A ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst as well as a preparation method and application thereof

Abstract

The application discloses a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst and a preparation method and application thereof. The steps comprise the following steps: (1) 300mg of ammonium molybdate tetrahydrate is dissolved in 100ml of water, 180mg of dopamine hydrochloride is added, stirring is carried out for 30 minutes, then 200ml of anhydrous ethanol and 2ml of a Tris-HCl buffer solution (concentration: 1mol / L) are added, the pH value is adjusted to 8.5, and reaction is carried out for 12 hours; (2) after centrifugal cleaning and freeze-drying, calcination is carried out under an argon atmosphere at 650 DEG C for 2 hours and at 900 DEG C for 3 hours to obtain N-Mo2C; (3) 186mg of cobalt nitrate hexahydrate is dissolved in 5ml of methanol, 205mg of 2-methylimidazole is dissolved in 5ml of methanol at the same time, 100mg of N-Mo2C is dispersed in the 2-methylimidazole methanol solution, and the cobalt nitrate methanol solution is added dropwise, and reaction is carried out at room temperature for 4 hours; (4) after filtration and cleaning, freeze-drying is carried out, and calcination is carried out under an argon atmosphere at 750 DEG C for 3 hours to obtain the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction catalyst material. The method has low cost and simple process, and the material has excellent performances such as high rate and long cycle when applied in lithium-sulfur batteries.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of battery catalyst materials, and particularly relates to a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst, a preparation method thereof and application thereof in lithium-sulfur batteries. BACKGROUND

[0002] Lithium-sulfur batteries (Li-S batteries) have great potential to become the next generation of high-energy-density energy storage devices due to their high theoretical energy density, low material cost and environmental friendliness. Compared with traditional lithium-ion batteries, lithium-sulfur batteries have a theoretical specific capacity of 1675 mAh·g -1 , and sulfur as a positive electrode material is not only abundant in resources and low in price, but also has low toxicity and little impact on the environment. Lithium-sulfur batteries have wide application prospects in the fields of electric vehicles, portable electronic devices and energy storage systems.

[0003] However, the sulfur positive electrode of lithium-sulfur batteries has a serious volume expansion problem during the charging and discharging process, which may cause damage to the electrode structure. Sulfur has poor electrical conductivity, which limits the rate of its electrochemical reaction and requires the assistance of a conductive agent. In addition, lithium-sulfur batteries produce polysulfides during operation, which are easily dissolved in the electrolyte and diffuse to the lithium negative electrode, causing the "shuttle effect", which leads to active material loss, rapid capacity decay and a decrease in coulombic efficiency. In addition, the dendrite growth problem of the lithium metal negative electrode may cause short circuits and safety hazards. SUMMARY

[0004] The application aims to solve the "shuttle effect" problem of sulfides in the prior art and provides a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst and a preparation method thereof.

[0005] The technical solution adopted by the application is: a preparation method of a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst, comprising the following steps:

[0006] Dissolve ammonium molybdate tetrahydrate in water, add dopamine hydrochloride, stir thoroughly, then add anhydrous ethanol and a Tris-HCl buffer solution (concentration of 1 mol / L), adjust the pH to 8.5, and react for 12 hours; after centrifugal cleaning, freeze-drying is performed, and N-Mo2C is obtained by first calcination under an argon atmosphere;

[0007] Dissolve cobalt nitrate hexahydrate in methanol to obtain solution I, dissolve 2-methylimidazole in methanol to obtain solution II, disperse N-Mo2C in solution II, stir, and dropwise add solution I, and react at room temperature for 4 hours; after filtration and cleaning, freeze-drying is performed, and the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst is prepared by second calcination under an argon atmosphere.

[0008] Specifically, the mass ratio of the ammonium molybdate tetrahydrate and the dopamine hydrochloride is 3:1.5-2.

[0009] Further, the pH is adjusted to 8.5 by adding ammonia water.

[0010] In the above scheme, the first calcination comprises calcining at 650 DEG C for 2 hours at a temperature increasing rate of 5 DEG C / min, and then calcining at 900 DEG C for 3 hours.

[0011] Further, the second calcination comprises calcining at 750 DEG C for 3 hours at a temperature increasing rate of 5 DEG C / min.

[0012] A ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst is prepared by the preparation method.

[0013] The application provides application of the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst in preparation of a lithium-sulfur battery.

[0014] Further, the application comprises the following steps:

[0015] (1) mixing the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction catalyst and sulfur in a mortar at a mass ratio of 2:8, transferring into a reaction kettle in a glove box, heating at 155 DEG C in an argon atmosphere for 12 hours, and obtaining a sulfur positive electrode material;

[0016] (2) mixing the sulfur positive electrode material, conductive carbon black and a binder at a mass ratio of 8:1:1, adding the mixture into N-methylpyrrolidone, and coating on the surface of carbon paper after stirring for 12 hours;

[0017] (3) drying the electrode sheet obtained in step (2) in a vacuum drying box;

[0018] (4) cutting the electrode sheet dried in step (3) by using a cutting machine, and obtaining a positive electrode sheet;

[0019] (5) assembling a battery in sequence by using a positive electrode shell, the positive electrode sheet prepared in step (4), 40 muL of lithium-sulfur electrolyte, a polypropylene separator, a gasket and a negative electrode shell, and the sulfur loading of the positive electrode sheet is 1.0-2.0 mg / cm 2 .

[0020] The application further provides a positive electrode material of a lithium-sulfur battery, which comprises the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst and sulfur, and the mass ratio is 2:8.

[0021] The application has the following advantages and beneficial effects:

[0022] The application relates to a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst for lithium-sulfur batteries and a preparation method thereof. The heterojunction electrocatalyst with excellent electrochemical performance is prepared by combining a chemical solution method with a high-temperature calcination technology. The lithium-sulfur battery catalyst material in the application is applied to a lithium-sulfur battery positive electrode material, the chemical solution method and the high-temperature calcination technology produce a synergistic effect, can greatly improve the high-rate performance and cycle stability of the battery, accelerate the charge-discharge reaction kinetics, and obviously reduce the'shuttle effect' of polysulfides. The preparation process is simple, low in cost, and friendly to the environment, and has important application prospects and potential industrialization value. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a TEM image of a lithium-sulfur battery catalyst material in the embodiment of the application;

[0024] Figure 2 is a cyclic voltammogram of a lithium-sulfur battery using the catalyst material in the embodiment of the application at a scan rate of 0.1 mV.s -1

[0025] Figure 3 is a discharge specific capacity graph of a lithium-sulfur battery using the catalyst material in the embodiment of the application under a 1C rate condition;

[0026] Figure 4 is a charge-discharge curve graph of a lithium-sulfur battery using the catalyst material in the embodiment of the application under a 1C rate condition;

[0027] Figure 5 is a discharge specific capacity graph of a lithium-sulfur battery using the catalyst material in the embodiment of the application under different rate conditions;

[0028] Figure 6 is a Li2S deposition curve graph of a lithium-sulfur battery using the catalyst material in the embodiment of the application. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the application will be described in detail below with reference to the drawings in the embodiments of the application. The described embodiments are only some of the embodiments of the application.

[0030] ​This invention provides a method for preparing a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst, which can be used in the field of lithium-sulfur batteries, particularly addressing the problems of polysulfide "shuttle effect" and poor stability under high current conditions in lithium-sulfur batteries. The synthesis process of the electrocatalyst material in the embodiments of the invention includes the following steps: dissolving 300 mg of ammonium molybdate tetrahydrate in 100 mL of deionized water, adding 180 mg of dopamine hydrochloride and stirring for 30 minutes, then adding 200 mL of anhydrous ethanol and 2 mL of 1 mol / L Tris-HCl buffer solution and continuing stirring for 20 minutes, then adjusting the pH to 8.5 with ammonia and reacting for 12 hours. The resulting solution is centrifuged and washed with anhydrous ethanol and water, respectively, and then freeze-dried. Next, it is calcined in an argon atmosphere at a heating rate of 5 °C / min, first at 650 °C for 2 hours, then at 900 °C for 3 hours, to obtain N-Mo₂C. Subsequently, 186 mg of cobalt nitrate hexahydrate was dissolved in 5 mL of methanol, and 205 mg of 2-methylimidazole was dissolved in 5 mL of methanol. 100 mg of N-Mo₂C was dispersed in the 2-methylimidazole methanol solution, and then cobalt nitrate methanol solution was added dropwise. The reaction was carried out at room temperature for 4 hours. After filtration and washing, the mixture was freeze-dried. Finally, the mixture was calcined at 750 °C for 3 hours in a high-purity argon atmosphere at a heating rate of 5 °C / min to obtain the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst. The positive electrode, negative electrode, electrolyte, and separator of this invention were prepared according to methods commonly used by those skilled in the art (and therefore will not be described in detail here).

[0031] The application of the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst material in lithium-sulfur batteries in the above embodiments is as follows: (1) The catalyst obtained above is mixed with sulfur in a mortar at a ratio of 2:8, transferred to a reaction vessel, and heated at 155°C for 12 hours to obtain the active material; (2) The active material: conductive carbon black: binder is mixed evenly at a ratio of 8:1:1, a certain amount of NMP is added to the mixture, and after stirring for 12 hours, it is coated on the surface of carbon paper with a coating thickness of 7 micrometers; (3) The coated electrode is placed in a vacuum drying oven at 60°C for 12 hours to dry the electrode fully; (4) The electrode is cut into round pieces with a diameter of 12 mm using a cutting machine, and the mass of the electrode is weighed. (5) The positive electrode shell, the positive electrode sheet obtained in step (4), 40 μL of lithium-sulfur electrolyte, polypropylene separator, gasket and negative electrode shell are assembled into a battery in sequence, and the sulfur loading of the positive electrode sheet is 1.0-2.0 mg·cm³. -2 .

[0032] Depend on Figure 1It can be seen that the micro-morphology of ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalytic material has obvious metal particles, large metal particles are cobalt, and small metal particles are molybdenum, forming a heterostructure between the interfaces, which can effectively promote the reaction kinetics of sulfur conversion, reduce the "shuttle effect" caused by polysulfide dissolution, and improve the electrochemical performance of lithium-sulfur batteries.

[0033] After electrochemical performance test, Figure 2 0.1 mV·s -1 The cyclic voltammogram at a scan rate of 0.1 mV·s

[0034] Figure 3 The long cycle curve under the condition of 1C high rate is shown, and the first discharge capacity is 847.2 mAh·g -1 After 1000 cycles, the capacity still remains at 393 mAh·g -1 The average decay rate of each cycle is only 0.076%.

[0035] Figure 4 The charge-discharge curves of the battery at the 1st, 100th, 300th, 500th, 700th, and 900th cycles are shown, and the lithium-sulfur battery has obvious charge-discharge platforms.

[0036] Figure 5 The charge-discharge curves under the conditions of 0.2C, 0.5C, 1C, 2C, 5C, and 10C are shown, and the discharge capacity of the battery under the condition of 5C high rate is 518.2 mAh·g -1 Under the condition of 10C high rate, the discharge capacity of the battery is still 392.8 mAh·g -1 After returning to the low rate condition, the battery capacity can still maintain the same level as before.

[0037] Figure 6 The Li2S deposition curve on the catalyst electrode is shown, and the Li2S deposition capacity is 420.92 mAh·g -1 The catalytic ability for polysulfide conversion is significantly improved.

[0038] Through electrochemical testing, the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst exhibits excellent performance and application prospect, significantly improves the cycle life and energy density of the battery, and effectively suppresses the "shuttle effect" in lithium-sulfur batteries.

Claims

1. A method for preparing a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst, characterized in that, The method comprises the following steps: ammonium molybdate tetrahydrate is dissolved in water, dopamine hydrochloride is added, and the mixture is stirred and mixed uniformly, then anhydrous ethanol and Tris-HCl buffer solution are added, the pH is adjusted to 8.5, and reaction is carried out for 12 hours; after centrifugal washing, freeze-drying is carried out, and first calcination is carried out under an argon atmosphere to obtain N-Mo2C; solution I is obtained by dissolving cobalt nitrate hexahydrate in methanol, solution II is obtained by dissolving 2-methylimidazole in methanol, N-Mo2C is dispersed in solution II, stirring is carried out, and solution I is added dropwise, reaction is carried out at room temperature for 4 hours; after filtration and washing, freeze-drying is carried out, and second calcination is carried out under an argon atmosphere to obtain the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst.

2. The method for preparing a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst according to claim 1, characterized in that: The mass ratio of the ammonium molybdate tetrahydrate and the dopamine hydrochloride is 3:1.5-2.

3. The method for preparing a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst according to claim 1, characterized in that: The pH is adjusted to 8.5 by adding ammonia water.

4. The method for preparing a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst according to claim 1, characterized in that: The first calcination comprises first calcination at 650 DEG C for 2 hours at a temperature increasing rate of 5 DEG C / min, and then calcination at 900 DEG C for 3 hours.

5. The method for preparing a ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst according to claim 1, characterized in that: The second calcination comprises calcination at 750 DEG C for 3 hours at a temperature increasing rate of 5 DEG C / min.

6. A ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst, characterized in that: The ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst is prepared by the preparation method of any one of claims 1-5.

7. The use of the ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst in claim 6 in the preparation of a lithium-sulfur battery.

8. Use according to claim 7, characterized in that: The ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst is used to prepare a positive electrode sheet.

9. Use according to claim 7 or 8, characterized in that, The method comprises the following steps: (1) ZIF-67 / nitrogen-doped molybdenum carbide heterojunction catalyst and sulfur are mixed uniformly in a mortar at a mass ratio of 2:8, the mixture is transferred into a reaction kettle in a glove box, and heating is carried out at 155 DEG C in an argon atmosphere for 12 hours to obtain a sulfur positive electrode material; (2) the sulfur positive electrode material, conductive carbon black and a binder are mixed uniformly at a mass ratio of 8:1:1, the mixture is added to N-methyl pyrrolidone, stirring is carried out for 12 hours, and then the mixture is coated on the surface of carbon paper; (3) the sheet obtained in step (2) is dried in a vacuum drying box; (4) the sheet after drying in step (3) is cut by a sheet cutting machine to obtain a positive electrode sheet; (5) Assembling the positive shell, the positive electrode sheet prepared in step (4), 40 μL of lithium-sulfur electrolyte, a polypropylene separator, a gasket and a negative shell in order to form a battery, and the sulfur loading of the positive electrode sheet is 1.0-2.0 mg / cm 2 .

10. A positive electrode material for a lithium-sulfur battery, characterized by: The ZIF-67 / nitrogen-doped molybdenum carbide heterojunction electrocatalyst and sulfur in claim 6 are mixed at a mass ratio of 2:8.

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