Multilayer porous carbon material, preparation method thereof, cathode of lithium-sulfur battery, and lithium-sulfur battery

Abstract

The present invention belongs to the technical field of lithium-sulfur battery materials, and specifically discloses a multi-layered porous carbon material, a preparation method thereof, a positive electrode of a lithium-sulfur battery, and a lithium-sulfur battery. The preparation method of the multi-layered porous carbon material comprises the following steps: mixing graphite oxide, a calcium salt, and sodium citrate to form a colloid; subjecting the colloid to heat treatment to obtain a powder material; mixing magnesium powder with the powder material to obtain a mixed material, placing the mixed material in an anaerobic environment for calcination treatment, and subjecting the product obtained by calcination to pickling treatment to obtain the multi-layered porous carbon material. The multi-layered porous carbon material obtained by the preparation method of the present invention has abundant pores, small chambers, and imprints, is suitable for use as a carrier of elemental sulfur in a lithium-sulfur battery, and can effectively improve the electrochemical performance of the lithium-sulfur battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur battery materials, and particularly relates to a multi-layer porous carbon material, a preparation method thereof, a lithium-sulfur battery cathode, and a lithium-sulfur battery. Background Art

[0002] A lithium-sulfur battery is a lithium battery with sulfur as the battery cathode and metallic lithium as the anode. Its theoretical energy density is 2600 Wh·kg -1 , which is 5 to 10 times that of current commercial lithium-ion batteries. Moreover, the elemental sulfur as the battery cathode is inexpensive and widely sourced, and is considered to be the next-generation lithium-ion battery with the most potential. However, current lithium-sulfur batteries have not yet been commercialized. On the one hand, it is because the elemental sulfur and its discharge products have poor electrical conductivity, large volume changes during charge and discharge, and the active substances dissolve in the electrolyte. On the other hand, mass production has not yet been achieved. Therefore, the promotion of its commercialization is very slow. Currently, the most mainstream approach is to compound the active substance sulfur with a lightweight and highly conductive porous carbon material. Conventional preparation methods for porous carbon materials, which are the best carriers for the active substance elemental sulfur in lithium-sulfur batteries, include hard template method, wet chemical method, and high-temperature pyrolysis of polymers, etc. These preparation methods all have typical defects such as long preparation cycle, high cost, and difficult process control. At the same time, in recent years, graphene has received unprecedented attention in the field of electrochemical energy storage due to its ultra-high specific surface area and excellent electrical conductivity. Currently, the preparation of graphene-based porous materials generally also uses the wet chemical method and freeze-drying method. Among them, the wet chemical method has low yield and strong corrosion during the reaction process, while the freeze-drying method takes at least more than 20 hours, has a long time-consuming, and high production cost, which is not conducive to industrial mass production. Summary of the Invention

[0003] Aiming at the problems of high preparation cost, low efficiency, and difficulty in batch production of the carrier of the active substance of the lithium-sulfur battery cathode, the present invention provides a multi-layer porous carbon material and a preparation method thereof.

[0004] Another object of the present invention is to provide a preparation method of a lithium-sulfur battery cathode, a lithium-sulfur battery cathode, and a lithium-sulfur battery using the multi-layer porous carbon material as a carrier.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A preparation method of a multi-layer porous carbon material, comprising the following steps:

[0007] Mix graphite oxide, calcium salt, and sodium citrate to form a colloid;

[0008] Heat the colloid to obtain a powder material;

[0009] Mix the magnesium powder with the powder material to obtain a mixed material. Place the mixed material in an oxygen-free environment for calcination treatment, and perform pickling treatment on the product obtained by calcination to obtain a multi-layer porous carbon material.

[0010] Correspondingly, a multi-layer porous carbon material is prepared by the preparation method of the multi-layer porous carbon material as described above.

[0011] In addition, a preparation method of a lithium-sulfur battery cathode includes a sulfur infiltration step. The sulfur infiltration step is to use a sulfur infiltration method to attach elemental sulfur to the surface of the multi-layer porous carbon material to obtain a carbon-sulfur composite. The mass ratio of the multi-layer porous carbon material to elemental sulfur in the carbon-sulfur composite is 1:(2.5 - 5);

[0012] Wherein, the multi-layer porous carbon material is the multi-layer porous carbon material as described above.

[0013] Correspondingly, a lithium-sulfur battery cathode is prepared by the preparation method of the lithium-sulfur battery cathode as described above.

[0014] A lithium-sulfur battery includes a cathode, and the cathode is the lithium-sulfur battery cathode as described above.

[0015] The beneficial effects of the present invention are as follows:

[0016] For the preparation method of the multi-layer porous carbon material provided above, the raw materials are rich in sources and cheap, the process is simple, the reaction efficiency and yield are high. The obtained multi-layer porous carbon material is in a sheet structure, has rich pores, and the connected parts between the sheets are divided into several small chambers. There are rich imprints on the sheets, which is beneficial for filling other substances and can self-constrain the filled substances.

[0017] The multi-layer porous carbon material provided by the present invention is in a sheet structure, has rich pores formed by the pores between the sheets, and the connected parts between the sheets are divided into several small chambers. There are rich imprints on the sheets, which is beneficial for attaching elemental sulfur and constraining elemental sulfur. Therefore, it is suitable to be used as a carrier for the active material of the lithium-sulfur battery cathode.

[0018] For the preparation method of the lithium-sulfur battery cathode provided by the present invention, since the carrier of elemental sulfur uses the multi-layer porous carbon material with the aforementioned surface characteristics, elemental sulfur can be filled in the pores, small chambers and imprints of the multi-layer porous carbon material. Through the complex pores and imprints, elemental sulfur can be firmly bound, making elemental sulfur not easy to fall off and improving the loading capacity. More importantly, due to the imprints being nanoscale and having a good nano effect, under the action of a higher surface energy, elemental sulfur undergoes atomic adsorption with it, and the binding effect is more firm.

[0019] The lithium-sulfur battery provided by the present invention has a lithium-sulfur positive electrode prepared by the above method, with a multi-layered porous carbon material as the carrier. The multi-layered porous carbon material has abundant pores, small chambers, and imprints, which increases the loading amount of elemental sulfur, and the loaded elemental sulfur shows a state of being divided into countless small piles. On the one hand, it can increase its contact area with the conductive material, and on the other hand, it can effectively reduce the volume change of elemental sulfur during charge and discharge, resulting in a significant improvement in the electrochemical properties such as charge and discharge characteristics and cycling performance of the lithium-sulfur battery. Description of the Drawings

[0020] Figure 1 XRD pattern of the product obtained in the calcination step of the preparation method of the multi-layered porous carbon material provided in Comparative Example 1 of the present invention;

[0021] Figure 2 XRD pattern of the product obtained in the calcination step of the preparation method of the multi-layered porous carbon material provided in Example 1 of the present invention;

[0022] Figure 3 SEM image of the product obtained by calcining the preparation method of the multi-layered porous carbon material provided in Comparative Example 1 of the present invention;

[0023] Figure 4 SEM image of the product obtained after calcining the preparation method of the multi-layered porous carbon material provided in Example 1 of the present invention;

[0024] Figure 5 Another SEM image of the product obtained after calcining the preparation method of the multi-layered porous carbon material provided in Example 1 of the present invention;

[0025] Figure 6 TEM image of the product obtained after calcining the preparation method of the multi-layered porous carbon material provided in Example 1 of the present invention;

[0026] Figure 7 First charge-discharge curves of lithium-sulfur batteries made of the materials obtained in Example 1 and Comparative Example 1 of the present invention at 0.2C;

[0027] Figure 8 Cycling curves of lithium-sulfur batteries made of the materials obtained in Example 1 and Comparative Example 1 of the present invention at 0.5C. Detailed Description of the Invention

[0028] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0029] The present invention provides a preparation method for a multi-layered porous carbon material.

[0030] The preparation method of the multi-layered porous carbon material comprises the following steps:

[0031] 1). Dissolve graphite oxide in water to form a graphite oxide solution.

[0032] 2). Dissolve a calcium salt in water to obtain a calcium salt solution.

[0033] 3). Dissolve sodium citrate in water to obtain a sodium citrate solution.

[0034] 4). Mix the sodium citrate solution and the graphite oxide solution to obtain a mixed solution; subsequently, dissolve the calcium salt solution in the mixed solution to obtain a colloid.

[0035] 5). Heat-treat the colloid to obtain a powder material.

[0036] 6). Mix magnesium powder with the powder material to obtain a mixed material, and then place the mixed material in an anaerobic environment for calcination to obtain a calcined product; subsequently, perform pickling treatment on the product obtained by calcination to obtain the multi-layered porous carbon material.

[0037] Among them, the above-mentioned graphite oxide is also graphite acid, and the surface of the graphite layer may contain functional groups such as hydroxyl groups, carboxyl groups, and epoxy groups. Since graphite oxide has a lamellar structure of graphite, it provides a good lamellar structure basis for calcination. During calcination, the overall structure between the lamellae is not damaged, and only abundant pores are formed in the lamellae. The pores between the lamellae form abundant pore channels, and several small chambers are formed at the connecting parts between the lamellae. Preferably, the mass concentration of the graphite oxide solution is 1.5 - 5.0%, which is beneficial to the formation of a colloid.

[0038] The above-mentioned calcium salt and sodium citrate can also be directly added to the graphite oxide solution in solid form, but adding them in solution state has a higher dissolution efficiency and is beneficial to shortening the preparation time.

[0039] Preferably, the calcium salt is any one of soluble calcium salts such as calcium chloride, calcium nitrate, calcium sulfonate, and calcium acetate. The purpose of adding the calcium salt is to provide calcium ions, so that the calcium ions and citrate ions are adsorbed by graphite oxide and flocculate.

[0040] There is no special requirement for the addition order of the above-mentioned calcium salt and sodium citrate. If the calcium salt is added last and at the slow addition rate well-known in conventional laboratories, the flocculation effect will be better, and a milky colloid will be obtained.

[0041] Preferably, when the above-mentioned materials are fed, in terms of molar ratio, the calcium salt: sodium citrate = 2: (2.5 - 3.5). At this ratio, it is beneficial for calcium citrate to form and adhere to the surface of graphite oxide during flocculation.

[0042] Preferably, when the above materials are fed, by mass ratio, the graphene oxide: calcium salt = 1: (1 - 1.5).

[0043] Preferably, the temperature for heat treatment of the colloid is 110 - 150 °C. The main purpose of the heat treatment is to volatilize the moisture of the colloid, so as to facilitate the subsequent magnesium thermal reaction when mixed with magnesium powder.

[0044] Preferably, when the above graphene oxide and magnesium powder are mixed, by mass ratio, the ratio of the two components is graphene oxide: magnesium powder = 1: (1.5 - 2.5); mixing with excessive magnesium powder and the graphene oxide is beneficial to giving full play to the role of the magnesium thermal reaction, making the magnesium thermal reaction generate more heat, and promoting the decomposition of calcium carbonate decomposed from calcium citrate.

[0045] During the above calcination treatment, an anaerobic environment is required to avoid oxidation of the magnesium powder. The specific anaerobic environment can be a closed system or an inert atmosphere, such as an atmosphere of argon, nitrogen, etc.

[0046] Preferably, the temperature of the calcination treatment is 630 - 670 °C, and keep it warm for 3 - 5 h within this temperature range. At a temperature of more than 500 °C, the previously generated calcium citrate decomposes into calcium carbonate particles, and the calcium carbonate particles need to decompose at 600 °C during the magnesium thermal reaction to generate calcium oxide and carbon dioxide. The generated carbon dioxide makes the graphene oxide sheets generate abundant pores, and the pores between the sheets form pore channels.

[0047] Preferably, during the above calcination process, when the temperature reaches 500 °C during the heating-up process, control the heating-up rate ≤ 3 °C / min. Controlling the heating-up rate after 500 °C is beneficial to promoting the melting of the magnesium powder into a liquid state and penetrating into the interior of the graphene oxide to undergo the magnesium thermal reaction, which not only provides heat for the decomposition of calcium carbonate, but also reduces the oxygen-containing functional groups on the surface of the graphene oxide, enabling the obtained carbon material to have good conductivity. Part of the magnesium powder reacts with carbon dioxide, and the carbon dioxide is reduced to carbon monoxide, further creating pores in the generated carbon material, making the obtained multi-layer porous carbon material have a more abundant pore volume.

[0048] Preferably, the pickling treatment is to clean with dilute hydrochloric acid solution, dilute sulfuric acid solution, etc., so that the calcium oxide and magnesium oxide attached to the surface of the multi-layer porous carbon material are eluted.

[0049] After the pickling treatment, it also includes the process of repeatedly washing with deionized water until neutral, and then drying.

[0050] The multi-layered porous carbon material obtained by the above method maintains the gap between the lamellae while keeping the graphite lamellar structure, and makes the lamellae have abundant pores. The pores between the lamellae form connected pore channels, and there are several small chambers between the lamellae. Moreover, there are imprints on the surface of the obtained carbon material, which are mainly imprints of calcium oxide and magnesium oxide. The reason is that during the magnesiothermic reaction, oxygen-containing functional groups are reduced, and the organic carbides generated by the decomposition of calcium citrate are also transformed into carbon. The generated carbon not only adheres to the surface of the multi-layered porous carbon material but also partially covers the surfaces of calcium oxide and magnesium oxide. After elution, imprints are formed. After detection, the specific surface area of the obtained multi-layered porous carbon material is 500 - 700m 2 / g.

[0051] The preparation method of the present invention has rich raw material sources, low prices, simple preparation processes, high reaction efficiency and high yields. The obtained multi-layered porous carbon material has a lamellar structure, abundant pore channels, several small chambers are formed between the lamellae, and there are imprints of metal oxides on the lamellae, which is beneficial to filling other substances on its surface. Therefore, it can be used as a carrier for elemental sulfur in the positive electrode of a lithium-sulfur battery. When used as a carrier for elemental sulfur, the loading amount of elemental sulfur is significantly increased, and the binding ability to elemental sulfur is significantly enhanced.

[0052] Based on the above invention scheme, the present invention further provides a preparation method for the positive electrode of a lithium-sulfur battery.

[0053] The preparation method for the positive electrode of this lithium-sulfur battery is as follows: using the sulfur infiltration method, elemental sulfur is attached to the surface of the multi-layered porous carbon material obtained by the above preparation method to obtain a carbon-sulfur composite. The mass ratio of the multi-layered porous carbon material to elemental sulfur in the carbon-sulfur composite is 1:(2.5 - 5);

[0054] Mix the carbon-sulfur composite with a conductive agent, a binder, and a solvent to obtain a positive electrode slurry;

[0055] Coat the positive electrode slurry on the surface of a positive electrode current collector, and after drying, calendaring, and punching, a positive electrode of a lithium-sulfur battery is obtained.

[0056] Preferably, the sulfur infiltration method is as follows: subject the multi-layered porous carbon material to sulfur infiltration treatment at 153 - 157 °C for 10 - 15 h, and then keep it at 250 - 300 °C for 1 - 2 h. At 150 - 160 °C, elemental sulfur melts into a liquid and infiltrates into the surface and interior of the multi-layered porous carbon material, and fills into the aforementioned pore channels, small chambers, and imprints.

[0057] Preferably, when preparing the positive electrode slurry, by mass ratio, the carbon-sulfur composite: conductive agent: binder = (7.5 - 8.5):(0.8 - 1.5):(0.8 - 1.5). More preferably, the carbon-sulfur composite: conductive agent: binder = 8:1:1.

[0058] Preferably, the conductive agent is Super P, acetylene black, etc. The binder is polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyetherimide (PEI), etc.

[0059] The solvent involved in preparing the slurry can be deionized water or other common solvents used in the preparation process of lithium-ion batteries and lithium-sulfur battery slurries.

[0060] The positive electrode current collector can be aluminum foil.

[0061] Thus, a positive electrode of a lithium-sulfur battery can be obtained, and this positive electrode of the lithium-sulfur battery can be further made into a lithium-sulfur battery.

[0062] Therefore, the present invention also provides a lithium-sulfur battery, including a positive electrode, an electrolyte, a negative electrode, and a separator. Among them, the positive electrode is the positive electrode of the lithium-sulfur battery prepared as described above in the present invention.

[0063] The electrolyte, negative electrode, and separator involved in the lithium-sulfur battery provided by the present invention are all well-known in the technical field of lithium-sulfur batteries and will not be elaborated here.

[0064] For the lithium-sulfur battery manufactured using the positive electrode of the lithium-sulfur battery provided by the present invention, since the carrier of the positive electrode active material elemental sulfur uses the aforementioned multi-layered porous carbon material, the multi-layered porous carbon material has abundant pores, small chambers, and imprints, which increases the loading amount of elemental sulfur, and the loaded elemental sulfur presents a state of being divided into countless small accumulations. On the one hand, it can increase its contact area with the conductive material, and on the other hand, it can effectively reduce the volume change of elemental sulfur during charge and discharge, resulting in a significant improvement in the electrochemical performance such as the charge and discharge characteristics and cycle performance of the lithium-sulfur battery.

[0065] To better illustrate the technical solution of the present invention, the following uses multiple examples to illustrate the multi-layered porous carbon material and lithium-sulfur battery provided by the present invention.

[0066] Example 1

[0067] A preparation method of a multi-layered porous carbon material and a lithium-sulfur battery, wherein the carrier of the positive electrode active material of the lithium-sulfur battery is the multi-layered porous carbon material prepared by the preparation method of the multi-layered porous carbon material.

[0068] Among them, the preparation method of the multi-layered porous carbon material includes the following steps:

[0069] 1) Obtaining graphite oxide:

[0070] Mix 10 g of natural graphite and 230 mL of 98% concentrated sulfuric acid evenly in a three-necked flask. Place the three-necked flask in an ice-water bath and control the temperature below 20 °C.

[0071] Subsequently, add 50 g of potassium permanganate to it. The potassium permanganate is added in several portions while maintaining it in the ice-water bath with the temperature not exceeding 35 °C. After the addition of potassium permanganate is completed, raise the temperature to about 40 °C and keep the reaction for 30 min to obtain the reaction product.

[0072] Add hydrogen peroxide to the reaction product until the solution color turns golden yellow. Then filter, wash with hydrochloric acid, and wash with distilled water until the pH reaches about 7. Disperse it directly in deionized water to obtain a graphite oxide solution, and dilute it into a graphite oxide solution with a mass fraction of 2.0%.

[0073] 2) Preparation of multi-layer porous carbon material

[0074] Take 100 mL of the graphite oxide solution and add 8.8 g of sodium citrate dihydrate to it. After complete dissolution, add 2.2 g of anhydrous calcium chloride and stir until a milky colloid is formed. Dry the milky colloid on a heating platform at 120 °C into a powder.

[0075] Mix according to the mass ratio of graphite oxide:magnesium powder of 1:2. Then heat and calcine in a tubular furnace with argon flowing through. When the temperature is raised to 500 °C, control the heating rate at 3 °C / min. When the temperature is raised to 650 °C, keep the temperature for 3 h, and then cool in the argon atmosphere.

[0076] Repeatedly wash the product obtained after furnace cooling with dilute hydrochloric acid, and then repeatedly wash with deionized water until it is neutral, and dry to obtain the multi-layer porous carbon material.

[0077] 3) Preparation of lithium-sulfur battery.

[0078] Mix the multi-layer porous carbon material obtained in step 2) and elemental sulfur according to the mass ratio of 1:3. Then carry out sulfur infiltration treatment at 155 °C for 12 h, and then keep the temperature constant at 250 °C for 5 min to obtain a carbon-sulfur composite.

[0079] Mix into a slurry according to the mass ratio of carbon-sulfur composite:Super P:PVDF = 8:1:1, coat it on aluminum foil, and dry at 60 °C in vacuum to obtain the positive electrode of the lithium-sulfur battery.

[0080] Using a lithium sheet as the negative electrode, Ceglard 2500 as the separator, and an electrolyte prepared by dissolving 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a solvent with a volume ratio of 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) = 1:1, and adding 1 wt% LiNO3 to the electrolyte, a 2032-type button lithium-sulfur battery was fabricated for charge-discharge testing and cycling performance testing.

[0081] Comparative Example 1

[0082] A preparation method of a multi-layered porous carbon material and a lithium-sulfur battery, wherein the carrier of the positive active material of the lithium-sulfur battery is a multi-layered porous carbon material prepared by the preparation method of the multi-layered porous carbon material.

[0083] Among them, the preparation method of the multi-layered porous carbon material includes the following steps:

[0084] 1) Obtaining graphite oxide:

[0085] 10 g of natural graphite and 230 mL of 98% concentrated sulfuric acid were placed in a three-necked flask and mixed evenly. The three-necked flask was placed in an ice-water bath, and the temperature was controlled below 20 °C;

[0086] Subsequently, 50 g of potassium permanganate was added thereto. The potassium permanganate was added in multiple portions and kept in the ice-water bath with the temperature not exceeding 35 °C. After the addition of potassium permanganate was completed, the temperature was raised to about 40 °C and the reaction was kept for 30 min to obtain a reaction product;

[0087] Hydrogen peroxide was added to the reaction product until the solution color changed to golden yellow. Subsequently, it was filtered, washed with hydrochloric acid, and washed with distilled water until the pH reached about 7, and then directly dispersed in deionized water to obtain a graphite oxide solution, which was diluted to a graphite oxide solution with a mass fraction of 2.0%.

[0088] 2) Preparation of the multi-layered porous carbon material

[0089] 100 mL of the graphite oxide solution was taken, and 8.8 g of sodium citrate dihydrate was added thereto. After dissolution, 2.2 g of anhydrous calcium chloride was added thereto, and it was stirred until a milky colloid was formed. The milky colloid was dried into a powder on a heating platform at 120 °C;

[0090] The powder was placed in a tubular furnace with argon flowing through and heated and calcined. When the temperature was raised to 500 °C, the heating rate was controlled at 3 °C / min. When the temperature was raised to 650 °C, it was kept for 3 h, and then cooled in the argon atmosphere;

[0091] The product obtained after furnace cooling was repeatedly washed with dilute hydrochloric acid, and then repeatedly washed with deionized water until neutral, and dried to obtain a multi-layered porous carbon material.

[0092] 3) Preparation of the lithium-sulfur battery.

[0093] Mix the multi-layered porous carbon material obtained in step 2) with elemental sulfur in a mass ratio of 1:3, then carry out sulfur infiltration treatment at 155 °C for 12 h, and then keep it at a constant temperature of 250 °C for 5 min to obtain a carbon-sulfur composite;

[0094] Mix into a slurry according to the mass ratio of carbon-sulfur composite: Super P: PVDF = 8:1:1, coat it on an aluminum foil, and dry it in vacuum at 60 °C to obtain a positive electrode of a lithium-sulfur battery;

[0095] Use a lithium sheet as the negative electrode, Ceglard 2500 as the separator, and the electrolyte is 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a solvent with a volume ratio of 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) = 1:1. 1 wt% of LiNO3 is added to the electrolyte to make a 2032-type coin lithium-sulfur battery, and charge-discharge tests and cycle performance tests are carried out.

[0096] Example 2

[0097] A preparation method of a multi-layered porous carbon material and a lithium-sulfur battery, wherein the carrier of the positive electrode active material of the lithium-sulfur battery is a multi-layered porous carbon material prepared by the preparation method of the multi-layered porous carbon material.

[0098] Among them, the preparation method of the multi-layered porous carbon material includes the following steps:

[0099] 1) Obtaining graphite oxide:

[0100] Put 10 g of natural graphite and 230 mL of 98% concentrated sulfuric acid into a three-necked flask and mix evenly. Place the three-necked flask in an ice-water bath and control the temperature below 20 °C;

[0101] Then add 50 g of potassium permanganate to it. The potassium permanganate is added in several times and kept in the ice-water bath with the temperature not exceeding 35 °C. After the addition of potassium permanganate is completed, raise the temperature to about 40 °C and keep the reaction for 30 min to obtain a reaction product;

[0102] Add hydrogen peroxide to the reaction product until the solution color turns golden yellow, then filter, wash with hydrochloric acid, and wash with distilled water until the pH reaches about 7, and directly disperse it in deionized water to obtain a graphite oxide solution, which is diluted into a graphite oxide solution with a mass fraction of 2.0%.

[0103] 2) Preparation of multi-layered porous carbon material

[0104] Take 100 mL of the graphene oxide solution, add 8.8 g of sodium citrate dihydrate thereto, and after complete dissolution, add 2.2 g of anhydrous calcium chloride thereto. Stir until a milky colloid is formed, and place the milky colloid on a heating platform at 120 °C to dry into a powder;

[0105] Mix according to the mass ratio of graphene oxide:magnesium powder of 1:1, and then heat and calcine in a tubular furnace with argon flowing through. When the temperature is raised to 500 °C, control the heating rate to be 2 °C / min. When the temperature is raised to 650 °C, keep it at this temperature for 3 h, and then cool the furnace in an argon atmosphere;

[0106] Use dilute hydrochloric acid to repeatedly wash the product obtained by furnace cooling, and then use deionized water to repeatedly wash until neutral, and dry to obtain a multi-layer porous carbon material.

[0107] 3) Preparation of lithium-sulfur battery.

[0108] Mix the multi-layer porous carbon material obtained in step 2) and elemental sulfur according to the mass ratio of 1:3, and then carry out sulfur infiltration treatment at 155 °C for 12 h, and then keep it at a constant temperature of 250 °C for 5 min to obtain a carbon-sulfur composite;

[0109] Mix into a slurry according to the mass ratio of carbon-sulfur composite:Super P:PVDF = 75:15:10, coat it on an aluminum foil, and dry it in a vacuum at 60 °C to obtain a positive electrode of a lithium-sulfur battery;

[0110] Use a lithium sheet as the negative electrode, Ceglard 2500 as the separator, and the electrolyte is 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a solvent with a volume ratio of 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) = 1:1. 1 wt% of LiNO3 is added to the electrolyte to make a 2032-type coin lithium-sulfur battery, and charge-discharge tests and cycle performance tests are carried out.

[0111] Example 3

[0112] A preparation method of a multi-layer porous carbon material and a lithium-sulfur battery, wherein the carrier of the positive electrode active material of the lithium-sulfur battery is the multi-layer porous carbon material prepared by the preparation method of the multi-layer porous carbon material.

[0113] Among them, the preparation method of the multi-layer porous carbon material includes the following steps:

[0114] 1) Obtaining of graphene oxide:

[0115] Put 10 g of natural graphite and 230 mL of 98% concentrated sulfuric acid into a three-necked flask and mix evenly. Place the three-necked flask in an ice-water bath and control the temperature below 20 °C;

[0116] Subsequently, 50 g of potassium permanganate was added thereto. The potassium permanganate was added in several portions while maintaining it in an ice-water bath with a temperature not exceeding 35 °C. After the addition of potassium permanganate was completed, the temperature was raised to about 40 °C and the reaction was maintained for 30 min to obtain a reaction product;

[0117] Hydrogen peroxide was added to the reaction product until the color of the solution turned golden yellow. Subsequently, it was filtered, washed with hydrochloric acid, and washed with distilled water until the pH reached about 7, and then directly dispersed in deionized water to obtain a graphene oxide solution, which was diluted to a graphene oxide solution with a mass fraction of 2.0%.

[0118] 2) Preparation of multi-layer porous carbon material

[0119] Take 100 mL of the graphene oxide solution and add 13.2 g of sodium citrate dihydrate thereto. After complete dissolution, add 3.3 g of anhydrous calcium chloride thereto and stir until a milky colloid is formed. The milky colloid is placed on a heating platform at 120 °C and dried into a powder;

[0120] The magnesium powder and the graphene oxide were mixed in a mass ratio of 2:1, and then heated and calcined in a tubular furnace with argon flowing through. When the temperature was raised to 500 °C, the heating rate was controlled at 3 °C / min. When the temperature was raised to 650 °C, the temperature was maintained for 3 h, and then cooled in the argon atmosphere;

[0121] The product obtained after furnace cooling was repeatedly washed with dilute hydrochloric acid, and then repeatedly washed with deionized water until neutral, and dried to obtain a multi-layer porous carbon material.

[0122] 3) Preparation of lithium-sulfur battery.

[0123] The multi-layer porous carbon material obtained in step 2) and elemental sulfur were mixed in a mass ratio of 1:3, and then sulfur infiltration treatment was carried out at 155 °C for 10 h, and then kept at a constant temperature of 300 °C for 5 min to obtain a carbon-sulfur composite;

[0124] A slurry was mixed in a mass ratio of carbon-sulfur composite: Super P: PVDF = 80:10:10, coated on an aluminum foil, and dried in vacuo at 60 °C to obtain a positive electrode of a lithium-sulfur battery;

[0125] Using a lithium sheet as the negative electrode, Ceglard 2500 as the separator, and the electrolyte was 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in a solvent with a volume ratio of 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) = 1:1. 1 wt% of LiNO3 was added to the electrolyte to make a 2032-type coin lithium-sulfur battery, and charge-discharge tests and cycling performance tests were carried out.

[0126] To verify the performance of the multi-layered porous carbon material and lithium-sulfur battery obtained in the embodiments of the present invention, XRD, SEM, and TEM tests were carried out on the multi-layered porous carbon materials prepared in Example 1 and Comparative Example 1 to observe and analyze their physical properties, and then they were further fabricated into lithium-sulfur batteries for charge-discharge tests and cycle performance tests respectively. The specific tests and test results are as follows.

[0127] 1. XRD Test:

[0128] The test was carried out using a conventional X-ray diffractometer. The specific test results are as Figure 1 , 2 shown, where Figure 1 is the XRD of the material after calcination and without pickling in Comparative Example 1, Figure 2 is the XRD of Example 1 after calcination and without pickling. As can be seen from Figure 1 , when no magnesium powder was added, the material obtained by calcination mainly contained the CaCO3 phase; as can be seen from Figure 2 , after adding magnesium powder, all the calcium carbonate in the calcined material was converted into the calcium oxide phase, and the magnesium powder was completely converted into magnesium oxide.

[0129] 2. SEM and TEM Tests:

[0130] Morphology analysis was carried out using a conventional scanning electron microscope and transmission electron microscope, and the results are as Figures 3 - 6 shown. Among them, Figure 3 is the SEM image of Comparative Example 1. As can be seen from Figure 3 , the graphite oxide without adding magnesium powder showed a massive and dense structure; Figure 4 , 5 are the SEM images of Example 1. As can be seen from Figure 4 , 5 , after adding magnesium powder, a rich pore structure was shown while retaining the graphite flake structure; Figure 6 is the TEM image of Example 1. As can be seen from Figure 6 , the layers were divided into small chambers, and there were metal oxide imprints on its surface. It can be seen that the material obtained in Example 1 can load more other substances compared to the material obtained in Comparative Example 1.

[0131] 3. First Charge-Discharge Test:

[0132] Charge-discharge procedure: Constant current discharge at 0.2C to 1.6V, and stand for 10 min;

[0133] Constant current charge at 0.2C to 2.8V, and stand for 10 min.

[0134] The test results are as Figure 7 shown.

[0135] As can be seen fromFigure 7 It can be seen that the initial discharge specific capacity of the sample with magnesium powder added is 1265 mAh / g, while that of the sample without magnesium powder added is only 450 mAh / g, and the difference between the two is nearly three times. At the same time, the capacity of the first plateau of the sample with magnesium powder added is close to the theoretical value of 400 mAh / g, indicating that the sample has a good fixation effect on the flow, while the sample without magnesium powder added shows an obvious polarization phenomenon.

[0136] 4. Cycle performance test:

[0137] Specific test method: Constant current charge and discharge at a current of 0.5C, the charge and discharge voltage range is 1.6 - 2.8V, and it is left standing for 10 minutes after each cycle. The test results are as Figure 8 shown.

[0138] It can be seen from Figure 8 that during the 200 - cycle life process, the initial specific capacity of the sample with magnesium powder added at a current of 0.5C is 900 mAh / g, and its single - cycle capacity attenuation rate is 0.23%. On the contrary, the initial capacity of the comparative sample is only about 500 mAh / g, and the capacity drops to 400 mAh / g after 50 cycles, with an obvious capacity attenuation.

[0139] Since the multi - layer porous carbon material obtained by the preparation method of the present invention has good uniformity, when it is made into a lithium - sulfur battery, it also shows good battery performance. Compared with the comparative sample, its discharge capacity is almost twice that of the comparative sample. Therefore, the corresponding performance tests for Examples 2 and 3 are no longer carried out.

[0140] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a multi-layer porous carbon material, characterized in that, It includes the following steps: Mix graphite oxide, calcium salt, and sodium citrate to form a colloid. Heat the colloid to obtain a powder material. Mix magnesium powder with the powder material to obtain a mixed material. Place the mixed material in an anaerobic environment for calcination, and perform pickling treatment on the calcined product to obtain a multi-layer porous carbon material.

2. The method for preparing a multi-layer porous carbon material according to claim 1, characterized in that, In terms of molar ratio, the calcium salt: sodium citrate = 2: (2.5 - 3.5).

3. The method for preparing a multi-layer porous carbon material according to claim 1, characterized in that, The temperature of the calcination treatment is 630 - 670 °C, and keep it at this temperature range for 3 - 5 h.

4. The method for preparing a multi-layer porous carbon material according to claim 1, characterized in that, In terms of mass ratio, the graphite oxide: magnesium powder = 1: (1.5 - 2.5); and / or the graphite oxide: calcium salt = 1: (1 - 1.5).

5. A multi-layer porous carbon material, characterized in that, The multi-layered porous carbon material is prepared by using the preparation method of the multi-layered porous carbon material according to any one of claims 1 to 4; the specific surface area of the multi-layered porous carbon material is 500 to 700 m 2 / g, and the surface of the multi-layered porous carbon material has imprints.

6. A method for preparing a positive electrode of a lithium-sulfur battery, including a sulfur infiltration step, wherein the sulfur infiltration step is to use a sulfur infiltration method to attach elemental sulfur to the surface of a multi-layer porous carbon material to obtain a carbon-sulfur composite, characterized in that, The mass ratio of the multi-layer porous carbon material to elemental sulfur in the carbon-sulfur composite is 1: (2.5 - 5); Among them, the multi-layer porous carbon material is the multi-layer porous carbon material described in claim 5.

7. The method for preparing a positive electrode of a lithium-sulfur battery according to claim 6, characterized in that, The method of sulfur infiltration is: perform sulfur infiltration treatment on the multi-layer porous carbon material at 153 - 157 °C for 10 - 15 h, and then keep it at 250 - 300 °C for 1 - 2 h after sulfur infiltration treatment.

8. The method for preparing a positive electrode of a lithium-sulfur battery according to claim 6, characterized in that, It also includes the process of making the carbon-sulfur composite into a positive electrode slurry with a conductive agent and a binder. According to the mass ratio, the carbon-sulfur composite: conductive agent: binder = (7.5 - 8.5): (0.8 - 1.5): (0.8 - 1.5).

9. A positive electrode of a lithium-sulfur battery, characterized in that, The positive electrode of the lithium-sulfur battery is prepared by using any one of the preparation methods described in claims 6 - 8.

10. A lithium-sulfur battery, including a positive electrode, characterized in that, The positive electrode is the positive electrode of the lithium-sulfur battery described in claim 9.

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