Electrolyte, preparation method thereof and lithium-sulfur battery

Abstract

The application belongs to the field of batteries, and particularly relates to an electrolyte, a preparation method thereof and a lithium-sulfur battery. The electrolyte comprises a lithium salt, a solvent and an additive; the additive comprises a sulfonamide solvent with a high electron-withdrawing group and low Lewis basicity. The electrolyte provided by the application contains an additive, the additive comprises a sulfonamide solvent with a high electron-withdrawing group and low Lewis basicity, the high electrochemical and thermal stability and strong non-polar property of the additive are used to inhibit the dissolution of Li PS, the fluorine atom in the fluorosulfamide has a strong electron-withdrawing effect, significantly reduces the coordination ability of the O atom, so that the fluorosulfamide is difficult to coordinate with Li PS, the shuttling of Li PS between the positive and negative electrodes can be effectively inhibited, the solubility of Li PS on the negative side of the lithium-sulfur battery is reduced, the contact and reaction between Li PS and the metal lithium negative electrode are hindered, and the cycle life of the battery is prolonged.

Description

Technical Field

[0001] The present invention belongs to the field of batteries, and in particular relates to an electrolyte, a preparation method thereof, and a lithium-sulfur battery. Background Art

[0002] Lithium-sulfur batteries have a high theoretical capacity (1675 mA hg -1 ) and energy density (2600Wh kg -1 ), and has the characteristics of high natural abundance, environmental protection, non-toxicity, and low price, and is considered to be an ideal choice for the next generation of secondary batteries.

[0003] Lithium-sulfur batteries (LiS batteries) are lithium-ion batteries that use sulfur as the cathode material, lithium metal as the anode, and conventional ethers such as ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (DOL) as the electrolyte. However, the discharge process at the sulfur cathode in conventional ether-based electrolytes undergoes a complex chemical and multiphase electrochemical process involving sulfur redox reactions, known as a "solid-liquid-solid" process, producing high-valent intermediate lithium polysulfides (LiPS). This process is strongly influenced by the solvation structure of the polysulfide anions. Due to the strong solvation capacity of conventional ether-based electrolytes, these intermediate LiPS are highly soluble in most organic liquid electrolytes. Dissolved LiPS can shuttle between the cathode and anode, diffuse onto the lithium metal surface, induce side reactions, and further accelerate lithium corrosion, ultimately leading to low sulfur utilization, poor Coulombic efficiency, rapid capacity decay, and poor cycling stability. Furthermore, due to the inherently slow kinetics of solid-solid reactions, the reduction of insoluble intermediates (Li2S4 and Li2S2) poses a significant obstacle to LiS batteries, resulting in significant capacity reduction.

[0004] Various solutions have been explored to address these issues in lithium-sulfur batteries. However, one of the biggest challenges is selecting suitable electrolyte solutions and the solvation of LiPS in lithium-sulfur batteries. Electrolyte modification is the most convenient and effective method to improve battery performance. The electrolyte acts as a bridge between the sulfur positive electrode and the lithium negative electrode, not only acting as an ion conductor for mass transfer but also extensively participating in the conversion reaction between lithium and sulfur. The different solubility of LiPS in various electrolyte solvents has a significant impact on the electrochemical behavior of lithium-sulfur batteries. Summary of the Invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide an electrolyte and a preparation method thereof and a lithium-sulfur battery.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] An electrolyte comprises a lithium salt, a solvent and an additive; the additive comprises a sulfonamide solvent containing a high electron-withdrawing group and having low Lewis basicity.

[0008] The sulfonamide solvent containing a high electron-withdrawing group and having a low Lewis basicity is one of N,N-dimethyltrifluoromethanesulfonamide and N,N-diethyltrifluoromethanesulfonamide, or a mixture thereof.

[0009] The additive is added to the electrolyte in an amount of 15-30%, preferably 25%.

[0010] The lithium salt includes one or a mixture of at least two of lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium trifluoromethanesulfonate, lithium difluorooxalatoborate, lithium difluorobis(oxalato)phosphate, lithium dioxalatoborate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium nitrate, and lithium perchlorate.

[0011] The lithium salt is a mixture of lithium bis(trifluoromethanesulfonyl imide) and lithium nitrate; preferably, the concentration of lithium bis(trifluoromethanesulfonyl imide) in the electrolyte is 0.5 mol / L-2 mol / L, preferably 1 mol / L; the mass concentration of lithium nitrate in the electrolyte is 1-5%, preferably 2%.

[0012] The solvent is an ether organic solvent; preferably one or a mixture of at least two of 1,3-dioxolane, 1,4-dioxane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

[0013] The solvent is a mixture of 1,3-dioxolane DOL and ethylene glycol dimethyl ether DME; preferably, the volume ratio of 1,3-dioxolane to ethylene glycol dimethyl ether is (0.5-1.5):1; more preferably 1:1.

[0014] The present invention also includes a method for preparing the electrolyte, comprising the following steps: mixing a solvent and a lithium salt in a glove box, then adding an additive, and stirring thoroughly to obtain the electrolyte.

[0015] The present invention also includes a lithium-sulfur battery comprising the electrolyte.

[0016] The lithium-sulfur battery comprises a positive electrode, a negative electrode, a separator and an electrolyte; the separator is a polypropylene film; the negative electrode is lithium metal;

[0017] Preferably, the positive electrode is prepared by the following method: grinding and mixing sulfur and a conductive agent, adding a binder, mixing, and then adding N-methylpyrrolidone to form a homogenous paste to form a positive electrode slurry; coating it on a carbon-coated aluminum foil, and drying it; the conductive agent is carbon nanotubes; the binder is PVDF; the mass ratio of the sulfur, conductive agent, and binder is 7:3:1.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The electrolyte provided by the present invention contains additives, which include sulfonamide solvents containing high electron-withdrawing groups and low Lewis basicity. The additives utilize their high electrochemical and thermal stability and strong non-polar properties to inhibit the dissolution of LiPS. The fluorine atoms in the fluorosulfonamide have a strong electron-withdrawing effect, which significantly reduces the coordination ability of the O atoms, thereby making it difficult for the fluorosulfonamide to coordinate with LiPS, effectively inhibiting the shuttling of LiPS between the positive and negative electrodes, thereby reducing the solubility of LiPS on the negative electrode side of the lithium-sulfur battery, hindering the contact and reaction between LiPS and the metallic lithium negative electrode, and extending the cycle life of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Graph showing the cyclic stability test results of Examples 1, 7 and Comparative Example 1;

[0021] Figure 2 Graph showing the rate performance test results of Examples 1, 7 and Comparative Example 1. DETAILED DESCRIPTION

[0022] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and the best embodiments.

[0023] Example 1

[0024] This embodiment provides a method for preparing a low-solvation electrolyte and a lithium-sulfur battery containing the same:

[0025] 1. Electrolyte configuration

[0026] In a glove box, the organic solvent DOL:DME was mixed with Li TFS I (1.0 M) in a volume ratio of 1:1, and then 2% of Li NO3 (an auxiliary additive, based on the total weight of the electrolyte) and 25% of N,N-dimethyltrifluoromethanesulfonamide additive (DOL:DME:N,N-dimethyltrifluoromethanesulfonamide volume ratio of 1.5:1.5:1) were added and stirred thoroughly to obtain the low-solvate lithium-sulfur battery electrolyte of the present invention.

[0027] 2. Preparation of Sulfur Cathode

[0028] Sulfur and carbon nanotubes (7:3 by mass) were ground and mixed, and PVDF binder (9:1 by mass) was added. An appropriate volume of N-methylpyrrolidone (NMP) was then added and stirred in a homogenizer for 15 minutes to form a stable and uniform cathode slurry. This slurry was then coated onto carbon-coated aluminum foil using a spatula and dried in a 60°C oven for 12 hours until the NMP evaporated completely.

[0029] 3. Lithium-sulfur button battery

[0030] In an argon-filled glove box, a lithium-sulfur battery was assembled using a polypropylene film (PP) as a separator, a sulfur / carbon composite material as a positive electrode, and lithium metal as a negative electrode. The above electrolyte was added to the battery cycler. The charge and discharge cycle test was carried out on a Neware battery cycler. The test conditions were constant current 0.1C charge and discharge, potential range 1.7-2.6V, and 100 cycles. Figure 1 As shown. The rate performance of lithium-sulfur batteries was evaluated at 0.1C, 0.2C and 0.3C. Figure 2 shown.

[0031] Examples 2-8 and Comparative Example 1

[0032] Compared with Example 1, the only difference is that the added components and contents of the electrolyte are different (see Table 1 for details), and the other parameters and preparation methods are the same as those of Example 1.

[0033] Table 1

[0034]

[0035]

[0036] Table 2 shows the test results of various embodiments and comparative examples;

[0037] Table 2

[0038]

[0039]

[0040] The test results show that compared with Comparative Example 1, the initial discharge specific capacities of Example 1 and Example 7 at a discharge rate of 0.1C are 1069 mAh / g and 1080 mAh / g, respectively, which are better than Comparative Example 1. At different rates of 0.1C, 0.2C, and 0.3C, Example 1 and Example 7 exhibit good rate performance.

[0041] The initial discharge specific capacity in Examples 1 to 8 increased by 70 to 120 mAh / g compared to Comparative Example 1, and the 100-cycle performance also increased from 49.3% to 61.4 to 81.3%.

[0042] Thus, the addition of the additive of the present invention can significantly improve the discharge specific capacity and cycle capacity retention rate. Through comparison of the examples, it is found that when the addition amount of the additive of the present invention is controlled at the preferred level of 15-30%, the addition effect can be further improved, especially when the addition amount is 25%, the addition effect can be further improved.

[0043] In summary, the electrolyte provided by the present invention contains additives, and the additives include sulfonamide solvents containing high electron-withdrawing groups and low Lewis basicity, which utilize their high electrochemical and thermal stability and strong non-polar properties to inhibit the dissolution of LiPS. The fluorine atoms in fluorosulfonamide have a strong electron-withdrawing effect, which significantly reduces the coordination ability of O atoms, thereby making it difficult for fluorosulfonamide to coordinate with LiPS, and can effectively inhibit the shuttling of LiPS between the positive and negative electrodes, thereby reducing the solubility of LiPS on the negative electrode side of the lithium-sulfur battery, hindering the contact and reaction between LiPS and the metallic lithium negative electrode, and extending the cycle life of the battery.

[0044] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. An electrolyte for a lithium-sulfur battery, characterized in that: The invention comprises a lithium salt, a solvent and an additive; the additive comprises a sulfonamide solvent containing a high electron-withdrawing group and having a low Lewis basicity; the sulfonamide solvent containing a high electron-withdrawing group and having a low Lewis basicity is one of N,N-dimethyltrifluoromethanesulfonamide and N,N-diethyltrifluoromethanesulfonamide, or a mixture thereof; The additive is added to the electrolyte in an amount of 15-30%; The lithium salt is a mixture of lithium bis(trifluoromethanesulfonyl)imide and lithium nitrate; the mass concentration of lithium nitrate in the electrolyte is 1-5%.

2. The electrolyte for lithium-sulfur batteries according to claim 1, characterized in that The additive is added in an amount of 25% in the electrolyte.

3. The electrolyte for lithium-sulfur batteries according to claim 1, characterized in that The concentration of lithium bis(trifluoromethanesulfonyl)imide in the electrolyte is 0.5 mol / L-2 mol / L.

4. The electrolyte for lithium-sulfur batteries according to claim 1, characterized in that The concentration of lithium bis(trifluoromethanesulfonyl)imide in the electrolyte is 1 mol / L.

5. The electrolyte for lithium-sulfur batteries according to claim 1, characterized in that The mass concentration of lithium nitrate in the electrolyte is 2%.

6. The electrolyte for lithium-sulfur batteries according to claim 1, characterized in that The solvent is an ether organic solvent.

7. The electrolyte for lithium-sulfur batteries according to claim 1, characterized in that The solvent is one of 1,3-dioxolane, 1,4-dioxane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, or a mixture of at least two of them.

8. The electrolyte for lithium-sulfur batteries according to claim 1, characterized in that The solvent is a mixed solution of 1,3-dioxolane DOL and ethylene glycol dimethyl ether DME.

9. The electrolyte for lithium-sulfur batteries according to claim 8, characterized in that The volume ratio of 1,3-dioxolane and ethylene glycol dimethyl ether is (0.5-1.5):

1.

10. The electrolyte for lithium-sulfur batteries according to claim 8, characterized in that The volume ratio of 1,3-dioxolane and ethylene glycol dimethyl ether is 1:

1.

11. A method for preparing the electrolyte for lithium-sulfur batteries according to any one of claims 1 to 10, characterized in that: The method comprises the following steps: mixing a solvent and a lithium salt in a glove box, then adding an additive, and fully stirring the mixture to obtain an electrolyte.

12. A lithium-sulfur battery, characterized in that: The invention comprises the electrolyte for lithium-sulfur batteries according to any one of claims 1 to 10.

13. The lithium-sulfur battery according to claim 12, characterized in that The invention comprises a positive electrode, a negative electrode, a separator and an electrolyte; the separator is a polypropylene film; and the negative electrode is lithium metal.

14. The lithium-sulfur battery according to claim 13, characterized in that The positive electrode is prepared by grinding and mixing sulfur and a conductive agent, adding a binder, mixing, and then adding N-methylpyrrolidone to form a positive electrode slurry; coating the slurry on a carbon-coated aluminum foil, and drying the slurry.

15. The lithium-sulfur battery according to claim 14, characterized in that The conductive agent is carbon nanotubes; the binder is PVDF; and the mass ratio of the sulfur, the conductive agent, and the binder is 7:3:1.

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