An electrolyte containing a bifunctional additive, a preparation method thereof, and a lithium-sulfur battery

Abstract

The present invention provides an electrolyte containing a bifunctional additive, a preparation method thereof, and a lithium-sulfur battery, relating to the technical field of high-energy batteries. The electrolyte includes an ether solvent, a lithium salt, and a bifunctional additive dissolved in the ether solvent. The bifunctional additive is a transition metal bromide-dimethoxyethane complex with a concentration of 0.001-0.02 mol / L. The preparation method of the transition metal bromide-dimethoxyethane complex includes the following steps: refluxing and reacting the transition metal bromide and the dimethoxyethane complex in the presence of a promoter for 20-30 h to obtain the transition metal bromide-dimethoxyethane complex. By using the bifunctional additive to interact with polysulfides in the lithium-sulfur battery, the present invention improves the utilization rate of active substances, inhibits the shuttle effect, and simultaneously forms a stable interface layer on the surface of the lithium negative electrode, thereby improving the positive and negative electrodes of the lithium-sulfur battery and effectively enhancing the rate performance, capacity retention rate, and cycle stability of the lithium-sulfur battery.

Description

Technical Field

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

[0002] In the face of the rapid development of new energy technologies and the increasing global energy demand, it is urgent to develop a new generation of high specific energy lithium-ion battery systems. Due to the rich reserves, low price, and environmental friendliness of sulfur in the cathode material, lithium-sulfur batteries have excellent theoretical specific capacity (1672 mAh·g -1 ) and a theoretical specific energy density as high as 2600 Wh·kg -1 , which is five times that of traditional lithium-ion batteries. Therefore, it is considered a battery system with ideal application prospects.

[0003] However, the multi-phase conversion of lithium-sulfur batteries is complex, and there are safety hazards in the lithium metal anode. During the charge and discharge process, intermediate polysulfides dissolve in the ether-based electrolyte. Under the action of the electric field and concentration gradient, the polysulfides dissolved in the electrolyte migrate back and forth between the positive and negative electrodes, forming a shuttle effect, which greatly reduces the utilization rate of the active material sulfur, resulting in battery capacity decay, low Coulomb efficiency, and lithium anode corrosion. In addition, during the battery cycle, lithium metal can spontaneously react with the electrolyte to form an unstable solid electrolyte interface layer (SEI) on the surface. Due to the repeated rupture and generation of the SEI layer, the current density is unevenly distributed on the surface of the negative electrode, causing uneven deposition of lithium metal and gradually forming dendrites. The volume of lithium dendrites increases infinitely during the cycle. On the one hand, it may pierce the separator and cause battery short circuit. On the other hand, lithium dendrites will also consume a large amount of electrolyte, reducing the utilization rate of lithium metal and Coulomb efficiency. Therefore, it is urgent to provide a solution to improve the above problems. Summary of the Invention

[0004] The purpose of the present invention is to provide an electrolyte containing a bifunctional additive, a preparation method thereof, and a lithium-sulfur battery. By using the bifunctional additive to interact with polysulfides in the lithium-sulfur battery, the utilization rate of active substances is improved, the shuttle effect is inhibited, and at the same time, a stable interface layer is formed on the surface of the lithium negative electrode, thereby improving the positive and negative electrodes of the lithium-sulfur battery and effectively improving the capacity retention rate and long cycle stability of the lithium-sulfur battery.

[0005] In a first aspect, an electrolyte containing a bifunctional additive provided by the present invention includes an ether solvent, a lithium salt, and a bifunctional additive dissolved in the ether solvent. The bifunctional additive is a transition metal bromide-dimethoxyethane complex with a concentration of 0.001-0.02 mol / L. The preparation method of the transition metal bromide-dimethoxyethane complex includes the following steps: refluxing and reacting the transition metal bromide with the dimethoxyethane complex in the presence of a promoter for 20-30 h to obtain the transition metal bromide-dimethoxyethane complex.

[0006] For the electrolyte containing a bifunctional additive provided by the present invention, since the transition metal bromide-dimethoxyethane complex is used as the bifunctional additive, it can interact with polysulfides in the lithium-sulfur battery, thereby accelerating the catalytic conversion of polysulfides and the uniform deposition of Li2S, improving the utilization rate of active substances, inhibiting the shuttle effect, and forming a stable LiBr interface layer on the surface of the lithium negative electrode in the lithium-sulfur battery, alleviating the corrosion of the polysulfides to the lithium negative electrode, and playing a protective role for the lithium negative electrode. Its beneficial effect is that the transition metal bromide-dimethoxyethane complex improves the positive and negative electrodes of the lithium-sulfur battery and can effectively improve the capacity retention rate and long-cycle stability of the lithium-sulfur battery.

[0007] Optionally, the transition metal bromide-dimethoxyethane complex includes one or more of nickel bromide-dimethoxyethane complex, cobalt bromide-dimethoxyethane complex, and iron bromide-dimethoxyethane.

[0008] Optionally, when refluxing and reacting the transition metal bromide with the dimethoxyethane complex in the presence of a promoter, the promoter includes one of triethyl orthoformate, trimethyl orthoformate, methyl formate, ethyl formate, methyl acetate, ethyl acetate, and isopropyl acetate. Its beneficial effect is that the promoter can improve the compatibility of the transition metal bromide and the dimethoxyethane, and can also provide a solvent environment and good reaction conditions for the reaction system, enabling the reaction components to be uniformly mixed and improving the reaction efficiency.

[0009] Optionally, when refluxing and reacting the transition metal bromide with the dimethoxyethane complex in the presence of a promoter, the reflux reaction atmosphere is controlled to be a non-oxidizing atmosphere.

[0010] Optionally, the concentration of the transition metal bromide-dimethoxyethane complex in the electrolyte is 0.003-0.015 mol / L. Its beneficial effect is that within such a concentration range, it is beneficial to be uniformly distributed in the electrolyte, ensuring its solubility in the electrolyte, and thus achieving the best catalytic effect.

[0011] Optionally, the ether solvent includes one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 2-methyltetrahydrofuran, tetrahydrofuran, and 1,3-dioxolane.

[0012] Optionally, the lithium salt includes one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(oxalato)borate, and lithium nitrate.

[0013] Optionally, the lithium salt in the electrolyte is lithium bis(trifluoromethanesulfonyl)imide and lithium nitrate, wherein the concentration of lithium bis(trifluoromethanesulfonyl)imide is 1.0 - 1.5 mol / L, and the concentration of lithium nitrate is 0.1 - 0.2 mol / L. Adding lithium nitrate to the electrolyte can form an interfacial layer on the surface of the lithium negative electrode, which can passivate the active surface of the negative electrode and protect the lithium negative electrode. This interfacial layer can effectively prevent the side reaction between polysulfides in the electrolyte and the lithium negative electrode, avoid the formation of irreversible lithium sulfide on the surface of the lithium negative electrode, and thus further improve the cycle performance and specific capacity of the lithium-sulfur battery.

[0014] Second, the present invention also provides a preparation method for any of the above optional electrolytes, including the following steps: under a non-oxidizing atmosphere, adding a lithium salt and a bifunctional additive to an ether solvent and stirring to obtain an electrolyte.

[0015] Third, the present invention also provides an application of any of the above optional electrolytes in a lithium-sulfur battery. Description of the Drawings

[0016] Figure 1 It is a performance graph of the lithium-sulfur batteries prepared in Example 1 and Comparative Example 1 of the present invention at different rates;

[0017] Figure 2 It is a performance graph of the lithium-sulfur batteries prepared in Examples 2, 3, and 4 of the present invention at different rates;

[0018] Figure 3 It is a performance graph of the lithium-sulfur batteries prepared in Example 1 and Comparative Example 2 of the present invention at different rates;

[0019] Figure 4 It is a cycle performance graph of the lithium-sulfur batteries prepared in Examples 1, 5, and 6 and Comparative Example 1 of the present invention at 0.2C;

[0020] Figure 5 It is a cycle performance graph of the lithium-sulfur batteries prepared in Example 1 and Comparative Example 1 of the present invention at 1C;

[0021] Figure 6 It is a SEM graph of the lithium negative electrode of the lithium-sulfur battery prepared in Example 1 of the present invention after 100 cycles at 0.2C;

[0022] Figure 7 SEM image of the lithium anode of the lithium-sulfur battery prepared in Comparative Example 1 of the present invention after 100 cycles at 0.2C. Detailed implementation manners

[0023] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Apparently, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein shall have the ordinary meanings understood by those of ordinary skill in the art in the field to which the present invention belongs. The words such as "including" used herein mean that the elements or items appearing before this word cover the elements or items listed after this word and their equivalents, without excluding other elements or items.

[0024] The embodiments of the present invention provide an electrolyte containing a bifunctional additive, which includes an ether solvent, a lithium salt and a bifunctional additive dissolved in the ether solvent. The bifunctional additive is a transition metal bromide-dimethoxyethane complex with a concentration of 0.001-0.02 mol / L.

[0025] After applying the electrolyte containing the bifunctional additive in a lithium-sulfur battery, the transition metal bromide-dimethoxyethane complex can interact with polysulfides, promote the catalytic conversion of polysulfides and the uniform deposition of Li2S, improve the utilization rate of active substances, thereby inhibiting the shuttle effect, and forming a stable lithium bromide interface layer on the surface of the lithium anode in the lithium-sulfur battery, alleviating the corrosion effect of polysulfides on the lithium anode. In this way, the capacity retention rate and long-cycle stability of the lithium-sulfur battery can be improved.

[0026] In fact, the preparation method of the transition metal bromide-dimethoxyethane complex includes the following steps: reacting the transition metal bromide with dimethoxyethane under the action of a promoter, and refluxing for 20-30 h to obtain the transition metal bromide-dimethoxyethane complex.

[0027] Specifically, the transition metal bromide can be at least one of nickel bromide, cobalt bromide, iron bromide, copper bromide, vanadium bromide and chromium bromide. In fact, the transition metal bromide can be the bromide of any transition metal element.

[0028] In some embodiments, the transition metal bromide-dimethoxyethane complex can be a single substance, such as one of nickel bromide-dimethoxyethane complex, cobalt bromide-dimethoxyethane complex and iron bromide-dimethoxyethane complex.

[0029] In some embodiments, the promoter may be one of triethyl orthoformate, trimethyl orthoformate, methyl formate, ethyl formate, methyl acetate, ethyl acetate, and isopropyl acetate.

[0030] In some embodiments, when preparing the transition metal bromide-dimethoxyethane complex, the reflux reaction system is controlled to proceed under a non-oxidizing atmosphere. Specifically, the non-oxidizing atmosphere may be a nitrogen atmosphere or an inert gas atmosphere.

[0031] In some embodiments, the concentration of the transition metal bromide-dimethoxyethane complex in the electrolyte containing the bifunctional additive is 0.003 - 0.015 mol / L.

[0032] In some embodiments, the ether solvent includes one or a mixture of solvents such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 2-methyltetrahydrofuran, tetrahydrofuran, and 1,3-dioxolane. In fact, when the ether solvent is a mixture of multiple solutions, for example, when the ether solvent is a mixture of ethylene glycol dimethyl ether and 1,3-dioxolane, ethylene glycol dimethyl ether and 1,3-dioxolane can be mixed in any ratio, and the preferred ratio is 1:1.

[0033] Optionally, the lithium salt further includes one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(oxalato)borate, and lithium nitrate. In fact, the lithium salts in the electrolyte are lithium bis(trifluoromethanesulfonyl)imide and lithium nitrate, where the concentration of lithium bis(trifluoromethanesulfonyl)imide is 1.0 - 1.5 mol / L and the concentration of lithium nitrate is 0.1 - 0.2 mol / L. Adding lithium nitrate to the electrolyte can form an interfacial layer on the surface of the lithium negative electrode that passivates the active surface of the negative electrode and protects the lithium negative electrode. This interfacial layer can effectively prevent the side reaction between polysulfides in the electrolyte and the lithium negative electrode, avoiding the formation of irreversible lithium sulfide on the surface of the lithium negative electrode, thereby further improving the cycle performance and specific capacity of the lithium-sulfur battery.

[0034] The present invention also provides a method for preparing the electrolyte containing the bifunctional additive in any of the above embodiments, including the following steps:

[0035] S0. Prepare the bifunctional additive: Under a non-oxidizing atmosphere, uniformly mix anhydrous transition metal bromide and dimethoxyethane in a promoter, and then heat to 90 - 100 °C for reflux reaction for 20 - 30 h, and then filter to collect the precipitate to obtain the transition metal bromide-dimethoxyethane complex;

[0036] S1. Preparation of electrolyte: Under a non-oxidizing atmosphere, a transition metal bromide-dimethoxyethane complex and a lithium salt are mixed and added to an ether solvent, and stirred in an environment of 20-30 °C until completely dissolved to form a clear solution to obtain the electrolyte.

[0037] Specifically, when performing step S1, the non-oxidizing atmosphere can be a nitrogen atmosphere and an inert gas atmosphere.

[0038] The present invention also provides a lithium-sulfur battery, which contains the electrolyte containing the bifunctional additive in any of the above embodiments.

[0039] Example 1:

[0040] The embodiment 1 of the present invention provides a preparation method of a lithium-sulfur battery, which includes the following steps:

[0041] S0. Preparation of bifunctional additive: In a nitrogen atmosphere, 10 g of anhydrous NiBr2, 120 mL of 1,2-dimethoxyethane (DME), and 5 mL of triethyl orthoformate are uniformly mixed, and then heated to reflux at 90-100 °C for 24 h. The precipitate is collected by filtration to obtain 8.1 g of a brick-red nickel bromide-dimethoxyethane complex, and the yield is calculated to be 81%;

[0042] S1. Preparation of electrolyte: In a glove box filled with argon, ethylene glycol dimethyl ether (DME) and 1,3-dioxolane (DOL) are stirred and mixed at a volume ratio of 1:1. Then, lithium bis(trifluoromethanesulfonyl)imide, lithium nitrate, and nickel bromide-dimethoxyethane complex are successively added to the mixed solution, and stirred in an environment of 25 °C for 8 h until a clear and stable solution is formed, that is, the electrolyte is prepared; among them, the addition amount of lithium bis(trifluoromethanesulfonyl)imide is such that the concentration of lithium bis(trifluoromethanesulfonyl)imide in the electrolyte is 1 mol / L, the addition amount of lithium nitrate is such that the concentration of lithium nitrate in the electrolyte is 0.2 mol / L, and the addition amount of nickel bromide-dimethoxyethane complex is such that the concentration of nickel bromide-dimethoxyethane complex in the electrolyte is 0.005 mol / L;

[0043] S2. Preparation of the positive electrode sheet: Sublimed sulfur powder, conductive carbon black, and a binder (PVDF) are uniformly mixed at a mass ratio of 6:3:1, and then N-methylpyrrolidone (NMP) is added as a solvent for mixing and ball milling to make a conductive paste. The conductive paste is uniformly coated on an aluminum foil and dried, and then cut into a disc with a diameter of 12 mm to obtain the positive electrode sheet;

[0044] S3. Assembly of the lithium-sulfur battery: In a glove box filled with argon, the positive electrode sheet, the separator, and the metallic lithium negative electrode are stacked and pressed in sequence, and the electrolyte is dropped on both sides of the separator, and then sealed to obtain the lithium-sulfur battery (CR2025); among them, the total amount of the electrolyte used is 60 μL.

[0045] Example 2:

[0046] The preparation method of the lithium-sulfur battery provided in Example 2 of the present invention is different from that in Example 1. In step S1, the addition amount of nickel bromide-dimethoxyethane complex is such that the concentration of nickel bromide-dimethoxyethane complex in the electrolyte is 0.003 mol / L.

[0047] Example 3:

[0048] The preparation method of the lithium-sulfur battery provided in Example 3 of the present invention is different from that in Example 1. In step S1, the addition amount of nickel bromide-dimethoxyethane complex is such that the concentration of nickel bromide-dimethoxyethane complex in the electrolyte is 0.008 mol / L.

[0049] Example 4:

[0050] The preparation method of the lithium-sulfur battery provided in Example 4 of the present invention is different from that in Example 1. In step S1, the addition amount of nickel bromide-dimethoxyethane complex is such that the concentration of nickel bromide-dimethoxyethane complex in the electrolyte is 0.015 mol / L.

[0051] Example 5:

[0052] The preparation method of the lithium-sulfur battery provided in Example 5 of the present invention is different from that in Example 1. In step S0 for preparing the bifunctional additive: in a nitrogen atmosphere, 10 g of anhydrous CoBr2, 120 mL of 1,2-dimethoxyethane (DME), and 5 mL of triethyl orthoformate are uniformly mixed and then heated to reflux at 90 - 100 °C for 24 h. The precipitate is filtered and collected to obtain 8.2 g of cobalt bromide-dimethoxyethane complex in blue, and the calculated yield is 82%; in step S1, the concentration of cobalt bromide-dimethoxyethane complex in the electrolyte is 0.005 mol / L.

[0053] Example 6:

[0054] The preparation method of the lithium-sulfur battery provided in Example 6 of the present invention is different from that in Example 1. In step S0 for preparing the bifunctional additive: in a nitrogen atmosphere, 10 g of anhydrous FeBr3, 120 mL of 1,2-dimethoxyethane (DME), and 5 mL of triethyl orthoformate are uniformly mixed and then heated to reflux at 90 - 100 °C for 24 h. The precipitate is filtered and collected to obtain 8.4 g of iron bromide-dimethoxyethane complex in brown, and the calculated yield is 84%; in step S1, the concentration of iron bromide-dimethoxyethane complex in the electrolyte is 0.005 mol / L.

[0055] Comparative Example 1:

[0056] Comparative Example 1 provides a method for preparing a lithium-sulfur battery. The difference from Example 1 is that in Comparative Example 1, step S0 is not carried out, and nickel bromide-dimethoxyethane complex is not added in step S1.

[0057] Comparative Example 2:

[0058] Comparative Example 2 provides a method for preparing a lithium-sulfur battery. The difference from Example 1 is that in Comparative Example 2, step S0 is not carried out, nickel bromide-dimethoxyethane complex is not added in step S1, and nickel bromide is added to make the concentration of nickel bromide in the electrolyte 0.005 mol / L.

[0059] Performance detection:

[0060] The lithium-sulfur batteries prepared in Example 1 and Comparative Example 1 were subjected to electrochemical performance tests at rates of 0.1, 0.2, 0.5, 1, 2, 3, and 4 C (1 C = 1675 mAh / g), and the voltage window was 1.7 - 2.8 V. The test results are as Figure 1 shown.

[0061] The lithium-sulfur batteries prepared in Examples 2, 3, and 4 were subjected to electrochemical performance tests at rates of 0.1, 0.2, 0.5, 1, 2, 3, and 4 C (1 C = 1675 mAh / g), and the voltage window was 1.7 - 2.8 V. The test results are as Figure 2 shown.

[0062] The lithium-sulfur batteries prepared in Example 1 and Comparative Example 2 were subjected to electrochemical performance tests at rates of 0.1, 0.2, 0.5, 1, 2, 3, and 4 C (1 C = 1675 mAh / g), and the voltage window was 1.7 - 2.8 V. The test results are as Figure 3 shown.

[0063] The lithium-sulfur batteries prepared in Examples 1, 5, and 6 and Comparative Example 1 were subjected to cycle performance tests at a rate of 0.2 C. The results are as Figure 4 shown.

[0064] The lithium-sulfur batteries prepared in Example 1 and Comparative Example 1 were subjected to cycle performance tests at a rate of 1 C. The results are as Figure 5 shown.

[0065] See Figure 1It can be seen that the lithium-sulfur battery prepared in Example 1 has better rate performance and higher discharge specific capacity compared with Comparative Example 1. At 0.1, 0.2, 0.5, 1, 2, 3, and 4C rates, the discharge specific capacities of the lithium-sulfur battery in Example 1 are 964.3, 849.7, 790.9, 757.1, 702.0, 661.7, and 602.9 mAh / h, respectively. Therefore, it can be seen that the addition of nickel bromide-dimethoxyethane complex can significantly improve the discharge specific capacity of the lithium-sulfur battery, especially with higher discharge specific capacity at high rates.

[0066] Combined with Figure 1 and Figure 2 It can be seen that after adding nickel bromide-dimethoxyethane complex in Examples 1-4, the rate performance of the lithium-sulfur battery can be significantly improved. At the same time, by comparing the rate curves in Examples 1-4, it can be seen that when the concentration of nickel bromide-dimethoxyethane complex in Example 1 is 0.005 mol / L, the lithium-sulfur battery shows relatively better rate performance. It may be because the concentrations of nickel bromide-dimethoxyethane complex in Examples 3 and 4 are relatively too high, resulting in uneven dispersion of nickel bromide-dimethoxyethane complex in the electrolyte, thus reducing the stability of nickel bromide-dimethoxyethane complex in the electrochemical process and the best catalytic effect. While the concentration of nickel bromide-dimethoxyethane complex in Example 2 is too low to achieve the best catalytic effect.

[0067] Refer to Figure 3 It can be seen that the lithium-sulfur battery prepared in Example 1 has better rate performance and higher discharge specific capacity compared with Comparative Example 2. Thus, it can be known that the addition of nickel bromide dimethoxyethane complex greatly increases its solubility in the electrolyte and exhibits the best catalytic effect in the electrolyte compared with nickel bromide.

[0068] Refer to Figure 4It can be seen that the initial discharge specific capacity of the lithium-sulfur battery prepared in Comparative Example 1 is 685.4 mAh / g. After 100 cycles at 0.2C, its discharge specific capacity decays to 335.9 mAh / g, the capacity retention rate is only 49.0%, and the single-cycle decay rate is 0.5%. While the initial discharge specific capacity of the lithium-sulfur battery prepared in Example 1 is 919.8 mAh / g. After 100 cycles, its discharge specific capacity decays to 821.6 mAh / g, the capacity retention rate is as high as 89.3%, and the single-cycle capacity decay rate is only 0.1%. At the same time, after 100 cycles, the capacity retention rates of the lithium-sulfur batteries prepared in Examples 5 and 6 are 88.2% and 83.0% respectively. It can be seen that due to the presence of the transition metal bromide-dimethoxyethane complex, it can interact with polysulfides, accelerate the catalytic conversion of polysulfides, improve the utilization rate of active substances, thereby inhibiting the shuttle effect, and can also form a stable LiBr interface layer on the surface of the lithium negative electrode, alleviating the corrosion of polysulfides to the lithium negative electrode, thus playing a protective role on the lithium negative electrode. Therefore, the addition of the transition metal bromide-dimethoxyethane complex can effectively improve the capacity retention rate and long-cycle stability of the lithium-sulfur battery.

[0069] See Figure 5 It can be seen that the discharge specific capacity of the lithium-sulfur battery prepared in Example 1 at 1C cycling is significantly higher than that of the lithium-sulfur battery prepared in Comparative Example 1. It is known that the initial discharge specific capacity of the lithium-sulfur battery prepared in Example 1 at 1C is 680.4 mAh / g. After 1000 cycles, the discharge specific capacity remains at 385.3 mAh / g, and its single-cycle capacity decay rate is only 0.04%. It can be seen that adding nickel bromide-dimethoxyethane complex to the electrolyte significantly improves the capacity and cycling performance of the lithium-sulfur battery and can maintain long-cycle stability for 1000 cycles at 1C.

[0070] See Figure 6 and Figure 7 It can be seen that after 100 cycles at 0.2C of the lithium-sulfur battery prepared in Example 1, the surface of the lithium negative electrode is flat and smooth without obvious cracks, while the surface of the lithium negative electrode in Comparative Example 1 shows severe fragmentation and a large number of cracks. Therefore, it can be known that the nickel bromide-dimethoxyethane complex can form a more stable interface layer on the surface of the lithium negative electrode, thereby inhibiting the growth of lithium dendrites and further playing a protective role on the lithium negative electrode.

[0071] Although the embodiments of the present invention have been described in detail above, it is obvious to those skilled in the art that various modifications and changes can be made to these embodiments. However, it should be understood that such modifications and changes fall within the scope and spirit of the present invention as described in the claims. Moreover, the present invention described herein can have other embodiments and can be implemented or realized in various ways.

Claims

1. An electrolyte for a lithium-sulfur battery containing a bifunctional additive, characterized in that, It includes an ether solvent, a lithium salt dissolved in the ether solvent, and a bifunctional additive. The bifunctional additive is a transition metal bromide-dimethoxyethane complex with a concentration of 0.001-0.02 mol / L, and the transition metal bromide-dimethoxyethane complex includes one or more of nickel bromide-dimethoxyethane complex, cobalt bromide-dimethoxyethane complex, and iron bromide-dimethoxyethane complex; The preparation method of the transition metal bromide-dimethoxyethane complex includes the following steps: reflux reacting the transition metal bromide with the dimethoxyethane complex for 20-30 h under a promoter to obtain the transition metal bromide-dimethoxyethane complex; the promoter includes one of triethyl orthoformate, trimethyl orthoformate, methyl formate, ethyl formate, methyl acetate, ethyl acetate, and isopropyl acetate.

2. The electrolyte according to claim 1, wherein When reflux reacting the transition metal bromide with the dimethoxyethane complex under a promoter, control the reflux reaction atmosphere to be a non-oxidizing atmosphere.

3. The electrolyte according to claim 1, wherein The concentration of the transition metal bromide-dimethoxyethane complex in the electrolyte is 0.003-0.015 mol / L.

4. The electrolyte according to claim 1, characterized in that, The ether solvent includes one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 2-methyltetrahydrofuran, tetrahydrofuran, and 1,3-dioxolane as a mixed solvent.

5. The electrolyte according to any one of claims 1 to 4, characterized in that, The lithium salt includes one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(oxalato)borate, and lithium nitrate.

6. The electrolyte according to claim 5, wherein The lithium salt in the electrolyte is lithium bis(trifluoromethanesulfonyl)imide and lithium nitrate, and the concentration of lithium bis(trifluoromethanesulfonyl)imide is 1.0-1.5 mol / L, and the concentration of lithium nitrate is 0.1-0.2 mol / L.

7. A method for preparing an electrolyte according to any one of claims 1 to 6, characterized in that, It includes the following steps: Under a non-oxidizing atmosphere, add the lithium salt and the bifunctional additive to the ether solvent and stir the solvent to obtain the electrolyte.

8. A lithium-sulfur battery, characterized in that, It includes the electrolyte according to any one of claims 1 to 6.

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