A metal phthalocyanine compound doped MXene material for lithium-sulfur battery positive electrode and its preparation method

Abstract

This invention discloses a metal phthalocyanine compound-doped MXene material for the cathode of lithium-sulfur batteries and its preparation method, belonging to the field of energy material preparation technology. The method involves mixing etched monolayer MXene with NiPC in solution, followed by freeze-drying and annealing to form a three-dimensional MX-NiPC catalytic material. The MX-NiPC is then composited with lithium sulfide to obtain Li₂S / MX-NiPC. This catalytic material provides strong adsorption of polysulfides through Ti-S bonds, and the rapid conversion of polysulfides is achieved through the catalytic ability of NiPC, avoiding the shuttle effect caused by increased polysulfide concentration and reducing capacity loss. Ultimately, compared with Li₂S / MX, Li₂S / MX-NiPC exhibits a 36% increase in specific capacity release at 0.1C (847 mAh / g) in the first cycle and a 38% increase in capacity after 100 cycles at 1C (400 mAh / g), significantly improving electrochemical performance and extending the cycle life of lithium-sulfur batteries.

Description

Technical Field

[0001] This invention relates to the field of energy material preparation technology, and in particular to a metal phthalocyanine compound-doped MXene material for lithium-sulfur battery cathodes and its preparation method. Background Technology

[0002] Lithium-sulfur batteries possess advantages such as high theoretical energy density, high theoretical specific capacity, low cost, and environmental friendliness, making them a key focus for researchers as a next-generation energy storage system. While lithium-sulfur batteries offer significant advantages, their inherent drawbacks cannot be ignored, including low cathode conductivity, lithium dendrite growth, and the shuttle effect. These are problems that must be overcome for lithium-sulfur batteries to achieve commercialization.

[0003] For example, patent publication number CN108232115B discloses a lithium-sulfur battery cathode material and its preparation method, as well as a lithium-sulfur battery and a lithium-sulfur battery. The provided lithium-sulfur battery cathode material is a composite material formed by cobaltates (magnesium cobaltate, nickel cobaltate, copper cobaltate, and zinc cobaltate) and elemental sulfur, with the elemental sulfur content being 60-90 wt%. The provided cobaltates have a strong adsorption effect on polysulfides, which can effectively inhibit the dissolution of lithium polysulfides in ether electrolytes, slow down the shuttle effect during battery charging and discharging, reduce the capacity decay of lithium-sulfur batteries, and improve battery life. The provided lithium-sulfur battery has an initial discharge capacity of 955 mAh / g (calculated based on the composite material) at a current of 0.1C, and a capacity of 722 mAh / g after 100 cycles, with a capacity retention rate of 75.6%.

[0004] MAX phases are a class of phases with the chemical formula M n+1 AX nMXenes are ternary layered compounds, where M represents a transition metal element, including Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, and Mo; A represents a group III A or IVA element; X represents C or N; and n has values ​​of 1, 2, and 3. The structural characteristic of the MAX phase is that M atoms and A atoms alternately arrange to form a near-close-packed hexagonal layered structure, with X atoms filling octahedral interstitial sites. The MA bond exhibits metallic bond characteristics, but its bond energy is weaker than that of the MX bond. Therefore, the A atom layer in the MAX phase is easily etched by HF solution. MXene materials are mainly prepared by selectively etching away the A atom layer in the MAX phase. In recent years, with the continuous exploration of the MXene family of materials, researchers have discovered that they possess unique structural and surface chemical properties, metallic conductivity, excellent hydrophilicity, and good mechanical stability, showing promising applications in many fields, such as energy storage, electromagnetic interference shielding, electrocatalytic hydrogen evolution, piezoresistive sensors, and membrane separation. With the advancement of research, more and more MXene materials are being applied in lithium-sulfur batteries. Besides the lithium-sulfur battery cathode materials and their preparation methods and lithium-sulfur batteries disclosed above, most existing research on the application of MXene materials in lithium-sulfur batteries focuses on structural design and chemical adsorption. There is still room for improvement in the catalytic conversion of polysulfides by MXene-based cathode materials. Summary of the Invention

[0005] The purpose of this application is to provide a metal phthalocyanine compound-doped MXene material for lithium-sulfur battery cathodes and a method for preparing the same, in order to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, this application provides the following technical solution: a method for preparing a metal phthalocyanine compound-doped MXene material for the cathode of a lithium-sulfur battery, comprising the following steps:

[0007] (1) Preparation of monolayer MXene colloidal solution by etching method;

[0008] (2) An ethanol solution of a metal phthalocyanine compound and an aqueous solution of carbon nanotubes were added to a monolayer MXene colloidal solution, and a mixed solution was prepared by magnetic stirring and sonication.

[0009] (3) The mixed solution was freeze-dried and then calcined at high temperature to obtain black powder MX-NiPC catalyst material.

[0010] As a further supplement to this solution, the etching method adopts one of the following: hydrofluoric acid etching, hydrochloric acid + lithium fluoride etching, and molten salt etching.

[0011] As a further supplement to this scheme, the specific method for preparing the monolayer MXene colloidal solution using the HCl + lithium fluoride etching method in step (1) is as follows:

[0012] 1) Under the condition of stirring at room temperature, lithium fluoride powder is added to concentrated HCl. After the lithium fluoride is completely dissolved, the MAX phase precursor material is slowly added.

[0013] 2) After stirring the solution obtained in step 1) in an oil bath at 35-45℃ for 22-26 hours, centrifuge and wash with water until the pH value is 7. After sonicating in an ice bath for 0.5-1 hour, centrifuge for 10 minutes and collect the upper suspension to obtain a monolayer MXene colloidal solution.

[0014] As a further supplement to this scheme, in step 1), the concentration of concentrated HCl is 8-12M, the concentration of lithium fluoride in concentrated HCl is (0.04-0.1) g / mL, and the mass ratio of lithium fluoride to MAX phase is (0.75-3.5):1.

[0015] As a further supplement to this scheme, the MAX phase precursor material is one or more of Ti3AlC2, Cr2AlC, V2AlC, Mo2TiAlC2, Ta2AlC, and Ta4AlC3.

[0016] As a further supplement to this scheme, in step 2), the centrifugation speed for washing is 3000-4500 r / min, the speed for centrifugation after sonication is 4000-6000 r / min, and the concentration of the monolayer MXene colloidal solution is 5-10 mg / mL.

[0017] As a further supplement to this scheme, the concentration of the carbon nanotube aqueous solution in step (2) is 40-60 mg / mL, and the volume ratio of the carbon nanotube aqueous solution to the monolayer MXene colloidal solution is 1:(5-20).

[0018] The metal phthalocyanine compound in step (2) is one or more of copper phthalocyanine, nickel phthalocyanine, zinc phthalocyanine, cobalt phthalocyanine, and iron phthalocyanine, and the concentration of the ethanol solution of the metal phthalocyanine compound is 0.5-4 mg / mL.

[0019] As a further supplement to this scheme, the ultrasonic treatment time for the mixed solution in step (2) is 0.5-1h.

[0020] As a further supplement to this scheme, in step (3), the freeze-drying time is 12-24 hours and the drying time is 36-48 hours.

[0021] The calcination temperature in step (3) is 300-400℃, the heating rate is 1-5℃ / min, the calcination time is 2-4h, and the atmosphere is argon or nitrogen.

[0022] A metal phthalocyanine compound-doped MXene material for use as a cathode in lithium-sulfur batteries, wherein the metal phthalocyanine compound-doped MXene material is prepared according to the preparation method of the metal phthalocyanine compound-doped MXene material for use as a cathode in lithium-sulfur batteries described above.

[0023] In summary, the technical effects and advantages of this invention are as follows:

[0024] 1. This invention synthesizes MXene-doped metal phthalocyanine compound materials using a freeze-drying and annealing method. MXene provides Ti-chemisorption groups, while the metal phthalocyanine compound provides active catalytic conversion groups. The synergistic effect of the two can play an adsorption-catalytic conversion role, thereby improving the utilization rate of active materials and suppressing the shuttle effect. Finally, the specific capacity of Li2S / MX-NiPC in the first cycle at 0.1C is as high as 872mAh / g, which is 40% higher than that of Li2S / MX. The capacity after 100 cycles at 1C is 35% higher than that of Li2S / MX, which greatly improves the electrochemical performance and cycle life of lithium-sulfur batteries.

[0025] 2. In this invention, the preparation method is scientific, reasonable, easy to implement, and low in cost. The resulting metal phthalocyanine compound-doped MXene material has the advantages of simple synthesis, easy adjustment of coordination composition, and can effectively reduce shuttle, improve charge and discharge capacity, and extend the cycle life of lithium sulfide-based lithium-sulfur batteries, providing an important reference for the next generation of high-performance lithium sulfide-based lithium-sulfur batteries. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 SEM images of the prepared metal phthalocyanine compound-doped MXene material;

[0028] Figure 2 The XRD pattern of the prepared metal phthalocyanine compound-doped MXene material;

[0029] Figure 3 The charge-discharge comparison diagram of the prepared Li2S / MX and Li2S / MX-NiPC at a current rate of 0.1C;

[0030] Figure 4 (a) Comparison of rate performance between Li2S / MX-NiPC and Li2S / MX; (b) Comparison of cycling performance between Li2S / MX-NiPC and Li2S / MX at 1C.

[0031] Figure 5 A comparison of the CV values ​​of Li2S / MX-NiPC and Li2S / MX. Detailed Implementation

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0033] Example 1

[0034] I. Preparation of MX-NiPC

[0035] (1) Add 40 mL of HCl to a polytetrafluoroethylene beaker, and while stirring the HCl solution, slowly add 3.2 g of lithium fluoride until it is completely dissolved. Weigh 2.0 g of MAX phase (Ti3AlC2) and slowly add it to the above solution. Stir and heat for 24 h at a temperature of 40 °C.

[0036] (2) Centrifuge the etched solution for 6 minutes at a speed of 3500 r / min. Pour out the supernatant, shake well and centrifuge again. Repeat the above steps 6 times until the pH of the supernatant is equal to 6-7.

[0037] (3) Add distilled water and shake well to obtain a black solution. Combine the solutions from the four test tubes into two test tubes, pass inert gas (argon) for 5 minutes to remove oxygen from the solution, and perform two ultrasonic treatments, each for 20 minutes.

[0038] (4) After sonication, the solution was centrifuged for 10 minutes at a speed of 6000 r / min. The solutions from the two test tubes were poured into two beakers (A and B), and the unreacted MAX solid at the bottom was discarded. 5 mL of carbon nanotube aqueous solution was added to beaker A and stirred for 30 minutes. 5 mL of carbon nanotube aqueous solution was added to beaker B. 0.05 g of nickel phthalocyanine (NiPc) was weighed and added to 5 mL of anhydrous ethanol and stirred to obtain a nickel phthalocyanine ethanol solution. The nickel phthalocyanine ethanol solution was added to beaker B and stirred for 30 minutes.

[0039] (5) After sonicating solutions A and B for 30 min, freeze for 24 h, freeze dry for 48 h, and then anneal in a tube furnace at 400 °C under an argon atmosphere for 2 h to obtain nickel phthalocyanine-doped MXene material (MX-NiPC) with a heating rate of 2 °C / min.

[0040] Figure 1The image shows the SEM image of MX-NiPC obtained in Example 1. The three-dimensional structure of MXene and the distribution of NiPC particles can be observed from the image. Its three-dimensional structure helps to accommodate active materials. The Ti contained in MXene can provide Ti-S bond formation for effective adsorption of polysulfides. The catalytic center provided by NiPC can quickly convert polysulfides to avoid the occurrence of dead sulfur or shuttle effect due to excessive polysulfide concentration, which would lead to permanent loss of active materials.

[0041] Figure 2 The XRD pattern of MX-NiPC obtained in Example 1 is shown below. Figure 2 As shown in (a), the peaks of the MAX phase used in this study correspond to the peaks of the PDF standard card of Ti3AlC2 and there are no obvious impurity peaks, indicating that the purity of the MAX phase used in this study is extremely high. Among them, the diffraction peaks at 9.6°, 34.1°, 38.9°, 42.0°, and 60.2° correspond to the (002), (101), (104), and (109) crystal planes of Ti3AlC2, respectively. The strongest diffraction peak (38.9°) in the XRD pattern of MXene after etching disappeared, indicating that the Al layer in the MAX phase was completely stripped away by etching. The diffraction peak at 9.6° shifted to the left to 7.5°. According to the Bragg equation, this indicates that the interplanar spacing increased. Further analysis shows that the Ti-Al bond of the MAX phase was destroyed during acid etching, and the Al layer was etched away, resulting in an increase in the interplanar spacing, indicating that monolayer MXene was successfully synthesized.

[0042] X-ray diffraction tests were performed on MX and MX-NiPC obtained by different doping of MXene, and the results are as follows: Figure 2 As shown in (b), both exhibit a characteristic peak at 7.5°, consistent with undoped MXene. Since NiPC is a trace doped material, no obvious diffraction peaks are visible in the XRD pattern.

[0043] Example 2

[0044] I. Preparation of MX-NiPC

[0045] (1) Add 40 mL of HCl to a polytetrafluoroethylene beaker, and while stirring the HCl solution, slowly add 3.2 g of lithium fluoride until it is completely dissolved. Weigh 2.0 g of MAX phase (Ti3AlC2) and slowly add it to the above solution. Stir and heat for 24 h at a temperature of 40 °C.

[0046] (2) Centrifuge the etched solution for 6 minutes at a speed of 3500 r / min. Pour out the supernatant, add 35 mL of distilled water, shake well and centrifuge. Repeat the above steps 6 times until the pH of the supernatant is equal to 6-7.

[0047] (3) Add distilled water and shake well to obtain a black solution. Combine the solutions from the four test tubes into two test tubes, pass inert gas (argon) for 5 minutes to remove oxygen from the solution, and perform two ultrasonic treatments, each for 20 minutes.

[0048] (4) After sonication, the solution was centrifuged for 10 minutes at a speed of 6000 r / min. The solutions from the two test tubes were poured into two beakers (A and B), and the unreacted MAX solid at the bottom was discarded. 5 mL of carbon nanotube aqueous solution was added to beaker A and stirred for 30 minutes. 5 mL of carbon nanotube aqueous solution was added to beaker B. 0.1 g of nickel phthalocyanine (NiPc) was weighed and added to 5 mL of anhydrous ethanol and stirred to obtain a cobalt phthalocyanine ethanol solution. The nickel phthalocyanine ethanol solution was added to beaker B and stirred for 30 minutes.

[0049] (5) After sonicating solutions A and B for 30 min, freeze for 24 h, freeze dry for 48 h, and then anneal in a tube furnace at 350 °C under a nitrogen atmosphere for 2 h to obtain nickel phthalocyanine-doped MXene material (MX-NiPC) with a heating rate of 1 °C / min.

[0050] II. Preparation of Electrodes and Cells

[0051] (1) MX-NiPC, Li2S, conductive carbon black and binder are ground and mixed in a ratio of 3:5:1:1. N-methylpyrrolidone (NMP) is added and stirred for 4-8 hours to form a uniform slurry. After coating, drying and cutting, Li2S / MX-NiPC positive electrode sheet is obtained.

[0052] (2) Assemble the Li2S / MX-NiPC positive electrode, Celgard 2400 separator, lithium sheet, positive and negative electrode shells into a Li-S battery, and perform electrochemical tests after standing for 8-12 hours.

[0053] Figure 3The figures show the initial charge-discharge activation curves of Li2S / MX and Li2S / MX-NiPC at 0.1C in Example 2. Since lithium sulfide is a product of complete lithiation of elemental sulfur and has poor conductivity, it needs to be charged to 3.6V during the initial charge to overcome its activation barrier. In subsequent charge-discharge processes, because trace amounts of lithium polysulfides already exist in the system and can undergo disproportionation reactions with lithium sulfide, the conversion reaction can be completed without overpotential. As can be seen from the figures, the activation barrier of Li2S / MX-NiPC is approximately 2.75V, while Li2S / MX, due to its lack of catalytic conversion active centers, has an activation barrier as high as 3.6V. This indicates that the addition of NiPC has a catalytic effect on the conversion reaction from Li2S to Li2S8. The reduction in the initial charge voltage is important for protecting the electrolyte, preventing electrolyte decomposition under high voltage. On the other hand, Li2S / MX-NiPC exhibits an initial specific capacity of 847 mAh / g, while Li2S / MX only has an initial discharge specific capacity of 622 mAh / g. Both show a long discharge plateau at 2.12 V, corresponding to the conversion reaction of liquid polysulfides. The first charge-discharge test results show that Li2S / MX-NiPC has a lower activation voltage and a higher discharge specific capacity, indicating that the addition of NiPC accelerates the conversion reaction and improves the utilization rate of active materials.

[0054] Figure 4 (a) is a comparison of the discharge specific capacity of Li2S / MX and Li2S / MX-NiPC at different current rates. The discharge specific capacities of Li2S / MX and Li2S / MX-NiPC at current rates of 0.1C, 0.2C, 0.5C, 1C, and 2C are 745.8 / 520.4, 667.8 / 475.3, 593.5 / 443.7, 528.8 / 414.6, and 465.7 / 348.7 mAh / g, respectively. When the current rate is restored to 0.1C, the discharge specific capacities of Li2S / MX and Li2S / MX-NiPC are 655.2 mAh / g and 482 mAh / g, respectively, with the Li2S / MX-NiPC cathode maintaining a high discharge specific capacity. Figure 4 (b) is a comparison of the cycling performance of Li2S / MX-NiPC and Li2S / MX at a current rate of 1C. It can be seen that the initial discharge specific capacity of Li2S / MX-NiPC and Li2S / MX is 522.8 mAh / g and 420.6 mAh / g, respectively. After 100 cycles, the discharge specific capacity decreases to 400.8 mAh / g and 289.6 mAh / g, respectively, with capacity retention rates of 76.6% and 68.8%, respectively. The specific capacity and cycling stability of the Li2S / MX-NiPC cathode are both superior to those of Li2S / MX.

[0055] Comparative Example 1

[0056] Compared to Example 1, the remaining experimental procedures of Comparative Example 1 remained unchanged, except that the metal phthalocyanine compound was removed during the preparation process, resulting in MX. Due to the lack of a catalytic center, the final specific capacity of Li2S / MX at 0.1C was only 621 mAh / g in the first cycle, a decrease of 28.8% compared to Li2S / MX-NiPC in Example 2. After 100 cycles at 1C, the capacity was 28.3% lower than that of Li2S / MX-NiPC in Example 2.

[0057] Figure 5 The cyclic voltammetry (CV) curves for Li₂S / MX-NiPC and Li₂S / MX are shown. The positions of the two reduction peaks and one oxidation peak are nearly identical, but the current of Li₂S / MX-NiPC is higher than that of Li₂S / MX. Analysis shows that the reduction and oxidation peak areas of Li₂S / MX-NiPC are slightly larger than those of Li₂S / MX, indicating that the utilization rate of the active material in Li₂S / MX-NiPC is higher than that in Li₂S / MX. CV tests show that at different scan rates, Li₂S / MX-NiPC maintains a better peak structure and a more concentrated voltage window for the electrochemical reaction, indicating a faster electrochemical reaction rate. Furthermore, Li₂S / MX-NiPC exhibits a higher reaction voltage in the reduction reaction, indicating that the reduction reaction requires less energy, confirming the rapid catalytic conversion capability of Li₂S / MX-NiPC.

[0058] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a metal phthalocyanine compound-doped MXene material for lithium-sulfur battery cathodes, characterized in that: Includes the following steps: (1) A monolayer MXene colloidal solution was prepared by etching. The etching method used was hydrochloric acid + lithium fluoride etching. The specific method for preparing a monolayer MXene colloidal solution by hydrochloric acid + lithium fluoride etching is as follows: 1) Under stirring conditions at room temperature, lithium fluoride powder is added to concentrated hydrochloric acid. After the lithium fluoride is completely dissolved, the MAX phase precursor material is slowly added. The MAX phase precursor material includes one or more of Ti3AlC2, Cr2AlC, V2AlC, Mo2TiAlC2, Ta2AlC, and Ta4AlC3. The concentration of concentrated hydrochloric acid is 8-12 mol / L, and the concentration of lithium fluoride in concentrated hydrochloric acid is (0.04-0.1) g / mL. The mass ratio of lithium fluoride to MAX phase precursor material is (0.75-3.5):

1. 2) After stirring the solution obtained in step 1) in an oil bath at 35-45 ℃ for 22-26 h, centrifuge and wash with water until the pH value is 7. After sonicating in an ice bath for 0.5-1 h, centrifuge for 10 min and collect the supernatant to obtain a monolayer MXene colloidal solution; the concentration of the monolayer MXene colloidal solution is 5-10 mg / mL. (2) Add an ethanol solution of a metal phthalocyanine compound and an aqueous solution of carbon nanotubes to a monolayer MXene colloidal solution, and prepare a mixed solution by magnetic stirring and sonication; the metal phthalocyanine compound includes one or more of nickel phthalocyanine (NiPc), cobalt phthalocyanine (CoPc), copper phthalocyanine (CuPc), iron phthalocyanine (FePc), manganese phthalocyanine (MnPc), and zinc phthalocyanine (ZnPc), the concentration of the ethanol solution of the metal phthalocyanine compound is 0.5-4 mg / mL; the concentration of the aqueous solution of carbon nanotubes is 10-80 mg / mL, and the volume ratio of the aqueous solution of carbon nanotubes to the monolayer MXene colloidal solution is 1:(5-20). (3) The mixed solution was freeze-dried and then calcined at high temperature to obtain black powder MX-NiPC catalyst material; the calcination temperature was 300-400 ℃, the heating rate was 1-5 ℃ / min, the calcination time was 2-4 h, and the atmosphere was argon or nitrogen. (4) Prepare Li2S / MX-NiPC cathode material and assemble it into a lithium-sulfur battery.

2. The method for preparing a metal phthalocyanine compound-doped MXene material for lithium-sulfur battery cathodes according to claim 1, characterized in that: In step 2), the centrifugation speed for washing is 3000-4500 r / min, the speed for centrifugation after sonication is 4000-6000 r / min, and the concentration of the monolayer MXene colloidal solution is 5-10 mg / mL.

3. The method for preparing a metal phthalocyanine compound-doped MXene material for lithium-sulfur battery cathodes according to claim 1, characterized in that: When treating the mixed solution in step (2), the ultrasonic time is 0.5-1 h.

4. The method for preparing a metal phthalocyanine compound-doped MXene material for lithium-sulfur battery cathode according to claim 1, characterized in that: In step (3), the freeze-drying time is 12-24 h and the drying time is 36-48 h.

5. A metal phthalocyanine compound-doped MXene material for use as a cathode in lithium-sulfur batteries, characterized in that: The metal phthalocyanine compound-doped MXene material is prepared according to the preparation method of the metal phthalocyanine compound-doped MXene material for lithium-sulfur battery cathode as described in claim 1.

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