Cobalt tungstate / tungsten dioxide heterojunction catalyst for positive electrode material of lithium-sulfur battery, preparation method of cobalt tungstate / tungsten dioxide heterojunction catalyst and positive electrode material

Abstract

The invention relates to the technical field of lithium-sulfur batteries, in particular to a cobalt tungstate / tungsten dioxide heterojunction catalyst for a positive electrode material of a lithium-sulfur battery, a preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst and the positive electrode material. The preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst comprises the following steps: respectively dissolving cobalt nitrate hexahydrate and sodium tungstate dihydrate in an aqueous solution to obtain a cobalt nitrate hexahydrate solution and a sodium tungstate dihydrate solution; adjusting the pH value of the sodium tungstate dihydrate solution, adding a cobalt nitrate hexahydrate solution, mixing and stirring, and sequentially carrying out hydrothermal reaction and first calcination treatment to obtain the cobalt tungstate / tungsten dioxide heterojunction catalyst. The cobalt tungstate / tungsten dioxide heterojunction catalyst has the advantage of high catalytic activity, plays a great role in improving the performance of a lithium-sulfur battery, and can be applied to a lithium-sulfur battery system to effectively improve the capacity retention ratio and coulombic efficiency of the battery and improve the cycling stability of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-sulfur batteries, and particularly to a cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery cathode material, a preparation method thereof, and a cathode material. Background Art

[0002] A lithium-sulfur battery is a secondary battery with metallic lithium as the anode and sulfur as the cathode. Due to its high theoretical specific capacity (1675 mAh / g) and high energy density (2600 Wh / kg), it is considered a strong candidate for the next generation of high-energy-density energy storage systems. However, the practical application of lithium-sulfur batteries still faces many challenges, such as the shuttle effect of intermediate polysulfides (such as lithium polysulfide) between the sulfur cathode and the lithium metal anode, resulting in poor cycle stability of lithium-sulfur batteries.

[0003] To improve the cycle stability of lithium-sulfur batteries, the prior art usually adopts the method of introducing a cathode catalyst to inhibit the shuttle effect, such as transition metal oxides, sulfides, and carbon-based materials. Among them, cobalt tungstate (CoWO4), as a transition metal oxide, shows significant advantages in catalyzing the conversion of polysulfides due to its unique d-electron configuration and stable crystal structure. However, single CoWO4 has problems such as poor conductivity and limited active sites, resulting in insufficient catalytic activity.

[0004] Heterojunction engineering provides a new idea to solve this bottleneck. Through the heterojunction interface, a built-in electric field can be formed to promote the directional separation of charges, significantly improving the catalytic activity.

[0005] Tungsten oxides (such as tungsten trioxide, tungsten dioxide, etc.) are an important class of semiconductor materials. Among them, tungsten dioxide (WO2) has metal-like properties, so it has good conductivity and can promote electron transport. It is an ideal heterojunction component. However, current research on tungsten oxides mainly focuses on tungsten trioxide (WO3), with less attention paid to WO2, and there is no research on the heterojunction between CoWO4 and WO2 and its application in lithium-sulfur batteries. Summary of the Invention

[0006] The present invention provides a preparation method of a cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery cathode material. Through this method, a cobalt tungstate / tungsten dioxide heterojunction catalyst with high catalytic activity can be prepared; and this cobalt tungstate / tungsten dioxide heterojunction catalyst plays a great role in improving the performance of lithium-sulfur batteries. Applying the cobalt tungstate / tungsten dioxide heterojunction catalyst of the present invention in a lithium-sulfur battery system can effectively inhibit the shuttle effect of polysulfides, improve the capacity retention rate and Coulombic efficiency of lithium-sulfur batteries, and improve the cycle stability of lithium-sulfur batteries.

[0007] The present invention also provides a cobalt tungstate / tungsten dioxide heterojunction catalyst, which is prepared by the above preparation method. Therefore, the cobalt tungstate / tungsten dioxide heterojunction catalyst has the characteristic of high catalytic activity. When the cobalt tungstate / tungsten dioxide heterojunction catalyst is applied to a lithium-sulfur battery, it can effectively inhibit the shuttle effect of polysulfides, improve the capacity retention rate and Coulomb efficiency of the lithium-sulfur battery, and improve the cycle stability of the lithium-sulfur battery.

[0008] The present invention also provides a cathode material, which includes a cathode active material obtained by grinding and second calcination treatment using the above cobalt tungstate / tungsten dioxide heterojunction catalyst and sulfur as raw materials. The cathode material contains the above cobalt tungstate / tungsten dioxide heterojunction catalyst. Therefore, when the cathode material containing the above cobalt tungstate / tungsten dioxide heterojunction catalyst is applied to a lithium-sulfur battery, it can effectively inhibit the shuttle effect of polysulfides, improve the capacity retention rate and Coulomb efficiency of the lithium-sulfur battery, and improve the cycle stability of the lithium-sulfur battery.

[0009] The present invention provides a preparation method of a cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery cathode material in a first aspect, including the following steps:

[0010] Dissolve cobalt nitrate hexahydrate and sodium tungstate dihydrate in an aqueous solution respectively to obtain a cobalt nitrate hexahydrate solution and a sodium tungstate dihydrate solution;

[0011] Use a pH regulator to adjust the pH value of the sodium tungstate dihydrate solution and then add the cobalt nitrate hexahydrate solution. After mixing and stirring, perform a hydrothermal reaction to obtain a solid precursor;

[0012] Perform a first calcination treatment on the solid precursor in an Ar / H2 atmosphere to obtain the cobalt tungstate / tungsten dioxide heterojunction catalyst.

[0013] For the preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery cathode material as described above, the pH value is 2-4.

[0014] For the preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery cathode material as described above, the pH regulator includes hydrochloric acid.

[0015] For the preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery cathode material as described above, the molar ratio of cobalt nitrate hexahydrate to sodium tungstate dihydrate is (1-3):(1-2).

[0016] For the preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery cathode material as described above, the temperature of the hydrothermal reaction is 170-200 °C and the time is 1-5 hours.

[0017] The preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst for the cathode material of a lithium-sulfur battery as described above, wherein the temperature of the first calcination treatment is 600 - 700 °C and the time is 1 - 4 hours.

[0018] In a second aspect, the present invention provides a cobalt tungstate / tungsten dioxide heterojunction catalyst, which is prepared by the preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst for the cathode material of a lithium-sulfur battery as described above.

[0019] In a third aspect, the present invention provides a cathode material, which includes a cathode active material obtained by grinding and a second calcination treatment using the cobalt tungstate / tungsten dioxide heterojunction catalyst and sulfur as raw materials.

[0020] For the cathode material as described above, in the cathode active material, the mass ratio of the cobalt tungstate / tungsten dioxide heterojunction catalyst to the sulfur is (1 - 2) : (3 - 4);

[0021] and / or, the temperature of the second calcination treatment is 150 - 160 °C and the time is 4 - 6 hours.

[0022] For the cathode material as described above, the cathode material further includes a conductive agent and a binder;

[0023] In the cathode material, the mass ratio of the cathode active material, the conductive agent and the binder is (7 - 8) : (1 - 2) : 1.

[0024] The solution of the present invention has at least the following effects:

[0025] The preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst for the cathode material of a lithium-sulfur battery provided by the present invention can prepare a cobalt tungstate / tungsten dioxide heterojunction catalyst with high catalytic activity through this method; this cobalt tungstate / tungsten dioxide heterojunction catalyst plays a great role in improving the performance of lithium-sulfur batteries. When applying the cobalt tungstate / tungsten dioxide heterojunction catalyst of the present invention in lithium-sulfur batteries, it has a good promoting effect on the catalytic conversion of polysulfides, reduces the energy barrier of the reaction from an electrochemical perspective, reduces polarization, and its adsorption characteristics for polysulfides can effectively inhibit the shuttle effect of polysulfides, improve the capacity retention rate and Coulomb efficiency of lithium-sulfur batteries, and improve the cycle stability of lithium-sulfur batteries; this preparation method has a simple process, and the raw materials are cheap and easily available, which is suitable for wide promotion and application.

[0026] The cobalt tungstate / tungsten dioxide heterojunction catalyst provided by the present invention has the advantages of high catalytic activity. It has high catalytic activity, can effectively promote the catalytic conversion of polysulfides, can effectively inhibit the shuttle effect of polysulfides, and improve the capacity retention rate, Coulomb efficiency and cycle stability of the battery; the cobalt tungstate / tungsten dioxide heterojunction catalyst is inexpensive, environmentally friendly, low-cost and resource-rich, providing a new solution for the preparation of cathode catalysts and having broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to these drawings.

[0028] Figure 1 XRD pattern of the cobalt tungstate catalyst (CoWO4) in Comparative Example 6 of the present invention;

[0029] Figure 2 XRD patterns and SEM images of the solid precursor (Co4W6O 21 (OH)2·4H2O) and the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention. Among them, Figure 2 a is the XRD pattern of the solid precursor (Co4W6O 21 (OH)2·4H2O) in Example 1 of the present invention, Figure 2 b is the XRD pattern of the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention, Figure 2 c is the SEM image of the solid precursor (Co4W6O 21 (OH)2·4H2O) in Example 1 of the present invention, Figure 2 d is the SEM image of the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention;

[0030] Figure 3 Particle size distribution diagram and specific surface area diagram of the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention. Among them, Figure 3 a is the particle size distribution diagram of the cobalt tungstate / tungsten dioxide heterojunction catalyst in Example 1 of the present invention, Figure 3 b is the specific surface area diagram of the cobalt tungstate / tungsten dioxide heterojunction catalyst in Example 1 of the present invention;

[0031] Figure 4TEM image of the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention;

[0032] Figure 5 Cyclic voltammetry curve (CV) of the lithium-sulfur button battery assembled with the cathode material in Example 1 of the present invention and a commercial lithium metal anode;

[0033] Figure 6 Charge-discharge cycling curves of the lithium-sulfur button batteries assembled with the cathode material in Example 1 of the present invention and the cathode materials in Comparative Examples 1-2 and a commercial lithium metal anode respectively;

[0034] Figure 7 Charge-discharge cycling curves of the lithium-sulfur button batteries assembled with the cathode material in Example 2 of the present invention and the cathode materials in Comparative Examples 3-4 and a commercial lithium metal anode respectively. Detailed implementation manners

[0035] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0036] For those technical or conditions not specified in the embodiments of the present invention, they shall be carried out according to the technologies or conditions described in the literature in the art or according to the product specifications. For the reagents or instruments not indicated with the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0037] It should be noted that the descriptions involving "first", "second", etc. in the present invention are used to distinguish similar objects, rather than to describe a specific order or sequence, and thus should not be construed as a limitation to the present invention.

[0038] The present invention provides a preparation method of a cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery cathode material in a first aspect, comprising the following steps:

[0039] Dissolve cobalt nitrate hexahydrate and sodium tungstate dihydrate in an aqueous solution respectively to obtain a cobalt nitrate hexahydrate solution and a sodium tungstate dihydrate solution;

[0040] Adjust the pH value of the sodium tungstate dihydrate solution with a pH regulator, then add the cobalt nitrate hexahydrate solution, mix and stir, and then carry out a hydrothermal reaction to obtain a solid precursor;

[0041] Carry out a first calcination treatment on the solid precursor in an Ar / H2 atmosphere to obtain the cobalt tungstate / tungsten dioxide heterojunction catalyst.

[0042] The present invention does not particularly limit the specific equipment for carrying out the above hydrothermal reaction. In some embodiments, the hydrothermal reaction can be carried out in a high-pressure reactor.

[0043] The present invention also does not particularly limit the specific equipment for carrying out the above first calcination treatment. In some embodiments, the first calcination treatment can be carried out in a tubular furnace.

[0044] The present invention does not particularly limit the specific time for the above mixing and stirring, as long as the cobalt nitrate hexahydrate solution and the adjusted sodium tungstate dihydrate solution are mixed evenly.

[0045] The above aqueous solution can use conventional types in the art. For example, the aqueous solution can be deionized water.

[0046] The object of the present invention for preparation is a cobalt tungstate / tungsten dioxide heterojunction catalyst. Specifically, first, cobalt nitrate hexahydrate and sodium tungstate dihydrate are respectively dissolved in an aqueous solution to obtain a cobalt nitrate hexahydrate solution and a sodium tungstate dihydrate solution; then, after using a pH regulator to adjust the pH value of the sodium tungstate dihydrate solution, the sodium tungstate dihydrate will dehydrate to form a specific tungstate anion to obtain the adjusted sodium tungstate dihydrate solution; then, the cobalt nitrate hexahydrate solution is added to the adjusted sodium tungstate dihydrate solution for mixing and stirring to obtain a mixed solution; then, the mixed solution is subjected to a hydrothermal reaction to obtain a solid precursor. The hydrothermal reaction can accelerate the formation of the crystal structure and is beneficial to the subsequent preparation of a cobalt tungstate / tungsten dioxide heterojunction catalyst with uniform particle size; finally, the solid precursor is subjected to a first calcination treatment in an Ar / H2 atmosphere to obtain a cobalt tungstate / tungsten dioxide heterojunction catalyst with high catalytic activity. This cobalt tungstate / tungsten dioxide heterojunction catalyst plays a great role in improving the performance of lithium-sulfur batteries. When applied in lithium-sulfur batteries, it can effectively inhibit the shuttle effect of polysulfides, improve the capacity retention rate and Coulomb efficiency of lithium-sulfur batteries, and improve the cycle stability of lithium-sulfur batteries; this preparation method has a simple process, cheap and easily available raw materials, and is suitable for wide promotion and application.

[0047] In some embodiments, before the solid precursor is subjected to the first calcination treatment in an Ar / H2 atmosphere, it also includes washing it to remove residual unreacted ions.

[0048] In a specific embodiment, the above pH value is 2-4.

[0049] When the pH value is within the above range, the coordination of tungstate anions and cobalt ions is achieved to form a solid precursor. The inventors have shown through research that if the pH value is greater than 4, the final catalyst prepared is pure phase cobalt tungstate (CoWO4), and the cobalt tungstate / tungsten dioxide heterojunction catalyst cannot be prepared; if the pH value is less than 2, no precipitation will be generated during the hydrothermal reaction, that is, no solid precursor can be obtained.

[0050] For example, the pH value may be any one of 2, 3, 4, or any two of them.

[0051] In one specific embodiment, the pH adjuster includes hydrochloric acid.

[0052] Using hydrochloric acid as a pH regulator can accurately adjust the pH and avoid interference from substances in other acids (if sulfuric acid is used, sulfate ions will be introduced), and the chloride ions in hydrochloric acid are easily removed during washing.

[0053] The present invention does not impose any particular limitation on the concentration of hydrochloric acid, as long as the pH value of the sodium tungstate dihydrate solution is adjusted to 2-4 using a pH adjuster.

[0054] In a specific embodiment, the molar ratio of the cobalt nitrate hexahydrate to the sodium tungstate dihydrate is (1-3):(1-2), for example, the molar ratio of the cobalt nitrate hexahydrate to the sodium tungstate dihydrate is 1:1, 1:2, 2:1, 2:2, 3:1 or 3:2, etc.

[0055] When the molar ratio of cobalt nitrate hexahydrate to sodium tungstate dihydrate is within the above range, a cobalt tungstate / tungsten dioxide heterojunction catalyst with high catalytic activity can be prepared.

[0056] In a specific embodiment, the temperature of the hydrothermal reaction is 170-200° C. and the time is 1-5 hours.

[0057] When the temperature and time parameters of the hydrothermal reaction are respectively within the above ranges, the hydrothermal reaction can proceed fully, which is beneficial to accelerate the formation of the crystal structure (ie, the solid precursor), thereby facilitating the subsequent preparation of a cobalt tungstate / tungsten dioxide heterojunction catalyst with uniform particle size.

[0058] Exemplarily, the temperature of the hydrothermal reaction may be in the range of any one of 170°C, 180°C, 190°C, 200°C, or any two of them;

[0059] The time may be any one of 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or a range consisting of any two of them.

[0060] In a specific embodiment, the temperature of the first calcination treatment is 600-700° C. and the time is 1-4 hours.

[0061] When the parameters of the temperature and time of the first calcination treatment are respectively within the above ranges, the lattice transformation of the solid precursor can be promoted to occur spontaneously, and overgrowth can be avoided, which is beneficial to the preparation of the above cobalt tungstate / tungsten dioxide heterojunction catalyst.

[0062] Exemplarily, the temperature of the first calcination treatment can be any one of 600 °C, 610 °C, 620 °C, 630 °C, 640 °C, 650 °C, 660 °C, 670 °C, 680 °C, 690 °C, 700 °C or the range composed of any two of them;

[0063] The time can be any one of 1 hour, 2 hours, 3 hours, 4 hours or the range composed of any two of them.

[0064] In a specific embodiment, the chemical formula of the above solid precursor is Co4W6O 21 (OH)2·4H2O.

[0065] In the second aspect of the present invention, a cobalt tungstate / tungsten dioxide heterojunction catalyst is provided, which is prepared by the preparation method of the cobalt tungstate / tungsten dioxide heterojunction catalyst for the cathode material of a lithium-sulfur battery. Therefore, the cobalt tungstate / tungsten dioxide heterojunction catalyst has the characteristics of high catalytic activity. When the cobalt tungstate / tungsten dioxide heterojunction catalyst is applied to a lithium-sulfur battery, the capacity retention rate and Coulombic efficiency of the lithium-sulfur battery can be effectively improved, and the cycle stability of the lithium-sulfur battery can be improved.

[0066] In the third aspect of the present invention, a cathode material is provided, which includes a cathode active material obtained by grinding and a second calcination treatment using the above cobalt tungstate / tungsten dioxide heterojunction catalyst and sulfur as raw materials.

[0067] Specifically, the cathode material includes a cathode active material, which is obtained by grinding and a second calcination treatment using the above cobalt tungstate / tungsten dioxide heterojunction catalyst and sulfur as raw materials. Therefore, the above cobalt tungstate / tungsten dioxide heterojunction catalyst is contained in the cathode material. When the cathode material containing the above cobalt tungstate / tungsten dioxide heterojunction catalyst is applied to a lithium-sulfur battery system, the capacity retention rate and Coulombic efficiency of the lithium-sulfur battery can be effectively improved, the cycle stability of the lithium-sulfur battery can be improved, and the shuttle effect of polysulfides can be effectively inhibited.

[0068] In the present invention, the above sulfur is a conventional type in the art. For example, the sulfur is sulfur S8.

[0069] In a specific embodiment, in the above cathode active material, the mass ratio of the cobalt tungstate / tungsten dioxide heterojunction catalyst to sulfur is (1-2):(3-4), such as the mass ratio of the cobalt tungstate / tungsten dioxide heterojunction catalyst to sulfur is 1:3, 1:4, 2:3 or 2:4, etc.

[0070] When the parameters of the mass ratio of the cobalt tungstate / tungsten dioxide heterojunction catalyst to sulfur are within the above ranges, the cobalt tungstate / tungsten dioxide heterojunction catalyst and sulfur in the positive electrode active material can be better matched, which is beneficial to improving the cycle stability of the lithium-sulfur battery.

[0071] In a specific embodiment, the temperature of the second calcination treatment is 150-160 °C and the time is 4-6 hours.

[0072] When the parameters of the temperature and time of the second calcination treatment are within the above ranges respectively, the cobalt tungstate / tungsten dioxide heterojunction catalyst and sulfur can be better compounded to obtain the positive electrode active material. Applying the positive electrode active material to the lithium-sulfur battery can significantly improve the capacity retention rate and Coulomb efficiency of the lithium-sulfur battery and improve the cycle stability of the lithium-sulfur battery.

[0073] Exemplarily, the temperature of the second calcination treatment can be any one of 150 °C, 151 °C, 152 °C, 153 °C, 154 °C, 155 °C, 156 °C, 157 °C, 158 °C, 159 °C, 160 °C or the range composed of any two of them;

[0074] The time can be any one of 4 hours, 5 hours, 6 hours or the range composed of any two of them.

[0075] In a specific embodiment, the above positive electrode material further includes a conductive agent and a binder; in the positive electrode material, the mass ratio of the positive electrode active material, the conductive agent and the binder is (7-8):(1-2):1, and preferably 7:2:1.

[0076] When the parameters of the mass ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode material are within the above ranges, the positive electrode active material, the conductive agent and the binder in the positive electrode material can be better matched, which is beneficial to improving the cycle stability of the lithium-sulfur battery.

[0077] The above conductive agent and binder can both use conventional types in the art. For example, the conductive agent can be selected from one or more of conductive carbon black, acetylene black, graphene, Ketjen black, carbon fiber; the binder can be selected from one or more of polyvinylidene fluoride, polyvinylidene difluoride, polytetrafluoroethylene.

[0078] In some embodiments, the above positive electrode material further includes a positive electrode current collector.

[0079] In the embodiments of the present invention, the positive electrode current collector can be a conventional positive electrode current collector in the art, for example, including one or more of aluminum foil and nickel foil.

[0080] In some embodiments, the preparation method of the above positive electrode material includes the following steps:

[0081] Mix the positive electrode active material, conductive agent, and binder to make a positive electrode paste;

[0082] Coat the positive electrode paste on the surface of the positive electrode current collector to obtain the positive electrode material.

[0083] The embodiments and comparative examples of the present invention are used to illustrate the implementation manners of the present invention in detail.

[0084] Example 1

[0085] The preparation method of the positive electrode material in this example includes the following steps:

[0086] 1. Preparation of cobalt tungstate / tungsten dioxide heterojunction catalyst

[0087] (1) Dissolve 1 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in 20 mL of deionized water to obtain a cobalt nitrate hexahydrate solution;

[0088] (2) Dissolve 1 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) in 20 mL of deionized water to obtain a sodium tungstate dihydrate solution, and use hydrochloric acid (HCl) to adjust the pH value of the sodium tungstate dihydrate solution to 4 to obtain the adjusted sodium tungstate dihydrate solution;

[0089] (3) Add the cobalt nitrate hexahydrate solution in (1) to the adjusted sodium tungstate dihydrate solution in (2), mix and stir for 40 minutes to obtain a mixed solution;

[0090] (4) Place the mixed solution in (3) in a high-pressure reactor at a temperature of 180 °C for hydrothermal reaction for 2 hours to obtain a reaction product, and wash the reaction product to remove residual unreacted ions to obtain a solid precursor (Co4W6O 21 (OH)2·4H2O);

[0091] (5) Under an Ar / H2 atmosphere, place the solid precursor in (4) in a tubular furnace at a temperature of 600 °C for calcination treatment for 2 hours to obtain a cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2).

[0092] 2. Preparation of positive electrode material

[0093] (1) Mix 100 mg of cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) and 300 mg of sulfur S8 to obtain a mixture, grind the mixture, and perform calcination treatment in an argon atmosphere at 155 °C for 6 hours to obtain a positive electrode active material (CoWO4 / WO2@S);

[0094] (2) Mix the cathode active material in (1) with conductive carbon black and polyvinylidene fluoride in a mass ratio of 7:2:1 to form a cathode paste;

[0095] (3) Uniformly coat the cathode paste in (2) on the surface of aluminum foil, and control the sulfur surface loading to be 1 mg / cm 2 to obtain the cathode material.

[0096] Example 2

[0097] The preparation method of the cathode material provided in this example is basically the same as that in Example 1, except that:

[0098] 2. Preparation of the cathode material

[0099] (3) Uniformly coat the cathode paste in (2) on the surface of aluminum foil, and control the sulfur surface loading to be 5 mg / cm 2 to obtain the cathode material.

[0100] Example 3

[0101] The preparation method of the cathode material in this example includes the following steps:

[0102] 1. Preparation of cobalt tungstate / tungsten dioxide heterojunction catalyst

[0103] (1) Dissolve 1 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in 40 mL of deionized water to obtain a cobalt nitrate hexahydrate solution;

[0104] (2) Dissolve 2 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) in 40 mL of deionized water to obtain a sodium tungstate dihydrate solution, and use hydrochloric acid (HCl) to adjust the pH value of the sodium tungstate dihydrate solution to 2 to obtain the adjusted sodium tungstate dihydrate solution;

[0105] (3) Add the cobalt nitrate hexahydrate solution in (1) to the adjusted sodium tungstate dihydrate solution in (2) and mix and stir for 20 minutes to obtain a mixed solution;

[0106] (4) Place the mixed solution in (3) in a high-pressure reaction kettle at a temperature of 170 °C for hydrothermal reaction for 1 hour to obtain a reaction product, and wash the reaction product to remove residual unreacted ions to obtain a solid precursor (Co4W6O 21 (OH)2·4H2O);

[0107] (5) Under an Ar / H2 atmosphere, place the solid precursor in (4) in a tubular furnace at a temperature of 600 °C for calcination treatment for 1 hour to obtain a cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2).

[0108] 2. Preparation of the positive electrode material

[0109] (1) Mix 100 mg of cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) and 300 mg of sulfur S8 to obtain a mixture. After grinding the mixture, conduct a calcination treatment in an argon atmosphere at 155 °C for 6 hours to obtain the positive electrode active material (CoWO4 / WO2@S);

[0110] (2) Mix the positive electrode active material in (1) with conductive carbon black and polyvinylidene fluoride in a mass ratio of 8:1:1 to make a positive electrode slurry;

[0111] (3) Uniformly coat the positive electrode slurry in (2) on the surface of aluminum foil, and control the sulfur surface loading to be 1 mg / cm 2 to obtain the positive electrode material.

[0112] Example 4

[0113] The preparation method of the positive electrode material in this example includes the following steps:

[0114] 1. Preparation of cobalt tungstate / tungsten dioxide heterojunction catalyst

[0115] (1) Dissolve 1 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in 30 mL of deionized water to obtain a cobalt nitrate hexahydrate solution;

[0116] (2) Dissolve 1.5 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) in 30 mL of deionized water to obtain a sodium tungstate dihydrate solution. Use hydrochloric acid (HCl) to adjust the pH value of the sodium tungstate dihydrate solution to 3 to obtain the adjusted sodium tungstate dihydrate solution;

[0117] (3) Add the cobalt nitrate hexahydrate solution in (1) to the adjusted sodium tungstate dihydrate solution in (2), and conduct a mixing and stirring for 30 minutes to obtain a mixed solution;

[0118] (4) Place the mixed solution in (3) in a high-pressure reactor at a temperature of 170 °C for a hydrothermal reaction for 2 hours to obtain a reaction product. Wash the reaction product to remove residual unreacted ions to obtain a solid precursor (Co4W6O 21 (OH)2·4H2O);

[0119] (5) Under an Ar / H2 atmosphere, place the solid precursor in (4) in a tube furnace at a temperature of 700 °C for a calcination treatment for 1 hour to obtain a cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2).

[0120] 2. Preparation of the positive electrode material

[0121] (1) Mix 100 mg of cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) and 300 mg of sulfur S8 to obtain a mixture. Grind the mixture and then calcine it in an argon atmosphere at 155 °C for 6 hours to obtain the positive electrode active material (CoWO4 / WO2@S);

[0122] (2) Mix the positive electrode active material in (1) with conductive carbon black and polyvinylidene fluoride in a mass ratio of 8:1:1 to make a positive electrode slurry;

[0123] (3) Uniformly coat the positive electrode slurry in (2) on the surface of aluminum foil, and control the sulfur surface loading to be 1 mg / cm 2 to obtain the positive electrode material.

[0124] Comparative Example 1

[0125] The preparation method of the positive electrode material provided in this comparative example includes the following steps:

[0126] (1) Mix 100 mg of cobalt tungstate (CoWO4) and 300 mg of sulfur S8 to obtain a mixture. Grind the mixture and then calcine it in an argon atmosphere at 155 °C for 6 hours to obtain the positive electrode active material (CoWO4@S);

[0127] (2) Mix the positive electrode active material in (1) with conductive carbon black and polyvinylidene fluoride in a mass ratio of 7:2:1 to make a positive electrode slurry;

[0128] (3) Uniformly coat the positive electrode slurry in (2) on the surface of aluminum foil, and control the sulfur surface loading to be 1 mg / cm 2 to obtain the positive electrode material.

[0129] Comparative Example 2

[0130] The preparation method of the positive electrode material provided in this comparative example includes the following steps:

[0131] (1) Mix 100 mg of tungsten dioxide (WO2) and 300 mg of sulfur S8 to obtain a mixture. Grind the mixture and then calcine it in an argon atmosphere at 155 °C for 6 hours to obtain the positive electrode active material (WO2@S);

[0132] (2) Mix the positive electrode active material in (1) with conductive carbon black and polyvinylidene fluoride in a mass ratio of 7:2:1 to make a positive electrode slurry;

[0133] (3) Uniformly coat the positive electrode slurry in (2) on the surface of aluminum foil, and control the sulfur surface loading to be 1 mg / cm 2 to obtain the positive electrode material.

[0134] Comparative Example 3

[0135] The preparation method of the cathode material provided in this comparative example includes the following steps:

[0136] (1) Mix 100 mg of cobalt tungstate (CoWO4) and 300 mg of sulfur (S8) to obtain a mixture. After grinding the mixture, perform a calcination treatment at 155 °C in an argon atmosphere for 6 hours to obtain a cathode active material (CoWO4@S);

[0137] (2) Mix the cathode active material in (1) with conductive carbon black and polyvinylidene fluoride in a mass ratio of 7:2:1 to make a cathode slurry;

[0138] (3) Uniformly coat the cathode slurry in (2) on the surface of aluminum foil, and control the sulfur surface loading to be 5 mg / cm 2 to obtain the cathode material.

[0139] Comparative Example 4

[0140] The preparation method of the cathode material provided in this comparative example includes the following steps:

[0141] (1) Mix 100 mg of tungsten dioxide (WO2) and 300 mg of sulfur (S8) to obtain a mixture. After grinding the mixture, perform a calcination treatment at 155 °C in an argon atmosphere for 6 hours to obtain a cathode active material (WO2@S);

[0142] (2) Mix the cathode active material in (1) with conductive carbon black and polyvinylidene fluoride in a mass ratio of 7:2:1 to make a cathode slurry;

[0143] (3) Uniformly coat the cathode slurry in (2) on the surface of aluminum foil, and control the sulfur surface loading to be 5 mg / cm 2 to obtain the cathode material.

[0144] Comparative Example 5

[0145] The preparation method of the cathode material provided in this comparative example is basically the same as that of Example 1, except that:

[0146] 1. Preparation of cobalt tungstate / tungsten dioxide heterojunction catalyst

[0147] (2) Dissolve 1 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) in 20 mL of deionized water to obtain a sodium tungstate dihydrate solution. Use hydrochloric acid (HCl) to adjust the pH value of the sodium tungstate dihydrate solution to 1 to obtain the adjusted sodium tungstate dihydrate solution.

[0148] The inventor of the present invention found that no precipitate will be formed during the hydrothermal reaction when the adjusted sodium tungstate dihydrate solution (pH = 1) in this comparative example is mixed with the cobalt nitrate hexahydrate solution, and no solid precursor can be obtained.

[0149] Comparative Example 6

[0150] The preparation method of the positive electrode material provided in this comparative example is basically the same as that of Example 1, except that:

[0151] 1. Preparation of cobalt tungstate / tungsten dioxide heterojunction catalyst

[0152] (1) Dissolve 1 mmol of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in 20 mL of deionized water to obtain a cobalt nitrate hexahydrate solution;

[0153] (2) Dissolve 1 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) in 20 mL of deionized water to obtain a sodium tungstate dihydrate solution, and use hydrochloric acid (HCl) to adjust the pH value of the sodium tungstate dihydrate solution to 6 to obtain the adjusted sodium tungstate dihydrate solution;

[0154] (3) Add the cobalt nitrate hexahydrate solution in (1) to the adjusted sodium tungstate dihydrate solution in (2), mix and stir for 40 minutes to obtain a mixed solution;

[0155] (4) Place the mixed solution in (3) in a high-pressure reactor at a temperature of 180 °C for hydrothermal reaction for 2 hours to obtain a reaction product, and wash the reaction product to remove residual unreacted ions to obtain a solid precursor;

[0156] (5) Under an Ar / H2 atmosphere, place the solid precursor in (4) in a tubular furnace at a temperature of 600 °C for calcination treatment for 2 hours to obtain a cobalt tungstate catalyst (CoWO4).

[0157] Figure 1 This is the XRD pattern of the cobalt tungstate catalyst (CoWO4) in Comparative Example 6 of the present invention.

[0158] As can be seen from Figure 1 using the adjusted sodium tungstate dihydrate solution (pH = 6) in this comparative example, a pure-phase cobalt tungstate catalyst (CoWO4) is finally prepared, and a cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) cannot be prepared.

[0159] The results show that

[0160] 1. The solid precursor and the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention were respectively tested by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the results are as shown in Figure 2 a-d of

[0161] As can be seen from Figure 2As can be seen from a of, the solid precursor matches the standard card of Co4W6O 21 (OH)2·4H2O completely, proving that the chemical formula of the solid precursor is Co4W6O 21 (OH)2·4H2O.

[0162] From Figure 2 b, it can be seen that the cobalt tungstate / tungsten dioxide heterojunction catalyst has good crystallization peaks, which completely match the standard diffraction peaks of CoWO4 and WO2.

[0163] From Figure 2 c, it can be seen that the solid precursor (Co4W6O 21 (OH)2·4H2O) has a granular morphology.

[0164] From Figure 2 d, it can be seen that the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) has uniform particle size and a small particle size distribution.

[0165] 2. The particle size test and specific surface area test were respectively carried out on the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention, and the results are as shown in Figure 3 a-b.

[0166] From Figure 3 a, it can be seen that the average particle size of the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) is 38 nm. The small particle size of CoWO4 / WO2 is beneficial to improving the catalytic effect.

[0167] From Figure 3 b, it can be seen that the specific surface area of the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) is 28.67 cm 2 g -1 , and its specific surface area is relatively large, which can expose more active sites and is beneficial to the progress of the catalytic reaction.

[0168] 3. The transmission electron microscope (TEM) test was carried out on the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention, and the results are as shown in Figure 4 .

[0169] From Figure 4 it can be seen that the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) provided by the present invention has a heterojunction interface, corresponding to different crystal planes of CoWO4 and WO2 respectively, indicating that it has a heterojunction between CoWO4 and WO2.

[0170] 4. The cathode material in Example 1 of the present invention was assembled with a commercial lithium metal anode to form a lithium-sulfur button battery, and cyclic voltammetry (CV) scanning tests were carried out. Among them, the electrolyte of the lithium-sulfur button battery was a commercial electrolyte (1.0 mol / L lithium bis(trifluoromethanesulfonyl)imide and 2 wt% LiNO3 in a mixture of 1,2-dimethoxyethane and 1,3-dioxolane (v / v = 1:1)). The results of the cyclic voltammetry (CV) scanning tests are as Figure 5 shown.

[0171] As can be Figure 5 seen, when the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) in Example 1 of the present invention was used as the cathode catalyst of the lithium-sulfur battery, a cyclic voltammogram with a scanning rate of 0.1 mV / s was carried out in the voltage range of 1.7 - 2.6 V, and obvious oxidation-reduction peaks were observed. This indicates that the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) provided by the present invention has a good promoting effect on the catalytic conversion of polysulfides. From an electrochemical perspective, it reduces the energy barrier of the reaction, reduces polarization, has good adsorption characteristics for polysulfides, and can effectively inhibit the shuttle effect of polysulfides.

[0172] 5. The cathode materials in Examples 1 - 2 of the present invention and the cathode materials in Comparative Examples 1 - 4 were respectively assembled with a commercial lithium metal anode to form lithium-sulfur button batteries, and charge-discharge cycling tests were carried out. Among them, the electrolyte of the lithium-sulfur button battery was a commercial electrolyte (1.0 mol / L lithium bis(trifluoromethanesulfonyl)imide and 2 wt% LiNO3 in a mixture of 1,2-dimethoxyethane and 1,3-dioxolane (v / v = 1:1)); Figure 6 are the charge-discharge cycling curves of the lithium-sulfur button batteries assembled with the cathode materials in Example 1 of the present invention and the cathode materials in Comparative Examples 1 - 2 and a commercial lithium metal anode, Figure 7 are the charge-discharge cycling curves of the lithium-sulfur button batteries assembled with the cathode materials in Example 2 of the present invention and the cathode materials in Comparative Examples 3 - 4 and a commercial lithium metal anode.

[0173] As can be Figure 6 seen, when a long charge-discharge cycling test was carried out at a rate of 1C, the initial capacity of the lithium-sulfur button battery (CoWO4 / WO2) assembled with the cathode material in Example 1 of the present invention and a commercial lithium metal anode was 1035.7 mAh g -1, and the capacity retention rate is 62.4% after 1000 cycles, and the Coulombic efficiency (CE) is stable at 98.7%. In contrast, the lithium-sulfur button battery (CoWO4) assembled with the cathode material in Comparative Example 1 and a commercial lithium metal anode had a capacity retention rate that dropped to 55.6% after 483 cycles. The lithium-sulfur button battery (WO2) assembled with the cathode material in Comparative Example 2 and a commercial lithium metal anode showed rapid capacity decay after 283 cycles, and the capacity finally dropped to 434.7 mAh g -1 , which may be due to limited catalytic activity. From Figure 7 , it can be seen that when performing charge-discharge cycle tests at a rate of 0.1 C, the initial capacity of the lithium-sulfur button battery (CoWO4 / WO2) assembled with the cathode material in Example 2 of the present invention and a commercial lithium metal anode is 646.5 mAh g -1 , and the capacity retention rate reaches 93.2% after 100 cycles, showing excellent cycle stability. The lithium-sulfur button battery (CoWO4) assembled with the cathode material in Comparative Example 3 of the present invention had a capacity retention rate of 84.6% after 59 cycles, and the lithium-sulfur button battery (WO2) assembled with the cathode material in Comparative Example 4 of the present invention had a capacity retention rate of 53.2% after 60 cycles, indicating that by using the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) provided in the embodiments of the present invention, even in the case of a high sulfur areal loading, the lithium-sulfur battery still has a high capacity retention rate and excellent cycle stability. The above results illustrate that the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) provided in the embodiments of the present invention has high catalytic activity. Applying it in a lithium-sulfur battery can effectively inhibit the shuttle effect of polysulfides, improve the capacity retention rate and Coulombic efficiency of the lithium-sulfur battery, and improve the cycle stability of the lithium-sulfur battery.

[0174] In summary, the cobalt tungstate / tungsten dioxide heterojunction catalyst (CoWO4 / WO2) provided by the present invention has the advantage of high catalytic activity. It plays a great role in improving the performance of lithium-sulfur batteries. Applying it in lithium-sulfur batteries has a good promoting effect on the catalytic conversion of polysulfides, can effectively inhibit the shuttle effect of polysulfides, improve the capacity retention rate and Coulombic efficiency of lithium-sulfur batteries, and improve the cycle stability of lithium-sulfur batteries.

[0175] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a cobalt tungstate / tungsten dioxide heterojunction catalyst for a lithium-sulfur battery positive electrode material, characterized in that: The steps include: Dissolving cobalt nitrate hexahydrate and sodium tungstate dihydrate in aqueous solution respectively to obtain cobalt nitrate hexahydrate solution and sodium tungstate dihydrate solution; After adjusting the pH value of the sodium tungstate dihydrate solution with a pH adjuster, the cobalt nitrate hexahydrate solution is added, mixed and stirred, and then a hydrothermal reaction is performed to obtain a solid precursor; The solid precursor is subjected to a first calcination treatment in an Ar / H2 atmosphere to obtain the cobalt tungstate / tungsten dioxide heterojunction catalyst.

2. The method for preparing the cobalt tungstate / tungsten dioxide heterojunction catalyst for lithium-sulfur battery positive electrode material according to claim 1, characterized in that: The pH value is 2-4.

3. The method for preparing the cobalt tungstate / tungsten dioxide heterojunction catalyst for lithium-sulfur battery positive electrode material according to claim 2, characterized in that: The pH adjuster includes hydrochloric acid.

4. The method for preparing the cobalt tungstate / tungsten dioxide heterojunction catalyst for lithium-sulfur battery positive electrode material according to claim 1, characterized in that: The molar ratio of the cobalt nitrate hexahydrate to the sodium tungstate dihydrate is (1-3):(1-2).

5. The method for preparing the cobalt tungstate / tungsten dioxide heterojunction catalyst for lithium-sulfur battery positive electrode material according to claim 1, characterized in that: The temperature of the hydrothermal reaction is 170-200° C. and the time is 1-5 hours.

6. The method for preparing the cobalt tungstate / tungsten dioxide heterojunction catalyst for lithium-sulfur battery positive electrode material according to claim 1, characterized in that: The temperature of the first calcination treatment is 600-700° C. and the time is 1-4 hours.

7. A cobalt tungstate / tungsten dioxide heterojunction catalyst, characterized in that: The catalyst is prepared by the method for preparing the cobalt tungstate / tungsten dioxide heterojunction catalyst for lithium-sulfur battery positive electrode material according to any one of claims 1 to 6.

8. A positive electrode material, characterized in that The positive electrode active material comprises the cobalt tungstate / tungsten dioxide heterojunction catalyst as claimed in claim 7 and sulfur as raw materials, which are ground and subjected to a second calcination treatment.

9. The positive electrode material according to claim 8, characterized in that In the positive electrode active material, the mass ratio of the cobalt tungstate / tungsten dioxide heterojunction catalyst and the sulfur element is (1-2): (3-4); And / or, the temperature of the second calcination treatment is 150-160° C. and the time is 4-6 hours.

10. The positive electrode material according to claim 8, characterized in that The positive electrode material also includes a conductive agent and a binder; In the positive electrode material, the mass ratio of the positive electrode active material, the conductive agent and the binder is (7-8): (1-2): 1.

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