A spherical carbon-coated ZnO-ZnS heterostructure material, a preparation method and application thereof

Abstract

This invention discloses a spherical carbon-coated ZnO-ZnS heterostructure material. Using zinc acetate as the zinc source and thiourea as the sulfur source, a spherical ZnO-ZnS heterostructure is prepared through a first hydrothermal reaction. Then, a second hydrothermal reaction yields a polydopamine-coated ZnO-ZnS heterostructure. Finally, ZnO-ZnS@NC is obtained through carbon calcination. In ZnO-ZnS, ZnO has a wurtzite structure, and ZnS has a zincblende structure. In ZnO-ZnS@NC, both ZnO and ZnS have wurtzite structures. A heterostructure interface exists between ZnO and ZnS. ZnO-ZnS has a smooth spherical structure. ZnO-ZnS@NC has a porous spherical structure. The peak-to-height ratio of ZnO-ZnS@NC is I. D / I G =0.717. Its preparation method includes the following steps: 1. One-step preparation of spherical ZnO-ZnS heterostructures; 2. Coating with polydopamine; 3. Carbonization of polydopamine. As a cathode material for lithium-sulfur batteries, under a current rate of 0.1C, the initial discharge specific capacity is 1200-1400 mA hg. ‑1 Under the condition of 100 cycles, the residual discharge specific capacity is 1000-1100 mA hg. ‑1 The capacity retention rate is 75-80%.

Description

Technical Field

[0001] This invention relates to the field of lithium-sulfur battery cathode support and catalysis technology, specifically to a spherical carbon-coated ZnO-ZnS heterostructure material, its preparation method, and its application. Background Technology

[0002] Lithium-sulfur batteries have a capacity of 1675 mA hg -1 It boasts an ultra-high theoretical capacity; based on an average discharge voltage of 2.15V, its theoretical energy density can reach 2600Wh / kg. -1 The energy density is much higher than that of traditional lithium-ion batteries. However, lithium-sulfur batteries suffer from severe volume expansion during charging and discharging, lithium dendrite growth at the negative electrode, and the dissolution of lithium polysulfides in the electrolyte during the reaction process, which causes them to shuttle back and forth across the membrane. This results in a significant decrease in energy density and rapid cycle degradation.

[0003] To address the cycling degradation caused by the shuttle effect, a key approach is to modify the carbon support by doping it with transition metal compounds, such as biomass-derived carbon, conductive polymers, carbon nanotubes, graphene oxide, and MXene. For example, in existing literature (K,B.,SR, and KN, Exploration of microporous bio-carbon scaffold for efficient utilization of sulfur in lithium-sulfur system. Electrochimica Acta, 2016.), K. Balakumar et al. prepared activated carbon using coconut pith with a fibrous structure via KOH activation. This activated carbon served as the conductive framework for the positive electrode material in lithium-sulfur batteries, achieving an initial discharge specific capacity of 1350 mA hg. -1 Under a current ratio of 0.1C, after 75 cycles, the remaining discharge specific capacity is 609 mAg h. -1 The capacity retention rate is 45.1%. Although this technical solution uses biomass as raw material to achieve a green and environmentally friendly effect, the carbon structure derived from biomass is irregular, making it difficult to control the usable structure and pore size. This results in uneven pore size distribution and makes it impossible to effectively select structural morphologies with the same pore size.

[0004] To address the issues of uneven pore size distribution, limited active sites, and difficulty in controlling the shuttle effect in disordered porous carbon, improvements can be achieved by loading materials with a specific pore size structure. For example, existing literature 2 ( H., et al., Rational design of cathode structure based on free-standing S / rGO / CNT nanocomposite for Li-S batteries. Synthetic Metals, 2020.) Flexible, layered, and freestanding S / rGO / CNT nanocomposites without binders or heat collectors were successfully prepared using different rGO / CNT ratios via vacuum filtration. An initial specific capacity of 1150 mA hg was achieved. -1 Under a current factor of 0.1C, the remaining specific capacitance after 300 cycles is 666 mA hg. -1 The technical solution achieves a capacity retention rate of 58%. While this solution fully utilizes the 3D porous advantage of CNTs, allowing for more complete reactions of the active materials, and the porous carbon structure provides some physical adsorption for lithium polysulfides, pure carbon CNTs cannot generate polar adsorption for long-chain lithium polysulfides. After prolonged charge and discharge, long-chain lithium polysulfides will pass through the separator and accumulate on the negative electrode, affecting lithium ion insertion and extraction, leading to a rapid decline in battery life. Therefore, positive electrodes prepared solely with carbon materials as carriers cannot achieve long-cycle charge and discharge.

[0005] To address the issue that pure carbon materials cannot effectively suppress lithium polysulfides in the electrolyte during battery reactions, metal oxides can be introduced onto regularly ordered carbon materials. The polar interaction between the metal oxides and lithium polysulfides can then suppress the shuttle effect. For example, existing literature (Zhao, Y., et al., Nitrogen-doped carbon nanotubes scoated with zinc oxide nanoparticles as sulfur encapsulator for high-performance lithium / sulfur batteries. Beilstein Journal of Nanotechnology, 2018.) prepared nitrogen-doped carbon nanotubes (ZnO@NCNT) coated with zinc oxide nanoparticles using a sol-gel method as the cathode for lithium-sulfur batteries, achieving an initial capacity of 1032 mA hg. -1 With a charge / discharge rate of 0.2C, the residual discharge capacity after 100 cycles is 665 mA hg. -1The capacity retention rate was 64.4%. Although this technical solution introduces ZnO into the 3D porous CNTs, ZnO has strong adsorption properties for lithium polysulfides, which can adsorb lithium polysulfides and thus suppress the shuttle effect. However, due to the low catalytic activity of ZnO for lithium polysulfides, it cannot improve the reaction kinetics of the battery. Therefore, as the reaction proceeds, ZnO adsorbs a large amount of lithium polysulfides, ultimately leading to a decrease in the utilization rate of the active material and a reduction in reaction kinetics.

[0006] To address the catalytic conversion problem of lithium polysulfides after adsorption on material surfaces, metal sulfides can be introduced onto regularly ordered carbon materials. The catalytic interaction between the metal sulfides and lithium polysulfides can accelerate the reaction kinetics, thereby suppressing the shuttle effect. For example, existing literature 4 (Shi, T., et al., A special core-shell ZnS-CNTs / S@NHcathode constructed to elevate electrochemical performances of lithium-sulfurbatteries. Journal of Colloid and Interface Science, 2021.) prepared ZnS-CNTs / S composite materials using a simple melt-diffusion method. The initial specific capacity of ZnS-CNTs / S as a cathode material was 779.2 mA hg. -1 After 150 cycles, the blood glucose level was 571.5 mA hg. -1 The capacity retention rate is 73.3%. Although ZnS has better catalytic activity for lithium polysulfides and can effectively accelerate the reaction kinetics inside the battery and accelerate the liquid-solid conversion during the reaction, the low anchoring ability of ZnS for lithium polysulfides means that it cannot effectively capture lithium polysulfides in the electrolyte. Therefore, as the reaction proceeds, the concentration of lithium polysulfides in the electrolyte increases, resulting in a severe shuttle effect and ultimately leading to a decrease in cycle life.

[0007] To address the challenge of a single material simultaneously possessing both adsorption capacity and catalytic activity for lithium polysulfides, a heterostructure can be fabricated by combining metal oxides and metal sulfides, thereby enabling the material to exhibit both adsorption capacity and catalytic activity. For example, in existing literature (Li, D., et al., CoS2-TiO2@C Core-Shell fibers ascathode host material for High-Performance Lithium-Sulfur batteries. Journal of Colloid and Interface Science, 2021.), Li et al. prepared CoS2-TiO2@carbon core-shell fibers using a combination of coaxial electrospinning and selective sulfidation. The resulting core-shell fibers effectively support sulfur, confine polysulfides, and accelerate the conversion of intermediate products. Under a current rate of 0.2C, the initial specific capacity of the S-CoS2-TiO2@C electrode was 1181.1 mA hg. -1 After 100 cycles, its capacity retention was 72.1%. Although this technical solution can retain the anchoring ability of metal oxides for lithium polysulfides in addition to the catalytic activity of metal sulfides for lithium polysulfides, thus possessing both anchoring ability and catalytic activity, and the presence of a built-in electric field increases the adsorption energy of lithium polysulfides, effectively capturing free long-chain lithium polysulfides and suppressing shuttle, thereby improving the reaction kinetics of lithium-sulfur batteries and achieving long cycle life, the lattice matching degree of the heterostructure formed by CoS2 and TiO2 is only 36.7%, and CoS2 and TiO2 belong to different crystal structures. The lattice distortion at the interface of the heterostructure is large, so the structure is unstable and has poor mechanical properties. During the reaction, the interface is easily destroyed, resulting in the loss of the built-in electric field's effect on lithium polysulfides.

[0008] In summary, existing lithium-sulfur batteries use sulfur (S) as the active material, which reacts with lithium (Li) at the negative electrode during charging and discharging to form long-chain lithium polysulfides (Li₂S). n (n=2,4,6,8), and dissolved in the electrolyte, shuttle between the two sides of the separator, causing the technical problem of battery cycle life degradation. Forming a heterostructure is an effective solution. The principle is that, based on the advantages of both materials at the same time - capturing free lithium polysulfides and converting captured lithium polysulfides - an additional built-in electric field is generated at the heterostructure interface. By applying a stronger polar effect to the lithium polysulfides in the high-energy transition state, the shuttle of lithium polysulfides is suppressed, the cycle degradation rate is reduced, and thus the long cycle life of lithium-sulfur batteries is improved. Summary of the Invention

[0009] The purpose of this invention is to provide a spherical carbon-coated ZnO-ZnS heterostructure material, its preparation method, and its application.

[0010] Since the heterostructure forms a boundary between two different materials, when selecting a heterostructure, in addition to choosing materials that have polar adsorption and catalytic activity for lithium polysulfides, it is also necessary to select materials with a certain degree of lattice matching, such as materials with the same crystal form and similar cell volume.

[0011] Therefore, this invention selects ZnO, which has a polar adsorption effect on lithium polysulfides, and ZnS, which has catalytic activity towards lithium polysulfides, to synthesize a heterostructure. Since wurtzite-type ZnS is difficult to maintain stability at low temperatures, but can undergo a phase transition at 800-1000℃, after preparing the dopamine-coated heterostructure, calcination occurs. Simultaneously with the carbonization of dopamine, the heterostructure undergoes an in-situ phase transition, transforming ZnS from a wurtzite structure to the same wurtzite structure as ZnO, forming a heterostructure with the same crystal form. To improve the mechanical properties of heterostructures, ZnO and ZnS are prepared with the same crystal structure as heterostructures. The lattice matching degree between ZnO and ZnS is 60-70%, which can reduce the impact of lattice distortion on the structure. However, heterostructures prepared by metal oxides and metal sulfides have poor conductivity and cannot meet the conductivity requirements of the positive electrode. Therefore, dopamine is coated on the surface of the heterostructure and carbonized to form dense carbon-coated ZnO-ZnS, namely ZnO-ZnS@NC.

[0012] To achieve the aforementioned objectives, the technical solution adopted by this invention is as follows:

[0013] A spherical carbon-coated ZnO-ZnS heterostructure material is disclosed. Using zinc acetate as the zinc source and thiourea as the sulfur source, the process involves: first, a first hydrothermal reaction to prepare a spherical ZnO-ZnS heterostructure, referred to as ZnO-ZnS; then, a second hydrothermal reaction to adjust the pH value with tris(hydroxymethyl)aminomethane to polymerize dopamine hydrochloride onto the surface of the ZnO-ZnS heterostructure, resulting in a polydopamine-coated ZnO-ZnS heterostructure, referred to as ZnO-ZnS@PDA; finally, carbon calcination to obtain the spherical carbon-coated ZnO-ZnS heterostructure material, referred to as ZnO-ZnS@NC.

[0014] The crystal structure of ZnO-ZnS is as follows: ZnO has a wurtzite structure and ZnS has a zincblende structure.

[0015] The crystal structure of ZnO-ZnS@NC is as follows: ZnO has a wurtzite structure and ZnS has a wurtzite structure.

[0016] In the ZnO-ZnS@NC, there is a heterogeneous interface between ZnO and ZnS;

[0017] The ZnO-ZnS has a smooth spherical structure with a flat surface and an average particle size of 2.0-2.1 μm;

[0018] The ZnO-ZnS@NC has a porous spherical structure with a rough surface and an average particle size of 1.8-1.9 μm.

[0019] In the Raman spectrum of the ZnO-ZnS@NC, the peak height ratio I D / I G =0.717.

[0020] A method for preparing spherical carbon-coated ZnO-ZnS heterostructure material includes the following steps:

[0021] Step 1, a one-step preparation of spherical ZnO-ZnS heterostructures: First, zinc acetate and thiourea are mixed in a certain mass ratio. Zinc acetate is placed in a mixed solvent of ethanol and water and sonicated to obtain solution A. Simultaneously, thiourea is placed in the mixed solvent and sonicated to obtain solvent B. Then, under certain conditions, solution A is stirred. After that, solution B is slowly added to solution A to obtain mixed solution A. Next, under certain conditions, the mixed solutions are mixed and stirred to obtain reaction solution A. Finally, reaction solution A undergoes a first hydrothermal reaction under certain conditions. The resulting product is centrifuged and washed under certain conditions, and then dried under certain conditions to obtain spherical ZnO-ZnS heterostructures, abbreviated as ZnO-ZnS.

[0022] The volume ratio of ethanol to deionized water in the mixed solvent is 1:1;

[0023] In step 1, the mass ratio of zinc acetate to thiourea is 2:1;

[0024] In step 1, the stirring conditions for solution A are: stirring temperature of 50-80℃, stirring time of 0.5-1h, and stirring for another 1h after the addition is complete.

[0025] In step 1, the conditions for preparing reaction solution A are: mixing and stirring for 20-24 hours at room temperature.

[0026] In step 1, the conditions for the first hydrothermal reaction are: the first hydrothermal temperature is 150-180℃ and the first hydrothermal time is 12h.

[0027] In step 1, the conditions for centrifugal washing are as follows: the washing solution is anhydrous ethanol, and the centrifugation speed is 7000-9000 r / min.

[0028] In step 1, the drying conditions are: drying temperature of 60-80℃ and drying time of 10-12h.

[0029] Step 2, polydopamine coating: First, ZnO-ZnS obtained in Step 1 and dopamine hydrochloride are mixed in a certain mass ratio. ZnO-ZnS is placed in a mixed solvent and stirred under certain conditions to obtain solution C. Then, dopamine hydrochloride is placed in solution C and stirred under certain conditions to obtain solution D. After that, under stirring conditions, tris(hydroxymethyl)aminomethane (Tris) is added to solution D to adjust the pH value of the solution to 8. After the addition is completed, the mixture is sonicated to obtain mixed solution B. Then, mixed solution B is stirred under certain conditions to obtain reaction solution B. Finally, reaction solution B is subjected to a second hydrothermal reaction under certain conditions. After the product is filtered, washed and dried, polydopamine-coated ZnO-ZnS heterostructure is obtained, abbreviated as ZnO-ZnS@PDA.

[0030] In step 2, all stirring is performed at room temperature.

[0031] In step 2, the mass ratio of ZnO-ZnS to dopamine hydrochloride is 1:3;

[0032] In step 2, the stirring conditions for solution C are as follows: stirring time is 15 min.

[0033] In step 2, the stirring conditions for solution D are: stirring time is 15 min;

[0034] In step 2, the ultrasonic conditions for mixed solution B are as follows: ultrasonic time is 15 min.

[0035] In step 2, the conditions for preparing reaction solution B are: mixing and stirring time of 24 hours;

[0036] In step 2, the conditions for the second hydrothermal reaction are: the second hydrothermal temperature is 150°C and the second hydrothermal time is 12 hours.

[0037] Step 3, carbonization of polydopamine: Under certain conditions, the ZnO-ZnS@PDA obtained in step 2 is calcined to achieve carbonization, thereby obtaining a spherical carbon-coated ZnO-ZnS heterostructure, referred to as ZnO-ZnS@NC;

[0038] In step 3, the calcination conditions are: first, calcination at a low temperature, and then calcination at a high temperature.

[0039] In step 3, the conditions for low-temperature calcination are 350°C and 1 hour, and the conditions for high-temperature calcination are 850°C and 3 hours.

[0040] The application of a spherical carbon-coated ZnO-ZnS heterostructure material as a cathode material for lithium-sulfur batteries shows an initial discharge specific capacity of 1200-1400 mA hg under a current rate of 0.1C. -1 Under the condition of 100 cycles, the residual discharge specific capacity is 1000-1100 mA hg -1 The capacity retention rate is 75-80%.

[0041] The technical effects of this invention have been tested experimentally, and the specific details are as follows:

[0042] The characteristics and functions of the spherical carbon-coated ZnO-ZnS heterostructure material obtained by the method of this invention were experimentally tested, and the results are as follows:

[0043] XRD analysis revealed that ZnO(W)-ZnS(S) contains characteristic peaks of both wurtzite ZnO(W) and zincblende ZnS(S); ZnO(W)-ZnS(W)@NC contains characteristic peaks of both wurtzite ZnO(W) and zincblende ZnS(W), while the original characteristic peaks of ZnS(S) have disappeared. The test results indicate that ZnO(W)-ZnS(W)@NC contains two isomorphous ZnO and ZnS.

[0044] SEM analysis revealed that ZnO(W)-ZnS(S) has a smooth spherical structure with a flat surface and an average particle size of 2.0-2.1 μm; ZnO-ZnS@NC has a porous spherical structure with a rough surface and an average particle size of 1.8-1.9 μm. This indicates that the crystal transformation leads to surface collapse and the formation of a rough surface.

[0045] EDS analysis revealed that ZnO-ZnS@NC contains Zn, O, and S elements, and the elements are evenly distributed.

[0046] Raman spectroscopy analysis revealed two distinct carbon characteristic peaks with a peak height ratio of I. D / I G =0.717, indicating that the carbonized PDA has a high degree of graphitization and good conductivity.

[0047] TEM testing confirmed the existence of a distinct heterostructure between ZnO and ZnS; the lattice fringe spacing between ZnO particles was 0.26 nm, corresponding to the (002) crystal plane; and the lattice fringe spacing between ZnS particles was 0.313 nm, corresponding to the (002) crystal plane, thus demonstrating the formation of the ZnO-ZnS@NC heterostructure.

[0048] The constant current cyclic charge-discharge test results show that, under a current rate of 0.1C, the initial discharge specific capacity is 1200-1400 mA hg. -1 Under the condition of 100 cycles, the residual discharge specific capacity is 1000-1100 mA hg -1 The capacity retention rate is 75-80%.

[0049] Cyclic voltammetry results show that there is one oxidation peak and two reduction peaks in the spectrum, indicating that the battery has good redox performance and good reversibility of reaction. After three cycles, the peaks basically overlap, indicating that the battery has excellent cycle performance and slow capacity decay rate. The sharp peaks indicate that sulfur in the composite material has good reactivity and exhibits fast reaction kinetics.

[0050] Therefore, the present invention has the following advantages over existing technologies in the cathode of lithium-sulfur batteries:

[0051] 1. This invention synthesizes a heterostructured material via a hydrothermal method. After calcination, a heterostructure with identical crystal forms is formed, exhibiting high lattice matching and stable interfaces, resulting in good mechanical properties.

[0052] 2. In this invention, the heterostructure coated with dopamine undergoes an in-situ phase transition during calcination to carbonize the dopamine, forming a heterostructure with the same crystal form. Due to the crystal form transformation, the surface structure of the grains collapses inward to form a porous structure, exposing more active sites. The crystal form transformation can introduce more vacancies and defects into the heterostructure, and the introduction of vacancies and defects can improve the ion mobility of the heterostructure.

[0053] 3. This invention uses dopamine to coat a heterostructure. The dopamine groups are combined with the heterostructure, and after polymerization, a dense polydopamine is formed on the surface of the heterostructure. After carbonization, a regular porous carbon structure is obtained, which restricts the volume shrinkage of the active material during charging and discharging and improves the conductivity of the positive electrode material. Attached image description:

[0054] Figure 1 The image shows the XRD pattern of the ZnO(W)-ZnS(S) heterostructure in step 1 of Example 1.

[0055] Figure 2 The image shows the SEM image of the microstructure of ZnO(W)-ZnS(S) in step 1 of Example 1.

[0056] Figure 3 The image shows the XRD pattern of ZnO(W)-ZnS(W)@NC in step 3 of Example 1.

[0057] Figure 4The image shown is a SEM image of ZnO(W)-ZnS(W)@NC in step 3 of Example 1.

[0058] Figure 5 This is the EDS test image of ZnO-ZnS@NC in step 3 of Example 1;

[0059] Figure 6 This is the Raman spectrum of dopamine carbonization in ZnO-ZnS@NC in step 3 of Example 1.

[0060] Figure 7 TEM tests were performed on the heterostructure interface of ZnO-ZnS@NC in step 3 of Example 1;

[0061] Figure 8 The TEM image of the (002) crystal plane lattice spacing in ZnO-ZnS@NC in step 3 of Comparative Example 1;

[0062] Figure 9 The TEM image of the (002) crystal plane lattice spacing in ZnS of ZnO-ZnS@NC in step 3 of Comparative Example 1;

[0063] Figure 10 The graph shows the cycle performance of the lithium-sulfur battery in step 2 of reference example 1, where NC is used as the positive electrode and the current rate is 0.1C.

[0064] Figure 11 In step 3 of Example 1, ZnO-ZnS@NC was assembled into a lithium-sulfur battery and the scan rate was 0.1-0.3 mV / s. -1 Cyclic voltammetry test diagram under the conditions;

[0065] Figure 12 The graph shows the cycle performance of the lithium-sulfur battery in step 3 of Comparative Example 2, where NC is used as the positive electrode and the current rate is 0.1C.

[0066] Figure 13 The graph shows the cycle performance of the lithium-sulfur battery in step 3 of Example 1, where ZnO@NC is used as the positive electrode and the current rate is 0.1C.

[0067] Figure 14 The graph shows the cycle performance of the lithium-sulfur battery in step 3 of Example 1, where ZnS@NC is used as the positive electrode and the current rate is 0.1C. Detailed Implementation

[0068] The present invention will be further described in detail through embodiments and with reference to the accompanying drawings, but this is not intended to limit the scope of the invention.

[0069] Example 1

[0070] A method for preparing spherical carbon-coated ZnO-ZnS heterostructure material includes the following steps:

[0071] Step 1: One-step preparation of spherical ZnO-ZnS heterostructures. First, with a zinc acetate to thiourea mass ratio of 2:1, 2g of zinc acetate was placed in a 100mL mixture of ethanol and water and sonicated for 15min to obtain solution A. Simultaneously, 1g of thiourea was placed in 100mL of the mixed solvent and sonicated for 15min to obtain solvent B. Then, solution A was stirred at 80℃ for 30min. After that, solution B was slowly added to solution A to obtain mixed solution A. After the addition was complete, stirring was continued for 1h. Next, under room temperature conditions and with a mixing and stirring time of 24 hours, the mixed solution was mixed and stirred to obtain reaction solution A. Finally, reaction solution A was subjected to a first hydrothermal reaction at a first hydrothermal temperature of 150℃ and a first hydrothermal time of 12 hours. The obtained product was centrifuged and washed with anhydrous ethanol as the washing liquid at a centrifugation speed of 9000 r / min, and then dried at a drying temperature of 60℃ and a drying time of 12 hours to obtain a spherical ZnO-ZnS heterostructure, abbreviated as ZnO-ZnS.

[0072] The volume ratio of ethanol to deionized water in the mixed solvent of the present invention is 1:1;

[0073] To verify the composition and crystal form of the ZnO-ZnS obtained in step 1, XRD tests were performed. The test results are as follows: Figure 1 As shown, ZnO-ZnS simultaneously contains characteristic peaks of both wurtzite ZnO and zincblende ZnS. Since zincblende ZnS has a cubic crystal system, its structure is relatively loose, resulting in a larger full width at half maximum (FWHM) of its XRD diffraction peaks. For ease of distinction, zincblende structure is denoted by S, and wurtzite structure by W; that is, ZnO with wurtzite structure is abbreviated as ZnO(W), and ZnS with zincblende structure is abbreviated as ZnS(S). Furthermore, any subsequent appearance of wurtzite ZnS is abbreviated as ZnS(W). Test results indicate that the ZnO-ZnS prepared in step 1 is ZnO(W)-ZnS(S).

[0074] To verify the microstructure of the ZnO(W)-ZnS(S) obtained in step 1, SEM testing was performed. The test results are as follows: Figure 2 As shown, ZnO(W)-ZnS(S) has a smooth spherical structure with a flat surface and an average particle size of 2.0-2.1 μm.

[0075] Step 2, polydopamine coating: First, with a mass ratio of ZnO-ZnS obtained in Step 1 to dopamine hydrochloride of 1:3, 1g of ZnO-ZnS was placed in 100mL of mixed solvent and stirred for 15min to obtain solution C. Then, 3g of dopamine hydrochloride was placed in solution C and stirred for 15min to obtain solution D. After that, under stirring conditions, tris(hydroxymethyl)aminomethane (Tris) was added to solution D to adjust the pH value to 8. After the addition was completed, the solution was sonicated for 15min to obtain mixed solution B. Then, mixed solution B was stirred for 24h to obtain reaction solution B. Finally, reaction solution B was subjected to a second hydrothermal reaction at a second hydrothermal temperature of 150℃ and a second hydrothermal time of 12h. After the product was filtered, washed and dried, the polydopamine-coated ZnO-ZnS heterostructure was obtained, abbreviated as ZnO-ZnS@PDA.

[0076] In step 2, all stirring is performed at room temperature.

[0077] Step 3, carbonization of polydopamine: Under argon conditions, the ZnO-ZnS@PDA obtained in step 2 is calcined to achieve carbonization by first calcining at low temperature and then at high temperature, thus obtaining spherical carbon-coated ZnO-ZnS heterostructure material, abbreviated as ZnO-ZnS@NC.

[0078] The conditions for low-temperature calcination in step 3 are: calcination temperature of 350℃ and calcination time of 1 hour; and the conditions for high-temperature calcination are: calcination temperature of 850℃ and calcination time of 3 hours.

[0079] To verify the composition and crystal form of the ZnO-ZnS@NC obtained in step 3, XRD tests were performed. The test results are as follows: Figure 3 As shown, ZnO-ZnS@NC simultaneously contains the characteristic peaks of both ZnO(W) and ZnS(W), while the original characteristic peak of ZnS(S) disappears. Test results indicate that step 3, while maintaining the wurtzite structure of ZnO, achieves the transformation of ZnS from a zincblende structure to a wurtzite structure, thus proving that the ZnO-ZnS@NC prepared in step 3 is indeed ZnO(W)-ZnS(W)@NC.

[0080] The mechanism by which the transformation of crystal structure affects composite materials is that the in-situ phase transition transforms the heterogeneous crystal structure of two substances into the same type, which can not only effectively improve the stability at the interface and maintain the mechanical properties of the material, but also, due to the transformation of crystal structure, vacancies and various defects are generated inside the crystal, providing a fast transfer channel for ion migration, which can improve the ion mobility in the material and thus improve the reaction kinetics of the battery.

[0081] To verify the microstructure of the ZnO-ZnS@NC obtained in step 3, SEM testing was performed. The test results are as follows: Figure 4 As shown, ZnO-ZnS@NC has a porous spherical structure with a rough surface and an average particle size of 1.8-1.9 μm. Compared with ZnO-ZnS obtained in step 1, ZnO(W)-ZnS(W)@NC has a larger specific surface area. This is because during the transformation of ZnS from a zincblende structure to a wurtzite structure, the structure changes from loose to compact, leading to inward collapse, which roughens the surface and exposes more active sites.

[0082] To verify the elemental composition of the ZnO-ZnS@NC obtained in step 3, EDS analysis was performed. The test results are as follows: Figure 5 As shown, ZnO-ZnS@NC contains Zn, O and S elements, and the elements are evenly distributed.

[0083] To demonstrate the degree of graphitization after dopamine carbonization in the ZnO-ZnS@NC obtained in step 3, Raman spectroscopy was performed. The test results are as follows: Figure 6 As shown, two distinct carbon characteristic peaks appear in the Raman spectrum, with a peak height ratio of I. D / I G =0.717. The test results show that ZnO-ZnS@NC has a high degree of graphitization, which indicates that the carbonized dopamine has good conductivity.

[0084] To further confirm that the ZnO-ZnS@NC obtained in step 3 is a heterostructure, TEM testing was performed. The results are as follows: Figure 7 , Figure 8 and Figure 9 As shown,

[0085] pass Figure 7 It can be confirmed that there is a clear heterogeneous interface between ZnO and ZnS;

[0086] Further through such Figure 8 It can be confirmed that the lattice stripe spacing between ZnO is 0.26 nm, which corresponds to the (002) crystal plane;

[0087] Through such Figure 9 It can be confirmed that the lattice fringe spacing between ZnS is 0.313 nm, which corresponds to the (002) crystal plane;

[0088] Based on the TEM test results above, it can be seen that ZnO and ZnS have similar lattice matching and low distortion energy during grain growth, thus exhibiting similar tropism. Therefore, ZnO and ZnS can nucleate and grow together. The crystal planes of ZnO and ZnS grain growth can be observed through TEM, resulting in a clear heterostructure interface.

[0089] The XRD, SEM, EDS and TEM tests above confirm that the material obtained in step 3 is a spherical ZnO(W)-ZnS(W)@NC, which is a heterostructure formed by ZnO(W) on the (002) crystal plane and ZnS(W) on the (002) crystal plane.

[0090] To demonstrate the performance of ZnO-ZnS@NC as a cathode material for lithium-sulfur batteries, lithium-sulfur batteries were assembled and subjected to electrochemical performance tests, including constant current charge-discharge cycle tests and cyclic voltammetry tests.

[0091] The specific method for assembling a lithium-sulfur battery is as follows: First, the sample to be tested is ground with sulfur powder, and a carbon-sulfur composite material is obtained using a conventional melt diffusion method. Then, using the carbon-sulfur composite material as the active material, conductive carbon black as the conductive agent, polyvinylidene fluoride (PVDF) as the binder, and N-methylpyrrolidone (NMP) as the solvent, a slurry is prepared with the active material, conductive agent, and binder in a mass ratio of 7:2:1. Finally, the slurry is uniformly coated onto aluminum foil to obtain an electrode sheet, and a coin cell is assembled by adding conventional lithium-sulfur battery electrolyte in a glove box. In specific embodiment 1, the positive electrode material of the lithium-sulfur battery is prepared using ZnO-ZnS@NC obtained in step 3, and the assembled battery is named S1.

[0092] The results of constant current charge-discharge cycle test are as follows: Figure 10 As shown, under a current multiplier of 0.1C, the initial discharge specific capacity is 1200-1400 mA hg. -1 Under the condition of 100 cycles, the residual discharge specific capacity is 1000-1100 mA hg -1 The capacity retention rate is 75-80%.

[0093] Cyclic voltammetry performance test results are as follows Figure 11 As shown, at a scan rate of 0.1-0.3 mV / s -1 Under the specified conditions, S1 exhibits two cathode peaks at 1.99 V and 2.25 V, while the main charge peak at 2.40 V and the small peak at 2.36 V are anodic peaks. Furthermore, the shape of the electrochemical window remains essentially unchanged at different scan rates in the CV curve. The test results demonstrate that ZnO-ZnS@NC exhibits good electrochemical reversibility and cycling stability during the cycling process. In addition, the sharp peaks indicate that sulfur in the composite material has good reactivity and exhibits rapid reaction kinetics.

[0094] To demonstrate the influence of heterostructure on electrochemical performance, namely the respective roles of ZnO and ZnS in composite materials, Comparative Example 1 and Comparative Example 2 are provided, in which composite materials prepared by introducing only ZnO or ZnS are used as cathode materials for lithium-sulfur batteries. Meanwhile, Reference Example 1 is provided, in which neither ZnO nor ZnS is introduced, and PDA carbonization is used directly as the cathode material for lithium-sulfur batteries as the basic reference example.

[0095] Reference ratio 1

[0096] A method for preparing PDA carbonization as a cathode material for lithium-sulfur batteries, the steps not specifically described are the same as those in Example 1, the difference being that step 1 is not performed, and in step 2, ZnO-ZnS is not added, the resulting material is named NC, and the resulting battery is named S2.

[0097] The constant current charge-discharge cycle test results of S2 are as follows: Figure 12 As shown, under the condition of a current ratio of 0.1C, the initial discharge specific capacity is 950 mA h / g; under the condition of 100 cycles, the remaining discharge specific capacity is 457 mA h / g, and the capacity retention rate is 48.1%.

[0098] A comparison with Example 1 shows that adding a ZnO-ZnS heterostructure can effectively improve the residual specific capacity after cycling. This is because when pure carbon is used as the cathode material, Li ions react with S ions through the membrane to produce long-chain lithium polysulfides, namely Li₂S. n (n=2,4,6,8), pure carbon materials have poor binding force to lithium polysulfides due to their stable structure, and cannot effectively anchor lithium polysulfides, causing lithium polysulfides to shuttle between the two sides of the separator, which seriously affects the cycle life of the battery.

[0099] However, when a ZnO-ZnS heterostructure is introduced into pure carbon,

[0100] ZnO has a good adsorption effect on lithium polysulfides and can effectively inhibit the shuttle of lithium polysulfides.

[0101] ZnS exhibits good catalytic activity for lithium polysulfides and can accelerate the catalytic reaction of Li2S. n (n=2,4,6,8) are converted to Li2S;

[0102] Furthermore, after ZnO and ZnS form a heterostructure, a built-in electric field is formed at the interface, which can adsorb lithium polysulfides in a high-energy transition state, thereby achieving long cycling, which is macroscopically reflected in the high residual capacity retention rate after cycling.

[0103] Comparative Example 1

[0104] A composite material that introduces only ZnO as the positive electrode material for a lithium-sulfur battery is described. The steps are the same as those in Example 1 unless otherwise specified. The difference is that in step 1, only zinc acetate is added, and thiourea is not added as a sulfur source. The resulting material is named ZnO@NC, and the resulting battery is named S3.

[0105] The constant current charge-discharge cycle test results of S3 are as follows: Figure 13 As shown, under the condition of a current ratio of 0.1C, the initial discharge specific capacity is 1384 mA h / g; under the condition of 100 cycles, the remaining discharge specific capacity is 725 mA h / g, and the capacity retention rate is 52.3%.

[0106] A comparison with the reference ratio shows that the introduction of ZnO can effectively improve the remaining specific capacity of lithium-sulfur batteries after cycling. This is mainly because ZnO has a good adsorption effect on lithium polysulfides, which can effectively inhibit lithium polysulfide shuttle. However, ZnO has low catalytic activity for lithium polysulfides and cannot promptly remove Li₂S₂. n The conversion of (n=2,4,6,8) to Li2S results in lithium polysulfides being adsorbed only on ZnO without improving reaction kinetics. As the reaction proceeds, excessive accumulation of lithium polysulfides on the positive electrode leads to a decrease in reaction kinetics, thereby affecting the cycle life of the battery.

[0107] However, a further comparison with Example 1 shows that introducing ZnO alone cannot obtain a ZnO-ZnS heterostructure, thus failing to effectively improve catalytic activity. As the reaction proceeds, the internal reaction kinetics of the battery decrease significantly, which proves that introducing ZnS to form a heterostructure can improve catalytic activity and enhance the reaction kinetics of the battery.

[0108] Comparative Example 2

[0109] A composite material that introduces only ZnS is used as the cathode material for lithium-sulfur batteries. The steps, unless otherwise specified, are the same as in Example 1, except that in step 1, ZnS is synthesized by mixing zinc acetate and thiourea in a volume ratio of 1:2. The sample and sublimed sulfur are then mixed and ground in a mass ratio of 1:1 and placed in a tube furnace under an atmosphere of H2 / Ar2 to remove excess ZnO. The resulting material is named ZnO@NC, and the resulting battery is named S4.

[0110] The constant current charge-discharge cycle test results of S4 are as follows: Figure 14 As shown, under the condition of a current ratio of 0.1C, the initial discharge specific capacity is 1451 mA h / g; under the condition of 100 cycles, the remaining discharge specific capacity is 763 mA h / g, and the capacity retention rate is 52.5%.

[0111] By comparing with the reference ratio, it can be seen that the introduction of ZnS can effectively improve Li2Sn The catalytic activity of (n=2,4,6,8) conversion to Li2S is improved by the introduction of ZnS, which effectively enhances the reaction kinetics of long-chain lithium polysulfides. In short-term reactions, it can effectively suppress the shuttle of long-chain lithium polysulfides. However, ZnS has a small adsorption effect on lithium polysulfides and a low adsorption energy. In continuous reactions, it cannot anchor lithium polysulfides. When the concentration of lithium polysulfides in the electrolyte is too high, the catalytic sites of ZnS are occupied, resulting in a decrease in catalytic activity, which leads to a decrease in reaction kinetics and a reduction in battery cycle life.

[0112] However, a further comparison with Example 1 shows that introducing ZnS alone cannot obtain a ZnO-ZnS heterostructure, thus lithium polysulfides cannot be effectively anchored, resulting in a severe shuttle effect. This proves that introducing ZnO to form a heterostructure can improve the anchoring effect of ZnS on lithium polysulfides, and the presence of the built-in electric field can effectively suppress lithium polysulfides that are originally in a high-energy transition state.

[0113] Comparative Examples 1 and 2 show that the technical effect obtained by introducing ZnO and ZnS to form a heterostructure is different from the technical effect obtained by introducing ZnO or ZnS separately. That is, it is not a simple one plus one equals two effect. In other words, forming a heterostructure achieves a one plus one greater than two effect.

[0114] The reason is that forming a heterostructure not only allows one to simultaneously obtain the characteristics of two materials, but also creates a built-in electric field, adsorbing lithium polysulfides in a high-energy transition state, thus realizing Li2S. n (n=2,4,6,8) Li2S conversion occurs at the ZnS interface, suppressing the shuttling of lithium polysulfides.

Claims

1. A spherical carbon-coated ZnO-ZnS heterostructure material, characterized in that: Using zinc acetate as the zinc source and thiourea as the sulfur source, a spherical ZnO-ZnS heterostructure (ZnO-ZnS) was first prepared via a hydrothermal reaction. Then, a second hydrothermal reaction was performed using tris(hydroxymethyl)aminomethane to adjust the pH, resulting in the polymerization of dopamine hydrochloride onto the surface of the ZnO-ZnS heterostructure, yielding a polydopamine-coated ZnO-ZnS heterostructure (ZnO-ZnS@PDA). Finally, carbon calcination was used to obtain a spherical carbon-coated ZnO-ZnS heterostructure material (ZnO-ZnS@NC). The Raman spectrum of ZnO-ZnS@NC shows a peak-to-height ratio of I... D / I G =0.

717.

2. The spherical carbon-coated ZnO-ZnS heterostructure material according to claim 1, characterized in that: The crystal structure of ZnO-ZnS is as follows: ZnO has a wurtzite structure and ZnS has a zincblende structure. The crystal structure of ZnO-ZnS@NC is as follows: ZnO has a wurtzite structure and ZnS has a wurtzite structure. In the ZnO-ZnS@NC, there is a heterogeneous interface between ZnO and ZnS.

3. The spherical carbon-coated ZnO-ZnS heterostructure material according to claim 1, characterized in that: The ZnO-ZnS has a smooth spherical structure with a flat surface and an average particle size of 2.0-2.1 μm; The ZnO-ZnS@NC has a porous spherical structure with a rough surface and an average particle size of 1.8-1.9 μm.

4. The method for preparing spherical carbon-coated ZnO-ZnS heterostructure material according to claim 1, characterized in that, The preparation method includes the following steps: Step 1, a one-step preparation of spherical ZnO-ZnS heterostructures: First, zinc acetate and thiourea are mixed in a certain mass ratio. Zinc acetate is placed in a mixed solvent of ethanol and water and sonicated to obtain solution A. Simultaneously, thiourea is placed in the mixed solvent and sonicated to obtain solvent B. Then, under certain conditions, solution A is stirred. After that, solution B is slowly added to solution A to obtain mixed solution A. Next, under certain conditions, the mixed solutions are mixed and stirred to obtain reaction solution A. Finally, reaction solution A undergoes a first hydrothermal reaction under certain conditions. The resulting product is centrifuged and washed under certain conditions, and then dried under certain conditions to obtain spherical ZnO-ZnS heterostructures, abbreviated as ZnO-ZnS. Step 2, polydopamine coating: First, ZnO-ZnS obtained in Step 1 and dopamine hydrochloride are mixed in a certain mass ratio. ZnO-ZnS is placed in a mixed solvent and stirred under certain conditions to obtain solution C. Then, dopamine hydrochloride is placed in solution C and stirred under certain conditions to obtain solution D. After that, under stirring conditions, tris(hydroxymethyl)aminomethane (Tris) is added to solution D to adjust the pH value of the solution to 8. After the addition is completed, the mixture is sonicated to obtain mixed solution B. Then, mixed solution B is stirred under certain conditions to obtain reaction solution B. Finally, reaction solution B is subjected to a second hydrothermal reaction under certain conditions. After the product is filtered, washed and dried, polydopamine-coated ZnO-ZnS heterostructure is obtained, abbreviated as ZnO-ZnS@PDA. Step 3, carbonization of polydopamine: Under certain conditions, the ZnO-ZnS@PDA obtained in step 2 is calcined to achieve carbonization, thereby obtaining a spherical carbon-coated ZnO-ZnS heterostructure, abbreviated as ZnO-ZnS@NC.

5. The preparation method according to claim 4, characterized in that: In step 1, the mass ratio of zinc acetate to thiourea is 2:1; In step 2, the mass ratio of ZnO-ZnS to dopamine hydrochloride is 1:3; The volume ratio of ethanol to deionized water in the mixed solvent is 1:1; In step 2, all stirring is performed at room temperature.

6. The preparation method according to claim 4, characterized in that: In step 1, the stirring conditions for solution A are: stirring temperature of 50-80℃, stirring time of 0.5-1h, and stirring for another 1h after the addition is complete. In step 1, the conditions for preparing reaction solution A are: mixing and stirring for 20-24 hours at room temperature. In step 1, the conditions for the first hydrothermal reaction are: the first hydrothermal temperature is 150-180 ℃ and the first hydrothermal time is 12 h. In step 1, the conditions for centrifugal washing are as follows: the washing liquid is anhydrous ethanol, and the centrifugation speed is 7000-9000 r / min. In step 1, the drying conditions are: drying temperature of 60-80 ℃ and drying time of 10-12 h.

7. The preparation method according to claim 4, characterized in that: In step 2, the stirring conditions for solution C are: stirring time is 15 min. In step 2, the stirring conditions for solution D are: stirring time is 15 min. In step 2, the ultrasonic conditions for mixed solution B are as follows: ultrasonic time is 15 min. In step 2, the conditions for preparing reaction solution B are: mixing and stirring time of 24 h; In step 2, the conditions for the second hydrothermal reaction are: the second hydrothermal temperature is 150℃ and the second hydrothermal time is 12 h.

8. The preparation method according to claim 4, characterized in that: In step 3, the calcination conditions are: first, calcination at a low temperature, and then calcination at a high temperature. In step 3, the conditions for low-temperature calcination are 350°C and 1 h, and the conditions for high-temperature calcination are 850°C and 3 h.

9. The application of the spherical carbon-coated ZnO-ZnS heterostructure material according to claim 1 as a cathode material for lithium-sulfur batteries, characterized in that: Under a current rate of 0.1C, the initial discharge specific capacity is 1200-1400 mA hg. -1 .

10. The application of the spherical carbon-coated ZnO-ZnS heterostructure material according to claim 1 as a cathode material for lithium-sulfur batteries, characterized in that: Under the condition of 100 cycles, the residual discharge specific capacity is 1000-1100 mA hg -1 The capacity retention rate is 75-80%.

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