Preparation method of hierarchical porous sulfur-carbon composite material and preparation method of lithium-sulfur battery positive pole piece

Abstract

The invention relates to a preparation method of a hierarchical porous sulfur-carbon composite material, which is characterized by comprising the following steps: mixing a single-walled carbon nanotube with sulfur powder, and melting in an inert atmosphere to form a sulfur-carbon primary compound; and carrying out ball-milling mixing on the obtained sulfur-carbon primary compound and a multi-walled carbon nanotube, and then carrying out secondary melting, so as to prepare the hierarchical porous sulfur-carbon composite material. The invention also discloses a preparation method of the positive pole piece of the lithium-sulfur battery. The secondary melting and low-rotating-speed ball-milling process significantly improves the electrode performance: the sulfur dispersion is uniform, the conductivity is good, the cycling stability is strong, and the mechanical property of the pole piece is good.

Description

Technical Field

[0001] The present invention relates to a composite material of carbon nanotubes and sulfur powder, which is applied to the positive electrode of a lithium battery. The present invention also relates to the application of a mixture of the composite material of carbon nanotubes and sulfur powder and a conductive agent in the positive electrode of a lithium battery, belonging to the technical field of lithium-sulfur batteries. Background Art

[0002] Lithium-sulfur batteries are considered an important direction for the next generation of energy storage technology due to their theoretical energy density of up to 2600Wh / kg. However, the insulation of the sulfur positive electrode, the shuttle effect of polysulfides (LiPS), and the collapse of the electrode structure under high sulfur loading seriously hinder their application.

[0003] Current research mostly uses single carbon materials (such as porous carbon and carbon nanotubes) combined with sulfur, but there are inherent defects: although porous carbon can physically adsorb LiPS, its microporous structure easily collapses during the sulfur melting process, resulting in a sulfur loading capacity generally lower than 5 mg / cm 2 , and the conductivity is insufficient; single-walled carbon nanotubes (SWCNTs) have a high specific surface area (> 1000m 2 / g) and conductivity, but excessive addition will sacrifice volume energy density and uneven sulfur distribution. In addition, single-walled carbon nanotubes are extremely difficult to disperse and are difficult to apply in lithium-sulfur batteries through simple processes. The tube confinement effect of multi-walled carbon nanotubes (MWCNTs) can inhibit the diffusion of LiPS, but when the tube diameter is too large (>20nm), sulfur is easy to escape, and the high aspect ratio makes it difficult to disperse the slurry.

[0004] Ketjen black has an ultra-high specific surface area (~1400m 2 / g) and porous structure, it is often used as a high-end conductive agent in electrodes, which can greatly improve the performance of the electrode, and has a strong adsorption capacity, which can improve the shuttle effect of polysulfides. However, when pure Ketjen black is used in lithium-sulfur batteries, Ketjen black will absorb a large amount of water and binder molecules in the water-based binder (such as CMC / SBR), resulting in a sharp increase in the viscosity of the slurry, poor leveling, and local stress concentration and cracking during the drying process. In addition, Ketjen black forms a highly conductive network through long-chain aggregates, but its rigid structure is difficult to adapt to volume shrinkage when the electrode is drying, and is prone to brittle fracture. Super P is often used as a lithium-sulfur positive electrode conductive agent. Its spherical particle structure and low specific surface area can reduce the adsorption of water and binders and improve the fluidity of the slurry. However, Super P has a small particle size and a large specific surface area, and is prone to agglomeration. After agglomeration, it will cause uneven dispersion in the electrode material, making it difficult to form an effective conductive network. The conductivity of some areas is poor, affecting the charge and discharge efficiency and consistency of the battery. Super P itself does not have the ability to adsorb or fix polysulfides, and cannot effectively inhibit the shuttling of polysulfides between the positive and negative electrodes.

[0005] In summary, developing a preparation method that combines multi-carbon synergy, high sulfur loading and structural stability, and matching it with a suitable conductive / adhesive system and low-damage process has become a key path to breaking through the technical bottleneck of lithium-sulfur batteries. Summary of the Invention

[0006] The first technical problem to be solved by the present invention is to provide a method for preparing a multi-level porous sulfur-carbon composite material with stable structure and good electrode performance in response to the above-mentioned technical status quo.

[0007] The second technical problem to be solved by the present invention is to provide a method for preparing a positive electrode sheet for a lithium-sulfur battery with a stable structure and good electrode performance in response to the above-mentioned technical status quo.

[0008] The technical solution adopted by the present invention to solve the above-mentioned first technical problem is: a method for preparing a multi-level pore sulfur-carbon composite material, characterized in that it includes the following steps: mixing single-walled carbon nanotubes and sulfur powder in a mass ratio of 1:10 to 1:20, melting at 150 to 160°C under an inert atmosphere for 12 to 15 hours to form a sulfur-carbon primary composite material; ball-milling the obtained sulfur-carbon primary composite material with multi-walled carbon nanotubes in a mass ratio of 7:1 to 3, and then melting for a second time at 150 to 160°C for 6 to 15 hours to obtain a multi-level pore sulfur-carbon composite material.

[0009] Preferably, the specific surface area of ​​the single-walled carbon nanotubes is greater than 1000 m 2 / g, tube diameter 1~2nm.

[0010] Preferably, the multi-walled carbon nanotubes have a diameter of 10 to 20 nm and an aspect ratio greater than 1000.

[0011] Preferably, the ball milling speed is 300 to 500 rpm, and the ball milling time is 2 to 4 hours.

[0012] The technical solution adopted by the present invention to solve the above second technical problem is: a method for preparing a positive electrode sheet of a lithium-sulfur battery, characterized by comprising the following steps:

[0013] The multi-level porous sulfur-carbon composite material, Super P and Ketjen black are mixed in a mass ratio of 9:1.2-2.0:0.5-1.0, and a transparent adhesive solution of sodium carboxymethyl cellulose is added, and the solid content is adjusted to 25%-50%. After dispersion, the mixture is coated on aluminum foil and vacuum dried at 60-80°C for 6-12 hours.

[0014] Preferably, the mixing of the multi-level pore sulfur-carbon composite material, Super P and Ketjen Black comprises the following steps:

[0015] First, Super P and Ketjen Black were premixed to obtain a mixed conductive agent; then the mixed conductive agent and the multi-level porous sulfur-carbon composite material were added into the stirring tank in three batches, with a dispersion speed of 200-400 rpm and a total dispersion time of 1 hour.

[0016] Preferably, the transparent glue solution of sodium carboxymethyl cellulose is prepared by the following steps:

[0017] Sodium carboxymethyl cellulose powder and deionized water are pre-dissolved in a mass ratio of 1:50 to 1:100, stirred at a speed of 500 to 800 rpm for 2 to 4 hours to obtain a transparent adhesive solution.

[0018] Preferably, the degree of substitution of the transparent sodium carboxymethyl cellulose glue is 0.8-1.2, the weight percentage of the aqueous solution is 2%, and the viscosity at 25° C. is 1500-2500 mPa·s.

[0019] Compared with the existing technology, the advantages of the present invention are: through the coordinated design of single-walled / multi-walled carbon nanotubes and the ratio of gradient conductive agents, a multi-level conductive-confined network construction under high sulfur load is achieved, and the proportion of Super P in the composite conductive agent is increased, which can be used as a "lubricant" to fill the porous network formed by Ketjen black, reduce the sudden change in slurry viscosity, and make the shear stress distribution more uniform during coating, thereby inhibiting cracking. While greatly retaining the conductivity and adsorption properties of Ketjen black, it effectively improves the film-forming performance of Ketjen black. Combined with the secondary melting and low-speed ball milling process, the electrode performance is significantly improved: sulfur is evenly dispersed, the conductivity is good, and the cycle stability is strong (1C cycle 200 times capacity retention rate>85%), and the electrode has good mechanical properties. This composite formula and process greatly solves the problems of lithium-sulfur battery shuttle effect and rapid capacity decay in traditional solutions. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The graph is a cycle performance curve of Example 2, Comparative Example 1 and Comparative Example 2 at a rate of 0.5C. DETAILED DESCRIPTION

[0021] The present invention will be described in further detail below with reference to the accompanying drawings and embodiments.

[0022] Example 1, a gradient composite multiple sulfur melting method for lithium battery positive electrode active material and composite carbon material pole piece preparation, its specific preparation and components are as follows: single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (SWCNT), the specific surface area of ​​the SWCNT is greater than 1000m 2 / g, tube diameter 1-2nm; MWCNT tube diameter 10-20nm, aspect ratio>100,

[0023] (1) Single-walled carbon nanotubes (SWCNTs) and sulfur powder were mixed in a mass ratio of 1:10 and melted at 155 °C for 12 h under an inert atmosphere to form a SWCNT / sulfur primary composite;

[0024] (2) The obtained SWCNT / sulfur composite was mixed with multi-walled carbon nanotubes (MWCNTs) in a mass ratio of 7:2 by ball milling at a ball milling speed of 300 rpm for 4 hours, and then melted for a second time at 155°C for 15 hours to obtain a hierarchical sulfur-carbon composite material.

[0025] (3) Proportion: CMC (sodium carboxymethyl cellulose) powder and deionized water were pre-dissolved in a mass ratio of 1:20, stirred at a speed of 500 rpm for 4 hours to obtain a transparent CMC glue solution.

[0026] (4) Super P (particle size 30-50 nm, specific surface area 60-80 m 2 / g) and Ketjen black (particle size 100-200nm, specific surface area 1400-1600m 2 / g) were pre-mixed in a mass ratio of 3:1 to prepare a composite conductive agent;

[0027] (5) The premixed conductive agent and sulfur-carbon composite material were added into a stirring tank containing transparent CMC glue in three batches, and the solid content of the slurry was controlled to be 40%, the dispersion speed was 400 rpm, and the total dispersion time was 1 hour.

[0028] (6) The slurry formed in the above process was coated on aluminum foil and vacuum dried at 60°C for 12 hours to obtain a positive electrode sheet for a lithium-sulfur battery.

[0029] Example 2, a gradient composite multiple sulfur melting method for lithium battery positive electrode active material and composite carbon material pole piece preparation, its specific preparation and components are as follows: single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (SWCNT), the specific surface area of ​​the SWCNT is greater than 1000m 2 / g, tube diameter 1-2nm; MWCNT tube diameter 10-20nm, aspect ratio>100,

[0030] (1) Single-walled carbon nanotubes (SWCNTs) and sulfur powder were mixed in a mass ratio of 1:15 and melted at 155 °C for 12 h under an inert atmosphere to form a SWCNT / sulfur primary composite;

[0031] (2) The obtained SWCNT / sulfur composite was mixed with multi-walled carbon nanotubes (MWCNTs) in a mass ratio of 7:2 by ball milling at a ball milling speed of 500 rpm for 4 hours, and then melted for a second time at 155°C for 15 hours to obtain a hierarchical sulfur-carbon composite material.

[0032] (3) Proportion: CMC powder and deionized water were pre-dissolved in a mass ratio of 1:20, stirred at 500 rpm for 4 hours to obtain a transparent CMC glue solution.

[0033] (4) Super P (particle size 30-50 nm, specific surface area 60-80 m 2 / g) and Ketjen black (particle size 100-200nm, specific surface area 1400-1600m 2 / g) were pre-mixed in a mass ratio of 3:1 to prepare a composite conductive agent;

[0034] (5) The premixed conductive agent and sulfur-carbon composite material were added into a stirring tank containing transparent CMC glue in three batches, and the solid content of the slurry was controlled to be 40%, the dispersion speed was 400 rpm, and the total dispersion time was 1 hour.

[0035] (6) The slurry formed in the above process was coated on aluminum foil and vacuum dried at 60°C for 12 hours to obtain a positive electrode sheet for a lithium-sulfur battery.

[0036] Example 3, a gradient composite multiple sulfur melting method for lithium battery positive electrode active material and composite carbon material pole piece preparation, its specific preparation and components are as follows: single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (SWCNT), the specific surface area of ​​the SWCNT is greater than 1000m 2 / g, tube diameter 1-2nm; MWCNT tube diameter 10-20nm, aspect ratio>100,

[0037] (1) Single-walled carbon nanotubes (SWCNTs) and sulfur powder were mixed in a mass ratio of 1:20 and melted at 155 °C for 12 h under an inert atmosphere to form a SWCNT / sulfur primary composite;

[0038] (2) The obtained SWCNT / sulfur composite was mixed with multi-walled carbon nanotubes (MWCNTs) in a mass ratio of 7:2 by ball milling at a ball milling speed of 500 rpm for 4 hours, and then melted for a second time at 155°C for 15 hours to obtain a hierarchical sulfur-carbon composite material.

[0039] (3) Proportion: CMC powder and deionized water were pre-dissolved in a mass ratio of 1:20, stirred at 500 rpm for 4 hours to obtain a CMC transparent adhesive solution.

[0040] (4) Super P (particle size 30-50 nm, specific surface area 60-80 m 2 / g) and Ketjen black (particle size 100-200nm, specific surface area 1400-1600m 2 / g) were pre-mixed in a mass ratio of 3:1 to prepare a composite conductive agent;

[0041] (5) The premixed conductive agent and sulfur-carbon composite material were added into a stirring tank containing transparent CMC glue in three batches, and the solid content of the slurry was controlled to be 40%, the dispersion speed was 400 rpm, and the total dispersion time was 1 hour.

[0042] (6) The slurry formed in the above process was coated on aluminum foil and vacuum dried at 60°C for 12 hours to obtain a positive electrode sheet for a lithium-sulfur battery.

[0043] Comparative Example 1: A lithium-sulfur battery positive electrode plate, comprising a multi-walled carbon nanotube sulfur-carbon composite, a conductive agent, and a binder, wherein the binder is the CMC binder described in Example 1. Multi-walled carbon nanotubes (MWCNTs) and sublimed sulfur were ball-milled in a mass ratio of 7:2 at a speed of 500 rpm for 4 hours, followed by secondary melting at 155°C for 15 hours to produce the multi-walled carbon nanotube sulfur-carbon composite. Super P and Ketjen Black were premixed in a mass ratio of 3:1. The premixed conductive agent and the multi-walled carbon nanotube sulfur-carbon composite were added to a ball mill at a speed of 400 rpm for 4 hours. The weight ratio of the sulfur / MWCNT composite to the composite conductive agent was 9:2. The CMC transparent adhesive described in Example 1 was added to form a slurry, with a solids content of 40%. The slurry was then coated on aluminum foil and vacuum-dried at 60°C for 12 hours to produce a lithium-sulfur battery positive electrode plate.

[0044] Comparative Example 2: A lithium-sulfur battery positive electrode plate comprises a multi-level pore sulfur-carbon composite material, a conductive agent, and a binder. The binder is the CMC binder described in Example 3. Single-walled carbon nanotubes (SWCNTs) and sulfur powder were mixed in a mass ratio of 1:15 and melted at 155°C under an inert atmosphere for 12 hours to form a primary SWCNT / sulfur composite. The resulting SWCNT / sulfur composite was then ball-milled with multi-walled carbon nanotubes (MWCNTs) in a mass ratio of 7:2 at 500 rpm for 4 hours, followed by a secondary melt-melting at 155°C for 15 hours to form a multi-level pore sulfur-carbon composite material. Super P conductive agent and the multi-level pore sulfur-carbon composite material were separately added to a ball mill at 400 rpm for 4 hours. The weight ratio of the multi-level pore sulfur-carbon composite material to the Super P conductive agent was 9:2. The CMC transparent adhesive described in Example 1 was added to form a slurry, and the solid content of the slurry was controlled to be 40%. The slurry was then coated on an aluminum foil and vacuum dried at 60° C. for 12 h to obtain a positive electrode sheet for a lithium-sulfur battery.

[0045] Performance test, comparison of the lithium-sulfur battery positive electrode sheets prepared in Example 2 and Comparative Examples 1 and 2 in the cycle performance of Li-S batteries. Figure 1As shown in the figure, the cycling performance changes of different sulfur-carbon composite materials are presented. As the number of cycles increases, the specific capacity of each system gradually decays. Among them, the "multi-level pore sulfur-carbon composite material + composite conductive agent" (red curve) has a high initial specific capacity and the slowest decay rate. It still maintains a high specific capacity at 200 cycles and has the best cycling stability; the "multi-level pore sulfur-carbon composite material + Super P conductive agent" (black curve) has a weaker specific capacity and a slower decay rate. At the same rate, the specific capacity is lower than that of the multi-level pore sulfur-carbon composite material + composite conductive agent system; the "multi-walled carbon nanotube sulfur-carbon composite material + composite conductive agent" (blue curve) has the fastest decay rate and the least help in capacity decay, but it is better than the multi-level pore sulfur-carbon composite material + Super P conductive agent system in terms of initial discharge capacity.

Claims

1. A method for preparing a multi-level pore sulfur-carbon composite material, characterized in that The method comprises the following steps: mixing single-walled carbon nanotubes and sulfur powder in a mass ratio of 1:10 to 1:20, melting the mixture at 150 to 160° C. for 12 to 15 hours under an inert atmosphere to form a sulfur-carbon primary composite; and ball-milling the obtained sulfur-carbon primary composite with multi-walled carbon nanotubes in a mass ratio of 7:1 to 3, followed by secondary melting at 150 to 160° C. for 6 to 15 hours to prepare a hierarchical pore sulfur-carbon composite material.

2. The method for preparing a positive electrode active material for a lithium battery according to claim 1, wherein The specific surface area of ​​the single-walled carbon nanotubes is greater than 1000 m 2 / g, tube diameter 1~2nm.

3. The method for preparing a positive electrode active material for a lithium battery according to claim 1, wherein The multi-walled carbon nanotubes have a diameter of 10 to 20 nm and an aspect ratio greater than 1000.

4. The method for preparing a positive electrode active material for a lithium battery according to claim 1, wherein The ball mill has a rotation speed of 300 to 500 rpm and a ball milling time of 2 to 4 hours.

5. A method for preparing a positive electrode sheet for a lithium-sulfur battery using the multi-level pore sulfur-carbon composite material according to any one of claims 1 to 4, characterized in that The steps include: The multi-level porous sulfur-carbon composite material, Super P and Ketjen black are mixed in a mass ratio of 9:1.2-2.0:0.5-1.0, and a transparent adhesive solution of sodium carboxymethyl cellulose is added, and the solid content is adjusted to 25%-50%. After dispersion, the mixture is coated on aluminum foil and vacuum dried at 60-80°C for 6-12 hours.

6. The preparation method according to claim 5, characterized in that The mixing of the multi-level pore sulfur-carbon composite material, Super P and Ketjen Black comprises the following steps: First, Super P and Ketjen Black were premixed to obtain a mixed conductive agent; then the mixed conductive agent and the multi-level porous sulfur-carbon composite material were added into the stirring tank in three batches, with a dispersion speed of 200-400 rpm and a total dispersion time of 1 hour.

7. The preparation method according to claim 5, characterized in that The transparent glue solution of sodium carboxymethyl cellulose is prepared by the following steps: Sodium carboxymethyl cellulose powder and deionized water are pre-dissolved in a mass ratio of 1:50 to 1:100, stirred at a speed of 500 to 800 rpm for 2 to 4 hours to obtain a transparent adhesive solution.

8. The preparation method according to claim 7, characterized in that The degree of substitution of the transparent sodium carboxymethyl cellulose glue is 0.8-1.2, the weight percentage of the aqueous solution is 2%, and the viscosity at 25° C. is 1500-2500 mPa·s.

9. The preparation method according to claim 5, characterized in that Multi-level pore sulfur-carbon composite material, Super P, and Ketjen Black are mixed in a mass ratio of 9:1.5.0:0.5.

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