MXene ZIF-67 composite coating on surface of negative electrode of magnesium battery and preparation method of MXene ZIF-67 composite coating

Abstract

The invention discloses a preparation method of an MXene ZIF-67 composite coating on the surface of a negative electrode of a magnesium battery. The preparation method comprises the following steps: grinding and polishing a magnesium alloy matrix, cleaning and drying, and removing oil; etching the Ti3AlC2 by using hydrofluoric acid to obtain MXene powder; mXene powder and cobalt nitrate hexahydrate are dissolved in a methanol solution to serve as a solution A; dissolving a 2-methylimidazole solution in a methanol solution to obtain a solution B; the solution A is poured into the solution B, hexadecyl trimethyl ammonium bromide is added, and MXene (at) ZIF-67 powder is obtained; dissolving in methanol; graphite is used as an anode, and the deoiled magnesium alloy is used as a cathode; and a constant voltage is applied, the cathode magnesium sheet is taken out and dried, and the MXene-coated ZIF-67 composite coating is obtained. The composite coating can reduce the voltage lag time and electrochemical impedance of the magnesium battery, improve the discharge voltage platform of the magnesium battery and improve the comprehensive discharge performance of the magnesium battery.

Description

Technical Field

[0001] The present invention relates to the technical field of artificial protective composite coatings on the negative electrode surface of magnesium batteries, and in particular to a MXene@ZIF-67 composite coating on the negative electrode surface of a magnesium alloy and a preparation method thereof. Background Art

[0002] Magnesium and its alloys have excellent physical properties and good safety. They are a kind of high energy density battery negative electrode material with good development prospects. Magnesium metal has an extremely high volumetric capacity (3383 mAh / cm 3 ), low standard potential (-2.37vs SHE), no dendrites and other advantages, which have attracted widespread attention to the research of magnesium batteries. Magnesium is chemically active and easily undergoes electrochemical reactions, but has poor corrosion resistance and is prone to side reactions with the electrolyte. This accompanying self-corrosion hydrogen evolution reaction leads to reactive power consumption of the magnesium negative electrode. At the same time, the generated hydrogen poses a safety hazard, and a passivation film is easily formed on the surface of the magnesium negative electrode, which hinders the electrochemical reaction and slows down the battery response, causing the battery hysteresis effect. The above defects of magnesium batteries affect the use and storage life of the battery.

[0003] Prior patent CN117577797A discloses a "sandwich" MXene Ti3C2T x / Hollow ZIF-67 / S lithium-sulfur battery positive electrode material preparation method, which is to prepare MXene@ZIF-67 and then etch it with tannic acid to obtain hollow ZIF-67, and then further prepare MXene / hollow ZIF-67 / S as a lithium-sulfur battery positive electrode material by melt diffusion. In contrast, the present invention prepares a protective film layer of a magnesium negative electrode, and the hollow ZIF-67 will inevitably collapse in the process of electrophoretic deposition.

[0004] The prior patent CN115821355A discloses a MXene film layer on the surface of a magnesium alloy electrode and its preparation method. By preparing a multilayer of titanium carbide, a magnesium alloy sheet is used as the cathode and anode of the electrodeposition, and a constant voltage is applied for constant voltage anodic electrophoresis electrodeposition to obtain a MXene film layer on the surface of the magnesium alloy electrode. However, the MXene after etching is negatively charged, so during the electrodeposition process, the film layer will be deposited at the anode under the action of the electric field force. Therefore, during the synthesis process of the protective film layer in the electrophoretic solution, the magnesium anode will inevitably undergo an oxidation reaction and corrosion of the magnesium anode will occur. Only the film layer formed by MXene will have the characteristics of structural instability and easy collapse. After the MXene film layer is formed, it is assembled into a magnesium battery. Due to the structural characteristics of MXene, the film layer is prone to change during the electrochemical reaction. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a new method for protecting the negative electrode of magnesium batteries, which creates an artificial protective layer on the surface of magnesium alloy. First, MXene (Ti3AlC2) is etched with hydrofluoric acid to obtain an accordion-shaped Ti3C2T X . Then, through in-situ growth, MXene with a huge conductive skeleton and metal organic framework material (ZIF-67) are composited to generate a MXene@ZIF-67 composite material with a core-shell structure with MXene as the growth center. Finally, an artificial protective layer with a high specific surface area is uniformly grown on the surface of the magnesium alloy by cathode electrophoretic deposition: MXene@ZIF-67 composite coating. The synergistic effect of MXene and ZIF-67 can not only inhibit the side reaction between the negative electrode and the electrolyte of the magnesium battery, but also promote the transport of magnesium ions between the electrode and the electrolyte.

[0006] The purpose of the present invention is to provide a MXene@ZIF-67 composite coating with an artificial protective film layer on the surface of a magnesium alloy and a preparation method thereof. The MXene@ZIF-67 composite coating on the surface of a magnesium alloy of the present invention is obtained by combining a hydrofluoric acid etching method, an in-situ growth method, and a cathode electrophoretic deposition method to obtain MXene (Ti3C2T x ), MXene@ZIF-67 composite materials and MXene@ZIF-67 composite coating on magnesium alloy surface.

[0007] The objective of the present invention is achieved through the following technical solutions:

[0008] <First aspect>

[0009] The present invention provides a method for preparing a MXene@ZIF-67 composite coating on a magnesium alloy surface, comprising the following steps:

[0010] S1. Etching of Ti3AlC2: Etching Ti3AlC2 with hydrofluoric acid, diluting and filtering until the filtered solution is neutral; vacuum drying to obtain the successfully etched MXene (Ti3C2T x )powder;

[0011] S2. Preparation of MXene@ZIF-67 composite material: The MXene powder and cobalt nitrate hexahydrate are dissolved in methanol, and the solution is black-red, which is used as solution A; 2-methylimidazole is dissolved in methanol as solution B; solution A is poured into solution B, and the solution turns blue. After the reaction, hexadecyltrimethylammonium bromide is added; stirring and centrifugation are performed, and the lower layer powder is taken out by suction filtering, and black-purple MXene@ZIF-67 powder is obtained after vacuum drying;

[0012] S3. Preparation of MXene@ZIF-67 composite coating: The MXene@ZIF-67 powder is dissolved in methanol and subjected to ultrasonic treatment to completely suspend the powder in the methanol; graphite is used as an anode and a magnesium alloy is used as a cathode, and they are immersed face to face in the suspension; a constant voltage of 10 to 60 V is applied to the two electrodes (using a regulated current), and the magnesium alloy at the cathode is removed after 1 to 20 minutes, and a MXene@ZIF-67 composite coating is obtained on the surface of the magnesium alloy after drying.

[0013] As an embodiment of the present invention, in step S1, etching aluminum carbide with hydrofluoric acid includes adding aluminum carbide to a mixed solution of hydrofluoric acid and water for stirring and etching. When aluminum carbide precursor powder is added, small bubbles are generated in the hydrofluoric acid solution.

[0014] Furthermore, the usage ratio of the aluminum carbide, hydrofluoric acid and water is 1.00 g~3.00 g:5~15 ml:2~10 ml;

[0015] And / or, the stirring etching has a temperature of 20-30° C., a rotation speed of 200-1000 r / min, and a time of 20-48 h.

[0016] As an embodiment of the present invention, in step S1, the vacuum drying temperature is 50-70°C.

[0017] As an embodiment of the present invention, in step S2, the dilution is performed with 500-1000 ml of deionized water.

[0018] As an embodiment of the present invention, in step S2, the vacuum drying temperature is 50-70°C.

[0019] In some specific embodiments, step S1 includes the following specific steps: using aluminum titanium carbide (Ti3AlC2) as an etching precursor, etching with hydrofluoric acid at room temperature, adding 1.00 g to 3.00 g of aluminum titanium carbide to a mixed solution of 5 to 15 ml of hydrofluoric acid and 2 to 10 ml of water, and etching at room temperature at a stirring speed of 200 to 1000 r / min for 20 to 48 h; small bubbles will be generated in the hydrofluoric acid solution when the precursor powder is added; after the etching is completed, 500 to 1000 ml of deionized water is added for dilution, and then the diluted solution is filtered until the filtered solution is neutral; after the filter paper with the etching product is vacuum dried at 50-70 ° C, the successfully etched MXene (Ti3C2T x )powder.

[0020] As an embodiment of the present invention, in step S2, the usage ratio of the MXene powder, cobalt nitrate hexahydrate, and methanol is: 0.1-0.6 g: 0.1-1 mmol: 5-20 ml;

[0021] And / or, in step S2, the usage ratio of 2-methylimidazole to methanol is 1-16 mmol:5-20 mL;

[0022] And / or, the molar ratio of the cobalt nitrate hexahydrate to 2-methylimidazole is 1:4-1:12.

[0023] As an embodiment of the present invention, in step S2, the reaction time of solution A and solution B is 1 to 3 hours.

[0024] As an embodiment of the present invention, in step S2, the mass ratio of hexadecyltrimethylammonium bromide to MXene powder is 1:300-1:2.5;

[0025] And / or, the stirring time is 5 to 20 h;

[0026] And / or, the centrifugal speed is 5000-10000 r / min, and the number of times is 2-5 times;

[0027] And / or, the vacuum drying temperature is 50-80°C and the time is 10-20 h.

[0028] In some specific embodiments, step S2 specifically includes the following steps: weighing the etched dry MXene (Ti3C2T x ) powder 0.1~0.6 g, cobalt nitrate hexahydrate 0.1~1 mmol, dissolved in 5~20 ml methanol solution, the solution is black-red, as solution A; weigh 1~16 mmol 2-methylimidazole and dissolve it in 5~20 mL methanol solution as solution B, pour solution A into solution B, the solution changes from black-red to blue, add 2~40 mg hexadecyltrimethylammonium bromide after 1~3 h; after stirring for 5~20 h, centrifuge at 5000~10000 r / min, centrifuge 2~5 times, filter out the lower layer of powder, and vacuum dry at 50~80 ℃ for 10~20 h to obtain black-purple MXene@ZIF-67 powder.

[0029] As an embodiment of the present invention, the magnesium alloy in step S3 is pre-treated; the pre-treatment steps specifically include: grinding and polishing the magnesium alloy substrate, drying after cleaning, degreasing in an alkaline degreasing solution, and then ultrasonically cleaning with ethanol and deionized water respectively and then drying.

[0030] Further, the alkaline degreasing solution includes 15-20 g / L NaOH, 30-40 g / L Na2CO3 and 15-20 g / LNa3PO4;

[0031] And / or, the degreasing treatment is performed at a temperature of 60-80° C. and for a time of 1-5 min.

[0032] Furthermore, the magnesium alloy matrix is ​​an AZ91 magnesium alloy matrix.

[0033] Furthermore, the grinding and polishing of the magnesium alloy substrate includes sequentially grinding the magnesium alloy substrate with 180#, 600#, 1000# and 1500# metallographic sandpaper.

[0034] Furthermore, the cleaning includes ultrasonically cleaning the magnesium alloy substrate with ethanol and deionized water for 10 to 15 minutes respectively.

[0035] As an embodiment of the present invention, in step S3, the usage ratio of the MXene@ZIF-67 powder to methanol is 0.05~0.1 g:5~30 ml;

[0036] And / or, the ultrasonic treatment time is 3 to 5 minutes;

[0037] And / or, the depth of immersion of the anode and cathode in the suspension is 1-3 cm, and the interval between the anode and cathode is 2-4 cm;

[0038] And / or, the drying temperature is 20-30°C.

[0039] Preferably, the anode and cathode are immersed in the suspension to a depth of 2 cm, and the distance between the anode and cathode is 3 cm.

[0040] In some specific embodiments, step S3 specifically includes the following steps: weighing 0.05~0.1 g of MXene@ZIF-67 powder obtained in step S2, dissolving it in 5~30 ml of methanol, and ultrasonically treating it for 3~5 min so that the powder is completely suspended in the methanol; the anode is graphite, and the cathode is a magnesium alloy sheet after degreasing treatment, which are immersed in the suspension face to face at a depth of 2 cm and a spacing of 3 cm; using a regulated current power supply to apply a constant voltage of 10~60 V to the two electrodes; after 1~20 min, take out the magnesium alloy at the cathode and dry it at room temperature to obtain a MXene@ZIF-67 composite coating on the surface of the magnesium alloy.

[0041] As an embodiment of the present invention, the above-mentioned aluminum carbotitanium, hydrofluoric acid, cobalt nitrate hexahydrate, 2-methylimidazole, hexadecyltrimethylammonium bromide, methanol, ethanol, NaOH (sodium hydroxide), Na2CO3 (sodium carbonate) and Na3PO4 (sodium phosphate) are all chemically pure or above.

[0042] <Second Aspect>

[0043] The present invention provides a MXene@ZIF-67 composite coating on the surface of a magnesium alloy, which is prepared by any of the methods described above.

[0044] <Third Aspect>

[0045] The present invention provides a magnesium battery, wherein the negative electrode is a magnesium alloy, and the MXene@ZIF-67 composite coating is deposited on the surface of the magnesium alloy.

[0046] As an embodiment of the present invention, the preparation method of the positive electrode of the magnesium battery is: cutting the nickel foam into long strips, grinding manganese dioxide, acetylene black and polyvinylidene fluoride in a mortar at a ratio of 7.5:1.5:0.1; using N-methylpyrrolidone as a solvent, mixing with the ground powder to form a paste, evenly applying it on the nickel foam, and using it as the positive electrode of the magnesium battery after drying for 5 hours.

[0047] As an embodiment of the present invention, the electrolyte of the magnesium battery is a magnesium sulfate solution.

[0048] The starting point of the present invention is to solve the problem of long voltage hysteresis time and serious self-corrosion hydrogen evolution reaction of magnesium negative electrode. Considering: (1) using the huge conductive skeleton of MXene, an artificial SEI film layer is formed on the magnesium negative electrode, which can stably transmit magnesium ions while avoiding direct contact between the magnesium negative electrode and the electrolyte; (2) MXene is a two-dimensional layered material, and the layers are connected by van der Waals force, so stacking will inevitably occur; (3) MOF materials have adjustable pore size, large specific area, high porosity, good stability, and corrosion resistance. Therefore, starting from the fundamental defects of magnesium negative electrode, MXene@ZIF-67 is synthesized to stably exist ZIF-67 between MXene layers. While using the excellent conductivity of MXene, the uniform and stable ZIF-67 can reduce the self-stacking of MXene between MXene layers, and the presence of ZIF-67 can improve the corrosion resistance of composite MXene@ZIF-67. In addition, the present invention uses an electrophoretic deposition method to form a film layer on the surface of the magnesium negative electrode for MXene@ZIF-67, and utilizes its characteristics to modify and protect the magnesium negative electrode instead of using it as a positive electrode material.

[0049] Methods for modifying magnesium negative electrodes include electrodeposition, hydrothermal method, self-assembly method, spraying method, etc. Among the methods for modifying the surface of magnesium metal, the most common method is to add a binder for spraying, and the addition of the binder will inevitably increase the impedance of the magnesium battery. The present invention selects the electrodeposition method to modify the magnesium negative electrode, and can synthesize MXene@ZIF-67 at room temperature without a binder, thereby maximizing the effect of MXene@ZIF-67.

[0050] The present invention adopts a method of cathode deposition to prepare a composite coating on a magnesium alloy substrate. A method of first electrodepositing MXene and then continuing to grow MOF (metal organic framework material) has been envisioned, but first the electrodeposition of MXene is deposited at the anode. This method will inevitably cause the magnesium negative electrode to undergo an oxidation reaction with the electrolyte at the anode; and in the case of subsequent MOF regrowth, the particle size of ZIF-67 and the interlayer spacing will not match because the growth source of ZIF-67 is not between the MXene layers.

[0051] In the early stage of the experimental design, the MXene@ZIF-67 heterostructure was designed. MXene was first electrodeposited, and then ZIF-67 was electrodeposited. As a result, only the surface of MXene was composited with ZIF-67 and MXene. And the film layer produced in this way is a layered structure, which is contrary to the original intention of adding ZIF-67. ZIF-67 cannot enter the interlayer and cannot stabilize the layered structure of MXene. In addition, the step-by-step electrodeposition will also cause the particle size of ZIF-67 and the interlayer spacing of MXene to be mismatched. From the electron microscope image of comparative example 1 ( Figure 6 ) It can be clearly seen that this step-by-step electrodeposition has an obvious defect in the electron microscope. Since ZIF-67 is only electrodeposited in the second step, the crystal particle size and the interlayer spacing of MXene will not match, and ZIF-67 does not grow in MXene with a smaller spacing.

[0052] During the experimental exploration phase, water was also used as a structural directing agent for ZIF-67 to perform in-situ growth of MXene@ZIF-67 and then electrophoretic deposition. However, using water as a structural directing agent revealed that ZIF-67 can be evenly distributed in the MXene layer, but its crystal size is very small and cannot complete the task of supporting and protecting the MXene layered structure.

[0053] Finally, the method of the present invention is adopted, and methanol is used as a structure-directing agent. After the in-situ growth of MXene@ZIF-67, it can be ensured that ZIF-67 can grow evenly and stably between MXene layers, and the particle size is uniform and appropriate. In addition, the electrical property of MXene@ZIF-67 is positive, and electrophoretic deposition can be achieved at the cathode instead of the anode, thereby avoiding oxidation of the magnesium electrode.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] 1) The preparation process of the present invention is simple and does not require high temperature and high pressure;

[0056] 2) The MXene@ZIF-67 composite coating has good adhesion to the magnesium alloy substrate and can be evenly distributed on the surface of the magnesium alloy;

[0057] 3) ZIF-67 is stably present between MXene layers. While utilizing the excellent conductivity of MXene, the uniform and stable ZIF-67 can reduce the self-stacking of MXene between MXene layers. In addition, the presence of ZIF-67 can improve the corrosion resistance of the composite MXene@ZIF-67.

[0058] 4) MXene@ZIF-67 is deposited on the cathode of the magnesium battery, which will not oxidize the magnesium electrode, which is beneficial to increase the bonding strength of the film layer and inhibit the occurrence of magnesium alloy side reactions during electrophoretic deposition;

[0059] 5) Through structural morphology testing and electrochemical performance testing of the assembled MXene@ZIF-67 / Mg||MnO2 full battery, it was found that the composite coating can reduce the voltage hysteresis time and electrochemical impedance of the magnesium battery, increase the discharge voltage platform of the magnesium battery, and improve the comprehensive discharge performance of the magnesium battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0061] Figure 1 SEM images of MXene before etching (1a), MXene after etching (1b), MXene@ZIF-67 composite coating (1c), and ZIF-67 on MXene@ZIF-67 composite coating (1d) in Example 1;

[0062] Figure 2 TEM images of the MXene@ZIF-67 composite coating prepared in Example 1 (2a-2c are composite coatings with different MXene layer spacings, respectively) and EDS element distribution diagrams (2d-2i);

[0063] Figure 3 The XRD spectrum of the phase characterization of the MXene@ZIF-67 composite coating prepared in Example 1;

[0064] Figure 4 The voltage hysteresis curve (4a), voltage hysteresis time (4b), discharge curve (4c), and electrochemical impedance spectrum (4d) of the battery assembled in Example 1;

[0065] Figure 5 SEM images of MXene@ZIF-67 composite coatings prepared with different dosage ratios of cobalt nitrate hexahydrate and dimethylimidazole;

[0066] Figure 6 This is a SEM image of the MXene@ZIF-67 composite coating prepared in Comparative Example 1;

[0067] Figure 7 This is the SEM image of the MXene@ZIF-67 composite coating prepared in Comparative Example 2. DETAILED DESCRIPTION

[0068] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0069] Example 1

[0070] (1) Magnesium alloy pretreatment:

[0071] The AZ91 magnesium alloy substrate was polished with 180#, 600#, 1000#, and 1500# metallographic sandpaper, cleaned, and dried; then it was placed in an alkaline degreasing solution (20 g / L NaOH, 30 g / L Na2CO3, 20 g / LNa3PO4) for degreasing at 60 °C for 1 min, followed by ultrasonic cleaning with ethanol solution and deionized water for 10 min respectively, and dried for later use.

[0072] (2) Etching of Ti3AlC2:

[0073] Aluminum titanium carbide (Ti3AlC2) was used as an etching precursor and etched with hydrofluoric acid at room temperature. 1.00 g of aluminum titanium carbide was added to a mixed solution of 7.5 ml of hydrofluoric acid and 2.5 ml of water, and etched at a speed of 200 r / min for 48 hours (small bubbles will be generated in the hydrofluoric acid solution when the precursor powder is added); after etching, 500 ml of deionized water was added for dilution, and the diluted solution was then filtered until the filtered solution was neutral; the filter paper with the etching product was vacuum dried at 70 °C to obtain the successfully etched MXene (Ti3C2T x )powder.

[0074] (3) Preparation of MXene@ZIF-67 composite materials:

[0075] Weigh 0.3 g of etched dry MXene powder and 1 mmol of cobalt nitrate hexahydrate, dissolve them in 8 ml of methanol solution, and the solution turns black and red, which is called solution A; weigh 8 mmol of dimethylimidazole and dissolve it in 8 ml of methanol solution to prepare solution B, pour A into solution B, and the solution turns from black and red to blue; add 2 mg of hexadecyltrimethylammonium bromide after 2 h; stir for 8 h, centrifuge at 8000 r / min, centrifuge twice, filter out the lower layer of powder, and dry it in vacuum at 60 °C for 12 h to obtain black and purple MXene@ZIF-67 composite material powder.

[0076] (4) Preparation of MXene@ZIF-67 composite coating:

[0077] Weigh 0.05 g of the above MXene@ZIF-67 composite material powder and dissolve it in 15 ml of methanol. Ultrasonic treatment is performed for 3 min to make the powder completely suspended in the methanol. The anode is graphite and the cathode is the magnesium alloy pretreated in step (1). They are immersed in the suspension face to face at a depth of 2 cm and a distance of 3 cm. A constant voltage of 60 V is applied to the two electrodes using a regulated current power supply. After 2 min, the magnesium alloy sheet of the cathode is removed and dried at room temperature. A MXene@ZIF-67 composite coating can be obtained on the surface of the magnesium alloy, and a magnesium alloy with a MXene@ZIF-67 composite coating grown on the surface is obtained.

[0078] (5) Preparation of positive electrode materials and assembly of batteries:

[0079] The nickel foam was cut into strips of 3 cm × 2 cm; manganese dioxide, acetylene black and polyvinylidene fluoride were ground in a mortar at a ratio of 7.5:1.5:0.1; N-methylpyrrolidone was used as a solvent and mixed with the ground powder to form a paste, which was evenly applied on the nickel foam and used as the positive electrode after drying for 5 h; the magnesium alloy with the surface growth MXene@ZIF-67 composite coating obtained in step (4) was used as the negative electrode, 1 M magnesium sulfate was used as the electrolyte, and Rbp double-sided pulp paper was used as the separator to assemble the MXene@ZIF-67 / Mg||MnO2 full battery.

[0080] The sodium hydroxide, sodium carbonate, sodium phosphate, aluminum carbotitanium, hydrofluoric acid, cobalt nitrate hexahydrate, 2-methylimidazole, hexadecyltrimethylammonium bromide, manganese dioxide, acetylene black, polyvinylidene fluoride, N-methylpyrrolidone, methanol and ethanol used in the above steps are all chemically pure or above.

[0081] Example 2

[0082] The remaining steps are consistent with Example 1, and only the ratio of cobalt nitrate hexahydrate and dimethylimidazole used in the preparation of MXene@ZIF-67 composite material in step (3) is explored. The dosage ratio of cobalt nitrate hexahydrate and dimethylimidazole is set to 1:2, 1:4, 1:8 and 1:16, respectively, to prepare MXene@ZIF-67 composite material. The amount of hexahydrate and cobalt nitrate used is 1mmol, and the dosage of dimethylimidazole is adjusted to 2mmol, 4mmol, 8mmol, and 16mmol to meet the dosage ratio of 1:2-1:16.

[0083] From the SEM images ( Figure 5 ) It can be seen that at 1:2, the surface of ZIF-67 crystals is rough and irregular, and a complete crystal structure cannot be obtained; at 1:4 and 1:8, the crystal structure gradually becomes regular, and the particle size is more uniform; but when it continues to increase to 1:16, the increase in ligands is not conducive to the formation of a clear crystal morphology of ZIF-67. In addition, the presence of excessive ligands will increase the friction between particles, resulting in surface damage to the crystal. Therefore, 1:8 is the best dosage ratio of cobalt nitrate hexahydrate and dimethylimidazole, which can make ZIF-67 particle size suitable and clear in morphology, and can stably exist between MXene layers.

[0084] Further experimental verification showed that when the dosage ratio of cobalt nitrate hexahydrate and dimethylimidazole was 1:4-1:12, ZIF-67 with a relatively clear morphology and suitable particle size could be synthesized, and it could stably exist between MXene layers.

[0085] Comparative Example 1

[0086] The sequence of electrophoretic deposition of MXene@ZIF-67 was explored. The specific steps include:

[0087] (1) Follow the steps (1) of Example 1.

[0088] (2) Referring to step (2) of Example 1, a successfully etched MXene (Ti3C2T x )powder.

[0089] (3) The MXene powder obtained in the anodic electrophoretic deposition step (2) was weighed, 0.5 g of the etched MXene was added to 15 ml of deionized water and ultrasonically stirred. With graphite as the cathode and magnesium sheet as the anode, a constant voltage of 10 V was applied at room temperature for 20 minutes to obtain a magnesium alloy with a MXene film layer deposited on the surface;

[0090] (4) Pure ZIF-67 powder was synthesized according to the method of step (3) in Example 1 without adding MXene. Specifically, the following steps were included: 1 mmol of cobalt nitrate hexahydrate was dissolved in 8 ml of methanol solution, and the solution was red, which was used as solution A; 8 mmol of dimethylimidazole was weighed and dissolved in 8 ml of methanol solution to prepare solution B, and solution A was poured into solution B, and the solution changed from red to blue; 2 mg of hexadecyltrimethylammonium bromide was added after 2 h; after stirring for 8 h, the solution was centrifuged at 8000 r / min, and after centrifugation twice, the lower layer of powder was filtered out, and vacuum dried at 60 °C for 12 h to obtain ZIF-67 powder.

[0091] Weigh 0.05 g of the synthesized ZIF-67 powder and dissolve it in 15 ml of methanol solution. Ultrasonic stirring is performed to make ZIF-67 evenly distributed in the electrophoresis solution. Graphite is used as the anode and the magnesium sheet on which the MXene film is deposited in step (3) is used as the cathode. A constant voltage of 60 V is applied to perform constant voltage electrophoresis deposition for 1 minute to obtain a MXene@ZIF-67 composite coating on the surface of the magnesium alloy.

[0092] From the SEM images ( Figure 6 ) It can be seen that only the surface of MXene is partially composited with ZIF-67, and this step-by-step deposition also has an obvious defect, which is that it will cause a mismatch between the ZIF-67 crystal particle size and the interlayer spacing of MXene. There is no ZIF-67 grain inserted between the MXene layers with smaller spacing, and the original intention of adding ZIF-67 to stabilize the MXene structure cannot be achieved.

[0093] Comparative Example 2

[0094] The structure directing agent for synthesizing MXene@ZIF-67 was explored. Comparative Example 2 includes the following steps:

[0095] Steps (1) and (2) are carried out with reference to steps (1) and (2) of Example 1;

[0096] The methanol solvent in steps (3) and (4) was replaced with water. In step (3), the amount of cobalt nitrate hexahydrate was 0.16 mmol, the amount of dimethylimidazole was 5.12 mmol, the ratio of cobalt nitrate hexahydrate to dimethylimidazole was 1:32, and the concentration of dimethylimidazole was 0.64 mol / L. The remaining steps were carried out with reference to steps (3) and (4) in Example 1 to prepare a MXene@ZIF-67 composite coating.

[0097] From the SEM images ( Figure 7 ) It can be seen that ZIF-67 can be evenly distributed between MXene layers, but because water is used as a structure-directing agent, the ZIF-67 crystal size is too small, and ZIF-67 in MXene@ZIF-67 cannot stably support the layered structure of MXene. This is inconsistent with our original intention of designing the core-shell stable structure MXene@ZIF-67.

[0098] Comparative Example 3

[0099] The MXene film layer on the surface of the magnesium alloy electrode was prepared by referring to the method in the embodiment of the prior patent CN115821355A. The performance of the MXene@ZIF-67 composite coating on the surface of the magnesium alloy prepared in Example 1 of the present invention was compared. According to the records, the hysteresis time of the MXene film layer prepared is 2.1s; Figure 4 From the results, it can be seen that the lag time of the MXene@ZIF-67 composite coating prepared in Example 1 of the present invention is only 1.3 s; and the discharge platform of the magnesium battery modified by the MXene@ZIF-67MXene@ZIF-67 composite coating prepared in Example 1 of the present invention is also significantly higher than the MXene film layer prepared in Example CN115821355A.

[0100] Therefore, the composite MXene@ZIF-67 coating of the present invention can continuously and stably protect the magnesium negative electrode, keep the magnesium ion transmission channel stable, and stably maintain the high voltage platform.

[0101] Test Case

[0102] 1. The composite material and coating in Example 1 were tested using a field emission electron scanning microscope (SEM). The results are as follows: Figure 1 As shown:

[0103] pass Figure 1 a and Figure 1 b It can be seen that after MXene is etched with hydrofluoric acid, it forms an accordion-shaped Ti3C2Tx ;pass Figure 1 c and Figure 1 d It can be seen that after in-situ growth and cathode electrophoretic deposition of the MXene@ZIF-67 composite coating, ZIF-67 grows between the MXene layers and plays a supporting role.

[0104] 2. The composite coating in Example 1 was tested by transmission electron microscopy (TEM) and EDS analysis. The results are as follows: Figure 2 As shown:

[0105] Figure 2 a~ Figure 2 c is the TEM image of different MXene interlayer spacings, and ac interlayer spacings range from 30 nm to 350 nm. Because MXene layers are connected by van der Waals forces, after hydrofluoric acid etching, the inhomogeneity of van der Waals forces will cause interlayer spacings of MXene of different sizes, so the interlayer spacing is a range. As can be seen from the figure, the size of the MXene interlayer spacing will not affect the uniform growth of ZIF-67 between MXene layers. MXenes with different interlayer spacings can make ZIF-67 grow uniformly between MXene layers, and by Figure 2 ac and Figure 1 b Comparison shows that after the growth of ZIF-67, the layered structure of MXene does not change and continues to maintain the accordion morphology.

[0106] Figure 2 As shown in di, the Co element is evenly distributed between the MXene layers, proving that ZIF-67 is successfully supported between the MXene layers; the stable distribution of Ti and C elements also indicates the stable existence of MXene.

[0107] 3. The composite coating in Example 1 was scanned by X-ray and analyzed by X-ray diffraction phase analysis (XRD). The results are as follows: Figure 3 As shown:

[0108] Figure 3 It can be seen that the composite coating mainly contains MXene, ZIF-67 and Mg, and can correspond to the standard spectrum.

[0109] 4. The electrochemical performance of the MXene@ZIF-67 composite coating in Example 1 was characterized by using a CHI760 electrochemical workstation and a NEWARE CT2001 charge and discharge tester to test the assembled battery using chronopotentiometry, electrochemical impedance spectroscopy and discharge curves. Figure 4 As shown:

[0110] Depend on Figure 4 a- Figure 4c It can be seen that after the MXene@ZIF-67 composite coating on the negative electrode surface of the magnesium battery is modified, the voltage hysteresis time is greatly reduced to 1.3 s, and the discharge time can maintain a high voltage (1.5~1.6V) platform for more than 14 h;

[0111] Depend on Figure 4 d It can be seen that the magnesium battery has the smallest electrochemical impedance after MXene@ZIF-67 modification, so the magnesium alloy negative electrode is more conducive to the transmission of magnesium ions under the protection of the MXene@ZIF-67 composite coating, which is conducive to the electrode reaction and comprehensively improves the discharge performance of the magnesium battery.

[0112] Electrodeposition on the magnesium anode forms a MXene@ZIF-67 film. Due to the addition of ZIF-67, the electrochemical impedance should inevitably increase (due to Figure 4 d shows that the impedance of a single ZIF-67 film layer is much greater than that of a single MXene, so, according to common sense, the impedance should increase when ZIF-67 is added to MXene). However, during the entire experimental verification phase, it was found that MXene@ZIF-67 was significantly better than MXene and ZIF-67 in terms of hysteresis time, and the electrochemical impedance of MXene@ZIF-67 was smaller than that of MXene and ZIF-67. From the discharge curve of the battery, it can be seen that the discharge time of MXene@ZIF-67 is better than that of MXene and ZIF-67, indicating that the composite structure is stable during the entire discharge process.

[0113] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a MXene@ZIF-67 composite coating on a magnesium alloy surface, characterized in that: The steps include: S1. Etching of aluminum titanium carbide: Etching aluminum titanium carbide with hydrofluoric acid, diluting and filtering until the filtered solution is neutral; vacuum drying to obtain successfully etched MXene powder; S2. Preparation of MXene@ZIF-67 composite material: The MXene powder and cobalt nitrate hexahydrate are dissolved in methanol, and the solution is black-red, which is used as solution A; 2-methylimidazole is dissolved in methanol as solution B; solution A is poured into solution B, and the solution turns blue. After the reaction, hexadecyltrimethylammonium bromide is added; stirring and centrifugation are performed, and the lower layer powder is taken out by suction filtering, and black-purple MXene@ZIF-67 powder is obtained after vacuum drying; S3. Preparation of MXene@ZIF-67 composite coating: The MXene@ZIF-67 powder is dissolved in methanol and subjected to ultrasonic treatment to completely suspend the powder in the methanol; graphite is used as an anode and a magnesium alloy is used as a cathode, and they are immersed face to face in the suspension; a constant voltage of 10 to 60 V is applied to the two electrodes, and the magnesium alloy at the cathode is removed after 1 to 20 min, and the MXene@ZIF-67 composite coating is obtained on the surface of the magnesium alloy after drying.

2. The method according to claim 1, characterized in that In step S1, etching aluminum carbide with hydrofluoric acid includes adding aluminum carbide to a mixed solution of hydrofluoric acid and water for stirring etching, wherein the amount ratio of aluminum carbide, hydrofluoric acid and water is 1.00 g to 3.00 g: 5 to 15 ml: 2 to 10 ml, the stirring etching temperature is 20 to 30° C., the rotation speed is 200 to 1000 r / min, and the time is 20 to 48 h; And / or, the vacuum drying temperature is 50-70°C.

3. The method according to claim 1, characterized in that In step S2, the usage ratio of the MXene powder, cobalt nitrate hexahydrate, and methanol is: 0.1-0.6 g: 0.1-1 mmol: 5-20 ml; And / or, in step S2, the usage ratio of 2-methylimidazole to methanol is 1-16 mmol:5-20 mL; And / or, the molar ratio of the cobalt nitrate hexahydrate to 2-methylimidazole is 1:4-1:

12.

4. The method according to claim 1, characterized in that: In step S2, the mass ratio of hexadecyltrimethylammonium bromide to MXene powder is 1:300-1:2.5; And / or, the stirring time is 5 to 20 h; And / or, the centrifugal speed is 5000-10000 r / min, and the number of times is 2-5 times; And / or, the vacuum drying temperature is 50-80°C and the time is 10-20 h.

5. The method according to claim 1, characterized in that The magnesium alloy in step S3 is pre-treated; the pre-treatment steps specifically include: grinding and polishing the magnesium alloy substrate, drying after cleaning, degreasing in an alkaline degreasing solution, and then ultrasonically cleaning with ethanol and deionized water respectively and then drying.

6. The method according to claim 5, characterized in that The alkaline degreasing solution includes 15-20 g / L NaOH, 30-40 g / L Na2CO3 and 15-20 g / L Na3PO4; And / or, the degreasing treatment is performed at a temperature of 60-80° C. and for a time of 1-5 min.

7. The method according to claim 1, characterized in that In step S3, the dosage ratio of the MXene@ZIF-67 powder to methanol is 0.05-0.1 g: 5-30 ml; And / or, the ultrasonic treatment time is 3 to 5 minutes; And / or, the depth of immersion of the anode and cathode in the suspension is 1-3 cm, and the interval between the anode and cathode is 2-4 cm; And / or, the drying temperature is 20-30°C.

8. The method according to any one of claims 1 to 7, characterized in that: The aluminum carbotitanium, hydrofluoric acid, cobalt nitrate hexahydrate, 2-methylimidazole, hexadecyltrimethylammonium bromide, methanol, ethanol, and NaOH, Na2CO3 and Na3PO4 are of chemical purity or above.

9. A MXene@ZIF-67 composite coating on a magnesium alloy surface, characterized in that: The method is prepared by the method according to any one of claims 1 to 8.

10. A magnesium battery, wherein the negative electrode is a magnesium alloy, characterized in that: The surface of the magnesium alloy is deposited with the MXene@ZIF-67 composite coating as claimed in claim 9.

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