Electrolyte for lithium-sulfur battery as well as preparation method and application of electrolyte

Abstract

The invention discloses an electrolyte for a lithium-sulfur battery as well as a preparation method and application of the electrolyte, and relates to the technical field of batteries. The electrolyte comprises a lithium salt electrolyte, an organic additive and an organic solvent; the organic additive comprises an amide organic compound, and the amide organic compound has the following structural formula: # imgabs0 #; r1, R2, R3, R4, R5, R6 and R7 are at least one of halogen atoms and hydrogen atoms, and the halogen atoms comprise F, Cl, Br and I; the preparation method comprises the following steps: adding the lithium salt electrolyte and the organic additive into the organic solvent in a glove box filled with argon, and uniformly mixing to obtain the electrolyte. The additive has the beneficial effects that the shuttling of polysulfide in the lithium-sulfur battery can be improved and eliminated by adding the additive, the impedance of the battery and the surface tension of the electrolyte are reduced, and the wettability between the electrolyte and a diaphragm and the cycle performance and the rate capability of the lithium-sulfur battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to an electrolyte for a lithium-sulfur battery, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-sulfur batteries have a high theoretical specific energy and are considered to be one of the next-generation energy storage systems with great development prospects. However, the problems of polysulfide generated at the sulfur cathode and the interfacial instability of the lithium anode seriously hinder the practical application of lithium-sulfur batteries. There are complex multiphase transformations during the charge and discharge process of lithium-sulfur batteries. The generated polysulfides are easily soluble in the electrolyte, causing the shuttle effect and reducing the utilization rate of active sulfur. At the same time, the polysulfides shuttle to the lithium anode, leading to the corrosion of metallic lithium and reducing the stability of the lithium anode. During the lithium plating / stripping process, the metallic lithium deposits unevenly, and the generated lithium dendrites may even pierce the separator, resulting in short circuits in the battery and causing safety problems.

[0003] Therefore, in order to solve the above technical problems in the prior art, it is urgent to optimize the components of the electrolyte. By introducing electrolyte additives into the traditional electrolyte of lithium-sulfur batteries, further explore the mechanism of action and electrochemical performance of electrolyte additives in regulating the positive and negative electrodes of lithium-sulfur batteries. Summary of the Invention

[0004] The purpose of the present invention is to solve at least one of the technical problems existing in the prior art, and provide an electrolyte for a lithium-sulfur battery, a preparation method thereof, and an application thereof, so as to solve the problem of poor cycling performance of lithium-sulfur batteries.

[0005] The technical solution of the present invention is as follows: The first aspect of the present invention provides an electrolyte for a lithium-sulfur battery, comprising a lithium salt electrolyte, an organic additive, and an organic solvent; The organic additive includes an amide organic compound, and the amide organic compound has the following structural formula: ; Wherein, R1, R2, R3, R4, R5, R6, and R7 are at least one of a halogen atom and a hydrogen atom, and the halogen atom includes F, Cl, Br, and I.

[0006] By adding an amide organic compound as an organic additive to the electrolyte, the present invention can improve and solve the problem of poor cycling performance of lithium-sulfur batteries. Specifically, first, the nitrogen atom in the amide additive is electron-rich and there is a strong interaction with the sulfur-containing lithium salt, which can form a stable Li-N bond. Among them, as the number of fluorine atoms participating in the electronic effect increases, the interaction strength is effectively enhanced. The addition of the amide additive helps to improve the interfacial properties of the battery, reduce the charge transfer resistance and interfacial resistance, and promote faster kinetic behavior. Second, the nitrogen in the amide additive coordinates with the lithium in the polysulfide, providing a favorable pathway for electron transfer, thereby improving the reaction rate and current density. Third, the battery with the amide additive can provide a higher capacity for the lithium-sulfur battery and promote the conversion of polysulfides. Fourth, the intermediate formed by the amide compound additive and the polysulfide will exhibit higher conductivity and faster electron transfer, thereby improving the performance and redox kinetic behavior of the lithium-sulfur battery.

[0007] In a preferred embodiment of the present invention, R1, R2, R3, R4, R5, R6, and R7 are all F or Cl or Br or I.

[0008] In a preferred embodiment of the present invention, the amide organic compound is heptafluorobutyramide.

[0009] In a preferred embodiment of the present invention, the molar concentration of the lithium salt electrolyte in the electrolyte is 0.1 to 4 mol / L, and the molar concentration of the organic additive in the electrolyte is 1 mmol / L to 10 mmol / L. Preferably, the molar concentration of the lithium salt electrolyte in the electrolyte is 0.1 to 2 mol / L, and the molar concentration of the organic additive in the electrolyte is 1 mmol / L to 5 mmol / L. More preferably, the molar concentration of the lithium salt electrolyte in the electrolyte is 1 to 2 mol / L, and the molar concentration of the organic additive in the electrolyte is 2 mmol / L to 4 mmol / L.

[0010] In a preferred embodiment of the present invention, the lithium salt electrolyte includes a fluorinated lithium salt and lithium nitrate, and the fluorinated lithium salt is at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonate, and lithium hexafluorophosphate.

[0011] In a preferred embodiment of the present invention, the molar ratio of the fluorinated lithium salt to lithium nitrate is (2 to 10):1. Preferably, the molar ratio of the fluorinated lithium salt to lithium nitrate is (4 to 10):1; more preferably, the molar ratio of the fluorinated lithium salt to lithium nitrate is (6 to 10):1. Specifically, it can be 10:1.

[0012] In a preferred embodiment of the present invention, the organic solvent comprises a first organic component and a second organic component with a volume ratio of (1~5):2; The first organic component comprises at least one of 1,3-dioxolane (DOL), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and dimethyl trisulfide (DMTS); The second organic component comprises at least one of ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether (DEE), and ethylene glycol dibutyl ether (DBE).

[0013] Preferably, the volume ratio of the first organic component to the second organic component is 1~3:2. The volume ratio of the first organic component to the second organic component is 1:1.

[0014] A second aspect of the present invention provides a method for preparing an electrolyte, comprising the following steps: In a glove box filled with argon, a lithium salt electrolyte and an organic additive are added to the organic solvent and mixed evenly to obtain the electrolyte.

[0015] A third aspect of the present invention provides a lithium-sulfur battery comprising an electrolyte, and the electrolyte is the above-mentioned electrolyte.

[0016] A preferred embodiment of the present invention further comprises a positive electrode and a negative electrode. The positive electrode material of the lithium-sulfur battery comprises sublimed sulfur, and the negative electrode material is lithium metal. The method for preparing the positive electrode comprises: Mix sublimed sulfur, multi-walled carbon nanotubes, acetylene black, and PVDF, and then add N-methylpyrrolidone for ball milling to form a stable and uniform positive electrode slurry; Apply the positive electrode slurry evenly on an aluminum foil paper and perform vacuum drying treatment to obtain the positive electrode; Among them, the ratio of sublimed sulfur, multi-walled carbon nanotubes, acetylene black, and PVDF is 5~7:1.5~2.5:1:1; The vacuum degree of the vacuum drying treatment is 9~11 kPa, the drying temperature is 55~65 °C, and the drying time is 5~7 h.

[0017] The present invention has at least one of the following beneficial effects: 1. An amide organic compound is added to the electrolyte of the present invention. By adding the above-mentioned additive, the shuttling of polysulfides in the lithium-sulfur battery can be improved and eliminated, the battery impedance and the surface tension of the electrolyte can be reduced, the wettability between the electrolyte and the separator can be improved, and the cycle performance and rate performance of the lithium-sulfur battery can be enhanced.

[0018] 2. The lithium salt electrolyte of the present invention includes a fluorinated lithium salt and lithium nitrate. The addition of lithium nitrate is to form a synergistic effect with the organic additive amide organic compound. On the one hand, the synergistic effect of the two helps to form a denser and more stable solid electrolyte interface (i.e., SEI film) on the lithium anode of the lithium-sulfur battery. For the battery assembled with the electrolyte without adding the amide organic compound of the present invention, lithium dendrites will be formed in the lithium anode, and the piercing of the separator by the dendrites will cause the battery to short-circuit. Therefore, the amide organic compound and lithium nitrate can synergistically extend the battery life. On the other hand, the synergistic effect of the two can reduce the shuttle effect caused by polysulfides, and the shuttle effect will lead to the attenuation of the battery capacity. Therefore, the amide organic compound and lithium nitrate can synergistically and effectively improve the discharge specific capacity.

[0019] 3. The organic solvent of the present invention includes a first organic component and a second organic component. Compared with the prior art that uses a single solvent, the first organic component and the second organic component of the present invention improve the cycle stability and capacity retention rate of the battery through synergistic effects.

[0020] 4. It has been verified that the battery assembled in the embodiment of the present invention has an initial capacity of 1001.84 mAh•g at 0.2C −1 , and after 150 cycles, the capacity retention rate is about 71.7%, while the capacity retention rate without addition is about 50.5%. This shows that adding amide organic compounds to the electrolyte helps to improve the cycle performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Examples 1-6 all showed different capacity retention rates after 150 cycles at 0.2C.

[0022] Figure 2 Examples 3 and the comparative example all showed different capacity retention rates after 150 cycles at 0.2C. DETAILED DESCRIPTION OF THE INVENTION

[0023] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] Example 1 (1)Electrolyte preparation: In a glove box filled with argon and with the water and oxygen content values both less than 1 ppm, the weighed LiTFI (lithium bis(trifluoromethanesulfonyl)imide, 1.0 M), LiNO3 (0.1 M), and heptafluorobutyramide (1 mM) were added to an organic solvent composed of ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (DOL) with a volume ratio of 1:1; stirred for 2 h to make it evenly mixed, thus obtaining the lithium-sulfur battery electrolyte; (2)Preparation of sulfur cathode: Sublimed sulfur, multi-walled carbon nanotubes, acetylene black, and PVDF were mixed according to a mass ratio of 6:2:1:1, and then an appropriate volume of N-methylpyrrolidone (NMP) was added and placed in a planetary ball mill and stirred at a speed of 300 r / min for 4 h to form a stable and uniform cathode slurry. The ball-milled cathode slurry was evenly coated on aluminum foil paper and placed in a vacuum drying oven for drying treatment (vacuum degree: 10000 Pa, drying temperature: 60 °C, drying time: 6 h).

[0025] (3)Battery assembly: A lithium metal sheet was taken as the negative electrode material, a polypropylene porous membrane was taken as the separator, and the lithium-sulfur battery electrolyte and sulfur cathode prepared in the previous steps were taken. The coin cell was made in the glove box.

[0026] Examples 2-6 and the comparative examples were all repeated in the same steps as Example 1, with the only difference being that the molar concentration of heptafluorobutyramide was changed or heptafluorobutyramide was not added, and the other steps were the same as those in Example 1. And the components of the lithium-sulfur battery electrolytes in Examples 2-6 and the comparative examples were carried out as shown in Table 1.

[0027] Table 1 Components of lithium-sulfur battery electrolytes in other examples and comparative examples Experimental result verification: The batteries prepared in Examples 1-6 and the comparative examples were placed on a charge-discharge instrument in a thermostat for testing, and cyclic testing was carried out at a current density of 0.2C, and the cut-off voltage range was 1.7~2.8 V. The cyclic performance of various lithium-sulfur batteries was measured, and the results are shown in Table 2 and Figures 1 - 2 as follows.

[0028] Table 2 Test results of the comparative example and Examples 1-6 after 150 cycles at 0.2C From Table 2, Figure 1 and Figure 2It can be seen that Examples 1-6 all showed different capacity retention rates after 150 cycles at 0.2C, and the capacity retention rates were between 64.2% and 71.7%, indicating good cycling performance. Compared with the comparative examples, a significant improvement in the capacity retention rate was observed after adding the amide compound-containing substance. Among them, Example 3 was particularly prominent. When 3 mmol / L of heptafluorobutyramide was added to the lithium-sulfur battery, the initial capacity at 0.2C was 1001.84 mAh•g −1 , and after 150 cycles, the capacity retention rate was about 71.7%. For the comparative example without heptafluorobutyramide added, the capacity retention rate was about 50.5%. This shows that the lithium-sulfur battery containing heptafluorobutyramide exhibits excellent cycling performance, indicating that adding heptafluorobutyramide to the electrolyte helps improve the cycling performance of the battery. In addition, in the comparison of different concentrations of heptafluorobutyramide, the concentration of 3 mmol / L was the best. The test results of Examples 1-6 can draw a conclusion. It can be found through the examples and comparative examples that when the additive is in the preferred range of 1-10 mmol / L, the addition effect can be improved. Especially when the addition amount is 3 mmol / L, the addition effect can be better improved, and the discharge specific capacity and cycling stability performance can be further improved.

[0029] As described above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its inventive concept, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. An electrolyte for a lithium-sulfur battery, characterized in that it comprises a lithium salt electrolyte, an organic additive and an organic solvent; the organic additive comprises an amide organic compound, and the amide organic compound has the following structural formula: ； wherein, R1, R2, R3, R4, R5, R6 and R7 are at least one of a halogen atom and a hydrogen atom, and the halogen atom includes F, Cl, Br and I.

2. The electrolyte according to claim 1, wherein The R1, R2, R3, R4, R5, R6 and R7 are all F or Cl or Br or I.

3. The electrolyte according to claim 1, wherein The amide organic compound is heptafluorobutyramide.

4. The electrolyte according to claim 1, wherein The molar concentration of the lithium salt electrolyte in the electrolyte is 0.1~4 mol / L, and the molar concentration of the organic additive in the electrolyte is 1 mmol / L ~10 mmol / L.

5. The electrolyte according to claim 1, characterized in that, The lithium salt electrolyte includes a fluorinated lithium salt and lithium nitrate, and the fluorinated lithium salt is at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonate and lithium hexafluorophosphate.

6. The electrolyte according to claim 5, characterized in that, The molar ratio of the fluorinated lithium salt to lithium nitrate is (2~10):

1.

7. The electrolyte according to claim 1, characterized in that the organic solvent includes a first organic component and a second organic component with a volume ratio of (1~5):2; the first organic component includes at least one of 1,3-dioxolane, ethylene carbonate, dimethyl carbonate, diethyl carbonate and dimethyl trisulfide; the second organic component includes at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether and ethylene glycol dibutyl ether.

8. The preparation method of the electrolyte according to any one of claims 1 to 7, characterized in that, It includes the following steps: In a glove box filled with argon, add the lithium salt electrolyte and the organic additive to the organic solvent and mix evenly to obtain the electrolyte.

9. A lithium-sulfur battery, characterized in that, It includes an electrolyte, and the electrolyte is the electrolyte according to any one of claims 1~7.

10. The lithium-sulfur battery according to claim 9, wherein It further includes a positive electrode and a negative electrode, and the preparation method of the positive electrode includes: Mix sublimed sulfur, multi-walled carbon nanotubes, acetylene black, and PVDF, and then add N-methylpyrrolidone for ball milling to form a stable and uniform positive electrode slurry; Apply the positive electrode slurry evenly on an aluminum foil paper and perform vacuum drying treatment to obtain the positive electrode; wherein, the ratio of sublimed sulfur, multi-walled carbon nanotubes, acetylene black, and PVDF is 5~7: 1.5~2.5: 1: 1; The vacuum degree of the vacuum drying treatment is 9~11 kPa, the drying temperature is 55~65 °C, and the drying time is 5~7 h.

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