Lithium-sulfur battery electrolyte, lithium-sulfur battery and activation method thereof

Abstract

The present application discloses a lithium-sulfur battery electrolyte, a lithium-sulfur battery and an activation method thereof. The lithium-sulfur battery electrolyte includes a solvent, a lithium salt and a nitrate additive. Among them, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms; in the electrolyte, the mass concentration of the nitrate additive is 1.0% to 8.0%, and the molar concentration of the lithium salt is 0.2 mol / L to 2.0 mol / L. For the lithium-sulfur battery electrolyte provided by the present application, the nitrate additive therein can generate a solid electrolyte interface rich in nitrogen oxides on the surface of the lithium metal negative electrode, inhibit the parasitic reaction between polysulfides and the lithium metal negative electrode and improve the uniformity of lithium deposition and stripping, thereby improving the cycle life of the lithium-sulfur battery.

Description

Technical Field

[0001] This application belongs to the technical field of rechargeable high specific energy secondary batteries, and particularly relates to a lithium-sulfur battery electrolyte, a lithium-sulfur battery, and an activation method thereof. Background Art

[0002] In the process of developing renewable energy, energy storage devices can be used to store and convert intermittent renewable energy. At present, energy storage devices are usually selected from lithium-ion batteries. Compared with traditional lithium-ion batteries, lithium-sulfur batteries have extremely high theoretical energy density (2600 Wh / kg) and actual energy density (>500 Wh / kg), and are a promising next-generation secondary battery system. However, the short cycle life of lithium-sulfur batteries is a bottleneck restricting their application. Summary of the Invention

[0003] Embodiments of this application provide a lithium-sulfur battery electrolyte, a lithium-sulfur battery, and an activation method thereof, to solve the technical problem of the short cycle life of existing lithium-sulfur batteries.

[0004] In a first aspect, embodiments of this application provide a lithium-sulfur battery electrolyte. The electrolyte includes a solvent, a lithium salt, and a nitrate additive. Among them, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms; in the electrolyte, the mass concentration of the nitrate additive is 1.0% - 8.0%, and the molar concentration of the lithium salt is 0.2 mol / L - 2.0 mol / L.

[0005] In some embodiments of this application, the nitrate additive is selected from one or more of butyl nitrate, amyl nitrate, isoamyl nitrate, isosorbide dinitrate, and pentaerythritol tetranitrate, preferably isosorbide dinitrate or butyl nitrate.

[0006] In some embodiments of this application, the solvent is selected from one or more of ethylene glycol dimethyl ether, 1,3-dioxolane, 1,2-dimethoxypropane, 1,3-dimethoxypropane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and tetrahydrofuran.

[0007] In some embodiments of this application, the lithium salt is selected from one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorooxalate borate, and lithium bis(oxalate).

[0008] In some embodiments of this application, the mass concentration of the nitrate additive is 3.0% - 6.0%, preferably 5.0% - 6.0%.

[0009] In some embodiments of this application, the molar concentration of the lithium salt is 0.5 mol / L - 1.0 mol / L.

[0010] In a second aspect, an embodiment of the present application provides a lithium-sulfur battery, which includes the lithium-sulfur battery electrolyte, a positive electrode, and a negative electrode in any one of the above embodiments.

[0011] In some embodiments of the present application, the number of cycles of the lithium-sulfur battery at a rate of 0.1C is greater than or equal to 70 cycles, preferably greater than or equal to 90 cycles, and more preferably greater than or equal to 100 cycles.

[0012] In a third aspect, an embodiment of the present application provides a method for activating a lithium-sulfur battery, including the following steps:

[0013] Providing and standing still the lithium-sulfur battery according to any one of the embodiments in the second aspect of the present application;

[0014] Performing constant current charge and discharge on the standing still lithium-sulfur battery, and performing pre-cycling at a rate of 0.03C to 0.07C to activate the lithium-sulfur battery.

[0015] In some embodiments of the present application, the number of pre-cycling is 2 to 3 cycles, preferably 2 cycles.

[0016] In some embodiments of the present application, the standing still time of the lithium-sulfur battery is 10h to 15h, preferably 12h.

[0017] In the lithium-sulfur battery electrolyte provided by the embodiment of the present application, the electrolyte includes a solvent, a lithium salt, and a nitrate additive. Among them, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms. First, the nitrate additive has a high reaction activity. The nitrate additive in the lithium-sulfur battery electrolyte can generate a solid electrolyte interface rich in nitrogen oxides on the surface of the metallic lithium negative electrode, inhibit the parasitic reaction between the polysulfide and the metallic lithium negative electrode, thereby avoiding the rapid failure of the metallic lithium negative electrode and improving the cycle life of the lithium-sulfur battery. Second, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms, and the solid electrolyte interface film formed by it is rich in nitrogen oxides. Nitrogen oxides are beneficial to the uniform transport of lithium ions, can promote the uniform deposition and extraction of lithium ions, and thus ensure the stability of the long-term cycle of the lithium-sulfur battery. In addition, the organic part in the nitrate additive molecule has good compatibility with the solvent in the electrolyte, has a high solubility in the electrolyte, has little influence on the ionic conductivity, and is easy to prepare the electrolyte. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required to be used in the embodiments of the present application. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative efforts.

[0019] Figure 1It is the cycling performance graph of the lithium-sulfur battery of Example 1 and Comparative Example 1 of the present application;

[0020] Figure 2 It is the graph of the constant current titration test results of the first-cycle discharge curve of the lithium-sulfur battery of Example 1 of the present application;

[0021] Figure 3 It is the graph of the constant current titration test results of the first-cycle discharge curve of the lithium-sulfur battery of Comparative Example 1 of the present application;

[0022] Figure 4 It is the effect diagram of the electrolyte of Example 1 of the present application;

[0023] Figure 5 It is the effect diagram after the electrolyte of Example 1 of the present application reacts with polysulfide;

[0024] Figure 6 It is the X-ray photoelectron spectrum of the solid electrolyte interface film formed in Example 1 of the present application;

[0025] Figure 7 It is the electron microscope image of the lithium deposition morphology of Example 1 of the present application;

[0026] Figure 8 It is the electron microscope image of the lithium deposition morphology of Comparative Example 1 of the present application. Detailed Description of the Invention

[0027] The features and exemplary embodiments of various aspects of the present application will be described in detail below. In the following detailed description, many specific details are set forth in order to provide a thorough understanding of the present application. However, it will be apparent to those skilled in the art that the present application may be practiced without some of these specific details. The following description of the embodiments is only intended to provide a better understanding of the present application by showing examples of the present application. In the drawings and the following description, at least some of the well-known structures and technologies are not shown in order to avoid unnecessarily obscuring the present application; and, for clarity, the dimensions of some structures may be exaggerated. In addition, the features, structures, or characteristics described below may be combined in any suitable manner in one or more embodiments.

[0028] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish an entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In the description of the present application, it should be noted that, unless otherwise specified, the meaning of "multiple" is more than two. Moreover, the term "include", "comprise" or any other variant thereof is intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements not only include those elements, but also include other elements not clearly listed, or also include elements inherent to such process, method, article or equipment. In the absence of more restrictions, the elements limited by the sentence "include..." do not exclude the existence of other identical elements in the process, method, article or equipment including the elements.

[0029] The short cycle life of lithium-sulfur batteries is a shortcoming that limits their application. During the cycle of lithium-sulfur batteries, soluble polysulfides are produced. Polysulfides have extremely high reactivity and will produce violent side reactions with metallic lithium, resulting in rapid failure of the metallic lithium negative electrode and rapid decay of the battery capacity. In addition, the unevenness of lithium metal deposition and extraction will also cause rapid consumption of metallic lithium, accelerating the decay of battery capacity. Studies have found that solid electrolyte interface films play an important role in shielding the reaction between polysulfides and the negative electrode and regulating the deposition and extraction behavior of lithium. The solid electrolyte interface film is generated by the reaction of electrolyte components and metallic lithium, which can prevent subsequent polysulfide corrosion of metallic lithium. Moreover, the deposition and extraction behavior of lithium is regulated by the solid electrolyte interface film. A uniform and stable solid electrolyte interface film can induce uniform deposition and extraction of lithium ions, thereby alleviating the growth of lithium dendrites and the generation of dead lithium, and thus reducing the continuous reaction between the electrolyte and metallic lithium. Therefore, constructing a stable solid electrolyte interface film is of great help in inhibiting the side reactions of polysulfides and metallic lithium. Among them, additives in the electrolyte are an effective strategy for regulating the solid electrolyte interface film. The inventors found that although lithium nitrate is currently one of the most effective additives in the electrolyte of lithium-sulfur batteries, the solid electrolyte interface film formed by lithium nitrate still cannot meet the long cycle requirements of practical lithium-sulfur batteries. Based on this, the inventors have conducted a lot of research to provide an electrolyte containing a new additive to construct a stable and uniform solid electrolyte interface film, improve the cycle stability of the metal lithium negative electrode, and thus improve the cycle life of the lithium-sulfur battery.

[0030] In order to solve the existing technical problems, the present application provides a lithium-sulfur battery electrolyte, a lithium-sulfur battery and an activation method thereof. The lithium-sulfur battery electrolyte provided by the present application is first introduced below.

[0031] An embodiment of the first aspect of the present application provides a lithium-sulfur battery electrolyte. The electrolyte includes a solvent, a lithium salt, and a nitrate additive. Among them, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms; in the electrolyte, the mass concentration of the nitrate additive is 1.0% - 8.0%, and the molar concentration of the lithium salt is 0.2 mol / L - 2.0 mol / L.

[0032] According to an embodiment of the present invention, the lithium-sulfur battery electrolyte mainly plays two roles inside the battery: conducting Li+ between electrodes + to form an internal circuit; dissolving and transporting the intermediate polysulfide near the positive electrode interface.

[0033] According to an embodiment of the present invention, the solvent uses an organic solvent. The single solvent component of the lithium-sulfur battery is mainly ether or sulfone. However, since it is difficult for the single solvent component to balance the stability of the lithium-sulfur battery electrolyte, ionic conductivity, solubility of polysulfide ions, etc., similar to lithium-ion batteries, the most commonly used electrolyte in lithium-sulfur batteries is also a binary or multi-component solvent combination.

[0034] According to an embodiment of the present invention, the lithium salt is the source of Li+ in the electrolyte + and usually requires a relatively high "free" Li+ + concentration, and balance the film-forming characteristics on the strongly reducing negative electrode side and the antioxidant ability on the positive electrode side of the anion.

[0035] Optionally, the molar concentration of the lithium salt is 0.2 mol / L - 2.0 mol / L. The concentration of the lithium salt can be determined considering ionic conductivity, etc., and is preferably 0.2 mol / L - 2.0 mol / L. When the molar concentration of the lithium salt is lower than the above range, it is difficult to ensure the ionic conductivity suitable for battery operation, and when the molar concentration is greater than the above range, due to the increase in the viscosity of the electrolyte, the mobility of lithium ions may decrease, and due to the increase in the decomposition reaction of the lithium salt itself, the battery performance may decline. Therefore, the molar concentration of the lithium salt is appropriately controlled within the above range.

[0036] According to an embodiment of the present invention, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms, which has a higher reactivity than polysulfide and will preferentially decompose on the lithium metal anode before polysulfide to form a solid electrolyte interface film, thereby preventing subsequent side reactions of polysulfide with lithium metal. Moreover, the highly reactive nitrate additive will also react with the polysulfide generated by the positive electrode. On the one hand, it reduces the side reaction of polysulfide with lithium metal. On the other hand, the reaction product of the nitrate additive and polysulfide will participate in the formation of a stable solid electrolyte interface film, increasing the formation amount of the solid electrolyte interface film. The solid electrolyte interface film formed by the participation of the nitrate additive is rich in nitrogen oxides, which are beneficial to the uniform transport of lithium ions and can promote the uniform deposition and extraction of lithium ions. Moreover, the nitrate additive belongs to organic matter, and the organic part in its molecule has good compatibility with the solvent. Therefore, the nitrate additive has a high solubility, has little influence on the ionic conductivity, and is easy to prepare the electrolyte.

[0037] The nitrate additive is prepared by esterifying nitric acid with an alcohol containing at least 4 carbon atoms. For example, isopentyl nitrate is directly esterified from isopentyl alcohol and nitric acid in the presence of urea; isosorbide dinitrate is prepared by esterifying the dehydrated and cyclized sorbitol solution with nitric acid. The nitrate additive contains at least 4 carbon atoms, that is, the nitrate additive has a long organic chain segment. Compared with short-chain nitrates, such as methyl nitrate, butyl nitrate, etc., it has a higher solubility in the electrolyte, can quickly dissolve in the electrolyte, form a solid electrolyte interface film faster, and reduce the influence on the ionic conductivity. Moreover, the nitrate additive with at least 4 carbon atoms reacts with the polysulfide generated by the positive electrode to form a stable solid electrolyte interface film, thereby further improving the cycle stability and cycle life of the lithium-sulfur battery.

[0038] Optionally, the mass concentration of the nitrate additive is 1.0% - 8.0%. When the mass concentration of the nitrate additive is lower than 1.0%, it is difficult to ensure the formation of a stable solid electrolyte interface film. When the mass concentration is greater than 8.0%, it will increase the absolute amount of the reaction between the nitrate additive and polysulfide, resulting in the loss of the positive electrode capacity and the possible decline of the battery performance. Therefore, the mass concentration of the nitrate additive is appropriately controlled within the above range.

[0039] In the lithium-sulfur battery electrolyte provided by the embodiments of the present application, the electrolyte includes a solvent, a lithium salt, and a nitrate additive. Among them, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms. First, the nitrate additive has high reactivity. The nitrate additive in the lithium-sulfur battery electrolyte can form a solid electrolyte interface rich in nitrogen oxides on the surface of the metallic lithium anode, inhibiting the parasitic reaction between polysulfides and the metallic lithium anode, thereby avoiding the rapid failure of the metallic lithium anode and improving the cycle life of the lithium-sulfur battery. Second, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms, and the solid electrolyte interface film formed by it is rich in nitrogen oxides. Nitrogen oxides are beneficial to the uniform transport of lithium ions, which can promote the uniform deposition and extraction of lithium ions, thereby ensuring the stability of the long-term cycle of the lithium-sulfur battery. In addition, the organic part in the nitrate additive molecule has good compatibility with the solvent in the electrolyte, has a high solubility in the electrolyte, has little effect on the ionic conductivity, and is easy to prepare the electrolyte.

[0040] In some embodiments, the nitrate additive is selected from one or more of butyl nitrate, amyl nitrate, isoamyl nitrate, isosorbide dinitrate, and pentaerythritol tetranitrate, and preferably isosorbide dinitrate or butyl nitrate.

[0041] According to the embodiments of the present invention, the nitrate additive is an ester of nitric acid and an alcohol containing at least 4 carbon atoms, such as butyl nitrate, amyl nitrate, isoamyl nitrate, isosorbide dinitrate, pentaerythritol tetranitrate, etc., so that the lithium-sulfur battery prepared with the electrolyte has a cycle number greater than or equal to 70 cycles at a 0.1C rate. The nitrate additive can be an organic compound or a composition of several organic compounds, such as isoamyl nitrate and amyl nitrate, isosorbide dinitrate and amyl nitrate, butyl nitrate and amyl nitrate, etc. Optionally, the nitrate additive is isosorbide dinitrate or butyl nitrate, and the cycle number is greater than or equal to 90 cycles.

[0042] In some embodiments, the solvent is selected from one or more of ethylene glycol dimethyl ether, 1,3-dioxolane, 1,2-dimethoxypropane, 1,3-dimethoxypropane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and tetrahydrofuran.

[0043] Optionally, the solvent is selected from a mixture of a first organic solvent and a second organic solvent. The first organic solvent is selected from one of ethylene glycol dimethyl ether, 1,3-dioxolane, 1,2-dimethoxypropane, 1,3-dimethoxypropane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and tetrahydrofuran. The second organic solvent is selected from one of ethylene glycol dimethyl ether, 1,3-dioxolane, 1,2-dimethoxypropane, 1,3-dimethoxypropane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane, and tetrahydrofuran, and the first organic solvent is different from the second organic solvent. More preferably, the solvent is selected from ethylene glycol dimethyl ether and 1,3-dioxolane, triethylene glycol dimethyl ether and 1,3-dioxolane, ethylene glycol dimethyl ether and 1,3-dioxolane. The volume ratio of the first organic solvent to the second organic solvent in the solvent is (0.2-3):1.

[0044] In some embodiments, the lithium salt is selected from one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorooxalate borate, and lithium bis(oxalate).

[0045] Optionally, the lithium salt is selected from one of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide.

[0046] In some embodiments, the mass concentration of the nitrate additive is 3.0% - 6.0%, preferably 5.0% - 6.0%.

[0047] In some embodiments, the molar concentration of the lithium salt is 0.5 mol / L - 1.0 mol / L.

[0048] In some embodiments, a method for preparing a lithium-sulfur battery electrolyte comprises the following steps: under the protection of an inert gas, adding a lithium salt to the solvent, and then adding a nitrate additive, and stirring well to obtain the lithium-sulfur battery electrolyte. Optionally, the inert gas is at least one of nitrogen, helium, and argon; the water content in the inert gas is <0.1 ppm, and the oxygen content is <0.1 ppm.

[0049] An embodiment of the second aspect of the present application provides a lithium-sulfur battery, which includes the lithium-sulfur battery electrolyte, a positive electrode, and a negative electrode in any one of the above embodiments.

[0050] According to an embodiment of the present invention, a lithium-sulfur battery includes the newly prepared lithium-sulfur battery electrolyte as described above. The nitrate additive in the electrolyte can form a solid electrolyte interface rich in nitrogen oxides on the surface of the lithium metal anode, inhibit the parasitic reaction between polysulfides and the lithium metal anode, improve the uniformity of lithium deposition and stripping, enhance the stability of the lithium metal anode, and increase the cycle life of the lithium-sulfur battery.

[0051] Optionally, the lithium-sulfur battery includes a positive electrode, a negative electrode, and the above-mentioned lithium-sulfur battery electrolyte, and further includes a separator.

[0052] Optionally, the positive electrode includes a positive electrode active material, a conductive agent, a current collector, and a binder. The negative electrode includes a negative electrode active material and a current collector. The positive electrode active material includes elemental sulfur. The conductive agent in the positive electrode includes at least one of carbon nanotubes, carbon fibers, porous carbon spheres, carbon shells, graphene oxide, graphene, and thin-layer graphite flakes. The current collector includes aluminum foil, copper foil, copper mesh, or carbon paper. The negative electrode active material includes lithium foil, lithium sheet, or lithium alloy.

[0053] The appearance of the lithium-sulfur battery can include a cylindrical shape, a square shape, a pouch shape, a coin shape using a can, etc., but is not particularly limited thereto.

[0054] In some embodiments, the lithium-sulfur battery has a cycle number greater than or equal to 70 cycles at a rate of 0.1C, preferably greater than or equal to 90 cycles, and more preferably greater than or equal to 100 cycles.

[0055] An embodiment of the second aspect of the present application provides a method for activating a lithium-sulfur battery, including the following steps:

[0056] (1) Provide and let stand a lithium-sulfur battery according to any embodiment of the second aspect of the present application;

[0057] (2) Perform constant current charge and discharge on the lithium-sulfur battery after standing, and perform pre-cycling at a rate of 0.03C to 0.07C to activate the lithium-sulfur battery.

[0058] According to an embodiment of the present invention, pre-cycling of the lithium-sulfur battery after standing is performed under low-rate conditions to activate and stabilize the battery, thereby improving the cycle performance of the lithium-sulfur battery during subsequent charge and discharge processes. During the low-rate pre-cycling at 0.03C to 0.07C, the highly active nitrate additive not only preferentially forms a solid electrolyte interface film on the lithium metal anode over polysulfides, preventing the side reaction of polysulfides with lithium metal, but also reacts with the polysulfides generated at the positive electrode to further form a solid electrolyte interface film, increasing the cycle life of the lithium-sulfur battery.

[0059] In some embodiments, the magnification of the pre-cycle is 0.03C to 0.07C, where the magnification can be 0.03C, 0.035C, 0.04C, 0.045C, 0.05C, 0.055C, 0.06C, 0.065C, 0.07C, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0060] In some embodiments, the number of pre-cycles is 2 to 3 cycles, preferably 2 cycles.

[0061] According to the embodiments of the present invention, before the conventional cycle of the lithium-sulfur battery, pre-cycle activation is performed. By controlling the specific charge-discharge magnification and the number of charge-discharge cycles, the cycle activation process is optimized. Through the optimized cycle activation process, the battery reaches a stable state, enabling the battery to perform excellently in subsequent conventional cycles, achieving the effect of improving the cycle performance. After this charge-discharge pre-cycle activation, when performing the conventional cycle test, the cycle performance can be significantly improved, and the cycle life of the lithium-sulfur battery can be increased.

[0062] In some embodiments, the standing time of the lithium-sulfur battery is 10h to 15h, preferably 12h.

[0063] Example

[0064] Hereinafter, the lithium-sulfur battery electrolyte, the lithium-sulfur battery, and its activation method of the present application will be described in more detail through examples, but the present application is in no way limited to these examples.

[0065] First, the components and contents of the lithium-sulfur battery electrolytes provided in Examples 1 to 10 and Comparative Example 1 are shown in Table 1.

[0066] Components and Contents of the Lithium-Sulfur Battery Electrolyte Provided in Table 1

[0067]

[0068]

[0069] Secondly, a lithium-sulfur battery is provided, which uses the lithium-sulfur battery electrolyte in any of the above embodiments or comparative examples. Among them, the preparation method of the lithium-sulfur battery includes: taking 35 μL of the lithium-sulfur battery electrolytes in Examples 1 to 10 and Comparative Example 1 above, and -2 the positive electrode sulfur with a loading of 4.0 mg cm

[0070] and the negative electrode lithium sheet with a thickness of 50 μm are assembled into a lithium-sulfur battery in an argon atmosphere.

[0071] (1) Obtain and stand the lithium-sulfur battery in any of the above embodiments or comparative examples;

[0072] (2) Perform constant current charge and discharge on the lithium-sulfur battery after standing still, and first perform pre-cycling to activate the lithium-sulfur battery.

[0073] Cycle the above-activated lithium-sulfur battery at a rate of 0.1C to measure the cycle life of the lithium-sulfur battery.

[0074] The main process technical parameters of the activation method of the lithium-sulfur battery and the cycle life of the lithium-sulfur battery after testing are shown in Table 2.

[0075] Table 2 Main process technical parameters of the activation method and cycle life

[0076] Static time (h) Pre-cycle magnification (C) Pre-cycle times (cycles) Cycle life (cycles) Example 1 12h 0.05C 2 cycles 100 cycles Example 2 12h 0.05C 2 cycles 96 cycles Example 3 13h 0.05C 2 cycles 92 cycles Example 4 12h 0.04C 2 cycles 85 cycles Example 5 11h 0.04C 2 cycles 83 cycles Example 6 12h 0.04C 3 cycles 76 cycles Example 7 12h 0.05C 2 cycles 82 cycles Example 8 13h 0.05C 3 cycles 80 cycles Example 9 13h 0.06C 3 cycles 72 cycles Example 10 12h 0.05C 2 cycles 87 cycles Comparative Example 1 12h 0.05C 2 cycles 50 cycles Comparative Example 2 12h -- -- 88 cycles Comparative Example 3 12h 0.2C 2 cycles 70 cycles Comparative Example 4 12h 0.05 10 cycles 78 cycles

[0077] As shown in Table 1 and Table 2, it can be seen that the cycle life of the lithium-sulfur batteries in Examples 1 to 10 reaches more than 70 cycles. Compared with the cycle life of the lithium-sulfur battery in Comparative Example 1, the electrolyte of the lithium-sulfur battery of the present application can significantly improve the cycle life of the lithium-sulfur battery. In addition, by comparing Example 1 with Comparative Example 1 and Comparative Example 2, it can be seen that compared with non-activation or high-rate activation, pre-cycling treatment at a low rate can activate and stabilize the battery, and the cycle performance of the lithium-sulfur battery during subsequent charge and discharge is improved. The cycle life is increased by 2 times compared with the cycle life of non-activation and 1.36 times compared with the cycle life of high-rate activation.

[0078] The following combines the attached Figures 1 - 8 , and further explains and illustrates Example 1 and Comparative Example 1.

[0079] Figure 1 shows the cycle performance diagram of the lithium-sulfur batteries of Example 1 and Comparative Example 1 provided by an embodiment of the present application.

[0080] As shown in Figure 1, the cycle life of the lithium-sulfur battery in Example 1 reaches 100 cycles and the capacity remains stable; while in the electrolyte of Comparative Example 1, due to the lack of the nitrate additive of the present application and the addition of lithium nitrate additive, the cycle capacity of the lithium-sulfur battery decays rapidly after 50 cycles. The above results show that the nitrate additive in the electrolyte can form a solid electrolyte interface film rich in nitrogen oxides, which can hinder the side reaction between polysulfides and the metal lithium negative electrode, improve the lithium deposition behavior, and extend the cycle life of the battery.

[0081] Figure 2 shows the constant current titration test result diagram of the first cycle discharge curve of the lithium-sulfur battery of Example 1 of the present application; Figure 3 shows the constant current titration test result diagram of the first cycle discharge curve of the lithium-sulfur battery of Comparative Example 1 of the present application.

[0082] As Figure 2 and Figure 3As shown, the first-cycle discharge curve of Example 1 shows three plateaus, where the first plateau is higher than 2.4 V. In contrast, the first-cycle discharge curve of Comparative Example 1 only has two plateaus, corresponding to the conversion of sulfur-polysulfide and polysulfide-lithium sulfide, respectively. The comparison shows that the reduction potential of the nitrate ester additive is higher, significantly superior to lithium nitrate, and the reduction reaction occurs prior to polysulfide, proving that the nitrate ester can decompose on the negative electrode surface to form a solid electrolyte interface film prior to polysulfide.

[0083] Figure 4 The effect diagram of the electrolyte of Example 1 of the present application is shown; Figure 5 The effect diagram after the electrolyte of Example 1 of the present application reacts with polysulfide is shown.

[0084] As Figure 4 shown, the nitrate ester additive of Example 1 can be quickly dissolved in the electrolyte. Compared with short-chain nitrate ester additives, such as methyl nitrate, etc., the nitrate ester additive in the present application has a longer organic chain segment and has better solubility in the electrolyte. The nitrate ester additive in the present application is clear and transparent when dissolved in the electrolyte. As Figure 5 shown, the nitrate ester additive and polysulfide Li 2 S 8 are mixed, and the two react to form a solid, proving that the oxidizing property of the nitrate ester additive is significantly higher than that of polysulfide. During the battery discharge process, the generated polysulfide will also react with the nitrate ester. On the one hand, it reduces the corrosion reaction of polysulfide on metallic lithium, and on the other hand, the reaction product of the two can participate in the formation of the solid electrolyte.

[0085] Figure 6 The X-ray photoelectron spectrum of the solid electrolyte interface film formed in Example 1 of the present application is shown.

[0086] The solid electrolyte interface film components on the metallic lithium negative electrode after cycling of the lithium-sulfur battery in Example 1 were analyzed by X-ray photoelectron spectroscopy. As Figure 6 shown, the solid electrolyte interface film of Example 1 is rich in nitrogen oxides LiN x O y , and this component can effectively shield the side reaction between polysulfide and metallic lithium negative electrode.

[0087] Figure 7 The electron microscope image of the lithium deposition morphology of Example 1 of the present application is shown; Figure 8 The electron microscope image of the lithium deposition morphology of Comparative Example 1 of the present application is shown.

[0088] As Figure 7 and Figure 8 shown, the nitrogen oxides in the solid electrolyte interface film are beneficial to the uniform transport of lithium ions, making the lithium deposition morphology of Example 1 more uniform, while the lithium deposition of Comparative Example 1 is uneven.

[0089] Although the present application has been described with reference to the preferred embodiments, various modifications can be made thereto and components thereof can be replaced with equivalents without departing from the scope of the present application. In particular, as long as there is no structural conflict, the technical features mentioned in each embodiment can be combined in any manner. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A lithium-sulfur battery electrolyte, characterized in that, the electrolyte comprises a solvent, a lithium salt and a nitrate additive, wherein, the nitrate additive is selected from one or more of butyl nitrate, amyl nitrate, isoamyl nitrate, isosorbide dinitrate, pentaerythritol tetranitrate; in the electrolyte, the mass concentration of the nitrate additive is 1.0% to 8.0%, and the molar concentration of the lithium salt is 0.5 mol / L to 2.0 mol / L; the lithium-sulfur battery prepared with the lithium-sulfur battery electrolyte has a cycle number of greater than or equal to 70 cycles at a 0.1C rate.

2. The lithium-sulfur battery electrolyte according to claim 1, characterized in that, the nitrate additive is selected from isosorbide dinitrate or butyl nitrate.

3. The lithium-sulfur battery electrolyte according to claim 1, characterized in that, the solvent is selected from one or more of ethylene glycol dimethyl ether, 1,3-dioxolane, 1,2-dimethoxypropane, 1,3-dimethoxypropane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,4-dioxane and tetrahydrofuran.

4. The lithium-sulfur battery electrolyte according to claim 1, characterized in that, the lithium salt is selected from one or more of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorooxalate borate, lithium bis(oxalate).

5. The lithium-sulfur battery electrolyte according to claim 1, characterized in that, the mass concentration of the nitrate additive is 3.0% to 6.0%.

6. The lithium-sulfur battery electrolyte according to claim 5, characterized in that, the mass concentration of the nitrate additive is 5.0% to 6.0%.

7. The lithium-sulfur battery electrolyte according to claim 1, characterized in that, the molar concentration of the lithium salt is 0.5 mol / L to 1.0 mol / L.

8. A lithium-sulfur battery, characterized in that, the lithium-sulfur battery comprises the lithium-sulfur battery electrolyte according to any one of claims 1 to 7, a positive electrode and a negative electrode.

9. The lithium-sulfur battery according to claim 8, characterized in that, the lithium-sulfur battery has a cycle number of greater than or equal to 70 cycles at a 0.1C rate.

10. The lithium-sulfur battery according to claim 8, characterized in that, the lithium-sulfur battery has a cycle number of greater than or equal to 90 cycles at a 0.1C rate.

11. The lithium-sulfur battery according to claim 8, characterized in that, the lithium-sulfur battery has a cycle number of greater than or equal to 100 cycles at a 0.1C rate.

12. An activation method for a lithium-sulfur battery, characterized in that, comprises the following steps: providing and standing the lithium-sulfur battery according to any one of claims 8 to 11; performing constant current charge and discharge on the standing lithium-sulfur battery, and performing pre-cycling at a rate of 0.03C to 0.07C to activate the lithium-sulfur battery.

13. The activation method for a lithium-sulfur battery according to claim 12, characterized in that, the number of pre-cycling is 2 to 3 cycles.

14. The activation method for a lithium-sulfur battery according to claim 12, characterized in that, the number of pre-cycling is 2 cycles.

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