A high-activity cathode material catalyst for lithium-sulfur batteries and a preparation method thereof

Abstract

The present invention discloses a highly active cathode material catalyst for lithium-sulfur batteries and a preparation method thereof. The catalyst comprises a flaky nitrogen-doped carbon-based support and platinum metal nanoclusters uniformly embedded on the support surface. The platinum metal nanoclusters are 1-10 nm in size and have a mass ratio of 10-20% on the support. The catalyst is then processed through molten sulfur to serve as the cathode material for lithium-sulfur batteries. The catalyst is prepared by physical mixing a cyanamide compound and an organometallic molybdenum salt, or by adding a defective carbon material, and then directly carbonizing the material in an inert atmosphere in a tube furnace. The high-surface-weight-weight-weight-weight-weight-weight-sheet flaky nitrogen-doped carbon conductive material, with uniform distribution of platinum metal nanoclusters, physically adsorbs and chemically catalyzes polysulfides formed during the charge and discharge process on the positive electrode side of the lithium-sulfur battery, capturing the polysulfides and eliminating the "shuttle effect" during charge and discharge, ultimately improving the electrochemical performance of the electrode material. The initial discharge specific capacity can reach up to 1314.2 mAh / g, and after 100 cycles, the specific capacity decay is only 20%. This catalyst material has broad application prospects for lithium-sulfur batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur battery positive electrode materials and relates to a high-activity positive electrode material catalyst for lithium-sulfur batteries and a preparation method thereof. Background Art

[0002] With the rapid development of industrial production and science and technology, humanity's demand for energy is increasing. Energy and the environment are two of the most important issues of the 21st century. Currently, 80% of our energy supply comes from fossil fuels. The use of traditional energy sources emits greenhouse gases, which are the main culprit for climate change, including global warming. The resulting pollutants cause significant environmental damage. The use of renewable energy can reduce the environmental damage caused by traditional energy sources. Therefore, research on new energy sources that can replace traditional energy sources has become a major trend. Among them, new energy sources such as wind, tidal, and solar energy are difficult to meet production needs due to their instability. To meet the growing demand for renewable energy storage systems, the development of batteries with high energy density, long life, and high safety has become an inevitable trend.

[0003] The invention and steady improvement of rechargeable lithium-ion batteries (LiBs) have brought hope for a fossil fuel-free society. However, due to the serious safety risks caused by the electrochemical structure of lithium intercalation and the formation of lithium dendrites, its commercial development has basically stagnated. With the increasing demand for large-scale energy storage systems, the development of high-capacity LiBs seems to have reached a bottleneck. Among them, lithium-sulfur batteries with sulfur as the positive electrode and metal lithium as the negative electrode have a very high theoretical energy density (2600W h kg -1 ) and theoretical specific capacity (1672mA hg -1 ) has begun to attract increasing attention. Furthermore, elemental sulfur, the raw material for the positive electrode of lithium-sulfur batteries, is naturally abundant, inexpensive, easily accessible, and environmentally friendly, making it a promising candidate for the next generation of secondary batteries with the greatest potential and large-scale application value. However, current lithium-sulfur batteries still suffer from low sulfur loading, low conductivity of elemental sulfur, and the "shuttle effect" on the positive electrode side. The shuttle effect, in particular, leads to a significant loss of active material in the battery, causing rapid capacity decay and unsatisfactory cycle life, limiting the practical application of lithium-sulfur batteries. Therefore, designing and optimizing the electrode structure to effectively suppress the shuttle effect while simplifying operation is of great significance to the commercial application of lithium-sulfur batteries. Summary of the Invention

[0004] In response to the above problems, the present invention proposes a highly active cathode material catalyst for lithium-sulfur batteries and a preparation method thereof. Cyanamide compounds and organic metal molybdenum salts are used as raw materials, or defective carbon materials are further added. Through sufficient physical mixing and direct carbonization in a tubular furnace under an inert atmosphere, a nitrogen-doped carbon conductive material with uniformly distributed molybdenum metal clusters (highly active cathode material catalyst for lithium-sulfur batteries) can be obtained.

[0005] The technical solutions of the present invention are as follows:

[0006] A high-activity cathode material catalyst for lithium-sulfur batteries comprises a flaky nitrogen-doped carbon-based carrier and molybdenum metal nanoclusters uniformly embedded on the carrier surface. The molybdenum metal nanoclusters have a size of 1-10 nm and a mass ratio of 10-20 wt% on the carrier.

[0007] The nitrogen-doped carbon-based support is a heat treatment product of an organic molybdenum metal salt and a cyanamide compound, or a heat treatment product of an organic molybdenum metal salt, a cyanamide compound and a defective carbon-based support, wherein the defective carbon-based support is one or more of porous carbon, graphene oxide and carboxylated carbon nanotubes; and the cyanamide compound is melamine, dicyandiamide (DCD), urea or thiourea.

[0008] The present invention also provides a method for preparing a high-activity cathode material catalyst for a lithium-sulfur battery, comprising the following steps:

[0009] (1) mechanically mixing an organic molybdenum metal salt and a cyanamide compound, or continuously adding a carbon-based carrier with defects and mechanically mixing them; the mass ratio of the organic metal molybdenum salt to the cyanamide compound is 1:10-1:1000;

[0010] (2) The mixture obtained in step (1) is heated in an inert atmosphere for heat treatment to obtain a catalyst having a nitrogen-doped carbon-based support and uniformly embedded molybdenum metal nanoclusters on the support surface (a high-activity positive electrode material catalyst for lithium-sulfur batteries).

[0011] Furthermore, the mechanical mixing is carried out by ball milling or manual grinding.

[0012] Furthermore, the mass ratio of the defective carbon-based support to the organic metal molybdenum salt is 1:5-1:50.

[0013] Furthermore, the cyanamide compound is dicyandiamide.

[0014] Furthermore, the organic metal molybdenum salt is molybdenum acetylacetonate.

[0015] Furthermore, the mass ratio of the organic metal molybdenum salt to the cyanamide compound is 1:50-1:500.

[0016] Furthermore, in step (2), the mixture is heated in an inert atmosphere for heat treatment, and the process is divided into three stages: the first stage is heat treated at a temperature of 250-350°C and a heat treatment time of 1-4 hours; the second stage is heat treated at a temperature of 450-700°C and a heat treatment time of 1-4 hours; and the third stage is heat treated at a temperature of 700-1000°C and a heat treatment time of 0.5-3 hours.

[0017] Furthermore, in step (2), the mixture is heated in an inert atmosphere for heat treatment, which is divided into three stages: the first stage is heat treated at a temperature of 300-350°C and a heat treatment time of 2-3 hours; the second stage is heat treated at a temperature of 550-650°C and a heat treatment time of 3-4 hours; and the third stage is heat treated at a temperature of 750-950°C and a heat treatment time of 1-2 hours.

[0018] Furthermore, the inert atmosphere is high-purity nitrogen.

[0019] Furthermore, the size of the molybdenum metal cluster is precisely controlled according to the ratio of the organic molybdenum metal salt to dicyandiamide.

[0020] The present invention also provides application of the catalyst in highly active positive electrode materials for lithium-sulfur batteries.

[0021] The beneficial effects of the present invention include:

[0022] The present invention uses cyanamide compounds and organic metal molybdenum salts as raw materials, or continues to add defective carbon materials, through sufficient physical mixing, under an inert atmosphere, directly carbonized in a tube furnace, and can obtain nitrogen-doped conductive carbon materials with uniformly distributed molybdenum metal clusters. The conductive carbon material prepared by the present invention has a rich pore structure and a three-dimensional conductive network in the macroscopic view, a microscopic morphology with a graphene-like structure, a high specific surface area, and a thin carbon layer. At the same time, the graphene-like curled carbon layer structure plays a physical adsorption role on polysulfides, and the molybdenum metal nanoparticles (nanoclusters) embedded on the surface of the carbon layer play a chemical adsorption and catalytic conversion role on polysulfides. It is conducive to the transfer of lithium ions and electrons, and has both physical adsorption and chemical catalysis on the polysulfides formed during the charge and discharge process on the positive side of the lithium-sulfur battery, effectively captures polysulfides, suppresses the shuttle effect of polysulfides, improves the battery's electrochemical stability, rate performance and coulomb efficiency, and ultimately improves the electrochemical performance of the electrode material.

[0023] The material is applied to lithium-sulfur batteries, effectively solving the serious shuttle effect and other problems in lithium-sulfur batteries, improving battery cycle stability and rate performance, and showing excellent electrochemical performance. When the material is used as the positive electrode of a lithium-sulfur battery, after 100 cycles at a current density of 0.2C, the specific capacity is 1038.4mA hg -1The capacity loss rate per cycle is 0.2%, and the coulombic efficiency is close to 100%. After 100 cycles of aluminum foil at a current density of 0.2C, the specific capacity is only 399.2mA hg -1 In the rate performance test, the electrode loaded with molybdenum metal nanoclusters and nitrogen-doped carbon materials maintained a specific capacity of 690.2 mA h g at a current density of 2.0C. -1 When the current density returned to 0.2C, the specific capacity could be maintained at 1086.6mA hg -1 The specific capacity of the aluminum foil electrode is maintained at 427.3 mA h g at a current density of 2.0C. -1 When the current density returned to 0.2C, the specific capacity could be maintained at 605.7 mAh g -1 , the specific capacity decreases rapidly. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a scanning electron microscope image of the Mo / NC material prepared in Example 1 at a magnification of 4000.

[0025] Figure 2 This is a scanning electron microscope image of the Mo / NC material prepared in Example 1 at a magnification of 15,000.

[0026] Figure 3 This is a transmission electron microscope image of the Mo / NC material prepared in Example 1 at a magnification of 1.0M.

[0027] Figure 4 This is a transmission electron microscope image of the Mo / NC material prepared in Example 1 at a magnification of 1.2M.

[0028] Figure 5 This is a cycling performance diagram of the lithium-sulfur battery assembled with Mo / NC electrodes in Example 1 and a comparative battery at a current density of 0.2C.

[0029] Figure 6 This is a rate performance diagram of the lithium-sulfur battery assembled with Mo / NC electrodes in Example 1 and a comparative battery.

[0030] Figure 7 This is a scanning electron microscope image of the Mo / NC material prepared in Example 2. DETAILED DESCRIPTION

[0031] The specific experimental scheme of the present invention is described in detail below with reference to specific examples, but the present invention is not limited to the examples listed. Unless otherwise specified, the methods described are conventional methods, and the raw materials and instruments used can be purchased on the market.

[0032] Example 1

[0033] Weigh 20g of dicyandiamide and 0.2g of acetylacetonate molybdenum in a mass ratio of (100:1) into a reagent bottle, add 200ml of ethanol, stir mechanically and disperse until the particles are evenly dispersed, centrifuge at 5000r / min, put in a blast drying oven at 60℃, and then use a ball mill for about 12h to mix the two evenly. Place in a tube furnace, use high-purity argon as an inert protective gas, and heat at a rate of 2℃ / min throughout the process, from the initial temperature of 30℃ to 300℃, heat treatment for 2 hours, then continue to heat to 600℃, continue heat treatment for 2 hours, then heat again to 900℃, heat treatment for 3 hours, and cool to room temperature under an inert atmosphere to obtain nitrogen-doped conductive carbon material (highly active positive electrode material catalyst for lithium-sulfur batteries) Mo / NC with uniformly distributed molybdenum metal clusters. Figure 1-4 As shown, Figure 1 This is a scanning electron microscope image of the Mo / NC material at a magnification of 4000, showing the overall morphology of the carbon-supported molybdenum metal nanocluster catalyst; Figure 2 This is a scanning electron micrograph of the Mo / NC material prepared in Example 1 at a magnification of 15,000. It can be seen that the material has a rich pore structure and a three-dimensional conductive network at a macroscopic level, a graphene-like structure at a microscopic level, and a high specific surface area. Figure 3 and Figure 4 These are transmission electron microscope images of the Mo / NC material prepared in Example 1 under ultra-high magnification. It can be clearly seen that the surface of the thin carbon layer with a graphene-like structure is uniformly inlaid with molybdenum metal clusters, and the size of the metal clusters is about 1-3 nm.

[0034] Sublimed sulfur and Mo / NC conductive material were mixed and ground in a mass ratio of 3:1, placed in a reactor and vacuumed. The reactor was transferred to a muffle furnace with a melting temperature of 155°C, a heating rate of 5°C / min, and a holding time of 12h to prepare Mo / NC@S composite material.

[0035] The prepared Mo / NC@S composite was mixed with Super-P and polyvinylidene fluoride (PVDF) in a mass ratio of 7:2:1. A slurry was prepared using N-methylpyrrolidone (NMP) as the solvent. This was then doctor-drawn onto aluminum foil. The resulting aluminum electrode sheet was dried in a vacuum oven for 10 hours and then punched out using a punch. Assembly was performed in an argon-filled glove box. Coin-type cells were assembled using the doctor-drawn slurry as the positive electrode, a lithium sheet as the negative electrode, Celgard 2325 polypropylene film as the separator, and a 1M LiTFSI / DOL:DMC (1:1) solution + 1% LiNO₃ as the electrolyte. A control cell was prepared by grinding sublimated sulfur with Super-P to form a Super-P@S composite. A slurry was prepared by mixing the Super-P@S composite with polyvinylidene fluoride (PVDF) in a mass ratio of 9:1. The remaining procedures remained unchanged and the cells were assembled in the same glove box. After the battery was left to rest for 12 hours, the constant current charge-discharge cycle performance and rate performance were completed using a blue electric test system with a test voltage window of 1.7 to 2.8 V. The current density of the rate performance test was 0.2C, 0.5C, 1.0C, 2.0C (1C = 1675 mA hg -1 ). Figure 5 The cycling performance diagram of the lithium-sulfur battery and the comparative battery with Mo / NC@S composite electrode assembled in Example 1 at a current density of 0.2C. After 100 cycles at a current density of 0.2C, the Mo / NC@S composite electrode has a specific capacity of 1038.4 mA h g -1 The capacity loss rate per cycle is 0.20%, and the coulombic efficiency is close to 100%. After 100 cycles of the aluminum foil electrode at a current density of 0.2C, the specific capacity is only 399.2mA hg -1 , the capacity loss rate per cycle is 0.50%, and the Coulomb efficiency is very low. Figure 6 As shown in the rate performance test, the specific capacity of the Mo / NC@S composite electrode was maintained at 690.2 mA h g at a current density of 2.0 C. -1 When the current density returned to 0.2C, the specific capacity could be maintained at 1086.6mA hg -1 The specific capacity of the aluminum foil electrode is maintained at 427.3 mA h g at a current density of 2.0C. -1 When the current density returned to 0.2C, the specific capacity could be maintained at 605.7mA hg -1 The above data show that evenly distributed molybdenum metal nanoclusters can significantly improve the electrochemical stability, rate performance and coulombic efficiency of batteries.

[0036] Example 2

[0037] Potassium citrate / sodium citrate (6.12 g potassium citrate and 5.16 g sodium citrate) were weighed into a porcelain boat and dried in an oven for 1 hour to remove moisture. The dried potassium citrate / sodium citrate was then pyrolyzed at 800°C for 1 hour in an argon atmosphere at a heating rate of 5°C / min. Inorganic impurities were removed using 1 M HCl solution and water (18.2 MΩ). After drying at 60°C, a porous carbon support with defects was obtained.

[0038] Weigh 20g of dicyandiamide and 0.2g of acetylacetonate molybdenum in a mass ratio of (100:1) into a reagent bottle, add 200ml of ethanol, stir mechanically and disperse, and until the particles are evenly dispersed, centrifuge at 5000r / min, put in a blast drying oven at 60℃, add porous carbon with a mass ratio of 1:5 to the organic metal molybdenum salt, transfer them together to a ball mill, and then use a ball mill for about 12h to mix the two evenly, place them in a tube furnace, use argon as an inert protective gas, and heat up at a rate of 2℃ / min from the initial temperature of 30℃ to 300℃, heat treat for 2 hours, then continue to heat up to 600℃, continue heat treatment for 2 hours, then heat up to 900℃ again, heat treat for 3 hours, and cool to room temperature under an inert atmosphere to obtain nitrogen-doped conductive carbon material (highly active positive electrode material catalyst for lithium-sulfur battery) Mo / NC with uniformly distributed molybdenum metal clusters. Figure 7 shown.

Claims

1. Application of a catalyst in a highly active cathode material for a lithium-sulfur battery, characterized in that: The catalyst comprises a flaky nitrogen-doped carbon-based carrier and platinum metal nanoclusters uniformly embedded on the carrier surface. The size of the platinum metal nanoclusters is 1-10 nm, and the mass ratio of the molybdenum metal nanoclusters on the carrier is 10-20 wt%.

2. Use of a catalyst according to claim 1 in a highly active cathode material for a lithium-sulfur battery, characterized in that: The catalyst is a heat-treated product of an organic molybdenum metal salt and a cyanamide compound, or a heat-treated product of an organic molybdenum metal salt, a cyanamide compound and a defective carbon-based carrier; the defective carbon-based carrier is one or more of porous carbon, graphene oxide and carboxylated carbon nanotubes; and the cyanamide compound is melamine, dicyandiamide, urea or thiourea.

3. Use of the catalyst according to claim 1 in a highly active cathode material for lithium-sulfur batteries, characterized in that: The following steps are involved: (1) mechanically mixing an organic molybdenum metal salt and a cyanamide compound, or continuously adding a defective carbon-based carrier and mechanically mixing them; the mass ratio of the organic metal molybdenum salt to the cyanamide compound is 1:10-1:1000; (2) heating the mixture obtained in step (1) in an inert atmosphere to perform heat treatment to obtain a highly active cathode material catalyst for a lithium-sulfur battery; The heat treatment process is divided into three stages. The first stage is at a heat treatment temperature of 250-350°C and a heat treatment time of 1-4 hours. The second stage is at a heat treatment temperature of 450-700°C and a heat treatment time of 1-4 hours. The third stage is at a heat treatment temperature of 700-1000°C and a heat treatment time of 0.5-3 hours.

4. Use of the catalyst according to claim 3 in a highly active cathode material for lithium-sulfur batteries, characterized in that: The organic metal molybdenum salt is molybdenum acetylacetonate.

5. Use of the catalyst according to claim 3 in a highly active cathode material for a lithium-sulfur battery, characterized in that: The mass ratio of the organic metal molybdenum salt to the cyanamide compound is 1:50-1:

500.

6. Use of the catalyst according to claim 3 in a highly active cathode material for a lithium-sulfur battery, characterized in that: The mass ratio of the defective carbon-based support to the organic metal molybdenum salt is 1:5-1:50.

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