Carbon-coated nano lithium sulfide-C3N4 composite material as well as preparation method and application thereof

Abstract

The invention discloses a carbon-coated nano lithium sulphide-C3N4 composite material as well as a preparation method and application of the carbon-coated nano lithium sulphide-C3N4 composite material. The preparation method comprises the following steps: mixing and reacting a lithium source, a sulfur source, an organic carbon precursor, a sulfur-containing C3N4 precursor and a solvent under the protection of an inert atmosphere to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor, and removing the solvent to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture; and under the protection of inert atmosphere, carrying out high-temperature calcination on the carbon-coated nano lithium sulfide-C3N4 composite material to prepare the carbon-coated nano lithium sulfide-C3N4 composite material. The carbon-coated nano lithium sulfide-C3N4 composite material prepared by the invention can be used as a positive electrode material of a lithium-sulfur battery, and nano lithium sulfide can provide discharge capacity as high as 1166mAh / g and is high in energy density; the in-situ generated carbon shell has high conductivity, can improve the charge transfer rate, inhibits overgrowth of lithium sulfide in the calcination process, and improves the battery performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to a carbon-coated nanometer lithium sulfide-C3N4 composite material and a preparation method and application thereof. BACKGROUND

[0002] At present, lithium ion batteries, as a common energy storage system, are widely used in portable electronic devices, electric vehicles and electric energy storage networks. However, the energy density of lithium ion batteries has reached the limit, and the safety is poor, so it is urgent to develop a new generation of secondary batteries. Lithium-sulfur batteries, as a new generation of secondary batteries, have outstanding advantages such as high mass energy density, high volume energy density and high specific capacity, and are one of the closest practical next-generation secondary battery systems at present, which has broad application prospects.

[0003] However, there are many problems in lithium-sulfur batteries that need to be solved, which hinder the industrialization of lithium-sulfur batteries. First, the intermediate product of the charging and discharging process, long-chain lithium polysulfide, is easily dissolved in the electrolyte, shuttles to the negative electrode region and reacts with the negative electrode metal lithium, resulting in loss of active material and capacity decay; second, the density difference between lithium sulfide and sulfur during the charging and discharging process will cause the volume expansion of the electrode to produce cracks, affecting the conductive path; third, the intrinsic conductivity of sulfur and lithium sulfide is low, and a conductive agent needs to be introduced into the electrode to improve the conductivity.

[0004] To solve the above problems, one solution is to composite lithium sulfide with carbon / catalyst to prepare lithium sulfide composite material. High-conductivity materials (such as carbon) can promote ion and electron transmission in the material; catalysts can promote the conversion between lithium sulfide and sulfur and inhibit the shuttle effect of polysulfides; at the same time, if a specific material structure (such as core-shell structure) can be constructed, not only can the volume expansion of the positive electrode during charging and discharging be inhibited, but also the direct contact between lithium sulfide and electrolyte can be avoided to the greatest extent through physical confinement or chemical adsorption, thereby inhibiting the dissolution of polysulfide lithium.

[0005] At present, the preparation of lithium sulfide composite material is generally to synthesize lithium sulfide, carbon and / or catalyst material respectively, and then mix the above-mentioned materials by ball milling or homogenization method to obtain lithium sulfide composite material. This method is complex, and it is difficult to directly composite lithium sulfide, carbon and catalyst uniformly through physical method, which is not conducive to the electrode cycle. For example, Chinese patent CN119750502A first reacts lithium source, sulfur source and sodium borohydride by ball milling to obtain lithium sulfide, then dissolves the lithium sulfide in ethanol and adds carbon-containing material for mixing, and finally obtains lithium sulfide / carbon composite material by freeze-drying. In this method, neither lithium sulfide nor carbon is generated in situ, and the uniformity of the two is poor, which will lead to low utilization rate of lithium sulfide in the composite material and poor battery performance. Therefore, it is crucial to find a method to generate lithium sulfide composite material in situ.

[0006] In addition, the particle size of lithium sulfide is related to the first cycle electrochemical activation barrier of lithium-sulfur batteries, and the smaller the particle size of lithium sulfide, the lower the barrier. However, the conventional method can generally only prepare micron-sized lithium sulfide, and it is difficult to directly obtain nanoscale lithium sulfide, because whether it is a carbon thermal reduction lithium sulfate method, a liquid phase method or a metathesis method, it needs to go through a high-temperature calcination step to remove the solvent. In the calcination process, small particle size lithium sulfide will spontaneously aggregate to form micron-sized lithium sulfide. Therefore, the method for preparing nanoscale lithium sulfide generally first prepares micron-sized lithium sulfide, and then performs particle size refinement treatment on the lithium sulfide through wet ball milling, electrostatic spraying and the like. This not only introduces an additional reaction section, but also increases the cost.

[0007] In summary, the current synthesis method of lithium sulfide composite material still has many problems, which leads to the difficulty of large-scale application of lithium-sulfur batteries and the high price. Therefore, it is of great significance to explore a method for in-situ generation of nanoscale lithium sulfide composite material. SUMMARY

[0008] The main purpose of the present application is to provide a carbon-coated nanoscale lithium sulfide-C3N4 composite material and a preparation method thereof to overcome the deficiencies in the prior art.

[0009] Another purpose of the present application is to provide the application of the carbon-coated nanoscale lithium sulfide-C3N4 composite material.

[0010] To achieve the above-mentioned purposes of the application, the technical solutions adopted by the present application include:

[0011] The present application provides a preparation method of a carbon-coated nanoscale lithium sulfide-C3N4 composite material, which comprises:

[0012] Under the protection of an inert atmosphere, a lithium source, a sulfur source, an organic carbon precursor, a sulfur-containing C3N4 precursor and a solvent are mixed and reacted to obtain a solution containing lithium sulfide, an organic carbon precursor and a sulfur-containing C3N4 precursor. The solvent is removed to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture;

[0013] Under the protection of an inert atmosphere, the lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is subjected to high-temperature calcination to obtain a carbon-coated nanoscale lithium sulfide-C3N4 composite material.

[0014] The present application also provides a carbon-coated nanoscale lithium sulfide-C3N4 composite material prepared by the above-mentioned preparation method. The carbon-coated nanoscale lithium sulfide-C3N4 composite material has a core-shell structure, which comprises a nanoscale lithium sulfide-C3N4 composite as a core, and a carbon shell layer coated on the surface of the core.

[0015] The application also provides application of the carbon-coated nanometer lithium sulfide-C3N4 composite material in preparation of a lithium-sulfur battery cathode material or a lithium-sulfur battery.

[0016] Correspondingly, the application also provides a lithium-sulfur battery cathode material, which comprises the carbon-coated nanometer lithium sulfide-C3N4 composite material.

[0017] Compared with the prior art, the application has at least the following beneficial effects:

[0018] The carbon-coated nanometer lithium sulfide-C3N4 composite material prepared by the application can be used as a lithium-sulfur battery cathode material. The nanometer lithium sulfide can provide a discharge capacity of up to 1166 mAh / g, and the energy density is high. In addition, the carbon-containing precursor can inhibit the excessive growth of lithium sulfide during the calcination process. The in-situ grown lithium sulfide has a small particle size and a small initial activated electrochemical barrier. The C3N4 has a large specific surface area and contains a large amount of pyridine nitrogen, which can form a Li-N bond with lithium polysulfide to inhibit the shuttle effect of lithium polysulfide through chemical adsorption. The in-situ generated carbon shell has high conductivity, which can improve the charge transfer rate and inhibit the excessive growth of lithium sulfide during the calcination process, thereby improving the battery performance. In addition, the carbon shell can inhibit the shuttle effect of lithium polysulfide and the volume expansion during the discharge process through physical confinement, thereby inhibiting the capacity attenuation and improving the battery performance. The sulfur-containing precursor can release hydrogen sulfide during the calcination process, which can convert the oxygen-containing impurities generated due to the presence of water in the solvent into lithium sulfide, thereby further improving the purity of lithium sulfide. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments of the application, and for those skilled in the art, other drawings can also be obtained from these drawings without creative labor.

[0020] Figure 1 A flow chart for preparing the carbon-coated nanometer lithium sulfide-C3N4 composite material (which can also be referred to as a nanometer lithium sulfide-C3N4@C composite) in a typical embodiment of the application;

[0021] Figure 2 is an X-ray diffraction (XRD) spectrum of the nanometer lithium sulfide-C3N4@C composite prepared in Example 1 of the application;

[0022] Figure 3 is a scanning electron microscope (SEM) image of the nanometer lithium sulfide-C3N4@C composite prepared in Example 1 of the application. DETAILED DESCRIPTION

[0023] In view of the deficiencies of the prior art, the present inventors have found, through long-term research and a large number of practices, that if a carbon-containing template is formed on the surface of lithium sulfide in advance, and the pre-formed core-shell structure is used to inhibit the agglomeration of lithium sulfide during calcination, smaller particle size lithium sulfide can be obtained. Therefore, the technical solutions of the present application are proposed.

[0024] The technical solutions, implementation processes and principles thereof will be further explained as follows. However, it should be understood that, within the scope of the present application, the above-mentioned technical features of the present application and the technical features specifically described in the following (Examples) can be combined with each other to form new or preferred technical solutions. Due to the limited space, they will not be listed one by one here.

[0025] As an aspect of the technical solutions of the present application, a preparation method of a carbon-coated nano lithium sulfide-C3N4 composite material includes:

[0026] Under the protection of an inert atmosphere, a lithium source, a sulfur source, an organic carbon precursor, a sulfur-containing C3N4 precursor and a solvent are mixed and reacted to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor. At the same time of removing the solvent, the organic carbon precursor is wrapped on the surface of the lithium sulfide and the sulfur-containing C3N4 precursor in the form of a cross-linked polymer to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture;

[0027] Under the protection of an inert atmosphere, the lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is subjected to high-temperature calcination to obtain the carbon-coated nano lithium sulfide-C3N4 composite material.

[0028] In some embodiments, the lithium source can be selected from a combination of one or more of lithium chloride, lithium nitrate, lithium sulfate, lithium bromide and lithium hydroxide, and is further preferably anhydrous lithium chloride, but is not limited thereto.

[0029] In some embodiments, the sulfur source can be selected from a combination of one or more of sodium sulfide, potassium sulfide, sodium thiosulfate and ammonium sulfide, and is further preferably anhydrous sodium sulfide, but is not limited thereto.

[0030] In some embodiments, the molar ratio of lithium atoms in the lithium source to -2 valence sulfur atoms in the sulfur source is (1-4):1.

[0031] In some more preferred embodiments, the molar ratio of the lithium source to the sulfur source is (1.9-2.1):1 in terms of lithium atoms:-2 valence sulfur atoms.

[0032] In some embodiments, the organic carbon precursor can be selected from a combination of one or more of polyvinylpyrrolidone, glucose, phenol formaldehyde resin, dopamine, polyimide, and the like, but is not limited thereto.

[0033] In some embodiments, the mass ratio of the organic carbon precursor to the lithium source is 1:5 to 5:1.

[0034] In some embodiments, the sulfur-containing C3N4 precursor can be selected from the group consisting of one or more of thiourea, thiocyanic acid, ammonium thiocyanate, N,N'-ethylene thiourea, N,N'-propylene thiourea, S-ethyl isothiourea, ethylene thiourea, thioamino triazine, thioamino urea, and the like, but not limited thereto.

[0035] In some embodiments, the mass ratio of the sulfur-containing C3N4 precursor to the lithium source is 1:10 to 5:1.

[0036] In some embodiments, the preparation method specifically comprises: under the protection of an inert atmosphere, dissolving the lithium source, the sulfur source, the organic carbon precursor, and the sulfur-containing C3N4 precursor in a solvent, respectively, and then mixing the above solutions and sufficiently stirring the reaction to obtain a suspension containing lithium sulfide, the organic carbon precursor, and the sulfur-containing C3N4 precursor.

[0037] Further, the solvent can be selected from the group consisting of one or more of methanol, ethanol, n-propanol, isopropanol, n-butanol, ethylene glycol, glycerol, dimethyl sulfoxide, ethyl acetate, and the like, but not limited thereto. Among them, the solvent should be soluble to the lithium source, the sulfur source, the organic carbon precursor, and the sulfur-containing C3N4 precursor, and the solvents used for dissolving different raw materials can be the same or different, but it is more optimal to select the same solvent, so that the raw materials can be fully mixed and reacted, and the reaction efficiency is high.

[0038] In some embodiments, the inert atmosphere is composed of an inert gas, which includes one or more of nitrogen, helium, neon, argon, krypton, and the like.

[0039] Further, the inert gas is nitrogen or argon.

[0040] In some embodiments, the preparation method specifically comprises: mixing the lithium source, the sulfur source, the organic carbon precursor, the sulfur-containing C3N4 precursor, and the solvent and sufficiently stirring to perform the reaction.

[0041] In some more preferred embodiments, the reaction is performed under stirring, and the stirring rate is 100 to 2000 rpm, preferably 500 to 1000 rpm.

[0042] In some embodiments, the reaction temperature is 0°C to the boiling point of the solvent, and preferably, the reaction temperature is 20 to 40°C.

[0043] In some embodiments, the reaction time is 0.5-48h, preferably, the reaction time is 2-24h.

[0044] In some embodiments, the preparation method specifically comprises: after the reaction is completed, performing solid-liquid separation on the obtained suspension containing lithium sulfide, organic carbon precursor and sulfur-containing C3N4 precursor to obtain a solution containing lithium sulfide, organic carbon precursor and sulfur-containing C3N4 precursor.

[0045] In some more preferred embodiments, the solid-liquid separation method includes but is not limited to any one of centrifugation, filtration.

[0046] In some embodiments, the solid-liquid separation is performed by centrifugation, and the centrifugation speed is 2000-15000 rpm, and the centrifugation time is 1-20 min.

[0047] In some embodiments, the solid-liquid separation is performed by filtration, and the filter paper used in the filtration has a pore size of 100 nm-10 μm, and the filtration form can be any one of normal pressure filtration, vacuum filtration or pressure filtration.

[0048] In some more preferred embodiments, the preparation method can further comprise: adding MXene material to the solution containing lithium sulfide, organic carbon precursor and sulfur-containing C3N4 precursor obtained by solid-liquid separation, and stirring, and then removing the solvent.

[0049] In some embodiments, the stirring rate is 100-2000 rpm, preferably 500-1000 rpm; the stirring temperature is 0℃ to the boiling point of the solvent, preferably 20-40℃; and the stirring time is 0.5-48h, preferably 2-24h.

[0050] Further, the MXene material accounts for 1-10% of the total mass of the solution containing lithium sulfide, organic carbon precursor and sulfur-containing C3N4 precursor.

[0051] In a preferred embodiment of the present application, after the MXene material (such as Ti3C2T x ) is added, the MXene material and C3N4 can form a synergistic effect, the Ti atoms in the MXene material can anchor polysulfides, and through chemical catalysis, promote the mutual transformation of sulfur and lithium sulfide during the charging and discharging process; C3N4 can assist in anchoring polysulfides through adsorption, and the two can synergistically avoid the polysulfides from staying too long in the polysulfide stage, thereby avoiding capacity decay caused by shuttling to the negative electrode.

[0052] In some embodiments, the solvent removal method includes evaporation, and the evaporation temperature is 30-150℃, preferably 60-100℃.

[0053] Furthermore, the evaporation time is 1 to 72 hours, preferably 2 to 12 hours.

[0054] Furthermore, the heating equipment used for the evaporation includes one or more combinations of heating platforms, tube furnaces, muffle furnaces, rotary evaporators, etc.

[0055] In some embodiments, the high-temperature calcination temperature is 200–800°C, and the time is 1–48 hours.

[0056] In some more preferred embodiments, the high-temperature calcination temperature is 300–600°C, preferably 400–600°C, and the time is 4–18 hours.

[0057] In some more specific implementation plans, such as Figure 1 As shown, the preparation method of the carbon-coated nano-lithium sulfide-C3N4 composite material (also referred to as "nano-lithium sulfide-C3N4@C composite") of the present invention specifically includes the following steps:

[0058] S1. Under an inert atmosphere, the lithium source, sulfur source, organic carbon precursor, and sulfur-containing C3N4 precursor are dissolved in solvents respectively.

[0059] S2, mixed solution, stirred thoroughly to react, to obtain a suspension of lithium sulfide, carbon organic precursor and sulfur-containing C3N4 precursor;

[0060] S3. The suspension is subjected to solid-liquid separation to obtain a solution of lithium sulfide, organic carbon precursor and sulfur-containing C3N4 precursor.

[0061] S4. Evaporate the solvent, and at the same time, the organic carbon precursor is wrapped on the surface of lithium sulfide and sulfur-containing C3N4 precursor in the form of cross-linked polymer to obtain a uniformly mixed lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture (also known as "lithium sulfide-sulfur-containing C3N4 precursor@organic carbon precursor mixture").

[0062] S5. Under an inert atmosphere, high-temperature calcination was carried out to obtain nano-lithium sulfide-C3N4@C composite.

[0063] In summary, this invention directly prepares composite materials in situ using a metathesis method. The integral composite materials prepared in situ have tightly connected structures and can form a specific core-shell structure, which is beneficial for improving battery performance.

[0064] As another aspect of the technical solution of the present application, it further relates to a carbon-coated nanometer lithium sulfide-C3N4 composite material prepared by the preparation method, wherein the carbon-coated nanometer lithium sulfide-C3N4 composite material has a core-shell structure, the core-shell structure comprises a nanometer lithium sulfide-C3N4 composite as a core, and a carbon shell layer coated on the surface of the core.

[0065] In some embodiments, the nanometer lithium sulfide-C3N4 composite comprises uniformly mixed nanometer lithium sulfide and C3N4.

[0066] Further, the particle size of the nanometer lithium sulfide is 300-500 nm.

[0067] Further, the thickness of the carbon shell layer is less than 100 nm. The carbon shell of the present application is coated outside the nanometer lithium sulfide-C3N4 composite, which inhibits the shuttle effect of polysulfides in the charging and discharging process through physical confinement, inhibits capacity decay, and improves battery performance. At the same time, the presence of the carbon shell can inhibit the volume increase phenomenon of lithium sulfide caused by agglomeration and adhesion during calcination, avoiding the too large electrochemical potential barrier of the first week discharge.

[0068] Further, the content of nanometer lithium sulfide in the carbon-coated nanometer lithium sulfide-C3N4 composite material is 20-80 wt%, the content of C3N4 is 10-30 wt%, and the content of the carbon shell layer is 1-20 wt%.

[0069] In some preferred embodiments, when the MXene material is added during the preparation process, the final prepared composite material further comprises a layered MXene material, and the carbon-coated nanometer lithium sulfide-C3N4 composite material is stacked on the surface of the layered MXene material.

[0070] As another aspect of the technical solution of the present application, it further relates to the application of the carbon-coated nanometer lithium sulfide-C3N4 composite material in preparing a lithium-sulfur battery cathode material or a lithium-sulfur battery.

[0071] Further, another embodiment of the present application further relates to a lithium-sulfur battery cathode material comprising the carbon-coated nanometer lithium sulfide-C3N4 composite material.

[0072] Correspondingly, another embodiment of the present application further relates to a lithium-sulfur battery using the carbon-coated nanometer lithium sulfide-C3N4 composite material as a cathode material.

[0073] In summary, the carbon-coated nanometer lithium sulfide-C3N4 composite material of the present application can be used as a positive electrode material of a lithium-sulfur battery. The nanometer lithium sulfide can provide a discharge capacity of up to 1166 mAh / g, and the energy density is high. The carbon-containing precursor can inhibit the excessive growth of lithium sulfide during the calcination process. The in-situ grown lithium sulfide has a small particle size and a small initial activated electrochemical barrier. The C3N4 has a large specific surface area and contains a large amount of pyridine nitrogen, which can form a Li-N bond with lithium polysulfide to inhibit the shuttle effect of lithium polysulfide through chemical adsorption. The in-situ generated carbon shell has high conductivity, which can improve the charge transfer rate and inhibit the excessive growth of lithium sulfide during the calcination process, thereby improving the battery performance. The carbon shell can also inhibit the shuttle effect of lithium polysulfide and the volume expansion during the discharge process through physical confinement, and inhibit the excessive agglomeration and growth of lithium sulfide during the calcination process, thereby inhibiting the capacity attenuation and improving the battery performance. The nanometer lithium sulfide has a small particle size, which is beneficial to reducing the electrochemical barrier. The sulfur-containing precursor releases hydrogen sulfide during the calcination process, which converts the oxygen-containing impurities generated due to the presence of water in the solvent into lithium sulfide, thereby further improving the purity of lithium sulfide.

[0074] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the embodiments. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0075] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs. The terms used in the specification of the present application herein are only for the purpose of describing specific embodiments and are not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more of the associated listed items.

[0076] Example 1

[0077] Step S1, in a glove box protected by nitrogen atmosphere, 4.239 g of anhydrous lithium chloride, 3.902 g of sodium sulfide, 4.595 g of polyvinylpyrrolidone with a weight average molecular weight of 1000000 and 2.298 g of thiourea were respectively dissolved in 250 mL of anhydrous ethanol to obtain an ethanol solution of lithium chloride, sodium sulfide, polyvinylpyrrolidone and thiourea;

[0078] Step S2, the above solutions were mixed, and the reaction was stirred sufficiently at a stirring speed of 1000 rpm and a reaction temperature of 30℃ for 12 h to obtain a suspension of lithium sulfide, polyvinylpyrrolidone and thiourea;

[0079] Step S3, centrifuging the above suspension at a centrifugal speed of 10000 rpm for 10 min to obtain a lithium sulfide, polyvinylpyrrolidone and thiourea ethanol solution;

[0080] Step S4, evaporating the solvent by using a rotary evaporator at a heating temperature of 80℃ for 6h under a vacuum degree of-0.098 MPa to obtain a lithium sulfide-thiourea@polyvinylpyrrolidone composite material;

[0081] Step S5, calcining the above composite material at a calcination temperature of 550℃ for 6h under an argon protective atmosphere to obtain a nano lithium sulfide-C3N4@C composite material.

[0082] The X-ray diffraction (XRD) spectrum of the nano lithium sulfide-C3N4@C composite material prepared in this example is shown in Figure 2 The XRD results show that, in addition to lithium sulfide and C3N4, there is no other crystalline impurity in the composite material.

[0083] The SEM of the nano lithium sulfide-C3N4@C composite material prepared in this example is shown in Figure 3 The results show that the particle size of lithium sulfide is 300-500 nm, and the thickness of the carbon shell is less than 100 nm. The obtained lithium sulfide is nano lithium sulfide.

[0084] Example 2

[0085] Step S1, in a glove box under a nitrogen atmosphere, 4.239 g of anhydrous lithium chloride, 3.902 g of sodium sulfide, 4.595 g of polyvinylpyrrolidone with a weight average molecular weight of 800000 and 2.298 g of thiourea were respectively dissolved in 250 mL of anhydrous ethanol to obtain a lithium chloride, sodium sulfide, polyvinylpyrrolidone and thiourea ethanol solution;

[0086] Step S2, mixing the above solutions and fully stirring the reaction at a stirring speed of 800 rpm and a reaction temperature of 40℃ for 10h to obtain a lithium sulfide, polyvinylpyrrolidone and thiourea suspension;

[0087] Step S3, filtering the above suspension by vacuum suction filtration with a filter paper filter hole diameter of 500 nm to obtain a lithium sulfide, polyvinylpyrrolidone and thiourea ethanol solution;

[0088] Step S4, evaporating the solvent by using a heating table at a heating temperature of 100℃ for 8h to obtain a lithium sulfide-thiourea@polyvinylpyrrolidone composite material;

[0089] Step S5, the composite material is high-temperature calcined under the protection of argon atmosphere, the calcination temperature is 500 DEG C, the calcination time is 5h, and the nano lithium sulfide-C3N4@C composite material is obtained.

[0090] Example 3

[0091] Step S1, in the glove box protected by argon atmosphere, 6.895g of lithium nitrate, 3.902g of sodium sulfide, 4.595g of glucose and 2.298g of thiourea are respectively dissolved in 500mL of anhydrous isopropyl alcohol to obtain isopropyl alcohol solutions of lithium nitrate, sodium sulfide, glucose and thiourea;

[0092] Step S2, the above solutions are mixed, and the reaction is fully stirred, the stirring speed is 600rpm, the reaction temperature is 40 DEG C, and the reaction time is 8h, to obtain a suspension of lithium sulfide, glucose and thiourea;

[0093] Step S3, the suspension is filtered, the filter form adopts normal pressure filtration, the diameter of the filter paper filter hole is 300nm, and the isopropyl alcohol solution of lithium sulfide, glucose and thiourea is obtained;

[0094] Step S4, the solvent is evaporated by using a tube furnace, the heating temperature is 120 DEG C, and the heating time is 8h, to obtain a lithium sulfide-thiourea@glucose composite material;

[0095] Step S5, the composite material is high-temperature calcined under the protection of argon atmosphere, the calcination temperature is 450 DEG C, the calcination time is 12h, and the nano lithium sulfide-C3N4@C composite material is obtained.

[0096] Example 4

[0097] Step S1, in the glove box protected by nitrogen atmosphere, 8.478g of lithium chloride, 11.026g of potassium sulfide, 4.595g of polyvinylpyrrolidone with a weight average molecular weight of 1300000 and 5.353g of trithiocyanic acid are respectively dissolved in 1L of ethylene glycol to obtain ethylene glycol solutions of lithium chloride, potassium sulfide, polyvinylpyrrolidone and trithiocyanic acid;

[0098] Step S2, the above solutions are mixed, and the reaction is fully stirred, the stirring speed is 2000rpm, the reaction temperature is 25 DEG C, and the reaction time is 16h, to obtain a suspension of lithium sulfide, polyvinylpyrrolidone and trithiocyanic acid;

[0099] Step S3, the suspension is centrifuged, the centrifugal speed is 10000rpm, and the centrifugal time is 8min, to obtain an ethylene glycol solution of lithium sulfide, polyvinylpyrrolidone and trithiocyanic acid;

[0100] Step S4, the solvent is evaporated by using a muffle furnace, the heating temperature is 75℃, and the heating time is 16h, to obtain a lithium sulfide-polyvinylpyrrolidone trithiocyanate composite material;

[0101] Step S5, the above-mentioned composite material is high-temperature calcined under a nitrogen protective atmosphere, the calcination temperature is 700℃, and the calcination time is 4h, to obtain a nano lithium sulfide-C3N4@C composite material.

[0102] Example 5

[0103] Step S1, in a glove box protected by an argon atmosphere, 10.994g of lithium sulfate and 11.026g of potassium sulfide, 4.595g of polyvinylpyrrolidone with a weight average molecular weight of 800000 and 3.126g of S-ethyl isothiourea are respectively dissolved in 1L of anhydrous methanol to obtain a methanol solution of lithium sulfate, potassium sulfide, polyvinylpyrrolidone and S-ethyl isothiourea;

[0104] Step S2, the above-mentioned solutions are mixed, and the reaction is fully stirred, the stirring speed is 400rpm, the reaction temperature is 40℃, and the reaction time is 18h, to obtain a suspension of lithium sulfide, polyvinylpyrrolidone and S-ethyl isothiourea;

[0105] Step S3, the above-mentioned suspension is centrifuged, the centrifugal speed is 12000rpm, and the centrifugal time is 5min, to obtain a methanol solution of lithium sulfide, polyvinylpyrrolidone and S-ethyl isothiourea;

[0106] Step S4, the solvent is evaporated by using a rotary evaporator, the heating temperature is 70℃, the heating time is 12h, and the vacuum degree is-0.095MPa, to obtain a lithium sulfide-S-ethyl isothiourea@polyvinylpyrrolidone composite material;

[0107] Step S5, the above-mentioned composite material is high-temperature calcined under a neon protective atmosphere, the calcination temperature is 500℃, and the calcination time is 10h, to obtain a nano lithium sulfide-C3N4@C composite material.

[0108] Example 6

[0109] Step S1, in a glove box protected by an argon atmosphere, 17.369g of lithium bromide and 6.814g of ammonium sulfide, 4.595g of phenolic resin with a weight average molecular weight of 3000 and 2.284g of ammonium thiocyanate are respectively dissolved in 1L of ethylene glycol to obtain an ethylene glycol solution of lithium bromide, ammonium sulfide, phenolic resin and ammonium thiocyanate;

[0110] Step S2, the above-mentioned solutions are mixed, and the reaction is fully stirred, the stirring speed is 100rpm, the reaction temperature is 0℃, and the reaction time is 18h, to obtain a suspension of lithium sulfide, phenolic resin and ammonium thiocyanate;

[0111] Step S3, the above suspension is filtered, the filtration form adopts vacuum filtration, the diameter of the filter hole is 5 μm, and a lithium sulfide, phenolic resin and ammonium thiocyanate ethylene glycol solution is obtained;

[0112] Step S4, the solvent is evaporated by using a rotary evaporator, the heating temperature is 70°C, the heating time is 12 h, and the vacuum degree is -0.095 MPa, and a lithium sulfide-ammonium thiocyanate@phenolic resin composite material is obtained;

[0113] Step S5, the above composite material is high-temperature calcined under a krypton protective atmosphere, the calcination temperature is 500°C, the calcination time is 24 h, and a nano lithium sulfide-C3N4@C composite material is obtained.

[0114] Example 7

[0115] Step S1, in a glove box protected by an argon atmosphere, 4.790 g of lithium hydroxide and 6.814 g of ammonium sulfide, 4.595 g of polyimide with a weight average molecular weight of 10000 and 3.065 g of N, N'-ethylene thiourea are respectively dissolved in 1 L of anhydrous n-propanol to obtain an n-propanol solution of lithium hydroxide, ammonium sulfide, polyimide and N, N'-ethylene thiourea;

[0116] Step S2, the above solutions are mixed, and the reaction is fully stirred, the stirring speed is 500 rpm, the reaction temperature is 40°C, and the reaction time is 24 h, and a lithium sulfide, polyimide and N, N'-ethylene thiourea suspension is obtained;

[0117] Step S3, the above suspension is filtered, the filtration form adopts normal pressure filtration, the diameter of the filter hole is 10 μm, and an n-propanol solution of lithium sulfide, polyimide and N, N'-ethylene thiourea is obtained;

[0118] Step S4, the solvent is evaporated by using a rotary evaporator, the heating temperature is 70°C, the heating time is 12 h, and the vacuum degree is -0.095 MPa, and a lithium sulfide-N, N'-ethylene thiourea@polyimide composite material is obtained;

[0119] Step S5, the above composite material is high-temperature calcined under a krypton protective atmosphere, the calcination temperature is 800°C, the calcination time is 48 h, and a nano lithium sulfide-C3N4@C composite material is obtained.

[0120] Example 8

[0121] Step S1, in a glove box protected by a nitrogen atmosphere, 4.239 g of lithium chloride, 3.902 g of sodium sulfide, 21.195 g of polyvinylpyrrolidone with a weight average molecular weight of 1000000 and 21.195 g of thiourea are respectively dissolved in 250 mL of anhydrous n-butanol to obtain an n-butanol solution of lithium chloride, sodium sulfide, polyvinylpyrrolidone and thiourea;

[0122] Step S2, mix the above solution, fully stirred reaction, stirring speed is 1000 rpm, reaction temperature is 20℃, reaction time is 48h, get lithium sulfide, polyvinyl pyrrolidone and thiourea suspension;

[0123] Step S3, the above suspension is centrifuged, the centrifugal speed is 15000 rpm, the centrifugal time is 1 min, get lithium sulfide, polyvinyl pyrrolidone and thiourea n-butanol solution;

[0124] Step S4, using rotary evaporator evaporate solvent, heating temperature is 30℃, heating time is 72h, vacuum degree is-0.098MPa, get lithium sulfide-thiourea@polyvinyl pyrrolidone composite material;

[0125] Step S5, the above composite material is calcined under argon protection atmosphere, calcination temperature is 200℃, calcination time is 18h, get nano lithium sulfide-C3N4@C composite material.

[0126] Example 9

[0127] Step S1, in the glove box under nitrogen atmosphere, 4.239g anhydrous lithium chloride, 3.902g sodium sulfide, 0.848g polyvinyl pyrrolidone with weight average molecular weight of 1000000 and 0.424g thiourea are respectively dissolved in 250mL anhydrous glycerol, get lithium chloride, sodium sulfide, polyvinyl pyrrolidone and thiourea glycerol solution;

[0128] Step S2, mix the above solution, fully stirred reaction, stirring speed is 1000 rpm, reaction temperature is 40℃, reaction time is 0.5h, get lithium sulfide, polyvinyl pyrrolidone and thiourea suspension;

[0129] Step S3, the above suspension is centrifuged, the centrifugal speed is 2000 rpm, the centrifugal time is 20min, get lithium sulfide, polyvinyl pyrrolidone and thiourea glycerol solution;

[0130] Step S4, MXene material Ti3C2T x The solution obtained in step S3 is added and stirred at 20℃ for 24h, the stirring speed is 2000rpm; Ti3C2T x 1% of the total mass of the solution obtained in step S3;

[0131] Step S4, using rotary evaporator evaporate solvent, heating temperature is 150℃, heating time is 1h, vacuum degree is-0.098MPa, get Ti3C2T x / lithium sulfide-thiourea@polyvinyl pyrrolidone composite material;

[0132] Step S5, high-temperature calcination of the above-mentioned composite material under an argon protective atmosphere, the calcination temperature is 600°C, and the calcination time is 1h, to obtain Ti3C2T x / nano lithium sulfide-C3N4@C composite material.

[0133] Example 10

[0134] Compared with Example 9, the difference is only that:

[0135] Step S3, filtering the above-mentioned suspension, and adopting normal pressure filtration for filtering form, the diameter of filter hole is 100nm, to obtain glycerol solution of lithium sulfide, polyvinylpyrrolidone and thiourea;

[0136] Step S4, adding MXene material Ti3C2T x to the solution obtained in step S3 and stirring at 40°C for 2h, the stirring speed is 1000rpm; Ti3C2T x accounts for 10% of the total mass of the solution obtained in step S3;

[0137] Step S4, evaporating the solvent by using a rotary evaporator, the heating temperature is 60°C, the heating time is 2h, and the vacuum degree is -0.098MPa, to obtain Ti3C2T x / lithium sulfide-thiourea@polyvinylpyrrolidone composite material;

[0138] Step S5, high-temperature calcination of the above-mentioned composite material under an argon protective atmosphere, the calcination temperature is 400°C, and the calcination time is 18h, to obtain Ti3C2T x / nano lithium sulfide-C3N4@C composite material.

[0139] It is tested that the nano lithium sulfide-C3N4@C composite material in the composite materials obtained in Examples 9 and 10 is stacked on the layered Ti3C2T x surface.

[0140] Example 11

[0141] Step S1, in a glove box protected by nitrogen atmosphere, 2.120g of anhydrous lithium chloride, 3.902g of sodium sulfide, 4.595g of polyvinylpyrrolidone with a weight average molecular weight of 1000000 and 2.298g of thiourea are respectively dissolved in 250mL of anhydrous ethanol, to obtain ethanol solution of lithium chloride, sodium sulfide, polyvinylpyrrolidone and thiourea;

[0142] Step S2, mixing the above-mentioned solutions, and fully stirring the reaction, the stirring speed is 1000rpm, the reaction temperature is 30°C, and the reaction time is 12h, to obtain suspension of lithium sulfide, polyvinylpyrrolidone and thiourea;

[0143] Step S3, centrifugation was performed on the above suspension at a centrifugal speed of 10000 rpm for 10 min, to obtain an ethanol solution of lithium sulfide, polyvinylpyrrolidone and thiourea;

[0144] Step S4, the solvent was evaporated by a rotary evaporator at a heating temperature of 80°C for 6 h under a vacuum degree of -0.098 MPa, to obtain a lithium sulfide-thiourea@polyvinylpyrrolidone composite material;

[0145] Step S5, high-temperature calcination was performed on the above composite material under an argon protective atmosphere at a calcination temperature of 550°C for 6 h, to obtain a nano lithium sulfide-C3N4@C composite material.

[0146] The X-ray diffraction pattern of the nano lithium sulfide-C3N4@C composite material prepared in this example was similar to Figure 2 The XRD results showed that, in addition to lithium sulfide and C3N4, there was no other crystalline impurity in the composite material.

[0147] The SEM of the nano lithium sulfide-C3N4@C composite material prepared in this example was similar to Figure 3 The results showed that the particle size of lithium sulfide was 300-500 nm, the thickness of the carbon shell was less than 100 nm, and the obtained lithium sulfide was nano lithium sulfide.

[0148] Example 12

[0149] Step S1, in a glove box under a nitrogen atmosphere, 8.478 g of anhydrous lithium chloride, 3.902 g of sodium sulfide, 4.595 g of polyvinylpyrrolidone with a weight average molecular weight of 800000 and 2.298 g of thiourea were respectively dissolved in 250 mL of anhydrous ethanol, to obtain an ethanol solution of lithium chloride, sodium sulfide, polyvinylpyrrolidone and thiourea;

[0150] Step S2, the above solutions were mixed and fully stirred to react at a stirring speed of 800 rpm and a reaction temperature of 40°C for 10 h, to obtain a suspension of lithium sulfide, polyvinylpyrrolidone and thiourea;

[0151] Step S3, the above suspension was filtered by vacuum suction filtration with a filter paper having a filter hole diameter of 500 nm, to obtain an ethanol solution of lithium sulfide, polyvinylpyrrolidone and thiourea;

[0152] Step S4, the solvent was evaporated by a heating table at a heating temperature of 100°C for 8 h, to obtain a lithium sulfide-thiourea@polyvinylpyrrolidone composite material;

[0153] Step S5, the above-mentioned composite material is high-temperature calcined under argon protective atmosphere, the calcination temperature is 500℃, the calcination time is 5h, and a nano lithium sulfide-C3N4@C composite material is obtained.

[0154] Comparative Example 1

[0155] Step S1, in a glove box under nitrogen atmosphere protection, 4.239g of anhydrous lithium chloride, 3.902g of sodium sulfide and 2.298g of thiourea are respectively dissolved in 250mL of anhydrous ethanol to obtain an ethanol solution of lithium chloride, sodium sulfide and thiourea;

[0156] Step S2, the above-mentioned solutions are mixed, fully stirred and reacted, the stirring speed is 1000rpm, the reaction temperature is 30℃, and the reaction time is 12h, so as to obtain a suspension of lithium sulfide and thiourea;

[0157] Step S3, the above-mentioned suspension is centrifuged, the centrifugal speed is 10000rpm, and the centrifugal time is 10min, so as to obtain an ethanol solution of lithium sulfide and thiourea;

[0158] Step S4, the solvent is evaporated by using a rotary evaporator, the heating temperature is 80℃, the heating time is 6h, and the vacuum degree is-0.098MPa, so as to obtain a lithium sulfide-thiourea composite material;

[0159] Step S5, the above-mentioned composite material is high-temperature calcined under argon protective atmosphere, the calcination temperature is 550℃, the calcination time is 6h, and a lithium sulfide-C3N4 composite material is obtained.

[0160] Comparative Example 2

[0161] Step S1, in a glove box under nitrogen atmosphere protection, 4.239g of anhydrous lithium chloride, 3.902g of sodium sulfide and 4.595g of polyvinylpyrrolidone with a molecular weight of 1000000 are respectively dissolved in 250mL of anhydrous ethanol to obtain an ethanol solution of lithium chloride, sodium sulfide and polyvinylpyrrolidone;

[0162] Step S2, the above-mentioned solutions are mixed, fully stirred and reacted, the stirring speed is 1000rpm, the reaction temperature is 30℃, and the reaction time is 12h, so as to obtain a suspension of lithium sulfide and polyvinylpyrrolidone;

[0163] Step S3, the above-mentioned suspension is centrifuged, the centrifugal speed is 10000rpm, and the centrifugal time is 10min, so as to obtain an ethanol solution of lithium sulfide and polyvinylpyrrolidone;

[0164] Step S4, the solvent is evaporated by using a rotary evaporator, the heating temperature is 80℃, the heating time is 6h, and the vacuum degree is-0.098MPa, so as to obtain a lithium sulfide@polyvinylpyrrolidone composite material;

[0165] Step S5, the above-mentioned composite material is high-temperature calcined under argon protective atmosphere, the calcination temperature is 550℃, the calcination time is 6h, and the lithium sulfide@C composite material is obtained.

[0166] Comparative Example 3

[0167] Step S1, in a glove box under nitrogen atmosphere protection, 4.239g of anhydrous lithium chloride and 3.902g of sodium sulfide are respectively dissolved in 250mL of anhydrous ethanol to obtain an ethanol solution of lithium chloride and sodium sulfide;

[0168] Step S2, the above-mentioned solutions are mixed, and the reaction is fully stirred at a stirring speed of 1000rpm and a reaction temperature of 30℃ for 12h to obtain a lithium sulfide suspension;

[0169] Step S3, the above-mentioned suspension is centrifuged at a centrifugal speed of 10000rpm for 10min to obtain an ethanol solution of lithium sulfide;

[0170] Step S4, the solvent is evaporated by using a rotary evaporator, the heating temperature is 80℃, the heating time is 6h, and the vacuum degree is-0.098MPa to obtain a lithium sulfide crude product;

[0171] Step S5, the above-mentioned composite material is high-temperature calcined under argon protective atmosphere, the calcination temperature is 550℃, the calcination time is 6h, and the lithium sulfide@C composite material is obtained.

[0172] Comparative Example 4

[0173] The difference between this comparative example and Example 1 is that the molar ratio of lithium atoms in the lithium source to-2 valence sulfur atoms in the sulfur source is 0.5:1.

[0174] The prepared composite material is excessive in the sulfur source, the obtained sample contains sulfur source impurities, has low purity, and has poor battery test performance.

[0175] Comparative Example 5

[0176] The difference between this comparative example and Example 1 is that the molar ratio of lithium atoms in the lithium source to-2 valence sulfur atoms in the sulfur source is 5:1.

[0177] The prepared composite material is excessive in the lithium source, the obtained sample contains lithium source impurities, has low purity, and has poor battery test performance.

[0178] Comparative Example 6

[0179] The difference between this comparative example and Example 1 is that the calcination temperature is 900℃.

[0180] The carbon shell of the finally prepared composite material is broken, cannot be shaped and exhibits a core-shell structure, carbon and lithium sulfide are combined together, and the melting of lithium sulfide leads to a large particle size; the nitrogen element in the nitrogen-containing precursor can leave the composite material due to high temperature, leading to the failure to achieve the purpose of introducing C3N4. The above reasons lead to poor battery performance.

[0181] Comparative Example 7

[0182] The present comparative example is compared with Example 1, the difference is that the calcination temperature is 100℃.

[0183] Low temperature makes the carbon shell unable to be shaped and unable to exhibit a core-shell structure; the lithium sulfide contains solvent molecules that cannot be removed; the nitrogen-containing precursor cannot form C3N4. The above three factors together lead to extremely poor battery performance.

[0184] The inventors of the present application also prepared the materials prepared in Examples 1-12 and Comparative Examples 1-7 as positive electrode materials, and carried out lithium-sulfur battery assembly and battery performance test, and the results are shown in Table 1:

[0185] Table 1 Performance of lithium-sulfur batteries assembled from Examples 1-12 and Comparative Examples 1-7

[0186] Sample Example Magnification (C) First discharge capacity (mAh / g) Example 1 0.5 986.2 Example 2 0.5 910.3 Example 3 0.5 869.5 Example 4 0.5 925.6 Example 5 0.5 884.1 Example 6 0.5 865.2 Example 7 0.5 857.7 Example 8 0.5 942.4 Example 9 0.5 1001.9 Example 10 0.5 1002.5 Example 11 0.5 803.2 Example 12 0.5 822.3 Comparative Example 1 0.5 718.2 Comparative Example 2 0.5 735.7 Comparative Example 3 0.5 628.3 Comparative Example 4 0.5 389.2 Comparative Example 5 0.5 443.8 Comparative Example 6 0.5 574.0 Comparative Example 7 0.5 394.1

[0187] From the test data of Examples 1-12 and Comparative Examples 1-7, it can be found that the nano lithium sulfide-C3N4@C composite material of the present application has good electrochemical performance, the lithium sulfide has high purity and small particle size, and the lithium-sulfur battery assembled with the material as the positive electrode has high discharge capacity and cycle performance. However, when lithium sulfide, lithium sulfide-C3N4 or lithium sulfide@C is directly used as an electrode material in Comparative Examples 1-3, the lithium sulfide is not coated with a carbon shell or does not introduce C3N4, and is limited by the shuttle effect of polysulfide, the lithium-sulfur battery capacity decays quickly, and thus has poor cycle performance.

[0188] By the above technical solution, the lithium sulfide prepared by the present application has higher purity and smaller particle size, and the sulfide solid-state electrolyte synthesized with the lithium sulfide as a raw material has higher ionic conductivity, and when applied to a lithium ion battery, exhibits better cycle and rate performance. C3N4 has a large specific surface area and contains a large amount of pyridine nitrogen, which can form a Li-N bond with lithium polysulfide, and inhibit the shuttle effect of lithium polysulfide through chemical adsorption; the in-situ generated carbon shell has high conductivity, which can improve the charge transfer rate, and can inhibit the excessive growth of lithium sulfide during calcination, improve the battery performance, and can inhibit the shuttle effect of lithium polysulfide and the volume expansion during the discharge process through physical confinement, inhibit the capacity decay, and improve the battery performance.

[0189] The above merely provides the preferred embodiments of the present application, and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the principles and technical scope of the present application shall fall into the scope of the present application.

Claims

1. A method for preparing a carbon-coated nanosulfurized lithium-C3N4 composite material, characterized in that, The application relates to a preparation method of a carbon-coated nano lithium sulfide-C3N4 composite material. The lithium source, the sulfur source, the organic carbon precursor, the sulfur-containing C3N4 precursor and the solvent are mixed and reacted under the protection of an inert atmosphere to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor; the solvent is removed to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture; The lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is high-temperature calcined under the protection of an inert atmosphere to obtain the carbon-coated nano lithium sulfide-C3N4 composite material.

2. The method of claim 1, wherein: The lithium source comprises a combination of one or more of lithium chloride, lithium nitrate, lithium sulfate, lithium bromide and lithium hydroxide, preferably anhydrous lithium chloride; and / or the sulfur source comprises a combination of one or more of sodium sulfide, potassium sulfide, sodium thiosulfate and ammonium sulfide, preferably anhydrous sodium sulfide; and / or the molar ratio of lithium atoms in the lithium source to -2 valence sulfur atoms in the sulfur source is (1-4):1, preferably (1.9-2.1):

1.

3. The method of claim 1, wherein: The organic carbon precursor comprises a combination of one or more of polyvinylpyrrolidone, glucose, phenolic resin and polyimide; And / or the mass ratio of the organic carbon precursor to the lithium source is 1:5-5:1; And / or the sulfur-containing C3N4 precursor comprises a combination of one or more of thiourea, thiocyanic acid, ammonium thiocyanate, N, N'-ethylene thiourea, N, N'-propylene thiourea, S-ethyl isothiourea, ethylene thiourea, sulfamoyltriazine and sulfamoylurea; And / or the mass ratio of the sulfur-containing C3N4 precursor to the lithium source is 1:10-5:

1.

4. The method of claim 1, wherein: The solvent comprises a combination of one or more of methanol, ethanol, n-propanol, isopropanol, n-butanol, ethylene glycol, glycerol and dimethyl sulfoxide; and / or the inert atmosphere is composed of an inert gas, the inert gas comprising a combination of one or more of nitrogen, helium, neon, argon and krypton, preferably nitrogen or argon. The application relates to a preparation method of a carbon-coated nano lithium sulfide-C3N4 composite material.

5. The production method according to claim 1, characterized by, The lithium source, the sulfur source, the organic carbon precursor, the sulfur-containing C3N4 precursor and the solvent are mixed and reacted under the protection of an inert atmosphere to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor; the solvent is removed to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture; The lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is high-temperature calcined under the protection of an inert atmosphere to obtain the carbon-coated nano lithium sulfide-C3N4 composite material.

6. The production method according to claim 5, characterized by, The lithium source, the sulfur source, the organic carbon precursor, the sulfur-containing C3N4 precursor and the solvent are mixed and reacted under the protection of an inert atmosphere to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor; the solvent is removed to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture; The lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is high-temperature calcined under the protection of an inert atmosphere to obtain the carbon-coated nano lithium sulfide-C3N4 composite material. The lithium source, the sulfur source, the organic carbon precursor, the sulfur-containing C3N4 precursor and the solvent are mixed and reacted under the protection of an inert atmosphere to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor; the solvent is removed to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture; The lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is high-temperature calcined under the protection of an inert atmosphere to obtain the carbon-coated nano lithium sulfide-C3N4 composite material. The lithium source, the sulfur source, the organic carbon precursor, the sulfur-containing C3N4 precursor and the solvent are mixed and reacted under the protection of an inert atmosphere to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor; the solvent is removed to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture; The lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is high-temperature calcined under the protection of an inert atmosphere to obtain the carbon-coated nano lithium sulfide-C3N4 composite material. The lithium source, the sulfur source, the organic carbon precursor, the sulfur-containing C3N4 precursor and the solvent are mixed and reacted under the protection of an inert atmosphere to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor; the solvent is removed to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture; The lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is high-temperature calcined under the protection of an inert atmosphere to obtain the carbon-coated nano lithium sulfide-C3N4 composite material. The lithium source, the sulfur source, the organic carbon precursor, the sulfur-containing C3N4 precursor and the solvent are mixed and reacted under the protection of an inert atmosphere to obtain a solution containing lithium sulfide, the organic carbon precursor and the sulfur-containing C3N4 precursor; the solvent is removed to obtain a lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture; The lithium sulfide-sulfur-containing C3N4 precursor / organic carbon precursor mixture is high-temperature calcined under the protection of an inert atmosphere to obtain the carbon-coated nano lithium sulfide-C3N4 composite material. and / or, the removing solvent is by evaporation, the temperature of the evaporation is 30-150℃, preferably 60-100℃, the time of the evaporation is 1-72h, preferably 2-12h; Preferably, the heating equipment for the evaporation includes one or more of the following: heating table, tube furnace, muffle furnace, rotary evaporator, or a combination thereof. and / or, the preparation method further comprises: adding MXene material to the solution obtained by the solid-liquid separation, and stirring, and then removing the solvent; preferably, the stirring rate is 100-2000rpm, preferably 500-1000rpm; the stirring temperature is 0℃ to the boiling point of the solvent, preferably 20-40℃; the stirring time is 0.5-48h, preferably 2-24h. Preferably, the MXene material accounts for 1-10% of the total mass of the solution containing lithium sulfide, organic carbon precursor and sulfur-containing C3N4 precursor.

7. The method of claim 1, wherein: The high-temperature calcination temperature is 200-800℃, and the time is 1-48h; preferably, the high-temperature calcination temperature is 300-600℃, and the time is 4-18h.

8. The carbon-coated nano lithium sulfide-C3N4 composite material prepared by the preparation method of any one of claims 1-7, wherein the carbon-coated nano lithium sulfide-C3N4 composite material has a core-shell structure, the core-shell structure comprises a nano lithium sulfide-C3N4 composite as a core, and a carbon shell layer coated on the surface of the core. Preferably, the nano lithium sulfide-C3N4 composite comprises uniformly mixed nano lithium sulfide and C3N4; and more preferably, the particle size of the nano lithium sulfide is 300-500nm. Preferably, the thickness of the carbon shell layer is less than 100nm. Preferably, the content of nano lithium sulfide in the carbon-coated nano lithium sulfide-C3N4 composite material is 20-80wt%, the content of C3N4 is 10-30wt%, and the content of the carbon shell layer is 1-20wt%. Preferably, the carbon-coated nano lithium sulfide-C3N4 composite material further comprises a layered MXene material, and the carbon-coated nano lithium sulfide-C3N4 composite material is stacked on the surface of the layered MXene material.

9. Use of the carbon-coated nano lithium sulfide-C3N4 composite material of claim 8 in the preparation of a lithium-sulfur battery cathode material or a lithium-sulfur battery.

10. A lithium-sulfur battery cathode material, characterized in that, Comprising: The carbon-coated nano lithium sulfide-C3N4 composite material of claim 8.

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