Ester electrolyte of sodium-sulfur battery as well as preparation method and application of ester electrolyte

Abstract

The invention belongs to the technical field of electrolyte, and particularly relates to ester electrolyte of a sodium-sulfur battery and a preparation method and application of the ester electrolyte, the ester electrolyte comprises a first solvent, a second solvent and sodium salt, 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, can inhibit dissolution and shuttling of polysulfide, and can also form a NaF-rich SEI film on the sodium negative electrode to inhibit growth of sodium dendrites.

Description

Technical Field

[0001] The present application relates to the technical field of electrolytes, and more specifically, to an ester electrolyte for a sodium-sulfur battery and a preparation method and application thereof. Background Art

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

[0003] The theoretical specific capacity of room temperature sodium-sulfur (RT Na-S) batteries is as high as 1274 Wh kg -1 , and the sulfur positive electrode and sodium negative electrode have abundant reserves and low cost, and are expected to become strong candidates for large-scale energy storage. However, in practical applications, it still faces problems such as slow reaction kinetics of the sulfur positive electrode, shuttling of polysulfides, and dendrite growth caused by unstable sodium negative electrode. In response to these problems, researchers have proposed a variety of coping strategies. For example, physical confinement and chemical adsorption methods are used on the sulfur positive electrode side to anchor polysulfides, thereby inhibiting their dissolution and shuttling; further, polar materials are introduced to catalyze the conversion process of polysulfides to achieve the purpose of accelerating the reaction kinetics. On the negative electrode side, Na is induced by designing a three-dimensional framework and constructing an artificial solid electrolyte interface (SEI). + Uniform deposition can inhibit the growth of sodium dendrites. However, the above strategies only have a positive effect on the sulfur positive electrode or the sodium negative electrode, and cannot modify the sulfur positive electrode and the sodium negative electrode at the same time. Summary of the invention

[0004] The present application is provided to solve the above defects existing in the prior art. An ester electrolyte for a sodium-sulfur battery and a preparation method and application thereof are needed, which can inhibit the dissolution and shuttling of polysulfides and can also inhibit the growth of sodium dendrites by regulating the SEI membrane components.

[0005] In a first aspect of the present application, an ester electrolyte for a sodium-sulfur battery is provided, wherein the ester electrolyte comprises 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] In a second aspect of the present application, a method for preparing the sodium-sulfur battery electrolyte described in any embodiment of the present application is provided, the preparation method comprising: dissolving a sodium salt in a first solvent at room temperature, and then adding a second solvent to obtain an ester electrolyte.

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

[0008] In a fourth aspect of the present application, a sodium-sulfur battery is provided, wherein the sodium-sulfur battery comprises the sodium-sulfur battery electrolyte, a sulfur positive electrode, a sodium negative electrode and a separator as described in any embodiment of the present application.

[0009] The ester electrolyte of the sodium-sulfur battery provided in each embodiment of the present application, and its preparation method and application, the first solvent ethyl trifluoroethyl carbonate and bis (2,2,2-trifluoroethyl) carbonate are obtained by fluorine substitution in diethyl carbonate, which can reduce the solvation ability of the solvent, have a weak solvation effect, and can reduce the solubility of polysulfides in the electrolyte. The ester electrolyte contains sodium salt, so that a NaF-rich SEI film can be formed at the sodium negative electrode, inhibiting the growth of sodium dendrites and improving the battery cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the drawings, which are not necessarily drawn to scale, the same reference numerals may describe similar parts in different views. The same reference numerals with letter suffixes or different letter suffixes may represent different instances of similar parts. The accompanying drawings generally illustrate various embodiments by way of example and not limitation, and together with the specification and claims, are used to illustrate the embodiments applied for. When appropriate, the same reference numerals are used throughout the drawings to refer to the same or similar parts. Such embodiments are illustrative and are not intended to be exhaustive or exclusive embodiments of the present apparatus or method.

[0011] Figure 1 The ester electrolyte according to Example 1 of the present application is assembled into a button cell at 200 mA·g -1 Cycling performance diagram under current density; Figure 2 The ester electrolyte according to Example 2 of the present application is assembled into a button cell at 200 mA·g -1 Cycling performance diagram under current density; Figure 3 The ester electrolyte according to Example 3 of the present application is assembled into a button cell at 200 mA·g -1 Cycling performance diagram under current density; Figure 4 The ester electrolyte according to Comparative Example 1 of the present application is assembled into a button cell at 200 mA·g -1 Cycling performance diagram at different current densities. DETAILED DESCRIPTION

[0012] In order to enable those skilled in the art to better understand the technical solution of the present application, the present application is described in detail below in conjunction with the accompanying drawings and specific implementation examples. The embodiments of the present application are further described in detail below in conjunction with the accompanying drawings and specific implementation examples, but are not intended to limit the present application.

[0013] The words "first", "second" and similar words used in this application do not indicate any order, quantity or importance, but are only used to distinguish. The words "include" or "comprise" and similar words mean that the elements before the word include the elements listed after the word, and do not exclude the possibility of including other elements.

[0014] According to an embodiment of the present application, an ester electrolyte for a sodium-sulfur battery is provided, wherein the ester electrolyte comprises 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.

[0015] The structural formula of diethyl carbonate is: ; The ethyl trifluoroethyl carbonate structural formula is: ; The bis(2,2,2-trifluoroethyl) carbonate structural formula is: ; The structural formula of 2,2,2-trifluoroethyl trifluoroethyl ester is: .

[0016] The reaction mechanism of sodium-sulfur battery is that the positive electrode sulfur combines with sodium ions to reduce to Na2S, and long-chain sodium polysulfide Na2S is generated in the middle. n (n=4, 6, 8), which is easily soluble in the electrolyte and shuttles to the sodium negative electrode, causing the loss of active sulfur and rapid decay of battery capacity. Ethyl trifluoroethyl carbonate is a methyl group on one side of diethyl carbonate replaced by F, bis (2,2,2-trifluoroethyl) carbonate is a methyl group on both sides of diethyl carbonate replaced by F, and 2,2,2-trifluoroethyl trifluoroethyl is a methyl group on both sides of ethyl propionate replaced by F. The first two substances can reduce the solubility of polysulfides and thus inhibit the shuttling of polysulfides.

[0017] The growth of sodium dendrites induced by uneven deposition of sodium ions on the sodium negative electrode 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 avoids the growth of sodium dendrites.

[0018] In some embodiments, the first solvent is ethyl trifluoroethyl carbonate or bis (2,2,2-trifluoroethyl) carbonate. Compared with 2,2,2-trifluoroethyl trifluoroethyl ester, ethyl trifluoroethyl carbonate and bis (2,2,2-trifluoroethyl) carbonate, combined with the second solvent, have a better effect on inhibiting the shuttling of polysulfides. Although 2,2,2-trifluoroethyl trifluoroethyl ester has a weak solvating effect, it cannot effectively inhibit the shuttling of polysulfides.

[0019] The ester electrolyte contains sodium salt and two solvents, which can form a NaF-rich SEI film on the sodium negative electrode, which can effectively inhibit the growth of sodium dendrites and improve the cycle stability of the battery.

[0020] In some embodiments, the volume ratio of the first solvent to the second solvent is 1:0.90-1.15. 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 has better performance in inhibiting polysulfide shuttling.

[0021] In some embodiments, the sodium salt concentration in the electrolyte is 0.8-1.25 mol·L -1 The concentration of sodium salt in the electrolyte helps to cooperate with the two solvents, so that the sodium negative electrode can form a stable solid electrolyte interface film.

[0022] In some embodiments, the sodium salt is selected from any one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, or sodium bis(trifluoromethylsulfonyl)imide. In some embodiments, the sodium salt is sodium bis(trifluoromethylsulfonyl)imide. The sodium salt is combined with the first solvent and the second solvent to form an ester electrolyte that can effectively inhibit polysulfide shuttling and improve reaction kinetics.

[0023] According to an embodiment of the present application, a method for preparing the sodium-sulfur battery electrolyte described in any embodiment of the present application is also provided, the preparation method comprising: dissolving a sodium salt in a first solvent at room temperature, and then adding a second solvent to obtain an ester electrolyte.

[0024] According to an embodiment of the present application, there is also provided an application of the sodium-sulfur battery electrolyte described in any embodiment of the present application or the preparation method described in any embodiment of the present application in a sodium-sulfur battery.

[0025] According to an embodiment of the present application, a sodium-sulfur battery is also provided, comprising the sodium-sulfur battery electrolyte, a sulfur positive electrode, a sodium negative electrode and a separator as described in any embodiment of the present application.

[0026] The raw materials used in the following examples and their CAS numbers 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(trifluorosulfonyl)imide NaFSI, CAS number: 100669-96-3; sodium bis(trifluoromethylsulfonyl)imide NaTFSI, CAS number: 91742-21-1.

[0027] Example 1 This embodiment discloses an ester electrolyte, and the preparation method includes dissolving 2 mmol sodium bis(trifluoromethylsulfonyl)imide NaTFSI in a mixed solution of 1 mL ethyl trifluoroethyl carbonate (ETFEC) and 1 mL fluoroethylene carbonate (FEC), and mixing the mixture thoroughly to obtain an ester electrolyte 1.

[0028] Example 2 This embodiment discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol sodium bistrifluoromethylsulfonyl imide NaTFSI in a mixed solution of 1 mL bis(2,2,2-trifluoroethyl) carbonate (TFEC) and 1 mL fluoroethylene carbonate (FEC), and mixing the mixture thoroughly to obtain an ester electrolyte 2.

[0029] Example 3 This embodiment discloses an ester electrolyte, and the preparation method includes dissolving 2 mmol sodium bis(trifluoromethylsulfonyl)imide NaTFSI in a mixed solution of 1 mL 2,2,2-trifluoroethyl trifluoroethyl ester and 1 mL fluoroethylene carbonate (FEC), and mixing the mixture thoroughly to obtain an ester electrolyte 3.

[0030] Example 4 This embodiment discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol sodium hexafluorophosphate NaPF6 in a mixed solution of 1 mL ethyl trifluoroethyl carbonate (ETFEC) and 1 mL fluoroethylene carbonate (FEC), and mixing the mixture thoroughly to obtain an ester electrolyte 4.

[0031] Example 5 This embodiment discloses an ester electrolyte, and the preparation method includes dissolving 2 mmol sodium bis(fluorosulfonyl)imide NaFSI in a mixed solution of 1 mL ethyl trifluoroethyl carbonate (ETFEC) and 1 mL fluoroethylene carbonate (FEC), and mixing the mixture thoroughly to obtain an ester electrolyte 5.

[0032] Comparative Example 1 This comparative example discloses an ester electrolyte, the preparation method of which includes dissolving 2 mmol sodium bistrifluoromethylsulfonyl imide NaTFSI in a mixed solution of 1 mL diethyl carbonate (DEC) and 1 mL fluoroethylene carbonate (FEC), and mixing the mixture thoroughly to obtain an ester electrolyte 6.

[0033] CR2032 button cells were used for evaluation. Four button cells were assembled in the order of positive electrode shell, sulfur positive electrode sheet, separator (Whatman, 1823-070), sodium negative electrode sheet and negative electrode shell, and the ester electrolytes obtained in Examples 1 to 3 and Comparative Example 1 were injected respectively. The electrolyte was charged at 200 mA g -1 The charge and discharge tests were carried out at a current density of , and the cut-off voltage range was 0.8~3.0V.

[0034] The battery cycle performance diagrams using the ester electrolytes of Examples 1, 2, 3 and Comparative Example 1 are shown as follows: Figure 1 , 2 As shown in Figures 3 and 4, the reversible capacity of the electrolyte of Example 1 reaches about 839 mAh·g after 100 cycles. -1 After 100 cycles, the reversible capacity of the electrolyte in Example 2 reached 905 mAh·g -1 After 100 cycles, the reversible capacity of the electrolyte of Example 3 reached about 499 mAh·g -1 After 100 cycles, the reversible capacity of the electrolyte in Comparative Example 1 reached about 574 mAh g -1. Therefore, the electrolyte of Example 2 has better cycle performance, and the reversible capacity is higher after 100 cycles. Although 2,2,2-trifluoroethyl trifluoroethyl ester has a fluorine substitution-induced weak solvation effect similar to ethyl trifluoroethyl carbonate (ETFEC) or bis (2,2,2-trifluoroethyl) carbonate (TFEC), it does not play a significant positive role in inhibiting polysulfide shuttles in sodium-sulfur batteries. It can be obtained that not all solvents with weak solvation can cooperate with sodium salts in sodium-sulfur batteries to have good ability to inhibit polysulfide shuttles and improve battery cycle stability. In the present application, it was found that ethyl trifluoroethyl carbonate (ETFEC) and bis (2,2,2-trifluoroethyl) carbonate (TFEC) can have good ability to inhibit polysulfide shuttles and improve battery cycle stability, which is related to their structure, and bis (2,2,2-trifluoroethyl) carbonate (TFEC) has a better effect.

[0035] Compared with diethyl carbonate (DEC), the electrolyte using ethyl trifluoroethyl carbonate (ETFEC) or a mixed solvent of bis(2,2,2-trifluoroethyl) carbonate (TFEC) and fluoroethylene carbonate (FEC) inhibits the dissolution and shuttling of polysulfides in it, improves the utilization rate of sulfur, and at the same time, generates a NaF-rich SEI layer on the negative electrode side, effectively inhibits the growth of sodium dendrites and improves the cycle stability of the entire battery.

[0036] In addition, although exemplary embodiments have been described herein, the scope includes any and all embodiments based on the present application with equivalent elements, modifications, omissions, combinations (e.g., various embodiments intersecting schemes), adaptations or changes. The elements in the claims are to be interpreted broadly based on the language adopted in the claims, and are not limited to the examples described in this specification or during the practice of this application, and the examples are to be interpreted as non-exclusive. Therefore, this specification and examples are intended to be considered as examples only, and the true scope and spirit are indicated by the following claims and the full scope of their equivalents.

[0037] The above description is intended to be illustrative rather than restrictive. For example, the above examples (or one or more schemes thereof) may be used in combination with each other. For example, a person of ordinary skill in the art may use other embodiments when reading the above description. In addition, in the above-mentioned specific embodiments, various features may be grouped together to simplify the present application. This should not be interpreted as an intention that a feature of an application that does not require protection is necessary for any claim. On the contrary, the subject matter of the present application may be less than all the features of an embodiment of a particular application. Thus, the following claims are incorporated herein in the specific embodiments as examples or embodiments, wherein each claim is independently a separate embodiment, and it is considered that these embodiments may be combined with each other in various combinations or arrangements. The scope of the present invention should be determined with reference to the appended claims and the full scope of equivalent forms granted by these claims.

[0038] The above embodiments are only exemplary embodiments of the present application and are not intended to limit the present invention. The protection scope of the present invention is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present invention within the essence and protection scope of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the protection scope of the present invention.

Claims

1. An ester electrolyte for a sodium-sulfur battery, characterized in that: The ester electrolyte comprises 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.

2. The ester electrolyte according to claim 1, characterized in that The first solvent is ethyl trifluoroethyl carbonate or bis(2,2,2-trifluoroethyl) carbonate.

3. The ester electrolyte according to claim 1, characterized in that The volume ratio of the first solvent to the second solvent is 1:0.90-1.

15.

4. The ester electrolyte according to claim 3, characterized in that: The volume ratio of the first solvent to the second solvent is 1:

1.

5. 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 .

6. The ester electrolyte according to claim 1, characterized in that: The sodium salt is selected from any one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide or sodium bis(trifluoromethylsulfonyl)imide.

7. The ester electrolyte according to claim 6, characterized in that: The sodium salt is sodium bis(trifluoromethanesulfonyl)imide.

8. The method for preparing an ester electrolyte according to any one of claims 1 to 7, characterized in that: The preparation method comprises: dissolving sodium salt in a first solvent at room temperature, and then adding a second solvent to obtain an ester electrolyte.

9. Use of the ester electrolyte according to any one of claims 1 to 7 or the preparation method of the ester electrolyte according to claim 8 in a sodium-sulfur battery.

10. A sodium-sulfur battery, characterized in that: The sodium-sulfur battery comprises the ester electrolyte according to any one of claims 1 to 7, a sulfur positive electrode, a sodium negative electrode and a separator.

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