Nitrogen-doped porous carbon for lithium-sulfur battery

Abstract

The invention relates to nitrogen-doped porous carbon for a lithium-sulfur battery. The preparation method comprises the following steps: dipping agricultural waste pomelo peel into a urea solution for 12-36 hours, then drying in a drying oven, then putting into a tube furnace, and carrying out heat treatment in an inert atmosphere to obtain nitrogen-doped pomelo peel carbon; mixing and grinding the obtained nitrogen-doped pomelo peel carbon and an activating agent potassium hydroxide according to a mass ratio, then putting into a tubular furnace, activating at 800 DEG C in an inert atmosphere, and washing and drying to obtain the nitrogen-doped porous carbon for the lithium-sulfur battery. Porous carbon with good conductivity and well-developed pore volume can be obtained through nitrogen doping and activating agent pore forming, the porous carbon is compounded with sulfur, the nitrogen-doped sulfur-carbon composite material can be obtained, and the nitrogen-doped sulfur-carbon composite material shows excellent electrochemical performance in a positive electrode of a lithium-sulfur battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon materials, and particularly relates to a nitrogen-doped porous carbon for lithium-sulfur batteries and its application in lithium-sulfur batteries. Background Art

[0002] Currently, the contradiction between increasing energy demand and the green and low-carbon transformation has made energy issues highly regarded. Exploring and developing new renewable clean energies and transforming the energy structure are important issues that need to be solved urgently. In the new energy system, secondary batteries, such as lead-acid batteries, nickel-cadmium batteries, lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, and zinc-ion batteries, have been widely studied in recent years. Especially the lithium-ion battery composed of a LiCoO 2 positive electrode and a graphite negative electrode was successfully commercialized in 1991. However, with the further growth of energy demand, lithium-ion batteries based on graphite negative electrodes (theoretical specific capacity of 372 mAh / g) and transition metal oxide positive electrodes can no longer meet the human demand for high-energy-density energy storage systems.

[0003] In the current new energy storage systems, lithium-sulfur batteries composed of sulfur positive electrodes and lithium metal negative electrodes have received attention and developed very rapidly. Different from the traditional lithium-ion insertion and extraction mechanisms, due to the relatively high theoretical energy density (2600 Wh / kg) and sulfur positive electrode specific capacity (1675 mAh / g), lithium-sulfur batteries are considered to be one of the development directions of the next-generation power battery systems. However, there are still many problems in the application of sulfur in lithium-ion batteries: for example, the conductivity of sulfur and its reduction product lithium sulfide is low, resulting in poor rate performance of the battery; the volume change of sulfur during the lithium insertion / delithium process will damage the structure of the electrode material, thereby affecting the charge and discharge performance of the battery. In the existing reports, the sulfur volume expansion problem is mainly solved by adsorbing elemental sulfur into carbon materials (graphene, carbon nanotubes, porous carbon, and carbon nanofibers) with a large specific surface area and high porosity to form carbon / sulfur composites. However, the conductivity still restricts the application of lithium-sulfur batteries. Nitrogen doping is an effective way to improve the conductivity of sulfur-carbon composites. Moreover, nitrogen is in the fourth main group of the periodic table and is adjacent to carbon, and its atomic radius is close to that of carbon atoms. Therefore, after nitrogen enters the lattice of carbon materials, the lattice of carbon materials will not change greatly, and the structure can remain relatively complete after nitrogen doping. The nitrogen elements entering the carbon lattice mainly exist in the forms of pyrrole-type nitrogen, pyridine-type nitrogen, and graphite-type nitrogen, etc. Among them, pyrrole-type nitrogen can form a carbon five-membered ring structure with carbon atoms, pyridine-type nitrogen is directly connected to the carbon six-membered ring, and graphite-type nitrogen is located inside the graphite structure in the carbon lattice and combines with three central carbon atoms to provide free electrons to increase the conductivity of carbon materials. Therefore, nitrogen doping can effectively improve the conductivity of sulfur-carbon composites. The present invention proposes a method for preparing sulfur-carbon composites using nitrogen-doped porous carbon. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the present invention provides a method for preparing nitrogen-doped porous carbon for lithium-sulfur batteries, which can achieve nitrogen doping and obtain nitrogen-doped porous carbon for lithium-sulfur batteries with high sulfur loading.

[0005] The technical solution adopted by the present invention is as follows: The steps of a nitrogen-doped porous carbon for lithium-sulfur batteries are as follows: (1) Immerse pomelo peel in a urea solution for 12 - 36 h, then dry it in an oven at 105 °C for 12 h. After drying, place it in a tube furnace for heat treatment, and after cooling, obtain nitrogen-doped pomelo peel carbon; (2) Mix and grind the nitrogen-doped pomelo peel carbon and the activator potassium hydroxide according to a mass ratio, then place it in a tube furnace and activate it at 800 °C under a nitrogen atmosphere. After washing and drying, nitrogen-doped porous carbon for lithium-sulfur batteries is obtained.

[0006] The concentration of the urea solution described in step (1) is 1 - 2 M.

[0007] The heat treatment process described in step (1) is under a nitrogen atmosphere, with a heating rate of 5 °C / min, a heat treatment temperature of 500 - 700 °C, and a heat treatment time of 30 - 180 min.

[0008] Furthermore, the heat treatment temperature is 600 - 700 °C and the heat treatment time is 120 - 180 min.

[0009] The mass ratio described in step (2) is 1:1 - 1:2.

[0010] The washing process described in step (2) is to immerse it in a 2 M hydrochloric acid solution for 2 h, and then rinse it with deionized water 3 - 5 times.

[0011] A nitrogen-doped porous carbon for lithium-sulfur batteries, characterized in that it is applied to the positive electrode of a lithium-sulfur battery.

[0012] The beneficial effects of the present invention are as follows: (1) Through the action of urea, nitrogen elements are doped into the pomelo peel while carbonization is completed. This method can control the content of nitrogen elements to improve the conductivity of porous carbon.

[0013] (2) The nitrogen-doped porous carbon prepared by the present invention has excellent specific capacity and rate performance when compounded with sulfur as the positive electrode material of a lithium-sulfur battery, and has great potential in the application of lithium-sulfur batteries. Brief Description of the Drawings

[0014] Figure 1 It is a transmission electron microscope image of the nitrogen-doped porous carbon prepared in Case 1; Figure 2Thermogravimetric diagram of the sulfur-carbon composite material obtained by compounding the nitrogen-doped porous carbon prepared in Case 4 with sulfur. Embodiment

[0015] The present invention will be further described below in conjunction with specific embodiments and the accompanying drawings, but is not limited thereto.

[0016] Meanwhile, in the following embodiments, the experimental methods are conventional methods unless otherwise specified; the raw materials and reagents can be obtained from commercial sources unless otherwise specified. Example 1

[0017] (1) Immerse the pomelo peel in a 2M urea solution for 24 h, then dry it in an oven at 105 °C for 12 h. After drying, place it in a tube furnace, introduce nitrogen, and then start heat treatment. The heating rate is 5 °C / min, the heat treatment temperature is 600 °C, and the heat treatment time is 180 min. After cooling, nitrogen-doped pomelo peel carbon is obtained; (2) Mix and grind the nitrogen-doped pomelo peel carbon and the activator potassium hydroxide in a mass ratio of 1:1, then place it in a tube furnace and activate it at 800 °C under a nitrogen atmosphere. After cooling, immerse the obtained activated material in a 2M hydrochloric acid solution for 2 h, then rinse it 5 times with deionized water, and obtain nitrogen-doped porous carbon after drying and grinding. Example 2

[0018] (1) Immerse the pomelo peel in a 2M urea solution for 36 h, then dry it in an oven at 105 °C for 12 h. After drying, place it in a tube furnace, introduce nitrogen, and then start heat treatment. The heating rate is 5 °C / min, the heat treatment temperature is 700 °C, and the heat treatment time is 120 min. After cooling, nitrogen-doped pomelo peel carbon is obtained; (2) Mix and grind the nitrogen-doped pomelo peel carbon and the activator potassium hydroxide in a mass ratio of 1:1, then place it in a tube furnace and activate it at 800 °C under a nitrogen atmosphere. After cooling, immerse the obtained activated material in a 2M hydrochloric acid solution for 2 h, then rinse it 5 times with deionized water, and obtain nitrogen-doped porous carbon after drying and grinding. Example 3

[0019] (1) Immerse the pomelo peel in a 2M urea solution for 24 h, then dry it in an oven at 105 °C for 12 h. After drying, place it in a tube furnace, introduce nitrogen, and then start heat treatment. The heating rate is 5 °C / min, the heat treatment temperature is 700 °C, and the heat treatment time is 160 min. After cooling, nitrogen-doped pomelo peel carbon is obtained; (2) Mix and grind the nitrogen-doped pomelo peel carbon and the activator potassium hydroxide in a mass ratio of 1:2, then place it in a tube furnace and activate it at 800 °C under a nitrogen atmosphere. After cooling, immerse the obtained activated material in a 2M hydrochloric acid solution for 2 h, then rinse it 5 times with deionized water, and obtain nitrogen-doped porous carbon after drying and grinding. Example 4

[0020] (1) Immerse the pomelo peel in 2M urea solution for 12 h, then dry it in an oven at 105 °C for 12 h. After drying, place it in a tube furnace, introduce nitrogen, and then start heat treatment. The heating rate is 5 °C / min, the heat treatment temperature is 600 °C, and the heat treatment time is 150 min. After cooling, nitrogen-doped pomelo peel carbon is obtained; (2) Mix and grind the nitrogen-doped pomelo peel carbon and the activator potassium hydroxide in a mass ratio of 1:2, then place it in a tube furnace and activate it at 800 °C under a nitrogen atmosphere. After cooling, immerse the obtained activated material in 2M hydrochloric acid solution for 2 h, then rinse it with deionized water 5 times, and obtain nitrogen-doped porous carbon after drying and grinding.

[0021] Preparation of sulfur-carbon composite material: Mix the nitrogen-doped porous carbon and sulfur evenly in a mass ratio of 1:3, obtain the sulfur-carbon composite material by the melt diffusion method, and use thermogravimetry to measure the sulfur content.

[0022] Preparation of lithium-sulfur battery: Use copper foil as the current collector of the negative electrode, use Celgard 3501 as the separator, coat the sulfur-carbon composite material and the copper current collector respectively to separate the positive and negative electrodes, then wrap the whole with an aluminum-plastic film, weld the electrode tabs and place them in a glove box, and finally install lithium strips, inject electrolyte, and seal to obtain a lithium-sulfur battery.

[0023] The following are the electrochemical performances of the nitrogen-doped porous carbon and sulfur composite prepared in the above case when applied to the positive electrode of a lithium-sulfur battery.

[0024] Source of nitrogen-doped porous carbon Nitrogen content of nitrogen-doped porous carbon % Sulfur content in sulfur-carbon composite material % Discharge specific capacity mAh / g applied to lithium-sulfur battery (at 0.1C) Rate performance mAh / g applied to lithium-sulfur battery (at 0.5C) Case 1 3.3 53.22 1176 705 Case 2 5.2 56.15 1365 955 Case 3 3.1 72.65 1249 749 Case 4 2.4 75.33 1387 693

Claims

1. A nitrogen-doped porous carbon for lithium-sulfur batteries, characterized in that, the preparation steps are as follows: (1) Immerse pomelo peel in a urea solution for 12 - 36 h, then dry it in an oven at 105 °C for 12 h. After drying, place it in a tube furnace for heat treatment. After cooling, nitrogen-doped pomelo peel carbon is obtained; (2) Mix and grind the nitrogen-doped pomelo peel carbon and the activator potassium hydroxide according to a mass ratio, then place it in a tube furnace and activate it at 800 °C under a nitrogen atmosphere. After washing and drying, the nitrogen-doped porous carbon for lithium-sulfur batteries is obtained.

2. The method for preparing a nitrogen-doped porous carbon for lithium-sulfur batteries according to claim 1, characterized in that, the concentration of the urea solution described in step (1) is 1 - 2 M.

3. The method for preparing a nitrogen-doped porous carbon for lithium-sulfur batteries according to claim 1, characterized in that, the heat treatment process described in step (1) is under a nitrogen atmosphere, with a heating rate of 5 °C / min, a heat treatment temperature of 500 - 700 °C, and a heat treatment time of 30 - 180 min.

4. The method for preparing a nitrogen-doped porous carbon for lithium-sulfur batteries according to claim 1, characterized in that, the mass ratio described in step (2) is 1:1 - 1:

2.

5. The method for preparing a nitrogen-doped porous carbon for lithium-sulfur batteries according to claim 1, characterized in that, the washing process described in step (2) is to immerse it in a 2 M hydrochloric acid solution for 2 h, and then rinse it with deionized water 3 - 5 times.

6. A nitrogen-doped porous carbon for lithium-sulfur batteries, characterized in that, it is applied to the positive electrode of a lithium-sulfur battery.

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