Preparation of cobalt-cobalt disulfide heterojunction embedded nitrogen-sulfur co-doped carbon nanocage cathode material and application in lithium-sulfur battery

Abstract

The application discloses a kind of cobalt-cobalt disulfide heterojunction embedded nitrogen-sulfur co-doped carbon nanocage positive electrode material preparation and application in lithium-sulfur battery, belong to lithium-sulfur battery positive electrode sulfur host preparation field;Among them, lithium-sulfur battery positive electrode material includes Co / CoS2@NSC nanocage and sublimed sulfur;Lithium-sulfur battery is assembled to lithium-sulfur battery positive electrode material using the application, and the electrochemical performance of each aspect is significantly improved, and the preparation process of the lithium-sulfur battery positive electrode material is simple, and battery assembling process is simple, and cost can be reduced.

Description

Technical Field

[0001] This invention belongs to the field of lithium-sulfur battery cathode material preparation, specifically relating to a lithium-sulfur battery cathode composite material Co / CoS2@NSC / S, its preparation method, and its application. Background Technology

[0002] Lithium-sulfur batteries (LSBs) are considered promising candidates for next-generation lithium-ion batteries due to their high capacity (1675 mAh g / L). -1 The ultra-high theoretical specific capacity and energy density, outstanding cost-effectiveness and environmental friendliness of lithium-sulfur batteries have been extensively studied. However, the large-scale commercial application of lithium-sulfur batteries has been greatly hindered. These challenges mainly focus on: (1) the volume change of active material during charge-discharge cycles can easily lead to the collapse of the electrode structure; (2) the low electron conduction efficiency between the conductive substrate and the active material, and the slow electrochemical reaction of polysulfides; (3) the accumulation of soluble lithium polysulfides (LiPSs) in the electrolyte reduces the ion diffusion rate and also produces a serious "shuttle effect", resulting in a decrease in reversible specific capacity and coulombic efficiency.

[0003] Transition metal sulfides (TMSs), due to their unique electronegativity and electronic structure, form numerous chalcophilic sites on their surfaces, enabling the formation of Metal-S and Li-S bonds that chemisorb lithium polysulfides and reduce Li-S ratios. + The diffusion barrier catalyzes the rapid conversion of LiPSs to Li2S2 / Li2S, which is beneficial for the capacity release and maintenance of high-performance lithium-sulfur batteries. However, transition metal sulfides have low conductivity, which is not conducive to the faster electron exchange between them and polysulfides, thus limiting the release of the catalytic potential of transition metal sulfides. Therefore, it is necessary to modify conventional TMSs materials, develop novel TMSs materials and apply them to cathode materials to fully utilize the advantages of TMSs, thereby significantly improving the electrochemical performance of high-sulfur-loaded lithium-sulfur batteries.

[0004] By incorporating heteroatoms into transition metal sulfides, the performance of TMSs can be further improved, thereby limiting the "shuttle effect." This is because heteroatoms can create defects or vacancies on the crystal surface of the original TMSs, better catalyzing the conversion of LiPSs. More importantly, heteroatom doping can also generate heterojunction structures. Heterojunctions are formed by the tight bonding of two different substances, and the two components easily exert a synergistic effect to jointly regulate the redox kinetics of polysulfides. In addition, due to the different electronic structures at the interface of the two different substances, a built-in electric field and a state of lattice disorder can be generated at the heterojunction interface, making the interaction between the heterojunction interface and LiPSs tighter and the electron transfer efficiency higher. This invention modulates the electronic structure of traditional CoS2 materials by constructing a Co / CoS2 heterojunction to prepare Co / CoS2@NSC / S cathode materials, which are then applied in lithium-sulfur batteries. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a lithium-sulfur battery cathode material Co / CoS2@NSC / S, its preparation method, and its applications. The lithium-sulfur battery cathode material comprises Co / CoS2@NSC and sublimed sulfur. The Co / CoS2@NSC has a highly hollow nanocage structure with an average particle size of approximately 250 nm and a large specific surface area. By applying this sulfide composite to the modification of lithium-sulfur battery cathodes, a lithium-sulfur battery cathode material can be prepared through a simple process. Lithium-sulfur batteries assembled using this cathode material exhibit significantly improved rate performance and cycle performance.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing Co / CoS2@NSC / S, a cathode material for lithium-sulfur batteries, includes the following steps:

[0008] (1) Weigh out zinc salt, cobalt salt, and hexadecyltrimethylammonium bromide and add them to deionized water. Stir until completely dissolved to obtain solution A.

[0009] (2) Weigh 2-methylimidazole and add it to deionized water, stir until completely dissolved, to obtain solution B;

[0010] (3) Mix solution A and solution B and stir until the reaction is uniform; centrifuge the resulting suspension to obtain the solid product, wash with anhydrous ethanol, and dry to obtain purple powder CoZn-ZIF;

[0011] (4) Add the purple powder CoZn-ZIF from step (3) to methanol, then add dopamine hydrochloride and stir until the reaction is uniform; centrifuge the resulting suspension to obtain the solid product, wash with methanol, and dry to obtain a brown powder.

[0012] (5) Place the brown powder from step (4) in a quartz boat and carbonize it under a nitrogen atmosphere to obtain Co@NC;

[0013] (6) Place the Co@NC from step (5) in the downstream quartz boat and place the sulfur powder in the upstream quartz boat. Sulfate under a nitrogen atmosphere to obtain Co / CoS2@NSC.

[0014] (7) After mixing the reaction product from step (6) with sulfur powder evenly, place it under an inert atmosphere. After heat treatment by melt sulfurizing method, Co / CoS2@NSC / S cathode material is obtained.

[0015] Further, in step (1), the zinc salt concentration in solution A is 0.002-2 mol / L; the zinc salt is one or more of Zn(NO3)2, ZnSO4, Zn(CH3COO)2, and ZnCl2. The cobalt salt concentration is 0.002-2 mol / L; the cobalt salt is one or more of Co(NO3)2, CoSO4, Co(CH3COO)2, and CoCl2, and the mass of hexadecyltrimethylammonium bromide is 0.01-0.05 g.

[0016] Furthermore, the concentration of 2-methylimidazole in solution B in step (2) is 0.05-5 mol / L.

[0017] Furthermore, the reaction time in step (3) is 4-16 h.

[0018] Furthermore, in step (4), the mass ratio of dopamine hydrochloride to CoZn-ZIF is 5:3-2:1, the volume of methanol is 30-120 mL, and the reaction time is 4-16 h.

[0019] Furthermore, the carbonization temperature in step (5) is 700-1300 ℃, the heating rate is 1-10 ℃ / min, and the reaction time is 2 hours.

[0020] Furthermore, in step (6), the vulcanization temperature is 300-500 ℃, the heating rate is 1-10 ℃ / min, and the reaction time is 2 hours.

[0021] Furthermore, in step (7), the heating rate of the melt sulfurizing method is 1 ℃ / min. The reaction temperature is 155 ℃, and the holding time is 8~12h.

[0022] The second objective of this invention is to disclose a lithium-sulfur battery comprising a positive electrode and a negative electrode, wherein a separator and an electrode solution are disposed between the positive and negative electrodes, and the positive electrode comprises the Co / CoS2@NSC / S composite material of this invention.

[0023] Furthermore, the positive electrode also includes aluminum foil.

[0024] Furthermore, the negative electrode is metallic lithium.

[0025] Furthermore, the diaphragm is a single-layer diaphragm made of polypropylene (PP).

[0026] Furthermore, the positive electrode is a Co / CoS2@NSC-aluminum foil composite positive electrode, and its preparation process includes the following steps:

[0027] (1) The obtained Co / CoS2@NSC / S composite material, conductive agent and binder are dispersed in N-methylpyrrolidone at a mass ratio of 8:1:1 and mixed evenly. The conductive agent can be Super p Li or carbon nanotube powder, and the binder is polyvinylidene fluoride.

[0028] (2) Coat the slurry obtained in step (1) onto the surface of aluminum foil;

[0029] (3) Place in a vacuum drying oven and dry at a temperature of 30-60℃ for 12-36 hours.

[0030] Application: The use of Co / CoS2@NSC / S, a cathode material for lithium-sulfur batteries, in the assembly of lithium-sulfur batteries.

[0031] The significant advantages of this invention are:

[0032] This invention provides a method for preparing the lithium-sulfur battery cathode material Co / CoS2@NSC / S. The excellent conductivity of Co / CoS2@NSC effectively promotes electron / ion migration and increases the electrochemical reaction rate. The hollow porous structure of Co / CoS2@NSC provides a large specific surface area, which improves the interfacial charge transport efficiency between the active material and the sulfur host, and also promotes the uniform deposition of the discharge product Li2S. The transition metal sulfides contained in Co / CoS2@NSC have a strong chemisorption effect on lithium polysulfides, effectively anchoring soluble lithium polysulfides and suppressing the shuttle effect. Co / CoS2@NSC possesses high catalytic activity, which can accelerate the conversion of lithium polysulfides and improve the electrochemical reaction kinetics. Therefore, lithium-sulfur batteries assembled using the cathode material of this invention show significantly improved rate performance and cycle performance. Attached Figure Description

[0033] Figure 1 The X-ray diffraction pattern of the sulfur host of the lithium-sulfur battery cathode obtained in Example 1;

[0034] Figure 2 This is a scanning electron microscope image of the sulfur host in the positive electrode of the lithium-sulfur battery prepared in Example 1;

[0035] Figure 3 The charge-discharge curves of the lithium-sulfur battery assembled from the lithium-sulfur battery cathode composite material prepared in Example 1 are shown.

[0036] Figure 4 The graph shows the cycle performance of a lithium-sulfur battery assembled from the lithium-sulfur battery cathode composite material prepared in Example 1.

[0037] Figure 5 The rate performance diagram is shown for the lithium-sulfur battery assembled from the lithium-sulfur battery cathode composite material prepared in Example 1.

[0038] Figure 6 This is a comparison chart of the electrochemical performance of lithium-sulfur batteries assembled from the lithium-sulfur battery cathode material prepared in Example 1 and the lithium-sulfur battery cathode material prepared in Comparative Example 1. Detailed Implementation

[0039] To make the above-mentioned features and advantages of the present invention more apparent and understandable, specific embodiments are described below in detail. Unless otherwise specified, the methods of the present invention are conventional methods in the art.

[0040] Example 1

[0041] A method for preparing Co / CoS2@NSC / S, a cathode material for lithium-sulfur batteries, comprising the following steps:

[0042] Step 1: Preparation of Co / CoS2@NSC / S composite cathode material

[0043] (1) Weigh zinc nitrate hexahydrate, cobalt nitrate hexahydrate, and 0.02 g cetyltrimethylammonium bromide into a beaker, add 40 mL of deionized water and stir until completely dissolved to form solution A (the concentration of zinc salt in solution A is 0.1 mol / L, and the concentration of cobalt salt is 0.007 mol / L). Weigh 2-methylimidazole into another beaker, add 280 mL of deionized water and stir until completely dissolved to form solution B (the concentration of 2-methylimidazole in solution B is 0.78 mol / L). Then slowly add solution A to solution B, stir and mix at room temperature to form a light purple solution, continue stirring for 12 h to form a dark purple suspension and then stop the reaction. Wash three times with anhydrous ethanol and centrifuge to obtain a light purple precipitate, then vacuum dry and grind to obtain CoZn-ZIF;

[0044] (2) The reaction product CoZn-ZIF from step (1) was added to 60 mL of methanol and ultrasonically stirred until uniformly dispersed. Then, 40 mg of dopamine hydrochloride was added and the reaction was stirred continuously at room temperature for 12 h. The product was washed three times with methanol and centrifuged to obtain a brown precipitate. Then, it was vacuum dried and ground to obtain CoZn-ZIF@PDA nanocubes.

[0045] (3) Place the reaction product CoZn-ZIF@PDA from step (2) in a quartz boat, then place the quartz boat in a tube furnace, and react at 5 °C / min to 900 °C for 2 h under an inert atmosphere to obtain Co@NC powder;

[0046] (4) Place the reaction product Co@NC and sulfur powder from step (3) into two quartz boats respectively. Place the quartz boat containing sulfur powder at the upper air vent of the tube furnace and place the quartz boat containing Co@NC in the center of the tube furnace. Heat under an inert gas atmosphere. After reacting at 400 ℃ for 2 h at a rate of 5 ℃ / min under an inert atmosphere, Co / CoS2@NSC powder is obtained.

[0047] (5) After uniformly mixing the reaction product Co / CoS2@NSC from step (4) with sulfur powder, the Co / CoS2@NSC / S cathode material was obtained by melting sulfur diffusion at a heating rate of 1 °C / min and holding at 155 °C for 8 h under an inert gas atmosphere.

[0048] Step 2: Preparation of Co / CoS2@NSC / S-aluminum foil composite cathode

[0049] (1) The obtained Co / CoS2@NSC / S composite material, conductive agent and binder are dispersed in N-methylpyrrolidone at a mass ratio of 8:1:1 and mixed evenly. The conductive agent is carbon nanotube powder and the binder is polyvinylidene fluoride.

[0050] (2) Coat the slurry obtained in step (1) onto the surface of the aluminum foil;

[0051] (3) Place it in a vacuum drying oven and dry it at 40°C for 12 hours to obtain Co / CoS2@NSC / S-aluminum foil composite cathode.

[0052] Step 3: Lithium-sulfur battery assembly

[0053] The Co / CoS2@NSC / S-aluminum foil composite positive electrode prepared in step 2 was used as the positive electrode, and lithium metal was used as the negative electrode. A commercial PP separator was placed between the positive and negative electrodes and placed in the battery case. Electrolyte was added dropwise to both sides of the separator, and the battery was pressurized and sealed to complete the assembly of the lithium-sulfur battery. The electrolyte was 1 M LiTFSI-DME / DOL (DME and DOL volume ratio = 1:1) and contained 1 wt% LiNO3.

[0054] Figure 1The X-ray diffraction pattern and specific surface area and pore size analysis of the sulfur host in the lithium-sulfur battery cathode prepared in Example 1 show that the diffraction peaks are very clear and distinct, indicating that the material has high crystallinity. The diffraction peaks correspond to the PDF card, indicating the accuracy of the phase composition and the correct synthesis of the material. Furthermore, the specific surface area of ​​Co / CoS2@NSC reaches 128.91 m². 2 g -1 Micropores and mesopores are the main components of its pore structure. The diameter of micropores is mainly concentrated in 0.8-1 nm, and the size of mesopores is 16-34 nm, indicating that it has a large specific surface area, which is beneficial to improve the contact area between the material and the electrolyte and increase the interfacial charge transport efficiency between the active material and the sulfur host. Figure 2 The image shows a scanning electron microscope image of the sulfur host of the lithium-sulfur battery cathode prepared in Example 1. The image shows that the material has a particle size of about 250 nm and exhibits a highly hollow porous nanocage structure.

[0055] Electrochemical performance testing

[0056] The Co / CoS2@NSC / S prepared by the above method was used as the positive electrode, lithium sheet was used as the negative electrode, commercial polypropylene (PP) material was used as the separator, and the battery was assembled in a glove box using a CR2025 battery case. Figure 3 The graph shows the charge-discharge curves of a lithium-sulfur battery assembled from the lithium-sulfur battery cathode material prepared in Example 1. It can be observed that Q in Comparative Example 1... H Q L -1 The value is 2.52, the polarization potential is 201.5 mV, and the Q value of Comparative Example 2 is... H Q L -1 The value is 2.48, and the polarization potential is 182.0 mV. Q in Example 1 H Q L -1 The value is 2.77 and the polarization potential is 151.9 mV, indicating that the cathode material of Example 1 is conducive to the strong adsorption and catalysis of polysulfide conversion, thereby suppressing the "shuttle effect" and reducing the loss of active material; Figure 4 The graph shows the cycle performance of a lithium-sulfur battery assembled from the lithium-sulfur cathode material prepared in Example 1. It can be seen that the battery capacity decreases gradually, with a coulombic efficiency exceeding 98% during cycling, and an initial specific capacity of 1247 mAh g⁻¹. -1 The reversible specific capacity after 100 cycles is 913 mAh g. -1 The capacity decay rate is 0.268% per cycle, indicating good cycle stability. Figure 5The graph shows the rate performance of a lithium-sulfur battery assembled from the lithium-sulfur battery cathode material prepared in Example 1. It can be seen that the battery still maintains a capacity of 803.0 mAh g⁻¹ at a high current density of 3 C. -1 The discharge specific capacity remains at 1023 mAh g⁻¹, and it is still able to maintain this capacity when the current density returns to 0.2 C. -1 It has excellent discharge capacity and rate performance.

[0057] Comparative Example 1

[0058] Method for preparing Co@NC / S cathode material for lithium-sulfur batteries:

[0059] Step 1: Preparation of Co@NC / S composite cathode material

[0060] (1) Weigh zinc nitrate hexahydrate, cobalt nitrate hexahydrate, and 0.02 g cetyltrimethylammonium bromide into a beaker, add 40 mL of deionized water and stir until completely dissolved to form solution A (the concentration of zinc salt in solution A is 0.1 mol / L, and the concentration of cobalt salt is 0.007 mol / L). Weigh 2-methylimidazole into another beaker, add 280 mL of deionized water and stir until completely dissolved to form solution B (the concentration of 2-methylimidazole in solution B is 0.78 mol / L). Then slowly add solution A to solution B, stir and mix at room temperature to form a light purple solution, continue stirring for 12 h to form a dark purple suspension and then stop the reaction. Wash three times with anhydrous ethanol and centrifuge to obtain a light purple precipitate, then vacuum dry and grind to obtain CoZn-ZIF;

[0061] (2) The reaction product CoZn-ZIF from step (1) was added to 60 mL of methanol and ultrasonically stirred until uniformly dispersed. Then, 120 mg of dopamine hydrochloride was added and the reaction was stirred continuously at room temperature for 12 h. The product was washed three times with methanol and centrifuged to obtain a brown precipitate. Then, it was vacuum dried and ground to obtain CoZn-ZIF@PDA nanocubes.

[0062] (3) Place the reaction product CoZn-ZIF@PDA from step (2) in a quartz boat, then place the quartz boat in a tube furnace, and react at 5 °C / min to 900 °C for 2 h under an inert atmosphere to obtain Co@NC powder;

[0063] (4) After uniformly mixing the reaction product Co@NC from step (3) and sulfur powder, the mixture is placed in a quartz boat and kept at 155 °C for 8 hours under an inert gas atmosphere by melt sulfurizing at a heating rate of 1 °C / min.

[0064] Step 2: Preparation of Co@NC / S-aluminum foil composite cathode

[0065] (1) The obtained Co@NC / S composite material, conductive agent and binder are dispersed in N-methylpyrrolidone at a mass ratio of 8:1:1 and mixed evenly. The conductive agent is carbon nanotube powder and the binder is polyvinylidene fluoride.

[0066] (2) Coat the slurry obtained in step (1) onto the surface of the aluminum foil;

[0067] (3) Place it in a vacuum drying oven and dry it at 40 ℃ for 12 hours to obtain Co@NC / S-aluminum foil composite cathode.

[0068] Step 3: Lithium-sulfur battery assembly

[0069] The Co@NC / S-aluminum foil composite positive electrode prepared in step 2 was used as the positive electrode, and lithium metal was used as the negative electrode. A commercial PP separator was placed between the positive and negative electrodes and placed in the battery case. Electrolyte was added to both sides of the separator, and the battery was pressurized and sealed to complete the assembly of the lithium-sulfur battery. The electrolyte was 1 M LiTFSI-DME / DOL (the volume ratio of DME to DOL = 1:1) and contained 1 wt% LiNO3.

[0070] Comparative Example 2

[0071] The specific steps for preparing the CoS2@NSC / S cathode for lithium-sulfur batteries are as follows:

[0072] Step 1: Preparation of CoS2@NSC / S composite cathode material

[0073] (1) Weigh zinc nitrate hexahydrate, cobalt nitrate hexahydrate, and 0.02 g cetyltrimethylammonium bromide into a beaker, add 40 mL of deionized water and stir until completely dissolved to form solution A (the concentration of zinc salt in solution A is 0.1 mol / L, and the concentration of cobalt salt is 0.007 mol / L). Weigh 2-methylimidazole into another beaker, add 280 mL of deionized water and stir until completely dissolved to form solution B (the concentration of 2-methylimidazole in solution B is 0.78 mol / L). Then slowly add solution A to solution B, stir and mix at room temperature to form a light purple solution, continue stirring for 12 h to form a dark purple suspension and then stop the reaction. Wash three times with anhydrous ethanol and centrifuge to obtain a light purple precipitate, then vacuum dry and grind to obtain CoZn-ZIF;

[0074] (2) The reaction product CoZn-ZIF from step (1) was added to 60 mL of methanol and ultrasonically stirred until uniformly dispersed. Then, 120 mg of dopamine hydrochloride was added and the reaction was stirred continuously at room temperature for 12 h. The product was washed three times with methanol and centrifuged to obtain a brown precipitate. Then, it was vacuum dried and ground to obtain CoZn-ZIF@PDA nanocubes.

[0075] (3) Place the reaction product CoZn-ZIF@PDA from step (2) in a quartz boat, then place the quartz boat in a tube furnace, and react at 5 °C / min to 900 °C for 2 h under an inert atmosphere to obtain Co@NC powder;

[0076] (4) Place the reaction product Co@NC and sulfur powder from step (3) into two quartz boats respectively. Place the quartz boat containing sulfur powder at the upper air vent of the tube furnace and place the quartz boat containing Co@NC in the center of the tube furnace. Heat under an inert gas atmosphere. After reacting at 400 ℃ for 3 h at a rate of 5 ℃ / min under an inert atmosphere, CoS2@NSC powder is obtained.

[0077] (5) After uniformly mixing the reaction product CoS2@NSC and sulfur powder in step (4), CoS2@NSC / S cathode material was obtained by melting sulfur diffusion method at a heating rate of 1 °C / min and holding at 155 °C for 8 h under an inert gas atmosphere.

[0078] Step 2: Preparation of CoS2@NSC / S-aluminum foil composite cathode

[0079] (1) The obtained CoS2@NSC / S composite material, conductive agent and binder are dispersed in N-methylpyrrolidone at a mass ratio of 8:1:1 and mixed evenly. The conductive agent is carbon nanotube powder and the binder is polyvinylidene fluoride.

[0080] (2) Coat the slurry obtained in step (1) onto the surface of the aluminum foil;

[0081] (3) Place it in a vacuum drying oven and dry it at 40 ℃ for 12 hours to obtain CoS2@NSC / S-aluminum foil composite cathode.

[0082] Step 3: Lithium-sulfur battery assembly

[0083] The CoS2@NSC / S-aluminum foil composite positive electrode prepared in step 2 was used as the positive electrode, and lithium metal was used as the negative electrode. A commercial PP separator was placed between the positive and negative electrodes and placed in the battery case. Electrolyte was added to both sides of the separator, and the battery was pressurized and sealed to complete the assembly of the lithium-sulfur battery. The electrolyte was 1 M LiTFSI-DME / DOL (DME and DOL volume ratio = 1:1) and contained 1 wt% LiNO3.

[0084] The lithium-sulfur battery assembled from the lithium-sulfur battery cathode material prepared by the above method was subjected to electrochemical performance testing using the same method described above. Figure 6This graph compares the electrochemical performance of lithium-sulfur batteries assembled with the cathode materials prepared in Example 1 and those prepared in Comparative Examples 1 and 2. It shows that after 100 cycles, the reversible specific capacities of Comparative Examples 1 and 2 are only 688 and 791 mAh g, respectively. -1 This indicates that Example 1 exhibits better cycle stability. Furthermore, at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 3.0 C, Example 1 achieved specific capacities of 1459, 1141, 1048, 957, 873, and 803 mAhg, respectively. -1 When the current density recovers to 0.2 C, its specific capacity can recover to 1023 mAh g⁻¹. -1 The performance is significantly higher than that of Comparative Example 1 and Comparative Example 2 at various rate capabilities. Furthermore, the comparison shows that the Co / CoS2@NSC / S cathode material for lithium-sulfur batteries can effectively catalyze the conversion of lithium polysulfides, accelerate reaction kinetics, and improve the electrochemical performance of the battery.

[0085] Example 1 exhibits superior performance compared to Comparative Examples 1 and 2. Firstly, the synthesized Co / CoS2@NSC nanocage possesses a highly hollow internal structure with numerous micropores and mesopores in its shell, which helps resist volume changes during cycling and ensures sufficient contact between the conductive substrate and the active material. This facilitates electrolyte wetting while achieving high sulfur loading. Secondly, the N,S co-doped carbon structure provides a more effective ability to anchor polysulfides, significantly suppressing the negative impact of the "shuttle effect" and improving sulfur utilization. In Comparative Example 1, single nitrogen doping and in Comparative Example 2, excessive sulfur doping lead to weaker and stronger adsorption capacities of the carbon matrix, respectively, which are detrimental to improving the electrochemical performance of lithium-sulfur batteries. Finally, the in-situ generated Co / CoS2 heterointerface in Co / CoS2@NSC exhibits good conductivity and shifts the d-band center of Co upwards to the Fermi level, resulting in stronger interactions with polysulfides and higher catalytic activity compared to single CoS2.

[0086] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for preparing Co / CoS2@NSC / S, a cathode material for lithium-sulfur batteries, characterized in that: Includes the following steps: (1) Weigh zinc salt, cobalt salt and hexadecyltrimethylammonium bromide, add deionized water and stir until completely dissolved to form solution A; weigh 2-methylimidazole, add deionized water and stir until completely dissolved to form solution B; then slowly add solution A to solution B, stir and mix at room temperature to form a light purple solution, continue stirring to form a dark purple suspension and then stop the reaction, wash three times with anhydrous ethanol and centrifuge to obtain a light purple precipitate, then vacuum dry and grind to obtain CoZn-ZIF; (2) The reaction product CoZn-ZIF from step (1) was added to methanol and ultrasonically stirred until it was uniformly dispersed. Then, dopamine hydrochloride was added and stirred continuously at room temperature to form a dark brown suspension. The reaction was stopped, washed three times with methanol and centrifuged to obtain a brown precipitate. Then, it was vacuum dried and ground to obtain CoZn-ZIF@PDA nanocubes. (3) Place the reaction product CoZn-ZIF@PDA from step (2) into a quartz boat, then place the quartz boat into a tube furnace and carbonize it under a nitrogen atmosphere to obtain Co@NC powder; (4) Place the reaction product Co@NC and sulfur powder from step (3) into two quartz boats respectively. Place the quartz boat containing sulfur powder at the upper air vent of the tube furnace and place the quartz boat containing Co@NC in the center of the tube furnace. Heat under an inert gas atmosphere and perform chemical vapor deposition to obtain Co / CoS2@NSC powder. (5) After uniformly mixing the reaction product Co / CoS2@NSC from step (4) with sulfur powder, place it in a quartz boat and obtain the Co / CoS2@NSC / S cathode material by melt sulfurizing under an inert gas atmosphere; In step (4), the mass ratio of sulfur powder to Co@NC is 1:1-10:1, the chemical vapor deposition reaction temperature is 300-500 ℃, the heating rate is 1-10 ℃ / min, and the reaction time is 2 hours.

2. The preparation method according to claim 1, characterized in that: In step (1), the concentration of zinc salt in solution A is 0.002-2 mol / L; the zinc salt is one or more of Zn(NO3)2, ZnSO4, (CH3COO)2Zn and ZnCl2; the concentration of cobalt salt is 0.002-2 mol / L; the cobalt salt is one or more of Co(NO3)2, CoSO4, Co(CH3COO)2 and CoCl2; the mass of hexadecyltrimethylammonium bromide is 0.01-0.05 g; the concentration of 2-methylimidazole in solution B is 0.05-5 mol / L; and the stirring time is 4-16 hours.

3. The preparation method according to claim 1, characterized in that: In step (2), the mass ratio of dopamine hydrochloride and CoZn-ZIF is 5:3-2:1, and the stirring time is 4-16 hours.

4. The preparation method according to claim 1, characterized in that: The carbonization reaction temperature in step (3) is 700-1300 ℃, the heating rate is 1-10 ℃ / min, and the reaction time is 2 hours.

5. The preparation method according to claim 1, characterized in that: The mass ratio of sulfur powder and Co / CoS2@NSC in step (5) is 7:3; the reaction temperature of the melt sulfur diffusion method is 155 ℃, the heating rate is 1 ℃ / min, and the holding time is 8~12 hours.

6. A lithium-sulfur battery cathode material prepared by the preparation method according to any one of claims 1-5.

7. The application of the lithium-sulfur battery cathode material as described in claim 6 in the assembly of lithium-sulfur batteries.

8. The application according to claim 7, characterized in that: The Co / CoS2@NSC / S composite material is used to prepare the positive electrode of a lithium-sulfur battery. The preparation method includes the following steps: 1) The Co / CoS2@NSC / S composite material, conductive agent and binder are dispersed in N-methylpyrrolidone at a mass ratio of 8:1:1 and mixed evenly to obtain a slurry; 2) Apply the slurry obtained in step 1) to the surface of the aluminum foil; 3) Place it in a vacuum drying oven and dry it at a temperature of 30-60 ℃ for 12-36 hours to obtain Co / CoS2@NSC / S-aluminum foil composite cathode.

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