An ester-based electrolyte for sodium-sulfur batteries, its preparation method and application

Abstract

This application belongs to the field of electrolyte technology, specifically relating to an ester-based electrolyte for sodium-sulfur batteries, its preparation method, and its application. The ester-based electrolyte includes a first solvent, a second solvent, and a sodium salt. The first solvent is ethyltrifluoroethyl carbonate, bis(2,2,2-trifluoroethyl) carbonate, or 2,2,2-trifluoroethyltrifluoroethyl ester. The second solvent is fluoroethylene carbonate, which can inhibit the dissolution and shuttle of polysulfides and can also form a NaF-rich SEI film on the sodium anode to inhibit the growth of sodium dendrites.

Description

Technical Field

[0001] This application relates to the field of electrolyte technology, and more specifically, to an ester electrolyte for sodium-sulfur batteries, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries are currently the most widely used electrochemical energy storage devices, but lithium reserves in the Earth's crust are extremely low and unevenly distributed, which cannot meet the needs of large-scale energy storage. Meanwhile, the popularization of electric vehicles has raised the requirements for battery energy density, and there is an urgent need to develop the next generation of battery systems with high resource abundance and high energy density.

[0003] The theoretical specific capacity of room temperature sodium-sulfur (RT Na-S) batteries is as high as 1274 Wh·kg. -1 Sulfur cathodes and sodium anodes are abundant and inexpensive, making them promising candidates for large-scale energy storage. However, their practical application faces challenges such as slow reaction kinetics at the sulfur cathode, polysulfide shuttle, and dendrite growth caused by instability at the sodium anode. To address these issues, researchers have proposed various strategies. For example, on the sulfur cathode side, physical confinement and chemical adsorption methods are used to anchor polysulfides, thereby inhibiting their dissolution and shuttle. Furthermore, polar materials are introduced to catalyze the polysulfide conversion process, accelerating reaction kinetics. On the anode side, methods such as designing a three-dimensional framework and constructing artificial solid electrolyte interfaces (SEIs) are used to induce Na… + Uniform deposition inhibits the growth of sodium dendrites. However, the above strategies only have a positive effect on one side of the sulfur cathode or sodium anode, and cannot simultaneously modify both the sulfur cathode and sodium anode. Summary of the Invention

[0004] This application addresses the aforementioned deficiencies in the prior art. There is a need for an ester-based electrolyte for sodium-sulfur batteries, its preparation method, and its application, capable of inhibiting the dissolution and shuttle of polysulfides, and also capable of suppressing the growth of sodium dendrites by regulating the SEI film composition.

[0005] In a first aspect, this application provides an ester electrolyte for a sodium-sulfur battery, the ester electrolyte comprising a first solvent, a second solvent, and a sodium salt, wherein the first solvent is ethyl trifluoroethyl carbonate, bis(2,2,2-trifluoroethyl) carbonate, or 2,2,2-trifluoroethyl trifluoroethyl ester; and the second solvent is fluoroethylene carbonate.

[0006] A second aspect of this application provides a method for preparing a sodium-sulfur battery electrolyte according to any embodiment of this application. The preparation method includes: dissolving a sodium salt in a first solvent at room temperature, and then adding a second solvent to obtain an ester electrolyte.

[0007] A third aspect of this application provides the application of the sodium-sulfur battery electrolyte or the preparation method described in any embodiment of this application in a sodium-sulfur battery.

[0008] A fourth aspect of this application provides a sodium-sulfur battery, the sodium-sulfur battery comprising the sodium-sulfur battery electrolyte, sulfur positive electrode, sodium negative electrode and separator as described in any embodiment of this application.

[0009] The sodium-sulfur battery ester electrolytes, their preparation methods, and applications provided in the various embodiments of this application use ethyl trifluoroethyl carbonate and bis(2,2,2-trifluoroethyl) carbonate as the first solvents. These are obtained by fluorine substitution of diethyl carbonate, which reduces the solubility of the solvent and has a weak solubilizing effect. This reduces the solubility of polysulfides in the electrolyte. Furthermore, the ester electrolyte contains sodium salts, thus forming a NaF-rich SEI film on the sodium anode, inhibiting the growth of sodium dendrites and improving the cycle stability of the battery. Attached Figure Description

[0010] In drawings that are not necessarily drawn to scale, the same reference numerals may describe similar parts in different views. The same reference numerals with or without letter suffixes may indicate different instances of similar parts. The drawings generally illustrate various embodiments by way of example rather than limitation and are used, together with the description and claims, to illustrate the claimed embodiments. Where appropriate, the same reference numerals are used in all drawings to refer to the same or similar parts. Such embodiments are illustrative and not intended to be exhaustive or exclusive embodiments of the apparatus or method.

[0011] Figure 1 The ester electrolyte according to Example 1 of this application is shown assembled into a button cell at 200 mA·g. -1 Cyclic performance at current density;

[0012] Figure 2 The ester electrolyte according to Example 2 of this application is shown assembled into a button cell at 200 mA·g. -1 Cyclic performance at current density;

[0013] Figure 3 The ester electrolyte according to Example 3 of this application is shown assembled into a coin cell at 200 mA·g. -1 Cyclic performance at current density;

[0014] Figure 4 The ester electrolyte of Comparative Example 1 according to this application is shown assembled into a coin cell at 200 mA·g. -1 Cyclic performance at current density. Detailed Implementation

[0015] To enable those skilled in the art to better understand the technical solutions of this application, the application will be described in detail below with reference to the accompanying drawings and specific embodiments. The embodiments of this application will be further described in detail below with reference to the accompanying drawings and specific examples, but these are not intended to limit the scope of this application.

[0016] The terms “first,” “second,” and similar terms used in this application do not indicate any order, quantity, or importance, but are merely used for distinction. Terms such as “including” or “comprising” mean that the element preceding the term covers the element listed after the term, and do not exclude the possibility of covering other elements as well.

[0017] According to an embodiment of this application, an ester electrolyte for a sodium-sulfur battery is provided. The ester electrolyte includes a first solvent, a second solvent, and a sodium salt. The first solvent is ethyl trifluoroethyl carbonate, bis(2,2,2-trifluoroethyl) carbonate, or 2,2,2-trifluoroethyl trifluoroethyl ester. The second solvent is fluoroethylene carbonate.

[0018] The structural formula of diethyl carbonate is:

[0019] ;

[0020] The structural formula of the ethyltrifluoroethyl carbonate is:

[0021] ;

[0022] The bis(2,2,2-trifluoroethyl) carbonate has the following structural formula:

[0023] ;

[0024] The structural formula of the 2,2,2-trifluoroethyltrifluoroethyl ester is:

[0025] .

[0026] The reaction mechanism of sodium-sulfur batteries is that sulfur at the positive electrode combines with sodium ions and is reduced to Na2S, in which long-chain sodium polysulfide Na2S is generated. n (n=4, 6, 8) These substances readily soluble in the electrolyte and shuttle to the sodium anode, causing the loss of active sulfur and rapid capacity decay of the battery. Ethyltrifluoroethyl carbonate is formed by replacing the hydrogen atom on one side of the methyl group of diethyl carbonate with an iron atom (F); bis(2,2,2-trifluoroethyl) carbonate is formed by replacing the hydrogen atom on both sides of the methyl group of diethyl carbonate with an iron atom (F); and 2,2,2-trifluoroethyltrifluoroethyl is formed by replacing the hydrogen atom on both sides of the methyl group of ethyl propionate with an iron atom (F). The first two substances can reduce the solubility of polysulfides, thereby inhibiting polysulfide shuttle.

[0027] The growth of sodium dendrites induced by uneven sodium ion deposition on the sodium anode side also leads to poor battery cycle stability. Ethyl trifluoroethyl carbonate and bis(2,2,2-trifluoroethyl) carbonate can also react with sodium metal to form a NaF-rich solid electrolyte interface film, which accelerates sodium ion transport and prevents sodium dendrite growth.

[0028] In some embodiments, the first solvent is ethyltrifluoroethyl carbonate or bis(2,2,2-trifluoroethyl) carbonate. Ethyltrifluoroethyl carbonate and bis(2,2,2-trifluoroethyl) carbonate, when combined with the second solvent, exhibit better suppression of polysulfide shuttle behavior compared to 2,2,2-trifluoroethyltrifluoroethyl ester. Although 2,2,2-trifluoroethyltrifluoroethyl ester has a weak solvation effect, it cannot effectively suppress polysulfide shuttle behavior.

[0029] Ester electrolytes contain sodium salt and two solvents, which enable the sodium anode to form a NaF-rich SEI film, effectively suppressing the growth of sodium dendrites and improving the cycle stability of the battery.

[0030] In some embodiments, the volume ratio of the first solvent to the second solvent is 1:0.90 to 1.15. In some embodiments, the volume ratio of the first solvent to the second solvent is 1:1. When the volume ratio of the first solvent to the second solvent is 1:1, the corresponding ester electrolyte exhibits better performance in suppressing polysulfide shuttle.

[0031] In some embodiments, the sodium salt concentration in the electrolyte is 0.8-1.25 mol·L⁻¹. -1 The concentration of sodium salt in the electrolyte helps it to cooperate with the two solvents, enabling the sodium anode to form a stable solid electrolyte interface film.

[0032] In some embodiments, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, or sodium bis(trifluoromethanesulfonyl)imide. In some embodiments, the sodium salt is sodium bis(trifluoromethanesulfonyl)imide. The ester electrolyte formed by the sodium salt in combination with the first solvent and the second solvent can effectively suppress polysulfide shuttle and improve reaction kinetics.

[0033] According to an embodiment of this application, a method for preparing a sodium-sulfur battery electrolyte according to any embodiment of this application is also provided. The preparation method includes: dissolving a sodium salt in a first solvent at room temperature, and then adding a second solvent to obtain an ester electrolyte.

[0034] According to embodiments of this application, the application of the sodium-sulfur battery electrolyte or the preparation method described in any embodiment of this application in a sodium-sulfur battery is also provided.

[0035] According to an embodiment of this application, a sodium-sulfur battery is also provided, the sodium-sulfur battery comprising the sodium-sulfur battery electrolyte, sulfur positive electrode, sodium negative electrode and separator as described in any embodiment of this application.

[0036] The raw materials and their CAS numbers used in the following embodiments include: ethyl trifluoroethyl carbonate (ETFEC), CAS number: 156783-96-9; bis(2,2,2-trifluoroethyl) carbonate (TFEC), CAS number: 1513-87-7; 2,2,2-trifluoroethyl trifluoroethyl ester, CAS number: 407-38-5; diethyl carbonate (DEC), CAS number: 105-58-8; fluoroethylene carbonate (FEC), CAS number: 114435-02-8; sodium hexafluorophosphate NaPF6, CAS number: 21324-39-0; sodium bis(fluorosulfonyl)imide NaFSI, CAS number: 100669-96-3; sodium bis(trifluoromethylsulfonyl)imide NaTFSI, CAS number: 91742-21-1.

[0037] Example 1

[0038] This embodiment discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in a mixed solution of 1 mL of ethyltrifluoroethyl carbonate (ETFEC) and 1 mL of fluoroethylene carbonate (FEC), and mixing thoroughly to obtain ester electrolyte 1.

[0039] Example 2

[0040] This embodiment discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in a mixed solution of 1 mL of bis(2,2,2-trifluoroethyl) carbonate (TFEC) and 1 mL of fluoroethylene carbonate (FEC), and mixing thoroughly to obtain ester electrolyte 2.

[0041] Example 3

[0042] This embodiment discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in a mixed solution of 1 mL of 2,2,2-trifluoroethyltrifluoroethyl ester and 1 mL of fluoroethylene carbonate (FEC), and mixing thoroughly to obtain ester electrolyte 3.

[0043] Example 4

[0044] This embodiment discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol of sodium hexafluorophosphate NaPF6 in a mixed solution of 1 mL of ethyl trifluoroethyl carbonate (ETFEC) and 1 mL of fluoroethylene carbonate (FEC), and mixing thoroughly to obtain ester electrolyte 4.

[0045] Example 5

[0046] This embodiment discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol of sodium difluorosulfonyl imide (NaFSI) in a mixed solution of 1 mL of ethyl trifluoroethyl carbonate (ETFEC) and 1 mL of fluoroethylene carbonate (FEC), and mixing thoroughly to obtain ester electrolyte 5.

[0047] Comparative Example 1

[0048] This comparative example discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in a mixed solution of 1 mL of diethyl carbonate (DEC) and 1 mL of fluoroethylene carbonate (FEC), and mixing thoroughly to obtain ester electrolyte 6.

[0049] The CR2032 coin cell was used for evaluation. Four coin cells were assembled in the following order: positive electrode shell, sulfur positive electrode, separator (Whatman, 1823-070), sodium negative electrode, and negative electrode shell. The ester electrolytes obtained in Examples 1-3 and Comparative Example 1 were injected into each cell, and the electrolytes were tested at 200 mA·g. -1 Charge and discharge tests were conducted at a current density of 0.8 to 3.0V, with a cutoff voltage range of 0.8 to 3.0V.

[0050] The battery cycle performance graphs using the ester electrolytes of Examples 1, 2, 3 and Comparative Example 1 are shown below. Figure 1 , 2 As shown in Figures 3 and 4, the electrolyte of Example 1 achieved a reversible capacity of approximately 839 mAh·g after 100 cycles. -1 The electrolyte in Example 2 achieved a reversible capacity of 905 mAh·g after 100 cycles. -1 The electrolyte in Example 3 achieved a reversible capacity of approximately 499 mAh·g after 100 cycles. -1 The electrolyte in Comparative Example 1 achieved a reversible capacity of approximately 574 mAh·g after 100 cycles. -1Therefore, the electrolyte of Example 2 exhibits superior cycle performance, with a higher reversible capacity after 100 cycles. Although 2,2,2-trifluoroethyl trifluoroethyl ester possesses a weak solvation effect induced by fluorine substitution, similar to ethyltrifluoroethyl carbonate (ETFEC) or bis(2,2,2-trifluoroethyl) carbonate (TFEC), it does not play a significant positive role in suppressing polysulfide shuttle in sodium-sulfur batteries. It can be seen that not all solvents with weak solvation effects can effectively suppress polysulfide shuttle and improve battery cycle stability in sodium-sulfur batteries. In this application, it was found that ethyltrifluoroethyl carbonate (ETFEC) and bis(2,2,2-trifluoroethyl) carbonate (TFEC) have a better ability to suppress polysulfide shuttle and improve battery cycle stability, which is related to their structures, with bis(2,2,2-trifluoroethyl) carbonate (TFEC) showing better performance.

[0051] Compared to diethyl carbonate (DEC), electrolytes using a mixture of ethyl trifluoroethyl carbonate (ETFEC) or bis(2,2,2-trifluoroethyl) carbonate (TFEC) and fluoroethylene carbonate (FEC) suppress the dissolution and shuttle of polysulfides, improving sulfur utilization. At the same time, a NaF-rich SEI layer is generated on the negative electrode side, effectively suppressing the growth of sodium dendrites and improving the cycle stability of the full cell.

[0052] Furthermore, although exemplary embodiments have been described herein, their scope includes any and all embodiments based on this application that have equivalent elements, modifications, omissions, combinations (e.g., schemes involving intersections of various embodiments), adaptations, or alterations. Elements in the claims will be interpreted broadly based on the language used in the claims and are not limited to the examples described in this specification or during the implementation of this application, which will be interpreted as non-exclusive. Therefore, this specification and examples are intended to be considered illustrative only, and the true scope and spirit are indicated by the following claims and the full scope of their equivalents.

[0053] The above description is intended to be illustrative and not restrictive. For example, the above examples (or one or more of them) can be used in combination with each other. Other embodiments may be used by those skilled in the art upon reading the above description. Furthermore, in the above detailed description, various features may be grouped together to simplify the application. This should not be construed as an intention that a feature of an unclaimed application is necessary for any claim. Rather, the subject matter of this application may be less than all the features of an embodiment of a particular application. Thus, the following claims are incorporated herein by reference as examples or embodiments, wherein each claim is independently considered as a separate embodiment, and these embodiments are contemplated as being possible in various combinations or arrangements. The scope of the invention should be determined by reference to the appended claims and the full scope of their equivalents.

[0054] The above embodiments are merely exemplary embodiments of this application and are not intended to limit the present invention. The scope of protection of the present invention is defined by the claims. Those skilled in the art can make various modifications or equivalent substitutions to the present invention within the spirit and scope of this application, and such modifications or equivalent substitutions should also be considered to fall within the scope of protection of the present invention.

Claims

1. An ester electrolyte for a sodium-sulfur battery, characterized in that, The ester electrolyte is composed of a first solvent, a second solvent, and a sodium salt. The first solvent is ethyl trifluoroethyl carbonate or bis(2,2,2-trifluoroethyl) carbonate; the second solvent is fluoroethylene carbonate. The volume ratio of the first solvent to the second solvent is 1:0.90~1.

15.

2. The ester electrolyte according to claim 1, characterized in that, The volume ratio of the first solvent to the second solvent is 1:

1.

3. The ester electrolyte according to claim 1, characterized in that, The sodium salt concentration in the electrolyte is 0.8-1.25 mol·L⁻¹.

4. The ester electrolyte according to claim 1, characterized in that, The sodium salt is selected from any one or more of sodium hexafluorophosphate, sodium difluorosulfonamide, or sodium ditrifluoromethylsulfonamide.

5. The ester electrolyte according to claim 4, characterized in that, The sodium salt is sodium bis(trifluoromethanesulfonyl)imide.

6. A method for preparing the ester electrolyte according to any one of claims 1-5, characterized in that, The preparation method includes: dissolving a sodium salt in a first solvent at room temperature, and then adding a second solvent to obtain an ester electrolyte.

7. The application of the ester electrolyte according to any one of claims 1-5 in a sodium-sulfur battery.

8. A sodium-sulfur battery, characterized in that, The sodium-sulfur battery comprises the ester electrolyte, sulfur positive electrode, sodium negative electrode, and separator as described in any one of claims 1-5.

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