Graphene coated lithium sulfide composite material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium battery materials, and particularly relates to a graphene coated lithium sulfide composite material as well as a preparation method and application thereof. The preparation method comprises the following steps: preparing a premix from a sulfur source, a lithium source and a liquid graphene precursor, pre-sintering the premix, namely curing the premix at low temperature to enable the graphene precursor to be trapezoidal to obtain a lithium sulfide precursor material uniformly coated with the graphene precursor, and finally sintering the lithium sulfide precursor material in a protective gas atmosphere. The final burning treatment is carbon thermal reduction, meanwhile, the graphene precursor is cyclized to form planar molecules, lithium sulfide obtained through reduction is subjected to in-situ coating, and the graphene-coated lithium sulfide composite material is obtained. The prepared graphene-coated lithium sulfide composite material can be applied to a lithium supplement agent, a lithium battery positive electrode material, a lithium-sulfur battery and a sulfide solid-state battery in the field of lithium batteries, and the graphene-coated lithium sulfide composite material is low in preparation cost, simple in preparation method and easy for industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery materials, and specifically relates to a graphene-coated lithium sulfide composite material and a preparation method and application thereof. Background Art

[0002] With the widespread adoption of electronic devices and vehicles like smartphones, drones, and new energy vehicles, the demand for battery energy density and safety is increasing. Traditional liquid lithium-ion batteries have limited energy density (typically 200-250 Wh / kg), and are no longer able to meet modern society's demand for high-energy-density batteries.

[0003] Lithium sulfide has important applications in lithium batteries, serving as a lithium supplement in lithium batteries, a cathode material for lithium-sulfur batteries, and a sulfide solid electrolyte. As a lithium supplement, lithium sulfide (Li2S) offers several advantages: its high theoretical capacity provides ample lithium ions for the battery; compared to lithium supplements such as metallic lithium, it is safer in batteries and does not induce lithium dendrite growth, thus avoiding the short-circuit problem caused by dendrites penetrating the separator. Lithium sulfide not only serves as a lithium supplement but also releases polysulfides to form an inorganic lithium-sulfur / fluoride-rich organic-inorganic composite SEI on the anode side, thereby suppressing the formation of metallic lithium dendrites and dead lithium. Furthermore, lithium-sulfur batteries, as a new type of high-energy-density battery system, offer significant advantages: the theoretical capacity of the sulfur cathode is as high as 1675 mAh / g, and the theoretical energy density of lithium-sulfur batteries can reach 2600 Wh / kg, far exceeding that of traditional lithium-ion batteries.

[0004] However, the electronic conductivity of lithium sulfide is poor, and its conductivity at room temperature is only 10 -13 S / cm, which hinders electron conduction between it and the current collector and reduces the electrochemical reaction rate. At the same time, lithium sulfide also has poor ionic conductivity, making it difficult to activate during the first charge and requiring a high activation barrier to overcome. As a result, the electrochemical performance of batteries (including initial coulombic efficiency and cycle stability) when lithium sulfide is used as a lithium supplement is poor, severely restricting its commercial application. Summary of the Invention

[0005] The object of the present invention is to provide a graphene-coated lithium sulfide composite material, a preparation method and application thereof. The graphene-coated lithium sulfide composite material provided by the present invention significantly improves the electronic conductivity of the material, has a high specific capacity, a high first coulombic efficiency and cycle stability, and can be applied to lithium supplements, lithium battery positive electrode materials, lithium-sulfur batteries and sulfide solid-state batteries in the field of lithium batteries. At the same time, the preparation cost of the present invention is low, the preparation method is simple, and it is easy to industrialize and produce.

[0006] In order to achieve the above object, the present invention provides the following technical solutions: The present invention provides a method for preparing a graphene-coated lithium sulfide composite material, comprising the following steps: A sulfur source, a lithium source, and a liquid graphene precursor are prepared into a premix, wherein the liquid graphene precursor comprises a liquid polyacrylonitrile oligomer, the sulfur source is calculated as a mole of the S element, the lithium source is calculated as a mole of the Li element, and the liquid graphene precursor is calculated as a mole of the C element, and the molar ratio of the sulfur source, the lithium source, and the liquid graphene precursor is (0.1-100):(0.1-100):(0.1-100); The premix is ​​heated and pre-sintered at a temperature of ≤400° C. to obtain a lithium sulfide precursor material uniformly coated with a graphene precursor; The lithium sulfide precursor material uniformly coated with the graphene precursor is subjected to a final calcination treatment in a protective gas atmosphere, wherein the temperature of the final calcination treatment is ≤1300°C and the temperature of the final calcination treatment is greater than the temperature of the pre-calcination treatment, to obtain the graphene-coated lithium sulfide composite material.

[0007] Preferably, the temperature of the final calcination treatment is 400-1300° C., the holding time is 0.5-30 h, and the heating rate from room temperature to the temperature of the final calcination treatment is 3-10° C. / min.

[0008] Preferably, the sulfur source includes one or more of sulfuric acid, sulfur and lithium sulfate; The lithium source includes one or more of lithium hydroxide, lithium sulfate, lithium carbonate and lithium oxide.

[0009] Preferably, the temperature of the pre-sintering treatment is 50-400° C., the time is 0.5-30 h, and the heating rate from room temperature to the pre-sintering treatment temperature is 3-10° C. / min.

[0010] The present invention provides a graphene-coated lithium sulfide composite material prepared by the preparation method described in the above technical solution.

[0011] The present invention provides the application of the graphene-coated lithium sulfide composite material described in the above technical solution in a lithium battery.

[0012] Preferably, the applications include lithium battery positive electrode material lithium supplement, lithium battery positive electrode material, lithium sulfur battery positive electrode material or sulfide solid electrolyte.

[0013] The present invention provides a lithium battery positive electrode material, comprising a positive electrode active component, a lithium supplement agent, a conductive agent and a binder, wherein the lithium supplement agent is the graphene-coated lithium sulfide composite material described in the above technical solution.

[0014] The present invention provides a lithium battery positive electrode sheet, comprising a current collector and a positive electrode material arranged on the surface of the current collector, wherein the positive electrode material is the lithium battery positive electrode material described in the above technical solution.

[0015] The present invention provides a lithium battery comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet is the lithium battery positive electrode sheet described in the above technical solution.

[0016] The present invention provides a preparation method of a graphene-coated lithium sulfide composite material, comprising the following steps: preparing a premix of a sulfur source, a lithium source and a liquid graphene precursor, wherein the liquid graphene precursor comprises a liquid polyacrylonitrile oligomer, the sulfur source is calculated in moles of the S element, the lithium source is calculated in moles of the Li element, the liquid graphene precursor is calculated in moles of the C element, and the molar ratio of the sulfur source, the lithium source and the liquid graphene precursor is (0.1-100):(0.1-100):(0.1-100); heating the premix to perform a pre-sintering treatment, wherein the pre-sintering temperature is ≤400°C, and the pre-sintering process is performed by low-temperature solidification. The graphene precursor is laddered, and laddering is to convert the linear polymer chain in the liquid graphene precursor into a polymer with a ladder structure or a step-like structure, laying the foundation for subsequent high-temperature carbonization, and obtaining a lithium sulfide precursor material uniformly coated with the graphene precursor; the lithium sulfide precursor material uniformly coated with the graphene precursor is subjected to a final calcination treatment in a protective gas atmosphere, the temperature of the final calcination treatment is ≤1300°C, and the temperature of the final calcination treatment is greater than the temperature of the pre-calcination treatment. The final calcination process is a carbon thermal reduction process, and the graphene precursor is cyclized at high temperature to form a planar molecule, and the reduced lithium sulfide is in situ coated to obtain the graphene-coated lithium sulfide composite material.

[0017] The present invention uses liquid graphene precursor, sulfur source and lithium source as raw materials, selects liquid polyacrylonitrile oligomer as liquid graphene precursor, optimizes the molar ratio of sulfur source, lithium source and liquid graphene precursor at the same time, solidifies and trapezoidizes the liquid graphene precursor by pre-sintering treatment in sequence, and then obtains planarized graphene molecules by high-temperature sintering method of lithium sulfide precursor material uniformly coated with graphene precursor by final sintering treatment, and at the same time, in situ coating is carried out on lithium sulfide precursor material obtained by carbon thermal reduction, and graphene in the obtained graphene-coated lithium sulfide composite material forms a three-dimensional conductive structure, which effectively improves the electronic conductivity of lithium sulfide material. Since the graphene in-situ coating layer is formed on the surface of lithium sulfide, it effectively isolates the air from direct contact with lithium sulfide, which helps to improve its chemical stability. The graphene-coated lithium sulfide composite material prepared by the present invention constructs an all-round, three-dimensional uniform coating layer, provides a "high-speed channel" for the transmission of electrons, and significantly improves the electronic conductivity of the material. In summary, the graphene-coated lithium sulfide composite material prepared by the present invention exhibits high specific capacity, high initial coulombic efficiency, and high cycling stability. It can be used in the field of lithium batteries as a lithium supplement, a lithium battery positive electrode material, a lithium-sulfur battery positive electrode material, or as a sulfide solid electrolyte. Furthermore, the present invention has low preparation cost and a simple preparation method, making it amenable to industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is an X-ray powder diffraction pattern of the graphene-coated lithium sulfide composite material prepared in Example 1 of the present invention; Figure 2 The X-ray powder diffraction comparison diagram of the graphene-coated lithium sulfide composite materials prepared in Examples 1 and 2 of the present invention and Comparative Examples 1 and 2; Figure 3 This is a charge-discharge curve of the first cycle of a button-type full battery at a rate of 0.2C, in which the graphene-coated lithium sulfide composite material prepared in Example 1 of the present invention is used as a lithium supplement in a lithium manganese oxide positive electrode material; Figure 4 This is a charge and discharge cycle diagram of a button-type full battery at a 1C rate in which the graphene-coated lithium sulfide composite material prepared in Example 1 of the present invention is used as a lithium supplement in a lithium manganese oxide positive electrode material, as well as a charge and discharge cycle diagram of a button-type full battery of a blank group (without lithium sulfide added) with a lithium manganese oxide positive electrode material at a 1C rate. DETAILED DESCRIPTION

[0019] The present invention provides a method for preparing a graphene-coated lithium sulfide composite material, comprising the following steps: A sulfur source, a lithium source, and a liquid graphene precursor are prepared into a premix, wherein the liquid graphene precursor comprises a liquid polyacrylonitrile oligomer, the sulfur source is calculated as a mole of the S element, the lithium source is calculated as a mole of the Li element, and the liquid graphene precursor is calculated as a mole of the C element, and the molar ratio of the sulfur source, the lithium source, and the liquid graphene precursor is (0.1-100):(0.1-100):(0.1-100); The premix is ​​heated and pre-sintered at a temperature of ≤400° C. to obtain a lithium sulfide precursor material uniformly coated with a graphene precursor; The lithium sulfide precursor material uniformly coated with the graphene precursor is subjected to a final calcination treatment in a protective gas atmosphere, wherein the temperature of the final calcination treatment is ≤1300°C and the temperature of the final calcination treatment is greater than the temperature of the pre-calcination treatment, to obtain the graphene-coated lithium sulfide composite material.

[0020] In the present invention, unless otherwise specified, all preparation raw materials / components are commercially available products well known to those skilled in the art.

[0021] The present invention prepares a premix by preparing a sulfur source, a lithium source and a liquid graphene precursor. The liquid graphene precursor comprises a liquid polyacrylonitrile oligomer. The sulfur source is calculated by mole of the S element, the lithium source is calculated by mole of the Li element, and the liquid graphene precursor is calculated by mole of the C element. The molar ratio of the sulfur source, the lithium source and the liquid graphene precursor is (0.1-100):(0.1-100):(0.1-100).

[0022] In the present invention, the sulfur source preferably includes one or more of sulfuric acid, sulfur, and lithium sulfate, and in embodiments, may be sulfuric acid or lithium sulfate. In embodiments of the present invention, the sulfuric acid is preferably used in the form of concentrated sulfuric acid, and the density of the concentrated sulfuric acid is preferably 1.84 g / cm³, and the mass fraction is preferably 98.3%. The lithium source preferably includes one or more of lithium hydroxide, lithium sulfate, lithium carbonate, and lithium oxide, and more preferably is lithium hydroxide or lithium sulfate. The lithium hydroxide preferably includes anhydrous lithium hydroxide and / or lithium hydroxide monohydrate, and in embodiments, may be anhydrous lithium hydroxide. In the present invention, the lithium sulfate preferably serves as both a sulfur source and a lithium source.

[0023] In the present invention, the liquid graphene precursor includes liquid polyacrylonitrile oligomer.

[0024] In the present invention, the liquid polyacrylonitrile oligomer (LPAN) is in liquid form, has a molecular weight of 100 to 100,000, and has the chemical structural formula: .

[0025] The present invention uses a liquid graphene precursor as a carbon source, and the conductive network coating formed by the liquid graphene precursor directly affects the electronic conductivity and ionic conductivity of the composite material. The present invention can significantly improve the electronic conductivity and ionic conductivity of the composite material by selecting a liquid polyacrylonitrile oligomer.

[0026] In the present invention, the sulfur source is calculated as the mole of the S element, the lithium source is calculated as the mole of the Li element, and the liquid graphene precursor is calculated as the mole of the C element. The molar ratio of the sulfur source, the lithium source and the liquid graphene precursor is (0.1~100):(0.1~100):(0.1~100), preferably (0.5~50):(0.5~50):(1~90), more preferably (0.8~30):(0.6~20):(1.5~60), further preferably (1~15):(0.8~10):(2~50), and in the embodiment it can be 1.5:1:2.5 or 0.5:1:2.5.

[0027] In the present invention, when the sulfur source and the lithium source are preferably substances other than lithium sulfate, the method for preparing the premix preferably includes: mixing the lithium source, sulfur source, water, and liquid graphene precursor to obtain a suspension; and removing the solvent from the suspension to obtain the premix. The mixing preferably includes: dissolving the lithium source in water to obtain a lithium source solution; mixing the lithium source solution and the sulfur source to obtain a mixed solution; and mixing the mixed solution with the liquid graphene precursor to obtain a suspension. The temperature for the desolventization is preferably 130-150°C. The present invention has no special requirements for the time of the desolventization, as long as the solvent is completely removed.

[0028] In the present invention, when the sulfur source and the lithium source are preferably lithium sulfate, the method for preparing the premix preferably includes: mixing the lithium sulfate and the liquid graphene precursor to obtain a suspension; and heat-treating the suspension to obtain the premix. The heat treatment temperature is preferably 130-150°C.

[0029] After obtaining the premix, the present invention heats the premix and performs a pre-sintering treatment, and the pre-sintering temperature is ≤400°C to obtain a lithium sulfide precursor material uniformly coated with a graphene precursor. In the present invention, the pre-sintering treatment is preferably carried out in a muffle furnace. The temperature of the pre-sintering treatment is preferably 50~400°C, more preferably 150~350°C, and in the embodiment it can be 300°C. The time of the pre-sintering treatment is preferably 0.5~30h, more preferably 1~24h, and in the embodiment it can be 4h. The heating rate from room temperature to the temperature of the pre-sintering treatment is preferably 3~10°C / min, more preferably 5~8°C / min. In the present invention, the role of the pre-sintering treatment is to perform low-temperature curing, trapezoidalize the graphene precursor, and obtain a lithium sulfide precursor material uniformly coated with the graphene precursor.

[0030] After obtaining the lithium sulfide precursor material uniformly coated with the graphene precursor, before performing the final calcination treatment, the present invention preferably further comprises crushing and screening the lithium sulfide precursor material uniformly coated with the graphene precursor in sequence to obtain a precursor powder, and then subjecting the precursor powder to the final calcination treatment. The specific implementation of the crushing preferably includes one or more of mechanical crushing, grinding crushing, ball milling crushing and air flow milling crushing. The sieve used for the screening is preferably 400 mesh. The present invention takes the 400 mesh sieve undersize to obtain the precursor powder. The mesh number of the precursor powder is preferably 400 mesh.

[0031] After obtaining the precursor powder (or the lithium sulfide precursor material uniformly coated with the graphene precursor), the present invention performs a final calcination treatment on the lithium sulfide precursor material uniformly coated with the graphene precursor in a protective gas atmosphere, wherein the temperature of the final calcination treatment is ≤1300°C, and the temperature of the final calcination treatment is greater than the temperature of the pre-calcination treatment, to obtain the graphene-coated lithium sulfide composite material.

[0032] In the present invention, the protective gas preferably includes nitrogen and / or a rare gas, more preferably one or more of nitrogen, argon and helium. The temperature of the final calcination treatment is preferably 400~1300°C, more preferably 500~1200°C, and in an embodiment it can be 700°C. The holding time of the final calcination treatment is 0.5~30h, preferably 5~24h, more preferably 10~12h, and in an embodiment it can be 8h. The heating rate from room temperature to the temperature of the pre-sintering treatment is preferably 3~10°C / min, more preferably 5~8°C / min. In the present invention, the role of the final calcination treatment is to perform carbon thermal reduction while the graphene precursor is cyclized at high temperature to form planar molecules, and the reduced lithium sulfide is in situ coated. In the present invention, the temperature of the final calcination treatment affects the degree of graphitization of the liquid graphene precursor as a carbon source and the degree of reduction to prepare lithium sulfide. By further optimizing the temperature of the final calcination treatment, the present invention constructs a full-dimensional, three-dimensional graphene coating layer, and reduces and prepares a stable graphene-coated lithium sulfide composite material at this temperature.

[0033] The graphene-coated lithium sulfide composite material provided by the present invention has low preparation cost, simple preparation method, and is easy to industrialize and produce.

[0034] The present invention provides a graphene-coated lithium sulfide composite material prepared by the preparation method described in the above technical solution. The graphene-coated lithium sulfide composite material prepared by the present invention has stable properties and can exhibit excellent performance in lithium-ion battery materials.

[0035] The present invention provides the application of the graphene-coated lithium sulfide composite material described in the above technical solution in a lithium battery.

[0036] In the present invention, the application preferably includes a lithium battery positive electrode material lithium supplement, a lithium battery positive electrode material, a lithium-sulfur battery positive electrode material or a sulfide solid electrolyte.

[0037] The present invention provides a lithium battery positive electrode material, comprising a positive electrode active component, a lithium supplement agent, a conductive agent and a binder, wherein the lithium supplement agent is the graphene-coated lithium sulfide composite material described in the above technical solution.

[0038] In the present invention, the positive electrode active component preferably includes lithium manganate. The conductive agent is preferably acetylene black, and the binder is preferably polyvinylidene fluoride (PVDF). The mass ratio of the positive electrode active component to the lithium supplement is preferably 95:0.1 to 0.8, and in embodiments, it can be 95:0.5. The mass ratio of the positive electrode active component to the conductive agent is preferably 95:3. The mass ratio of the positive electrode active component to the binder is preferably 95:2.

[0039] The present invention provides a lithium battery positive electrode sheet, comprising a current collector and a positive electrode material disposed on the surface of the current collector, wherein the positive electrode material is the lithium battery positive electrode material described in the above technical solution. In the present invention, the current collector is preferably an aluminum foil current collector.

[0040] The present invention provides a method for preparing a lithium battery electrode sheet according to the above technical solution, which preferably comprises the following steps: Mixing the positive electrode active component, the conductive agent, the binder, the lithium supplement agent and the organic solvent to obtain a positive electrode slurry; The positive electrode slurry is coated on the surface of the current collector and then dried to obtain the lithium battery electrode sheet.

[0041] In the present invention, the solid content of the positive electrode slurry is preferably 40-60%, more preferably 50%. The drying is preferably oven drying, the drying temperature is preferably 110-120°C, and the drying time is preferably 8-12 hours.

[0042] The present invention provides a lithium battery comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet is the lithium battery positive electrode sheet described in the above technical solution.

[0043] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0044] Example 1: 1 mol of anhydrous lithium hydroxide was dispersed in 200 mL of deionized water. Concentrated sulfuric acid (1.84 g / cm³, 98.3% mass fraction) was then slowly added. The H₂SO₄ content in the concentrated sulfuric acid was 1.5 mol. Finally, 100 g of a liquid graphene precursor (liquid polyacrylonitrile oligomer with a carbon content of 2.5 mol / 100 g) was added to obtain a suspension. The suspension was then dried at 150°C to obtain a premix. The premix was pre-sintered in a muffle furnace at 300°C for 4 hours. After cooling naturally, the premix was ground through a 400-grit sieve to produce a lithium sulfide precursor powder uniformly coated with the graphene precursor. The graphene-coated lithium sulfide precursor powder was then placed in a tube furnace and heated to 700°C under a nitrogen atmosphere at a heating rate of 5°C / min. The final calcination was performed at this temperature for 8 hours to obtain a graphene-coated lithium sulfide composite material.

[0045] Example 2: 0.5 mol of lithium sulfate was added to 100 g of a liquid graphene precursor (liquid polyacrylonitrile oligomer, wherein the carbon content of the liquid polyacrylonitrile oligomer was 2.5 mol / 100 g) to obtain a suspension. This suspension was then heated and dried at 150°C to obtain a premix. The premix was placed in a muffle furnace and pre-sintered at 300°C for 4 hours. After natural cooling, it was ground through a 400-mesh sieve to produce a lithium sulfide precursor powder uniformly coated with the graphene precursor. The lithium sulfide precursor powder uniformly coated with the graphene precursor was then placed in a tube furnace and heated to 700°C at a heating rate of 5°C / min under a nitrogen atmosphere. It was then held at this temperature for 8 hours for final calcination to obtain a graphene-coated lithium sulfide composite material.

[0046] The difference between Example 2 and Example 1 is that in Example 2, the sulfur source and the lithium source are both lithium sulfate, while in Example 1, the sulfur source is concentrated sulfuric acid and the lithium source is anhydrous lithium hydroxide.

[0047] Comparative Example 1: 1 mol of anhydrous lithium hydroxide was dispersed in 200 mL of deionized water, and concentrated sulfuric acid (1.84 g / cm³, 98.3% by mass) was slowly added. The H₂SO₄ content in the concentrated sulfuric acid was 1.5 mol. Finally, 100 g of a liquid graphene precursor (liquid polyacrylonitrile oligomer with a carbon content of 2.5 mol / 100 g) was added to obtain a suspension. The suspension was heated and dried at 150°C to obtain a premix. The premix was placed in a muffle furnace and pre-sintered at 300°C for 4 hours. After natural cooling, it was ground through a 400-mesh sieve to obtain a lithium sulfide precursor powder uniformly coated with the graphene precursor. The lithium sulfide precursor material powder uniformly coated with the graphene precursor is then placed in a tubular furnace, heated to 1500°C at a heating rate of 5°C / min under a nitrogen atmosphere, and finally sintered at a constant temperature for 8 hours to obtain a graphene-coated lithium sulfide composite material.

[0048] The difference between Comparative Example 1 and Example 1 is that the temperature of the final calcination treatment in Comparative Example 1 is 1500°C, while the temperature of the final calcination treatment in Example 1 is 700°C.

[0049] Comparative Example 2: 1 mol of anhydrous lithium hydroxide was dispersed in 200 mL of deionized water, and then concentrated sulfuric acid (1.84 g / cm³, 98.3% by mass) was slowly added. The H2SO4 content in the concentrated sulfuric acid was 1.5 mol. Finally, 100 g of a liquid graphene precursor (liquid polyacrylonitrile oligomer, wherein the carbon content of the liquid polyacrylonitrile oligomer was 2.5 mol / 100 g) was added to obtain a suspension. The suspension was heated and dried at 150°C to obtain a premix. The premix was then crushed and sieved through a 400-mesh sieve, placed in a tube furnace, and heated to 700°C at a heating rate of 5°C / min under a nitrogen atmosphere. The mixture was then calcined at this temperature for 8 hours to obtain a graphene-coated lithium sulfide composite material.

[0050] The difference between Comparative Example 2 and Example 1 is that the premix prepared in Comparative Example 2 is directly subjected to final firing without undergoing a pre-sintering step, while the premix prepared in Example 1 needs to undergo pre-sintering before final firing.

[0051] Test Example 1: The graphene-coated lithium sulfide composite materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were subjected to X-ray powder diffraction.

[0052] Figure 1 This is the X-ray powder diffraction pattern of the graphene-coated lithium sulfide composite material prepared in Example 1 of the present invention. Figure 2 1 and 2, and comparative X-ray powder diffraction patterns of the graphene-coated lithium sulfide composite materials prepared in Examples 1 and 2 of the present invention and Comparative Examples 1 and 2.

[0053] Depend on Figure 1 It can be seen that the four main peaks of the graphene-coated lithium sulfide composite material prepared in Example 1 of the present invention are located at 27°, 31°, 45°, and 53°, corresponding to the (111), (200), (220), and (311) crystal planes of Li2S, respectively. This shows that a high-purity graphene-coated lithium sulfide composite material can be obtained by the technical method of the present invention.

[0054] Depend on Figure 2It can be seen that the four main peaks of the graphene-coated lithium sulfide composite material prepared in Examples 1 and 2 of the present invention are located at 27°, 31°, 45° and 53°, corresponding to the (111), (200), (220) and (311) crystal planes of Li2S, respectively. This shows that a high-purity graphene-coated lithium sulfide composite material can be obtained by the preparation method provided by the present invention. Compared with Example 1, Comparative Example 1 shows that the final calcination temperature has an important influence on the performance of lithium sulfide in the graphene-coated lithium sulfide composite material. The XRD diffraction peak of Comparative Example 1 can no longer match the diffraction peak of Li2S. This shows that in the process of preparing the graphene-coated lithium sulfide composite material, if the final calcination temperature is too high, the formation of lithium sulfide will be affected. When the final calcination temperature is too high, part of the lithium sulfide is directly decomposed during the formation of lithium sulfide in the carbon thermal reduction process during the final calcination treatment, resulting in a reduction in the active component of lithium sulfide. Comparison between Comparative Example 2 and Example 1 shows that the carbon-coated lithium sulfide precursor curing step has an important influence on the performance of the lithium sulfide material. In Comparative Example 2, when the pre-sintering step is not performed, the graphene precursor coating layer is not cured and the ladder-shaped reaction is not performed, and a full-range and uniform carbon layer coating cannot be achieved. Therefore, when carbon thermal reduction is performed during the subsequent final firing process, insufficient reduction will result and the carbon coating layer cannot be effectively and uniformly coated, which is not conducive to the generation of lithium sulfide components.

[0055] Test Example 2: (1) The graphene-coated lithium sulfide composite material prepared in Examples 1 and 2 and Comparative Examples 1 and 2 was used as a lithium supplement for the lithium manganate positive electrode material. The following components were weighed in parts by mass: 95 parts by mass of lithium manganate (LiMn2O4) positive electrode material, 3 parts by mass of acetylene black, 2 parts by mass of PVDF, and 0.5 parts by mass of the graphene-coated lithium sulfide composite material lithium supplement; (2) dispersing the graphene-coated lithium sulfide composite material lithium supplement, lithium manganate positive electrode material, acetylene black and a binder in N-methylpyrrolidone, controlling the solid content to 50 wt % to adjust the slurry viscosity, and mixing them evenly under dry conditions to prepare a lithium battery positive electrode slurry with the graphene-coated lithium sulfide composite material as a lithium supplement; (3) coating on treated aluminum foil and drying at 120°C for 12 hours to obtain a lithium battery positive electrode sheet with a graphene-coated lithium sulfide composite material as a lithium supplement; (4) In an argon environment glove box, the dried positive and negative electrode sheets were cut into discs with a diameter of 14 mm and assembled into button-type full batteries. In this assembly, the graphite electrode was used as the negative electrode. The electrolyte used was a conventional laboratory electrolyte. The positive electrode sheet, electrolyte, diaphragm, and graphite negative electrode sheet were sequentially added to the positive electrode shell for battery assembly. After assembly, the battery was pressed and sealed in a fully automatic packaging machine. After standing for 10 hours, the battery performance was tested using a blue electric test system. The electrochemical test was carried out at a constant temperature of 25°C, and the battery test voltage range was 3.0~4.3V relative to (Li / Li + ), the cycle test current is 1C test cycle 100 cycles after the discharge capacity (mAh / g), the test results are shown in Table 1, Figure 3 and Figure 4 . Figure 3 This is a charge-discharge curve of the first cycle of a button-type full battery at a rate of 0.2C, in which the graphene-coated lithium sulfide composite material prepared in Example 1 of the present invention is used as a lithium supplement in a lithium manganese oxide positive electrode material; Figure 4 This is a charge and discharge cycle diagram of a button-type full battery at a rate of 1C in which the graphene-coated lithium sulfide composite material prepared in Example 1 of the present invention is used as a lithium supplement in a lithium manganate positive electrode material, and a charge and discharge cycle diagram of a button-type full battery of a lithium manganate positive electrode material in a blank group (without adding the graphene-coated lithium sulfide composite material lithium supplement) at a rate of 1C.

[0056] Table 1 Performance of button batteries obtained by using composite materials prepared in Examples and Comparative Examples as lithium supplements

[0057] As can be seen from Table 1, compared with Example 1 and Example 2, when lithium sulfate is selected as the sulfur source and the lithium source, the prepared graphene-coated lithium sulfide has the same excellent performance. Therefore, the prepared graphene composite lithium sulfide material is used as a lithium supplement for lithium manganese oxide positive electrode materials, which significantly improves its charge and discharge capacity, first coulomb efficiency, and long cycle effect.

[0058] Comparing Comparative Example 1 with Example 1, it can be seen that the temperature during the final calcination process has a significant impact on the performance of the lithium sulfide material. The first coulombic efficiency, first discharge specific capacity, and long cycle capacity retention rate of Comparative Example 1 are all much lower than those of Example 1. This indicates that in the process of preparing the graphene-coated lithium sulfide composite material, if the final calcination temperature is too high, the formation of lithium sulfide will be affected. When the temperature of the final calcination process is too high, part of the lithium sulfide is directly decomposed during the formation of lithium sulfide during the carbothermal reduction process during the final calcination process, resulting in a reduction in the active components of lithium sulfide. Therefore, the charge and discharge capacity, first coulombic efficiency, and long cycle performance during the charge and discharge process are all poor.

[0059] Comparing Comparative Example 2 with Example 1, it can be seen that the pre-sintering step of the carbon-coated lithium sulfide precursor has an important influence on the performance of the prepared graphene-coated lithium sulfide composite material. The first coulombic efficiency, first discharge specific capacity, and long cycle capacity retention rate of Comparative Example 2 are all much lower than those of Example 1, which shows that in the process of preparing the graphene-coated lithium sulfide composite material, the pre-sintering step is important for preparing a lithium sulfide precursor uniformly coated with a graphene precursor. When the pre-sintering step is not performed, the liquid graphene precursor is not effectively solidified and trapezoidal, and the graphene precursor cannot be fully and uniformly coated with a carbon layer. Therefore, the subsequent final carbon thermal reduction will lead to insufficient reduction and the carbon coating layer cannot be effectively and uniformly coated, the chemical stability of the sample decreases, and the electronic conductivity decreases. Therefore, when the graphene composite lithium sulfide material prepared by directly performing the final sintering without the pre-sintering step is used as a lithium replenisher for lithium manganate positive electrode material, its charge and discharge capacity, first coulombic efficiency, and long cycle effect are all poor.

[0060] Although the above embodiment provides a detailed description of the present invention, it is only a part of the embodiments of the present invention, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a graphene-coated lithium sulfide composite material, characterized in that: The following steps are involved: A sulfur source, a lithium source, and a liquid graphene precursor are prepared into a premix, wherein the liquid graphene precursor comprises a liquid polyacrylonitrile oligomer, the sulfur source is calculated as a mole of the S element, the lithium source is calculated as a mole of the Li element, and the liquid graphene precursor is calculated as a mole of the C element, and the molar ratio of the sulfur source, the lithium source, and the liquid graphene precursor is (0.1-100):(0.1-100):(0.1-100); The premix is ​​heated and pre-sintered at a temperature of ≤400° C. to obtain a lithium sulfide precursor material uniformly coated with a graphene precursor; The lithium sulfide precursor material uniformly coated with the graphene precursor is subjected to a final calcination treatment in a protective gas atmosphere, wherein the temperature of the final calcination treatment is ≤1300°C and the temperature of the final calcination treatment is greater than the temperature of the pre-calcination treatment, to obtain the graphene-coated lithium sulfide composite material.

2. The preparation method according to claim 1, characterized in that The temperature of the final sintering treatment is 400-1300° C., the holding time is 0.5-30 hours, and the heating rate from room temperature to the temperature of the final sintering treatment is 3-10° C. / min.

3. The preparation method according to claim 1, characterized in that The sulfur source includes one or more of sulfuric acid, sulfur and lithium sulfate; The lithium source includes one or more of lithium hydroxide, lithium sulfate, lithium carbonate and lithium oxide.

4. The preparation method according to claim 1, characterized in that The pre-sintering treatment temperature is 50-400° C., the time is 0.5-30 hours, and the heating rate from room temperature to the pre-sintering treatment temperature is 3-10° C. / min.

5. The graphene-coated lithium sulfide composite material prepared by the preparation method according to any one of claims 1 to 4.

6. Use of the graphene-coated lithium sulfide composite material according to claim 5 in a lithium battery.

7. The use according to claim 6, characterized in that The applications include lithium battery positive electrode material lithium supplement, lithium battery positive electrode material, lithium sulfur battery positive electrode material or sulfide solid electrolyte.

8. A lithium battery positive electrode material, characterized in that The invention comprises a positive electrode active component, a lithium supplement agent, a conductive agent and a binder, wherein the lithium supplement agent is the graphene-coated lithium sulfide composite material according to claim 5.

9. A lithium battery positive electrode plate, characterized in that: It comprises a current collector and a positive electrode material arranged on the surface of the current collector, and the positive electrode material is the lithium battery positive electrode material according to claim 8.

10. A lithium battery comprising a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, characterized in that: The positive electrode sheet is the lithium battery positive electrode sheet according to claim 9.

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