane to a total of the cyclic lithium salt and the linear lithium salt is 2.0 or more and 3.1 or less, and the solid electrolyte is a molecular crystal having a hexagonal crystal structure.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] For example, Patent Document 1 discloses a molecular crystal solid electrolyte containing an electron-donating sulfur-based organic compound and a lithium salt, which is said to be highly safe when used as an electrolyte solution in a battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-214510 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventional solid electrolytes have room for improvement in terms of ionic conductivity.

[0005] Therefore, an object of the present disclosure is to provide a solid electrolyte with excellent ion conductivity. [Means for solving the problem]

[0006] The present disclosure achieves the above object by the following means. (Aspect 1) sulfolane, cyclic lithium salts having the following structure, and linear lithium salts: [ka] the molar ratio of the sulfolane to the total of the cyclic lithium salt and the chain lithium salt is 2.0 or more and 3.1 or less; and It is a molecular crystal with a hexagonal crystal structure. solid electrolyte. (Aspect 2) the molar ratio of the cyclic lithium salt to the chain lithium salt is 2.3 or more; 2. The solid electrolyte of embodiment 1. (Aspect 3) The solid electrolyte according to aspect 1 or 2, wherein the chain lithium salt is one or both of the following: [ka] [ka] (Aspect 4) An electrochemical device having an ion conductor, The ionic conductor comprises the solid electrolyte of aspect 1, 2, or 3. Electrochemical devices. [Effects of the Invention]

[0007] According to the present disclosure, a solid electrolyte with excellent ion conductivity can be provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 shows a schematic diagram of an example of the configuration of an electrochemical device. [Figure 2] FIG. 2 shows the results of DSC measurements of the solid samples of Examples 5 and 6 and Comparative Examples 1 and 2. [Figure 3] FIG. 3 shows the X-ray diffraction patterns of the solid samples of Examples 1 to 6 and Comparative Example 1. [Figure 4] FIG. 4 is a graph showing the relationship between the molar ratio of sulfolane, LiCFSA, and chain lithium salt (solvent / LiCFSA / chain lithium salt=3 / 1−x / x) and ionic conductivity (ionic conductivity / Scm−1) in Examples 1 to 6 and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0009] ≪Solid electrolyte≫ The solid electrolyte of the present disclosure is sulfolane, a cyclic lithium salt having the following structure, and a chain lithium salt, [ka] the molar ratio of the sulfolane to the total of the cyclic lithium salt and the chain lithium salt is 2.0 or more and 3.1 or less; and It is a molecular crystal with a hexagonal structure.

[0010] The solid electrolyte can provide excellent ion conductivity.

[0011] 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 gist of the present disclosure.

[0012] <Lithium salt> The solid electrolyte of the present disclosure contains a predetermined cyclic lithium salt and a chain lithium salt.

[0013] (cyclic lithium salt) The cyclic lithium salt used in the solid electrolyte of the present disclosure is lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide (LiCFSA) represented by the above chemical formula (1). According to the findings of the present inventors, the ionic conductivity of the sulfolane alone and the cyclic lithium salt alone is difficult to measure due to high resistance. Only by combining the sulfolane and the cyclic lithium salt, as in the solid electrolyte of the present disclosure, does substantial ionic conductivity occur.

[0014] (chain lithium salt) Chain lithium salts have a higher ionic dissociation property than cyclic lithium salts. Therefore, adding chain lithium salts to a solid electrolyte increases the number of moles of mobile lithium ions contained in the solid electrolyte. However, if an excess of chain lithium salt is added to a solid electrolyte, the crystal structure cannot be maintained, and the electrolyte becomes liquid. Therefore, the amount of chain lithium salt added may be adjusted appropriately within a range in which the molecular crystal can maintain a hexagonal crystal structure.

[0015] In the present disclosure, the chain lithium salt is not particularly limited, and may be lithium bis(fluorosulfonyl)imide (LiFSA) represented by the following chemical formula (3) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSA) represented by the following chemical formula (4). [ka] [ka]

[0016] In the solid electrolyte of the present disclosure, the molar ratio of the cyclic lithium salt to the chain lithium salt (cyclic lithium salt / chain lithium salt) may be 2.3 or more. That is, the solid electrolyte of the present disclosure can contain 2.3 moles or more of the cyclic lithium salt per mole of the chain lithium salt. In the solid electrolyte of the present disclosure, the molar ratio may be 2.4 or more, or 2.5 or more, or may be 19 or less, or 18 or less.

[0017] <Sulfolane> The solid electrolyte of the present disclosure contains sulfolane. Sulfolane is a type of electron-donating sulfur-based organic compound and is represented by the following chemical formula (2). In the molecular crystal, the sulfolane represented by the following chemical formula (2) may or may not be chemically bonded to the lithium salt represented by the above chemical formula (1). The molecular crystal according to one embodiment may have a hexagonal crystal structure due to the orientation of each molecule while maintaining the molecular structure of sulfolane and the anionic molecular structure of the lithium salt. [ka]

[0018] In the solid electrolyte of the present disclosure, the molar ratio of sulfolane to the sum of the cyclic lithium salt and the chain lithium salt (sulfolane / total of cyclic lithium salt and chain lithium salt) is 2.0 or more and 3.1 or less. That is, the solid electrolyte of the present disclosure contains 2.0 to 3.1 moles of sulfolane per mole of lithium salt. According to the findings of the present inventors, if the molar ratio is too high, a portion of the sulfolane becomes liquid, making it impossible to maintain an appropriate morphology as a molecular crystal. Furthermore, if the molar ratio is too low, ionic conductivity tends to decrease. In the solid electrolyte of the present disclosure, the molar ratio of 2.0 to 3.1 or less specifically improves ionic conductivity. The molar ratio may be 2.0 to 3.0 or 2.0 to less than 3.0, 2.0 to less than 3.0, or 2.0 to 2.9 or less. The molar ratio of sulfolane to lithium salt in the solid electrolyte (sulfolane / lithium salt) can be determined using various analytical devices.

[0019] <Molecular crystal> The molecular crystal according to this embodiment may have a specific crystal structure by containing the sulfolane and the lithium salt. For example, the molecular crystal according to this embodiment may have a hexagonal crystal structure. In a molecular crystal having a hexagonal crystal structure, a Li layer is present in the xy plane direction of the hexagonal crystal structure, and components that hinder the diffusion of Li are substantially absent in the xy plane, making it easy for Li to diffuse. As a result, it is believed that excellent ionic conductivity is exhibited. The molecular crystal according to this embodiment may be composed of a single hexagonal crystal structure, or may have a hexagonal crystal structure and other crystal structures. In particular, it is believed that even more excellent ionic conductivity is exhibited when the molecular crystal is composed of a single hexagonal crystal structure.

[0020] <Other> The solid electrolyte of the present disclosure is not limited to the present disclosure. For example, by coating the surface of a filler such as silica with a molecular crystalline solid electrolyte, it is possible to effectively utilize only the direction in which ion conduction is fast in the hexagonal crystal structure. Furthermore, when combined with a sulfide SE, it is desirable to use a chain lithium salt with low anion reactivity (LiTFSA is less reactive with sulfide SE than LiFSA), but this is not a limitation.

[0021] ≪Electrochemical Devices≫ The solid electrolyte of the present disclosure has excellent ionic conductivity and can be used as a constituent material for various electrochemical devices. That is, an electrochemical device according to one embodiment includes an ionic conductor, and the ionic conductor has the solid electrolyte of the present disclosure. While a battery will be exemplified below as an electrochemical device, the molecular crystal of the present disclosure can also be used in electrochemical devices other than batteries.

[0022] As shown in FIG. 1 , a battery 100 according to one embodiment includes a positive electrode active material layer 20, an electrolyte layer 30, and a negative electrode active material layer 40. At least one of the positive electrode active material layer 20, the electrolyte layer 30, and the negative electrode active material layer 40 contains the molecular crystal of the present disclosure. That is, in the battery 100, the molecular crystal of the present disclosure can function as a solid electrolyte. In the battery 100, the configuration other than the molecular crystal as the solid electrolyte is the same as in the prior art. For example, the configuration described in JP 2014-186937 A, JP 2021-068556 A, etc. can be adopted.

[0023] As shown in FIG. 1, the battery 100 may include a positive electrode current collector 10 electrically connected to the positive electrode active material layer 20, and a negative electrode current collector 50 electrically connected to the negative electrode active material layer 40. A known configuration may be adopted as the configuration of the current collector. Furthermore, in addition to the above configuration, the battery 100 may also include general battery configurations, such as tabs and terminals.

[0024] The battery 100 may also be one in which the above components are housed inside an exterior body. Any known exterior body for a battery can be used as the exterior body. A plurality of batteries 100 may be electrically connected in any manner and stacked in any manner to form an assembled battery. Examples of the shape of the battery 100 include coin type, laminate type, cylindrical type, and prismatic type. The battery 100 may be a lithium ion battery. The battery 100 may be a primary battery or a secondary battery. The battery 100 may be manufactured, for example, through a process of dry or wet molding the above layers. [Example]

[0025] The technology of the present disclosure will be described in more detail below with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0026] Comparative Example 1 A cyclic lithium salt represented by the following chemical formula (1) and sulfolane represented by the following chemical formula (2) were weighed out so that the molar ratio of the sulfolane to the cyclic lithium salt was 3.0. The sulfolane was heated to form a melt, and the cyclic lithium salt was added thereto. The cyclic lithium salt was dissolved by stirring while heating. After stirring for 2 hours, the mixture was allowed to cool to room temperature in the air, thereby obtaining an evaluation sample according to Comparative Example 1. The obtained evaluation sample was a solid at room temperature (20°C) in an air atmosphere, and was a molecular crystal containing sulfolane and the cyclic lithium salt. [ka] [ka]

[0027] Example 1 As lithium salts, a cyclic lithium salt represented by the above chemical formula (1) and a chain lithium salt represented by the following chemical formula (3) were weighed out so that the molar ratio of the cyclic lithium salt to the chain lithium salt was 19.0. Furthermore, sulfolane represented by the above chemical formula (2) was weighed out so that the molar ratio of the sulfolane to the total of the cyclic lithium and the chain lithium salt was 3.0. Sulfolane was heated to form a melt, and the lithium salt was added thereto. The mixture was stirred while heating to dissolve the lithium salt. After stirring for 2 hours, the mixture was allowed to cool to room temperature in the air, thereby obtaining an evaluation sample according to Example 1. The obtained evaluation sample was a solid at room temperature (20°C) in an air atmosphere, and was a molecular crystal containing sulfolane and a lithium salt. [ka]

[0028] Example 2 An evaluation sample according to Example 2 was obtained in the same manner as in Example 1, except that the molar ratio of the cyclic lithium salt to the chain lithium salt was 9.0. The obtained evaluation sample was a solid at room temperature (20°C) in an air atmosphere, and was a molecular crystal containing sulfolane and a lithium salt.

[0029] Example 3 An evaluation sample according to Example 3 was obtained in the same manner as in Example 1, except that the molar ratio of the cyclic lithium salt to the chain lithium salt was 4.0. The obtained evaluation sample was a solid at room temperature (20°C) in an air atmosphere, and was a molecular crystal containing sulfolane and a lithium salt.

[0030] Example 4 An evaluation sample according to Example 4 was obtained in the same manner as in Example 1, except that the molar ratio of the cyclic lithium salt to the chain lithium salt was 2.3. The obtained evaluation sample was a solid at room temperature (20°C) in an air atmosphere, and was a molecular crystal containing sulfolane and a lithium salt.

[0031] Comparative Example 2 Except for the molar ratio of the cyclic lithium salt to the chain lithium salt being 0.4, an evaluation sample according to Comparative Example 2 was obtained in the same manner as in Example 1. The obtained evaluation sample was liquid at room temperature (20°C) in the air.

[0032] Comparative Example 3 Except for replacing the chain lithium salt with the chain lithium salt represented by the above chemical formula (3), an evaluation sample according to Comparative Example 3 was obtained in the same manner as in Comparative Example 1. The obtained evaluation sample was liquid at room temperature (20°C) in an air atmosphere.

[0033] Example 5 Except for replacing the chain lithium salt with the chain lithium salt represented by the following chemical formula (4), an evaluation sample according to Example 5 was obtained in the same manner as in Example 1. The obtained evaluation sample was solid at room temperature (20°C) in an air atmosphere, and was a molecular crystal containing sulfolane and a lithium salt. [ka]

[0034] Example 6 Except for replacing the chain lithium salt with the chain lithium salt represented by the above chemical formula (4), an evaluation sample according to Example 6 was obtained in the same manner as in Example 2. The obtained evaluation sample was a solid at room temperature (20°C) in an air atmosphere, and was a molecular crystal containing sulfolane and a lithium salt.

[0035] <Evaluation> The evaluation samples of Comparative Examples 1 to 3 and Examples 1 to 6 were evaluated and compared as follows.

[0036] (Thermal characteristic evaluation) The evaluation samples of Examples 5 and 6 and Comparative Examples 1 and 2 were subjected to DSC measurement from −50° C. to 200° C. at a temperature increase rate of 5° C. / min.

[0037] (Crystal structure analysis) Crystal structure analysis was performed using CuKα radiation with an X-ray diffractometer on the evaluation samples of Examples 1 to 6 and Comparative Example 1. The measurement conditions were as follows: Note that the electrolytes of Comparative Examples 2 and 3 were liquid at 25°C, so crystal structure analysis was not performed.

[0038] Measurement conditions: Scan speed 2° / min, step width 0.01°

[0039] (Ionic conductivity evaluation) For the electrolytes of Examples 1 to 6 and Comparative Examples 1 to 3, two-electrode symmetrical cells were prepared using Li metal for the electrodes and the various electrolyte solutions described above as the electrolyte solution, with the electrode area and inter-electrode distance fixed, and the resistance values ​​were measured by an AC impedance method at 25°C. The ionic conductivity was calculated from the obtained resistance value and the cell shape (electrode area, inter-electrode distance). The impedance measurement conditions were as follows:

[0040] Impedance measurement conditions: temperature 25°C, amplitude 10mV, frequency 1M~10mHz

[0041] (result) 2 shows the DSC measurement results for each of the evaluation samples of Examples 5 and 6 and Comparative Examples 1 and 2. Also, FIG. 3 shows the X-ray analysis patterns for each of the evaluation samples of Examples 1 to 6 and Comparative Example 1.

[0042] 2, Comparative Example 1 and Examples 5 and 6 have a melting peak of a hexagonal structure near 50°C, indicating that they are molecular crystals with a hexagonal structure. In contrast, Comparative Example 2 has a similar broad peak, indicating that the molecular crystals in Comparative Example 2 have low crystallinity and are liquid.

[0043] According to the X-ray diffraction results shown in Fig. 3, peaks attributable to the hexagonal crystal structure are observed around 12.2°, 13.7°, 15.8°, 18.4°, and 18.7° in Comparative Example 1. Similar peaks are also observed in Examples 1 to 6, which indicates that all of Examples 1 to 6 have a hexagonal crystal structure.

[0044] The measurement results of the ionic conductivity of each of the evaluation samples of Examples 1 to 6 and Comparative Examples 1 to 3 are shown in Fig. 4. The evaluation results are shown in Table 1.

[0045] [Table 1]

[0046] The results shown in Table 1 and FIG. 4 reveal the following.

[0047] When sulfolane represented by chemical formula (2) and cyclic lithium salt represented by chemical formula (1) were used, the evaluation sample according to Comparative Example 1, in which the molar ratio of sulfolane to cyclic lithium salt was 3.0, was a solid having a hexagonal crystal structure and had an ionic conductivity of 7.9 × 10 -7 S / cm was shown.

[0048] When sulfolane, a cyclic lithium salt represented by chemical formula (1), and a chain lithium salt represented by chemical formula (3) were used, the evaluation samples according to Examples 1 to 4, in which the molar ratio of the cyclic lithium salt to the chain lithium salt was 2.3 to 19.0 and the molar ratio of sulfolane to the total of the cyclic lithium salt and the chain lithium salt was 3.0, were solids with a hexagonal crystal structure and exhibited higher ionic conductivity than the evaluation sample according to Comparative Example 1. Furthermore, the smaller the molar ratio of the cyclic lithium salt to the chain lithium salt, the higher the ionic conductivity.

[0049] The evaluation samples of Comparative Examples 2 and 3, in which the molar ratio of cyclic lithium salt to chain lithium salt was 2.3 or less, were liquid. This shows that when the molar ratio of cyclic lithium salt to chain lithium salt is small, i.e., when the content of chain lithium salt is high, the electrolyte becomes liquid.

[0050] When the chain lithium salt represented by chemical formula (4) was used as the chain lithium salt, the evaluation samples according to Examples 5 and 6, in which the molar ratio of the cyclic lithium salt to the chain lithium salt was 2.3 to 19.0 and the molar ratio of sulfolane to the total of the cyclic lithium salt and the chain lithium salt was 3.0, were solids with a hexagonal crystal structure and exhibited higher ionic conductivity than the evaluation sample according to Comparative Example 1. [Explanation of symbols]

[0051] 10 Positive electrode current collector 20 Cathode active material layer 30 Electrolyte layer 40 Negative electrode active material layer 50 Negative electrode current collector 100 batteries

Claims

1. sulfolane, a cyclic lithium salt having the following structure, and a chain lithium salt, 【Chemical 1】 a molar ratio of the sulfolane to the total of the cyclic lithium salt and the chain lithium salt is 2.0 or more and 3.1 or less; and It is a molecular crystal with a hexagonal crystal structure. solid electrolyte.

2. a molar ratio of the cyclic lithium salt to the chain lithium salt is 2.3 or more; The solid electrolyte according to claim 1 .

3. The chain lithium salt is one or both of the following: The solid electrolyte according to claim 1: 【Chemistry 2】

4. An electrochemical device having an ion conductor, The ionic conductor comprises the solid electrolyte according to claim 1. Electrochemical devices.

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