A core-shell structure lithium-sulfur battery positive electrode material and a preparation method thereof

Abstract

The present application relates to a core-shell structure lithium-sulfur battery positive electrode material and a preparation method thereof, and the core-shell structure nitrogen-doped graphene-coated sulfur / carbon nanotube (or carbon nanofiber) composite material is synthesized by a simple liquid phase method in one step. The three-dimensional network formed by the carbon nanotube not only improves the utilization rate of sulfur and improves the rate performance, but also is located between the core and shell, which is beneficial to relieve the volume expansion of elemental sulfur and improve the cycle performance; the coating of graphene can slow down the "shuttle effect" and improve the rate performance, and the coordination of the nitrogen-containing functional groups on the graphene and the polysulfide will further improve the "shuttle effect" and improve the cycle life of the lithium-sulfur battery. The physicochemical synergistic effect of the nitrogen-doped graphene and the carbon nanotube improves the discharge specific capacity, the rate performance and the cycle life of the lithium-sulfur battery. The preparation method is simple and the material has excellent performance, and will have a wide application prospect in the fields of unmanned aerial vehicles and electric vehicles with high rate and high energy density requirements.

Description

Technical Field

[0001] This invention relates to a core-shell structured cathode material for lithium-sulfur batteries, which can improve the specific capacity, rate performance, and cycle life of lithium-sulfur batteries. The invention also relates to a method for preparing the aforementioned lithium-sulfur battery cathode material. Background Technology

[0002] With the rapid development of industries such as drones and electric vehicles, and the proposal of "carbon peaking" and "carbon neutrality" goals, the market demand for high-rate and high-energy-density batteries (≥350Wh / kg) is becoming increasingly strong. However, traditional battery materials are approaching their theoretical capacity and cannot meet the further development requirements of the market. Therefore, developing new lithium-ion rechargeable batteries with high performance, low cost, and environmental friendliness is of great strategic significance. Elemental sulfur has high theoretical specific capacity (1675mAh / g) and high theoretical specific energy (2600Wh / kg), as well as advantages such as abundant reserves, low price, and environmental friendliness. Lithium-sulfur batteries are considered one of the most promising battery systems to achieve an energy density of over 500Wh / kg. However, elemental sulfur itself has poor conductivity (at room temperature of 25 degrees Celsius, the conductivity is only 5×10⁻⁶). -30 The sulfur content (S / cm) of lithium-sulfur batteries is low, and the polysulfides formed with lithium ions during charging and discharging are easily soluble in organic electrolytes. This leads to poor cycle performance, low specific capacity, and poor rate performance in lithium-sulfur batteries constructed with elemental sulfur as the cathode, thus restricting their further marketization. Currently, many research institutes and companies at home and abroad are dedicated to the research and development of lithium-sulfur batteries. Common methods include improving sulfur utilization through conductive materials; using porous materials to fix and adsorb polysulfides; utilizing the coordination effect of chemical bonds with polysulfides; and using the physicochemical barrier effect of coatings and functional membranes to reduce the "shuttle effect" of lithium-sulfur batteries, thereby improving their discharge specific capacity and cycle life. Although these methods have achieved good results, research on high-rate and high-energy-density lithium-sulfur batteries is relatively limited. The high instantaneous power required for startup and acceleration in applications such as drones and electric vehicles is a current challenge for lithium-sulfur batteries. To secure a place in the future energy field, it is necessary to develop cathode materials for lithium-sulfur batteries with high rate capability, high specific capacity, and long cycle performance.

[0003] Therefore, it is particularly important to develop a lithium-sulfur battery cathode material that is simple to operate, highly feasible, and has excellent performance, as well as its preparation method. The development of high-performance lithium-sulfur battery cathode materials is of great significance to the industrialization of lithium-sulfur batteries, and even to promoting the industrialization of drones and new energy vehicles. Summary of the Invention

[0004] The first technical problem to be solved by this invention is to provide a lithium-sulfur battery cathode material with nitrogen-doped graphene coated with sulfur / carbon nanotubes (or carbon nanofibers) having a core-shell structure.

[0005] The second technical problem to be solved by the present invention is to provide a method for preparing the above-mentioned core-shell structured lithium-sulfur battery cathode material.

[0006] Using the preparation method of this invention, core-shell structured lithium-sulfur battery cathode materials with significantly improved rate performance and cycle life can be prepared in one step via a liquid phase method.

[0007] To solve the first technical problem mentioned above, the technical solution adopted by the present invention is as follows:

[0008] A core-shell structured lithium-sulfur battery cathode material is characterized by a three-dimensional conductive network composed of carbon nanotubes (or carbon nanofibers) dispersed inside sulfur particles and between nitrogen-doped graphene layers and sulfur particles, with the nitrogen-doped graphene layers covering the outside of the sulfur / carbon nanotubes (or carbon nanofibers) to form a core-shell structure.

[0009] The sulfur microparticles and the carbon nanotubes encapsulated within them constitute the core of the composite, while the outermost layer is covered by a nitrogen-doped graphene layer to form the shell of the composite. The three-dimensional network structure of carbon nanotubes between the core and shell plays a role in conducting electricity and buffering.

[0010] The three-dimensional conductive network constructed from carbon nanotubes (or carbon nanofibers) not only improves the utilization rate of active sulfur and enhances the rate performance of the battery, but its location between the core and shell also helps alleviate the volume expansion of elemental sulfur, improving the cycle performance of the battery. The graphene coating can mitigate the "shuttle effect" and improve the rate performance of the battery. The coordination between the nitrogen-containing functional groups on the graphene and the polysulfides will further improve the "shuttle effect" and increase the cycle life of the lithium-sulfur battery. The physicochemical synergistic effect of nitrogen-doped graphene and carbon nanotubes (or carbon nanofibers) improves the discharge specific capacity, rate performance, and cycle life of the sulfur cathode and the lithium-sulfur battery.

[0011] To solve the second technical problem mentioned above, the technical solution adopted by the present invention is as follows:

[0012] A method for preparing the above-mentioned core-shell structured lithium-sulfur battery cathode material is characterized by: preparing a nitrogen-doped graphene-coated sulfur / carbon nanotube (or carbon nanofiber) composite with a core-shell structure in one step via a liquid phase method.

[0013] Preferably, the preparation method specifically includes the following steps:

[0014] Carbon nanotubes (or carbon nanofibers), graphene oxide, urea (nitrogen source), and thiosulfate are added to an aqueous solution containing polyvinylpyrrolidone in a certain mass ratio and subjected to ultrasonic treatment. After ultrasonic treatment for 1 to 2 hours, an acid solution is slowly injected while the mixed solution is stirred until the reaction is completed. After filtration, washing, and drying, a core-shell structured composite of nitrogen-doped graphene-coated sulfur / carbon nanotubes (or carbon nanofibers) (a lithium-sulfur cathode material with a core-shell structure) is obtained.

[0015] The concentration of polyvinylpyrrolidone is 0.5% to 3%, the stirring rate is 500 rpm to 1000 rpm, and the reaction temperature of the mixed solution is 60 to 90 degrees Celsius.

[0016] The carbon nanotubes or carbon nanofibers have an outer diameter of 5 nm to 50 nm and a length of 0.5 μm to 5 μm.

[0017] The thickness of the graphene oxide is 3nm to 30nm, and the length is 2μm to 10μm.

[0018] The thiosulfate is one of sodium thiosulfate, ammonium thiosulfate, or potassium thiosulfate.

[0019] The mass ratio of the carbon nanotubes (or carbon nanofibers), graphene oxide, urea, and thiosulfate is 1–3:0.2–1:0.1–0.5:25–40.

[0020] The acid solution is one of hydrochloric acid, phosphoric acid, nitric acid, oxalic acid, citric acid, and ascorbic acid, with a concentration of 10% to 40%.

[0021] The principle of this invention is as follows: A core-shell structured nitrogen-doped graphene-coated sulfur / carbon nanotube (or carbon nanofiber) composite is synthesized in one step using a liquid-phase method. Redox and disproportionation reactions are used to prepare nitrogen-doped graphene from graphene oxide, as well as to form sulfur particles and coat the sulfur / carbon nanotubes (or carbon nanofibers) with graphene. Urea, the nitrogen source, forms nitrogen doping on the graphene through the reduction of graphene oxide. The three-dimensional network distribution of carbon nanotubes or carbon nanofibers within the sulfur particles effectively improves the utilization rate of active sulfur and enhances the discharge specific capacity and rate performance of the sulfur cathode. It effectively alleviates the volume expansion of sulfur particles between the sulfur particles and graphene, which is beneficial for improving the battery's cycle performance. The graphene coating not only mitigates the "shuttle effect" of polysulfides through physical barriers but also further reduces the diffusion loss of polysulfides through the chemical coordination effect between nitrogen-containing functional groups and polysulfides, improving the battery's capacity retention. Graphene can also further improve the electronic conductivity of sulfur, thereby further enhancing the discharge specific capacity and rate performance of the sulfur cathode. The synergistic effect of nitrogen-doped graphene and carbon nanotubes can effectively improve the discharge specific capacity, rate performance, and cycle life of sulfur cathodes and lithium-sulfur batteries. Furthermore, the preparation method of the described lithium-sulfur battery cathode material is simple and easy to scale up for production, thus promoting the industrial application of high-rate and high-energy-density lithium batteries.

[0022] Compared with the prior art, the advantages of the present invention are: (1) The preparation method is simple. The desired nitrogen-doped graphene-coated sulfur / carbon nanotube (or carbon nanofiber) composite with core-shell structure can be obtained in one step by a simple liquid phase method; (2) The effect is significant. Through the physicochemical synergistic effect of carbon nanotube (or carbon nanofiber) with three-dimensional network structure and nitrogen-doped graphene, the specific capacity, rate performance and cycle life of lithium-sulfur batteries can be effectively improved; (3) The operation is convenient and easy to scale up production. Attached Figure Description

[0023] Figure 1 This is a scanning electron microscope (SEM) image of the nitrogen-doped graphene-coated sulfur / carbon nanotube composite with a core-shell structure in Example 1.

[0024] Figure 2 This is a transmission electron microscope (TEM) image of the nitrogen-doped graphene-coated sulfur / carbon nanotube composite with a core-shell structure in Example 3.

[0025] Figure 3 This is a graph showing the cycling performance of the sulfur / carbon nanotube composite in Comparative Example 1 at a rate of 0.5C.

[0026] Figure 4 This is a cycling performance diagram of the nitrogen-doped graphene-coated sulfur / carbon nanotube composite with a core-shell structure in Example 1 at a rate of 0.5C. Detailed Implementation

[0027] The present invention will now be described in detail through exemplary embodiments.

[0028] The above-mentioned sulfur cathode material is prepared by a conventional liquid-phase method.

[0029] Examples of the preparation method are as follows.

[0030] Example 1

[0031] Preparation of cathode material: 2g of carbon nanotubes (outer diameter ~30nm, length 3~5μm), 0.5g of graphene oxide (thickness ~10nm, size 4~6μm), 0.5g of urea and 23g of sodium thiosulfate were added to 2L of 1% polyvinylpyrrolidone aqueous solution and sonicated for 1 hour. Then, while stirring the solution, a 25% hydrochloric acid solution was added dropwise until the reaction was completed. After filtration, washing and drying, a nitrogen-doped graphene-coated sulfur / carbon nanotube core-shell structure composite (lithium-sulfur battery cathode material) with a particle size of about 2.5μm was obtained. The sulfur content in the composite was about 64%. The reaction temperature of the solution was 80 degrees and the stirring rate was 600 rpm.

[0032] Preparation of lithium-sulfur batteries: A nitrogen-doped graphene-coated sulfur / carbon nanotube core-shell composite, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was then added dropwise, followed by ball milling at 400 rpm for 6 hours. The milled slurry was uniformly coated onto aluminum foil and vacuum dried (60°C for 8 hours) to serve as the positive electrode of the lithium-sulfur battery. The negative electrode was a lithium metal sheet, the battery separator was a polypropylene porous membrane, and the electrolyte was a 1 mol / L LiTFSI solution of DOL + DME (volume ratio 1:1) containing 1% LiNO3. After assembling the coin cells in an argon-atmospheric glove box, battery performance was tested using a charge-discharge apparatus. The voltage range for battery testing was 1.6V–2.8V.

[0033] Example 2

[0034] The only difference between this embodiment and Embodiment 1 is that:

[0035] Preparation of cathode material: 2g of carbon nanotubes, 0.5g of graphene oxide, 0.5g of urea and 20g of sodium thiosulfate were added to 2 liters of 1.5% polyvinylpyrrolidone aqueous solution and sonicated for 1 hour. Then, while stirring the solution, 25% hydrochloric acid solution was added dropwise until the reaction was completed. After filtration, washing and drying, a nitrogen-doped graphene-coated sulfur / carbon nanotube core-shell structure composite with a particle size of about 2 micrometers was obtained. The sulfur content in the composite was about 61%.

[0036] Example 3

[0037] The only difference between this embodiment and Embodiment 1 is that:

[0038] Preparation of cathode material: 5g of carbon nanotubes, 0.5g of graphene oxide, 0.5g of urea and 60g of sodium thiosulfate were added to 2.5L of 0.5% polyvinylpyrrolidone aqueous solution and sonicated for 1 hour. Then, while stirring the solution, a 30% hydrochloric acid solution was added dropwise until the reaction was completed. After filtration, washing and drying, a nitrogen-doped graphene-coated sulfur / carbon nanotube core-shell structure composite with a particle size of about 4.4 micrometers was obtained. The sulfur content in the composite was about 68%. The reaction temperature of the solution was 90 degrees Celsius.

[0039] Example 4

[0040] The only difference between this embodiment and Embodiment 1 is that:

[0041] Preparation of cathode material: 1.8g of carbon nanotubes, 0.3g of graphene oxide, 0.3g of urea and 20g of ammonium thiosulfate were added to 2 liters of 1% polyvinylpyrrolidone aqueous solution and sonicated for 2 hours. Then, while stirring the solution, a 30% hydrochloric acid solution was added dropwise until the reaction was completed. After filtration, washing and drying, a nitrogen-doped graphene-coated sulfur / carbon nanotube core-shell structure composite with a particle size of about 2.2 micrometers was obtained. The sulfur content in the composite was about 66%. The reaction temperature of the solution was 80 degrees Celsius.

[0042] Example 5

[0043] The only difference between this embodiment and Embodiment 1 is that:

[0044] Preparation of cathode material: 5g of carbon nanotubes, 2g of graphene oxide, 2g of urea and 55g of ammonium thiosulfate were added to 3.5L of 1.5% polyvinylpyrrolidone aqueous solution and sonicated for 2 hours. Then, while stirring the solution, 30% hydrochloric acid solution was added dropwise until the reaction was completed. After filtration, washing and drying, a nitrogen-doped graphene-coated sulfur / carbon nanotube core-shell structure composite with a particle size of about 1.6 micrometers was obtained. The sulfur content in the composite was about 62%. The reaction temperature of the solution was 90 degrees Celsius.

[0045] Example 6

[0046] The only difference between this embodiment and Embodiment 1 is that:

[0047] Preparation of cathode material: 5g of carbon nanotubes, 1g of graphene oxide, 1g of urea and 60g of ammonium thiosulfate were added to 4 liters of 2% polyvinylpyrrolidone aqueous solution and sonicated for 2 hours. Then, while stirring the solution, 30% hydrochloric acid solution was added dropwise until the reaction was completed. After filtration, washing and drying, a nitrogen-doped graphene-coated sulfur / carbon nanotube core-shell structure composite with a particle size of about 1.4 micrometers was obtained. The sulfur content in the composite was about 67%. The reaction temperature of the solution was 90 degrees Celsius.

[0048] Comparative Example 1

[0049] The comparative example differs from Example 1 in that:

[0050] Preparation of sulfur-carbon composite: Elemental sulfur and carbon nanotubes were ball-milled and mixed in a mass ratio of 13:7. After the mixture was homogeneous, it was placed in a hydrothermal reactor for melting treatment. The reaction temperature of the reactor was 155 degrees Celsius and the reaction time was 15 hours. After the reaction was completed, the sulfur / carbon nanotube composite was obtained.

[0051] Preparation of lithium-sulfur batteries: Sulfur / carbon nanotube composites, carbon black and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 8:1:1.

[0052] Test results

[0053] The nitrogen-doped graphene-coated sulfur / carbon nanotube composite with a core-shell structure prepared in this invention was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and its electrochemical performance was tested. Figure 1 The detection results of (Example 1) show that the particle size of the composite is approximately 2.5 micrometers, and the surface of the particles is covered with a wrinkled film layer, namely a nitrogen-doped graphene layer. From Figure 2(Example 3) The core-shell structure of the composite can be clearly seen. The outermost layer is nitrogen-doped graphene, the black part is sulfur microparticles and carbon nanotubes inside them, and a three-dimensional network structure of carbon nanotubes is distributed between the sulfur microparticles and the graphene layer.

[0054] Table 1 Examples 1-6, Comparative Example 1

[0055]

[0056]

[0057] The electrochemical performance test data are shown in Table 1. From the results of Examples 1-6 and Comparative Example 1, it can be seen that Comparative Example 1 (e.g.) Figure 3 As shown, the first-cycle discharge specific capacity at 0.5C rate is 909.1 mAh / g, the capacity retention after 100 cycles is 47.9%, and the coulombic efficiency is 90.6%; Example 1 (as shown) Figure 4 As shown, at a 0.5C rate, the first-cycle discharge specific capacity was 1127.4 mAh / g, the capacity retention after 200 cycles was 80.8%, and the coulombic efficiency was 97.7%. Similarly, compared with Examples 2-6, Comparative Example 1 had a discharge specific capacity at a 0.5C rate that was 78.3 mAh / g to 296.7 mAh / g lower, a coulombic efficiency that was 7.3% to 8.1% lower, and a capacity retention that was far lower than all the examples.

[0058] This demonstrates that the three-dimensional network constructed by carbon nanotubes (or carbon nanofibers) in the liquid-phase method effectively improves the utilization rate of active sulfur and enhances the rate performance of the battery. Therefore, all examples exhibit a discharge specific capacity of approximately 1100 mAh / g at a 0.5C rate, which is higher than that of the sulfur / carbon nanotube composite prepared by the melt method. The carbon nanotube network located between the sulfur particles and the graphene layer not only acts as a conductive bridge but also effectively alleviates the volume expansion problem of elemental sulfur. Therefore, the examples exhibit superior battery cycle performance compared to the comparative example. The graphene coating can mitigate the "shuttle effect" and improve the rate performance of the battery. The coordination of nitrogen-containing functional groups on graphene with polysulfides further improves the "shuttle effect" and enhances the cycle life of lithium-sulfur batteries. Therefore, the examples exhibit a much higher capacity retention rate after 200 cycles than the comparative example after only 100 cycles. At the same time, the examples demonstrate higher coulombic efficiency, which is beneficial for improving the cycle life of the battery. The electrochemical performance test results of all examples and comparative examples show that the physicochemical synergistic effect of nitrogen-doped graphene and carbon nanotubes (or carbon nanofibers) can effectively improve the discharge specific capacity, rate performance and cycle life of sulfur cathodes and lithium-sulfur batteries. The effect will become more and more obvious as the number of cycles increases.

Claims

1. A core-shell structured lithium-sulfur battery cathode material, characterized in that: A three-dimensional conductive network composed of carbon materials is dispersed inside sulfur particles and between nitrogen-doped graphene layers and sulfur particles. The nitrogen-doped graphene layers are coated on the outside of the sulfur / carbon materials, forming a core-shell structure. The carbon material referred to here refers to carbon nanotubes or carbon nanofibers.

2. The method for preparing the core-shell structured lithium-sulfur battery cathode material according to claim 1, characterized in that: Includes the following steps: Carbon materials, graphene oxide, urea, and thiosulfate were added to an aqueous solution containing polyvinylpyrrolidone in a certain mass ratio and subjected to ultrasonic treatment. After ultrasonic treatment for 1 to 2 hours, an acid solution was slowly injected while the mixture was stirred until the reaction was completed. After filtration, washing, and drying, a core-shell structured composite of nitrogen-doped graphene-coated sulfur / carbon materials was obtained. The stirring rate was 500 to 1000 rpm, the reaction temperature of the mixed solution was 60 to 90 degrees Celsius, and the concentration of polyvinylpyrrolidone was 0.5% to 3%.

3. The method for preparing the lithium-sulfur cathode material according to claim 2, characterized in that: The carbon nanotubes or carbon nanofibers have an outer diameter of 5 nm to 50 nm and a length of 0.5 μm to 5 μm.

4. The method for preparing the lithium-sulfur cathode material according to claim 2, characterized in that: The thickness of the graphene oxide is 3 nm to 20 nm, and the length is 2 μm to 10 μm.

5. The method for preparing the lithium-sulfur cathode material according to claim 2, characterized in that: The thiosulfate is one of sodium thiosulfate, ammonium thiosulfate, or potassium thiosulfate.

6. The method for preparing the lithium-sulfur cathode material according to claim 2, characterized in that: The mass ratio of the carbon material, graphene oxide, urea, and thiosulfate is 1–3:0.3–1:0.1–0.5:25–40. Carbon materials refer to carbon nanotubes or carbon nanofibers.

7. The method for preparing the lithium-sulfur cathode material according to claim 2, characterized in that: The acid solution is one of hydrochloric acid, phosphoric acid, nitric acid, oxalic acid, citric acid, and ascorbic acid, with a concentration of 10% to 40%.

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