Lithium ion conductor

Abstract

To provide a lithium ion conductor capable of achieving both a high lithium ion transport number and high ion conductivity.SOLUTION: The lithium ion conductor contains a polymer having a siloxane structure in a main chain and a sulfolane skeleton in a side chain, and a lithium salt.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present disclosure relates to lithium ion conductors. [Background technology]

[0002] Various techniques have been proposed regarding lithium ion conductors such as those disclosed in Patent Document 1. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-089875 [Non-patent literature]

[0004] [Non-Patent Document 1] Chihiro Doi et al., 2020 ECS Meeting Abstracts, Volume MA2020-02 Z01:General Student Poster Session 3527 “Ionic Transport Properties in Sulfone-Based Solid Polymer Electrolytes” Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 discloses a nonaqueous electrolyte containing sulfolane and a Li salt. It has been reported that electrolytes containing sulfolane exhibit a high lithium ion transport number due to hopping conduction, but when used as a solid electrolyte, the ionic conductivity is low, and there is room for improvement in taking advantage of the high lithium ion transport number.

[0006] The present disclosure has been made in view of the above circumstances, and has as its main object to provide a lithium ion conductor that can achieve both a high lithium ion transport number and high ionic conductivity. [Means for solving the problem]

[0007] That is, the present disclosure includes the following aspects. <1> A lithium ion conductor comprising: a polymer having a siloxane structure in its main chain and a sulfolane skeleton in its side chain; and a lithium salt.

[0008] <2> The lithium salt is at least one selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium fluorosulfonyl(trifluoromethanesulfonyl)imide. <1> The lithium ion conductor according to claim 1.

[0009] <3> the molar ratio (mol%) of the lithium salt to the monomer unit in the polymer is lithium salt:monomer unit=10:90 to 70:30; <1> or <2> The lithium ion conductor according to claim 1.

[0010] <4> The polymer is poly(methylpropyloxysulfolanesiloxane). <1> ~ <3> 1. The lithium ion conductor according to claim 1 .

[0011] <5> <1> ~ <4> A battery comprising the lithium ion conductor according to any one of the above items. [Effects of the Invention]

[0012] The lithium ion conductor of the present disclosure can achieve both a high lithium ion transport number and high ionic conductivity. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a graph showing the relationship between the concentration of the lithium salt and the glass transition temperature Tg of the lithium ion conductor in Example 1. [Figure 2] FIG. 2 is a graph showing the relationship between temperature and ionic conductivity σ of the lithium ion conductor in Example 1. [Figure 3] FIG. 3 is a graph showing the relationship between the concentration of the lithium salt and the glass transition temperature Tg of the lithium ion conductor in Example 2. [Figure 4] FIG. 4 is a graph showing the relationship between temperature and ionic conductivity σ of the lithium ion conductor in Example 2. [Figure 5] FIG. 5 is a graph showing the relationship between temperature and heat capacity ΔQ of the lithium ion conductor in Example 3. [Figure 6] FIG. 6 is a graph comparing the ionic conductivity×Li ion transport number (σ×tLi) of the lithium ion conductors of Examples 1 and 2 and Comparative Examples 1-1 and 2. [Figure 7] FIG. 7 is a graph showing the relationship between the Li ion transport number tLi and lnσ for the lithium ion conductors of Examples 1 and 2 and Comparative Examples 1-1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present disclosure will be described. It should be noted that matters other than those specifically mentioned in this specification that are necessary for implementing the present disclosure (for example, the general configuration and manufacturing process of a lithium ion conductor that do not characterize the present disclosure) can be understood as design matters for those skilled in the art based on prior art in the relevant field. The present disclosure can be implemented based on the contents disclosed in this specification and common general technical knowledge in the relevant field.

[0015] The present disclosure provides a lithium ion conductor including a polymer having a siloxane structure in its main chain and a sulfolane skeleton in its side chain, and a lithium salt.

[0016] Lithium ion conductors used as polymer electrolytes are expected to have the effects of improving the bonding strength between the electrodes and the solid electrolyte layer in solid-state batteries and suppressing cracking of the solid electrolyte layer from the viewpoints of safety, flexibility, formability, etc. However, conventional polyethylene oxide (PEO)-based polymer electrolytes have low lithium ion transport numbers and low ionic conductivity. On the other hand, poly(3-acryloylsulfolane) (abbreviated as PASL), which has a structure in which a sulfolane skeleton is introduced into the polymer side chain, exhibits a special conduction mechanism similar to that of a high-concentration electrolyte that uses sulfolane as a solvent, and can improve the low Li-ion transport number mentioned above. However, its ionic conductivity is low due to its high glass transition temperature Tg. In the present disclosure, by introducing siloxane into the polymer main chain, the glass transition temperature Tg of the polymer electrolyte can be lowered, and the ionic conductivity is improved compared to conventional polymer electrolytes that incorporate sulfolane while maintaining a high Li-ion transport number.

[0017] The lithium ion conductor of the present disclosure includes a polymer having a siloxane structure in the main chain and a sulfolane skeleton in the side chain, and a lithium salt. The lithium ion conductor of the present disclosure is used as an electrolyte in a battery.

[0018] The lithium salt may be at least one selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide (abbreviation: LiTFSA), lithium bis(fluorosulfonyl)imide (abbreviation: LiFSA), and lithium fluorosulfonyl(trifluoromethanesulfonyl)imide (abbreviation: LiFTA). The molar ratio (mol%) of the lithium salt to the monomer unit, which is a repeating unit in the polymer, may be lithium salt:monomer unit=10:90 to 70:30. That is, when the monomer unit in the polymer is considered to be one unit (1 mol), the molar fraction of the lithium salt in the lithium ion conductor may be 10 to 70 mol%. Specifically, this is intended to represent the molar ratio of the lithium salt to sulfolane, which is a monomer unit in the polymer.

[0019] The polymer may be any polymer having a siloxane structure in the main chain and a sulfolane skeleton in the side chain, and may be one represented by the following general formula (1), or may be poly(methylpropyloxysulfolanesiloxane) (abbreviation: PMSS).

[0020] [ka]

[0021] In general formula (1), R 1 is a hydrogen atom or a hydrocarbon group having 1 to 5 carbon atoms, and R 2 is a hydrocarbon group having 2 to 5 carbon atoms.

[0022] The battery of the present disclosure comprises a positive electrode, an electrolyte layer, and a negative electrode in this order. The battery of the present disclosure includes the lithium ion conductor of the present disclosure. The lithium ion conductor of the present disclosure may be included in the positive electrode, the electrolyte layer, or the negative electrode of the battery.

[0023] The type of battery is not particularly limited, but examples include lithium ion batteries. The battery may be a primary battery or a secondary battery. The battery may be a liquid battery using an electrolytic solution as an electrolyte, or a solid battery. In the present disclosure, a solid-state battery refers to a battery containing a solid electrolyte. The solid-state battery may be a semi-solid battery, which is a solid battery containing a solid electrolyte and a liquid-based material, or an all-solid-state battery, which is a solid battery containing no liquid-based material. The shape of the battery is not particularly limited, and may be, for example, a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, or a laminate type. Examples of uses of the battery include power sources for vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), electric vehicles (BEVs), gasoline-powered vehicles, and diesel-powered vehicles. In particular, the battery may be used as a driving power source for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or electric vehicles (BEVs). The battery may also be used as a power source for mobile objects other than vehicles (for example, trains, ships, and aircraft), and as a power source for electrical appliances such as information processing devices. [Example]

[0024] Example 1 [PMSS production] As shown in the following reaction formula (A), 3-hydroxysulfolane and allyl alcohol were reacted in the presence of KOH at 30°C for 24 hours to synthesize 3-allyloxysulfolane.

[0025] [ka]

[0026] As shown in the following reaction formula (B), poly(methylhydrosiloxane) (abbreviated as PMHS, molecular weight 1700 to 3200) and 3-allyloxysulfolane were reacted in an Ar atmosphere in the presence of a Karstedt catalyst at 90°C for 48 hours to synthesize PMSS. The glass transition temperature Tg of PMSS was -32.9°C. The brittle temperature Tb of PMSS was 360°C.

[0027] [ka]

[0028] [Fabrication of lithium ion conductors] PMSS was mixed with LiTFSA to prepare a lithium ion conductor, PMSS×LiTFSA.

[0029] Example 2 The lithium ion conductor PMSS×LiFSA was prepared by mixing PMSS with LiFSA.

[0030] Example 3 PMSS was mixed with LiFTA to prepare a lithium ion conductor, PMSS×LiFTA.

[0031] (Comparative Example 1) Instead of PMSS, PEO (molecular weight 5000) was prepared, and LiTFSA was mixed with PEO to prepare a lithium ion conductor PEO×LiTFSA.

[0032] (Comparative Example 2) Instead of PMSS, PASL was prepared and mixed with LiTFSA to produce a lithium ion conductor, PASL×LiTFSA. The glass transition temperature Tg of PASL was 117°C. The embrittlement temperature Tb of PASL was 300°C.

[0033] <Comparison of Tg and Tb in the solvent alone> Table 1 shows the glass transition temperature Tg and embrittlement temperature Tb of PMSS and PASL.

[0034] [Table 1]

[0035] As shown in Table 1, when the glass transition temperatures Tg of PMSS and PASL are compared, PMSS has a lower glass transition temperature Tg than PASL, and an improvement in ionic conductivity can be expected. As shown in Table 1, when comparing the embrittlement temperatures Tb of PMSS and PASL, it is clear that PMSS has a higher embrittlement temperature Tb than PASL, and is therefore more thermally stable.

[0036] <Measurement of Tg of PMSS×LiTFSA in Example 1> In Example 1, the glass transition temperatures Tg of a lithium ion conductor composed of 100 mol% PMSS and 0 mol% LiTFSA, a lithium ion conductor composed of 90 mol% PMSS and 10 mol% LiTFSA, a lithium ion conductor composed of 67 mol% PMSS and 33 mol% LiTFSA, a lithium ion conductor composed of 50 mol% PMSS and 50 mol% LiTFSA, and a lithium ion conductor composed of 30 mol% PMSS and 70 mol% LiTFSA were measured. The glass transition temperature Tg of the lithium ion conductor composed of 0 mol% PMSS and 100 mol% LiTFSA, i.e., pure LiTFSA, was estimated from the melting point of LiTFSA. The molar fraction (mol %) of PMSS in the lithium ion conductor means the molar fraction of PMSS monomer units. The results are shown in Figure 1 and Table 2. FIG. 1 is a graph showing the relationship between the concentration of the lithium salt and the glass transition temperature Tg of the lithium ion conductor in Example 1. As shown in FIG. 1 and Table 2, it can be seen that when the concentration of the lithium salt in the lithium ion conductor is 10 mol % to 70 mol %, the lithium ion conductor has a desired glass transition temperature Tg.

[0037] [Table 2]

[0038] <Measurement of ionic conductivity of PMSS×LiTFSA in Example 1> In Example 1, the ionic conductivities σ and logσ of a lithium ion conductor composed of 90 mol% PMSS and 10 mol% LiTFSA, a lithium ion conductor composed of 67 mol% PMSS and 33 mol% LiTFSA, a lithium ion conductor composed of 50 mol% PMSS and 50 mol% LiTFSA, and a lithium ion conductor composed of 30 mol% PMSS and 70 mol% LiTFSA were measured. The results are shown in Figure 2 and Table 3. FIG. 2 is a graph showing the relationship between temperature and ionic conductivity σ of the lithium ion conductor in Example 1. As shown in FIG. 2 and Table 3, it can be seen that when the molar fraction of the lithium salt in the lithium ion conductor is 10 mol % to 70 mol %, the lithium ion conductor has a desired ionic conductivity σ.

[0039] [Table 3]

[0040] <Measurement of Tg and ionic conductivity of PMSS×LiFSA in Example 2> In Example 2, the glass transition temperatures Tg of the lithium ion conductors containing 100 mol% PMSS and 0 mol% LiFSA, 90 mol% PMSS and 10 mol% LiFSA, 67 mol% PMSS and 33 mol% LiFSA, 50 mol% PMSS and 50 mol% LiFSA, and 30 mol% PMSS and 70 mol% LiFSA were measured. The glass transition temperature Tg of the lithium ion conductor containing 100 mol% LiFSA, i.e., pure LiFSA, was estimated from the melting point of LiFSA. The results are shown in Figure 3 and Table 4. FIG. 3 is a graph showing the relationship between the concentration of the lithium salt and the glass transition temperature Tg of the lithium ion conductor in Example 2. As shown in FIG. 3 and Table 4, it can be seen that when the molar fraction of the lithium salt in the lithium ion conductor is 10 mol % to 70 mol %, the lithium ion conductor has a desired glass transition temperature Tg.

[0041] [Table 4]

[0042] <Measurement of ionic conductivity of PMSS×LiFSA in Example 2> In Example 2, the ionic conductivities σ and logσ of a lithium ion conductor composed of 90 mol% PMSS and 10 mol% LiFSA, a lithium ion conductor composed of 67 mol% PMSS and 33 mol% LiFSA, a lithium ion conductor composed of 50 mol% PMSS and 50 mol% LiFSA, and a lithium ion conductor composed of 30 mol% PMSS and 70 mol% LiFSA were measured. The results are shown in Figure 4 and Table 5. FIG. 4 is a graph showing the relationship between temperature and ionic conductivity σ of the lithium ion conductor in Example 2. As shown in FIG. 4 and Table 5, it can be seen that when the molar fraction of the lithium salt in the lithium ion conductor is 10 mol % to 70 mol %, the lithium ion conductor has a desired ionic conductivity σ.

[0043] [Table 5]

[0044] <Measurement of Heat Capacity of PMSS×LiFTA in Example 3> In Example 3, the heat capacities ΔQ of a lithium ion conductor composed of 67 mol% PMSS and 33 mol% LiFTA, a lithium ion conductor composed of 50 mol% PMSS and 50 mol% LiFTA, a lithium ion conductor composed of 30 mol% PMSS and 70 mol% LiFTA, and a lithium ion conductor composed of 10 mol% PMSS and 90 mol% LiFTA were measured by differential scanning calorimetry (DSC). The results are shown in Figure 5. FIG. 5 is a graph showing the relationship between temperature and heat capacity ΔQ of the lithium ion conductor in Example 3. As shown in Figure 5, the heat capacity ΔQ of the lithium ion conductor increases as the concentration of lithium salt increases. The arrow in Figure 5 indicates the glass transition temperature Tg. The temperature at which the change in heat capacity ΔQ begins in Figure 5 was defined as the glass transition temperature.

[0045] <Comparison of ionic conductivity and Li-ion transport number> The ionic conductivity σ and lithium ion transport number (t ) were measured at a temperature of 90°C for the lithium ion conductor PMSS / LiTFSA containing 33 mol% of LiTFSA in Example 1, the lithium ion conductor PMSS / LiFSA containing 33 mol% of LiFSA in Example 2, the lithium ion conductor PEO / LiTFSA containing 9.1 mol% of LiTFSA in Comparative Example 1-1, the lithium ion conductor PEO / LiTFSA containing 23.1 mol% of LiTFSA in Comparative Example 1-2, and the lithium ion conductor PASL / LiTFSA containing 33 mol% of LiTFSA. Li ) was measured. Ion conductivity σ, lnσ, and Li ion transport number t of the lithium ion conductors of Examples 1 and 2 and Comparative Examples 1-1 and 1-2 Li , ionic conductivity × Li ion transport number (σ × t Li ) values ​​are shown in Table 6.

[0046] [Table 6]

[0047] FIG. 6 shows the relationship between the ionic conductivity and the Li ion transport number (σ×t Li ) is a graph comparing FIG. 7 shows the Li-ion transport numbers t Li 10 is a graph showing the relationship between lnσ and As shown in Table 6 and FIGS. 6 to 7, Example 1 (PMSS / LiTFSA) has a lithium ion conductivity equivalent to that of Comparative Example 1-1 (PEO / LiTFSA), and also has a higher Li ion transport number. It can be seen that Example 2 (PMSS / LiFSA) has a higher lithium ion conductivity and a higher Li ion transport number than Example 1 (PMSS / LiTFSA). On the other hand, Comparative Example 2 (PASL / LiTFSA) has a high Li ion transport number, but is found to have lower ionic conductivity than Examples 1 and 2 and Comparative Examples 1-1 and 1-2.

Claims

1. A lithium ion conductor comprising: a polymer having a siloxane structure in its main chain and a sulfolane skeleton in its side chain; and a lithium salt.

2. 2. The lithium ion conductor according to claim 1, wherein the lithium salt is at least one selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, and lithium fluorosulfonyl(trifluoromethanesulfonyl)imide.

3. 2. The lithium ion conductor according to claim 1, wherein a molar ratio (mol %) of the lithium salt to the monomer unit in the polymer is lithium salt:monomer unit=10:90 to 70:

30.

4. 10. The lithium ion conductor of claim 1, wherein the polymer is poly(methylpropyloxysulfolanesiloxane).

5. A battery comprising the lithium ion conductor of claim 1.

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