A method for preparing porous MXene by using lithium dendrite growth principle

Abstract

The application discloses a method for preparing porous MXene by using lithium dendrite growth principle, which comprises the following steps: placing a MXene nanosheet film peeled off between a polypropylene film and a dust-free paper as a diaphragm, assembling a lithium metal symmetric battery by taking a lithium sheet as a positive and negative electrode and taking lithium hexafluorophosphate as an electrolyte; placing the assembled battery in a blue electricity system to perform cyclic charging and discharging test, wherein lithium dendrites continuously grow from the negative electrode to the positive electrode during the charging and discharging process, until the battery is short-circuited by piercing the diaphragm and the MXene nanosheet film in the diaphragm interlayer; and disassembling the battery to recover the porous MXene nanosheet with holes. The method overcomes the disadvantages of material composition and structure damage caused by chemical etching and hole formation in the preparation of the porous MXene nanosheet, and has the advantages of simple method, mild conditions, low cost, and no damage to the composition and structure of the hole-formed nanolayer. In addition, the prepared porous MXene nanosheet as an electrode material of a super capacitor has rich electrochemical active sites and ion transmission channels, so that the material has high ion transmission, high specific capacity and excellent rate performance.

Description

Technical Field

[0001] This invention belongs to the field of two-dimensional nanomaterial preparation technology, specifically relating to a method for preparing porous MXene by using the principle of lithium dendrite growth to create pores in MXene nanosheets. Background Technology

[0002] With the rapid growth of energy demand and the overconsumption of fossil fuels, developing efficient and clean energy has become a strategic choice for countries worldwide, and efficient energy storage systems are an indispensable supporting system for this development. Supercapacitors, due to their high power density and long lifespan, play a crucial role in leading the future energy storage industry. Electrode materials are the core component of supercapacitors, directly affecting their energy, power density, and lifespan. MXenes are a class of two-dimensional layered materials mainly composed of alternating metal and carbon / nitrogen layers. Their unique structure, good conductivity, and high electron storage performance make MXenes one of the ideal candidate electrode materials for supercapacitors. However, MXene nanosheets are prone to tight stacking during self-assembly, often leading to a sharp decrease in the active surface area and a significant reduction in capacity. Constructing porous structures can provide channels for ion transport, which is an effective way to solve the ion transport problem during the assembly of MXene nanosheets.

[0003] Currently, chemical etching is a widely used method for creating channels within MXene nanosheets. The basic principle of chemical etching is to use oxidizing agents (H₂O₂, H₂SO₄, CuSO₄, etc.) to oxidize Ti₃C₂T₄. x Some Ti in the structure is catalytically oxidized to TiO2, or the C element in the MXene nanosheet structure is etched, followed by acid or heat treatment to obtain a porous MXene nanosheet structure. However, the main drawback of the chemical etching method for preparing porous MXene nanosheets is that the composition and structure of the prepared material are damaged to varying degrees, leading to a severe decrease in the hydrophilicity and electrochemical properties of the MXene nanosheet layers. Therefore, developing a physical technique to prepare porous MXene nanosheets is an effective strategy to alleviate the significant imbalance of the Ti / C ratio in MXene and maintain its hydrophilicity. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing porous MXene by using dendrite growth principle to create pores in MXene nanosheets.

[0005] To achieve the above objectives, the technical solution provided by this invention includes the following steps:

[0006] Step 1: The MXene nanosheet dispersion obtained by exfoliation is concentrated into a slurry by high-speed centrifugation. The slurry is then coated onto a polypropylene film (PP film) using a coating tool, or the MXene nanosheet dispersion obtained by exfoliation is directly filtered onto a filter membrane and then vacuum dried to form an MXene film on the PP film or filter membrane. The MXene film is then removed and placed between the polypropylene film and the cleanroom paper (AP) to form a PP / MXene / AP membrane.

[0007] Step 2: Using lithium sheets as positive and negative electrodes, lithium hexafluorophosphate (LiPF6) as electrolyte, and the PP / MXene / AP membrane from Step 1 as separator, with the PP membrane placed on the positive electrode side and the AP membrane placed on the negative electrode side, a lithium metal symmetric battery is assembled; then the battery is placed in the Blue Electric System for cyclic charging and discharging.

[0008] Step 3: After the battery is short-circuited, disassemble the battery, take out the porous MXene film and place it in anhydrous ethanol for 30-60 minutes. Then, disperse it ultrasonically in deionized water to finally obtain porous MXene (P-MXene) nanosheets.

[0009] In step 1 above, it is preferable that the concentration of MXene nanosheets in the MXene nanosheet dispersion obtained by exfoliation is 10-20 mg / mL.

[0010] In step 1 above, the preferred high-speed centrifugation rate is 13000-15000 rpm, and the centrifugation time is 20-30 min.

[0011] In step 1 above, the vacuum drying temperature is preferably 25°C and the time is 6 to 12 hours.

[0012] In step 2 above, the preferred cyclic charging and discharging current is 0.2 to 2 mA.

[0013] The beneficial effects of this invention are as follows:

[0014] 1. This invention utilizes the principle of lithium dendrite growth to create pores in MXene nanosheets, transforming harmful lithium dendrites into a beneficial "tool" for preparing P-MXene nanosheets. A lithium metal symmetric battery is assembled using a PP / MXene / AP membrane as the separator and lithium sheets as the positive and negative electrodes. The assembled battery is then placed in a blue battery system for cyclic charging and discharging. During the charging and discharging process, lithium dendrites continuously grow, piercing the separator and the MXene film between the layers, ultimately yielding P-MXene nanosheets.

[0015] 2. The method of the present invention does not damage the composition and structure of the porous nanolayer, overcoming the main drawback of the material composition and structure obtained by creating channels in the MXene nanosheets by chemical etching being damaged to varying degrees, resulting in a serious decrease in the hydrophilicity and electrochemical performance of the MXene nanosheets.

[0016] 3. The method for preparing P-MXene nanosheets in this invention has the advantages of simple operation, mild conditions and low cost. The prepared P-MXene nanosheets have good hydrophilicity, abundant electrochemical active sites and ion transport channels, and exhibit high ion transport, high specific capacity and excellent rate performance as electrode materials for supercapacitors. Attached Figure Description

[0017] Figure 1 It is Ti3C2T x Field emission transmission electron microscope image of nanosheets.

[0018] Figure 2 It is Ti3AlC2, Ti3C2T x Nanosheets and P-Ti3C2T prepared in Example 1 x X-ray diffraction pattern of nanosheets.

[0019] Figure 3 This is a schematic diagram of the lithium metal symmetric battery structure in Example 1.

[0020] Figure 4 This is a schematic diagram and optical photograph of lithium dendrite growth in Example 1.

[0021] Figure 5 It is Ti3C2T in Example 1 x and P-Ti3C2T x Optical photograph of the nanosheet dispersion.

[0022] Figure 6 It is Ti3C2T in Example 1 x and P-Ti3C2T x Raman spectrum of nanosheets.

[0023] Figure 7 P-Ti3C2T prepared under different charge-discharge cycles x Field emission transmission electron microscope image of nanosheets.

[0024] Figure 8 P-Ti3C2T fabricated under different current densities x Field emission transmission electron microscope image of nanosheets.

[0025] Figure 9 It is Ti3C2T x Fiber electrodes and P-Ti3C2T x EIS curve of fiber electrode in three-electrode system.

[0026] Figure 10 It is Ti3C2T x Fiber electrodes and P-Ti3C2Tx GCD curve of fiber electrode in three-electrode system. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0028] The Ti3C2T obtained in the following examples is stripped x The preparation method of the nanosheet dispersion is as follows: Weigh 1.6g of LiF and dissolve it in 20mL of 9mol / L HCl aqueous solution and stir for 15min. Then, slowly add 1g of 200-mesh Ti3AlC2 powder and stir the reaction at 50℃ for 48h. Subsequently, add deionized water, centrifuge at 10000rpm for 10min, remove the supernatant, and repeat centrifugation until the pH is approximately neutral to obtain neutral Ti3C2T. x Dispersion, at this time, multilayer Ti3C2T x Delamination occurs. To further improve Ti3C2T... x Stripping yield, neutral Ti3C2T x The dispersion was shaken for 30 min, then centrifuged at 2500 rpm for 20 min. The supernatant dispersion was collected, and the centrifugation was repeated three times to obtain a Ti3C2T concentration of 20 mg / mL. x Nanosheet dispersion.

[0029] Figure 1 Field emission transmission electron microscopy images show the Ti3C2T obtained by exfoliation. x The nanosheets exhibit irregular shapes and ultrathin, transparent morphology, indicating successful exfoliation of monolayer and few-layer Ti3C2T. x Nanosheets. Raw materials: Ti3AlC2 and Ti3C2T x X-ray diffraction (XRD) spectra of nanosheets are as follows: Figure 2 As shown, the characteristic diffraction peak of the Al layer in the Ti3AlC2 raw material at 2θ = 39.9° completely disappears, indicating that the Al atomic layer of Ti3AlC2 was successfully removed after etching and intercalation with HCl and LiF. Ti3C2T x The XRD pattern still shows the characteristic peak corresponding to the (002) crystal plane at 2θ = 5.8°, indicating that its layered structure is maintained. Simultaneously, the position of the (002) crystal plane shifts to a smaller angle, indicating that the interlayer forces weaken after Ti-Al fracture, and Ti3C2T... x After a large number of negatively charged functional groups are adsorbed on the surface, the positively charged Li in the solution... + It enters the interlayer space, thus increasing the interlayer spacing.

[0030] Example 1

[0031] Step 1: The Ti3C2T obtained by stripping has a concentration of 20 mg / mL. x The nanosheet dispersion was concentrated by high-speed centrifugation at 15000 rpm for 20 min to obtain a slurry. The slurry was then coated onto a PP film using a coating tool and subsequently vacuum dried at 25 °C for 12 h to form Ti3C2T on the PP film. x Thin film. Remove Ti3C2T. x The film was cut into 13mm diameter circular films and placed between a 19mm diameter PP film and a 19mm diameter AP film to form PP / Ti3C2T. x / AP membrane.

[0032] Step 2: Using lithium sheets as positive and negative electrodes and LiPF6 as the electrolyte, the PP / Ti3C2T prepared in Step 1... x The AP membrane is a separator, the PP membrane is placed on the positive electrode side, and the AP membrane is placed on the negative electrode side, assembled as follows: Figure 3 The lithium metal symmetric battery shown is then placed in the Blue Battery system and subjected to cyclic charging and discharging at a current of 0.5mA.

[0033] Step 3: After the battery short-circuits, disassemble the battery and remove the porous Ti3C2T. x Thin film, and porous Ti3C2T x The film was placed in anhydrous ethanol and allowed to stand for 1 hour to remove lithium dendrites. Then, it was ultrasonically dispersed in deionized water to obtain porous Ti3C2T. x (P-Ti3C2T x Nanosheets.

[0034] The AP used in this embodiment has lower strength than PP film, making it more susceptible to being pierced by lithium dendrites of different sizes and shapes, thus forming holes. Meanwhile, AP can protect Ti3C2T. x The integrity of the nanosheets and the PP film protect the battery from short circuits. Figure 4 The optical photographs show, macroscopically, that in Ti3C2T x A thick layer of metallic lithium with a metallic luster formed between the lithium and the PP film, indicating that a significant number of lithium dendrites pierced through the AP and Ti3C2T layers. x The film, but not through the PP film. (Passed through) Figure 2 The XRD pattern shows that Ti3C2T x Thin film and P-Ti3C2T x Nanosheets and Ti3C2T x The diffraction peaks of the nanosheets are basically consistent, further indicating the successful preparation of P-Ti3C2T. x Nanosheets. Furthermore, during coating, vacuum drying, and battery charge-discharge cycling, Ti3C2T...x None of them underwent significant oxidation and retained their original structure.

[0035] Through Ti3C2T x and P-Ti3C2T x Optical photograph of nanosheet dispersion ( Figure 5 It can be found that the prepared P-Ti3C2T x The nanosheets exhibit good dispersion, meaning that the surface functional groups of the electrode material are not significantly damaged, thus preserving the Ti3C2T composition. x The excellent hydrophilicity of nanosheets. Observation of Ti3C2T x and P-Ti3C2T x Raman spectrum of nanosheets ( Figure 6 ), found at 195 and 720cm -1 The vibrational band at that point corresponds to the A atoms of Ti and C. 1g Out-of-plane vibrations, 382 and 616 cm -1 The vibrational band at this point corresponds to the in-plane vibrations of Ti, C, and the Eg groups of the surface functional groups, indicating that P-Ti3C2T x The composition and structure of the nanosheets are similar to those of Ti3C2T. x The nanosheet phase is consistent. The above results clearly demonstrate that the prepared P-Ti3C2T x Nanosheets do not damage the raw material Ti3C2T x The structure and surface functional groups of nanosheets.

[0036] The lithium metal symmetric battery was assembled according to steps 1 and 2 in Example 1 above, and after being subjected to 10,000, 30,000 and 60,000 charge-discharge cycles at a current of 0.5 mA, the battery was disassembled and the porous Ti3C2T was removed. x The thin film was placed in anhydrous ethanol and allowed to stand for 1 hour to remove lithium dendrites, and then ultrasonically dispersed in deionized water. Figure 7 The obtained P-Ti3C2T x Field emission transmission electron microscopy (FESTEM) images of the nanosheets show that, as the number of battery charge-discharge cycles increases, the P-Ti3C2T nanosheets... x The pore morphology on the nanosheets gradually becomes more apparent. This indicates that lithium dendrites continuously grow with the battery's charging and discharging time before the battery short-circuit.

[0037] Similarly, a lithium symmetric battery was assembled according to the method in Example 1 above, and cyclic charge-discharge cycles were performed at currents of 0.25mA, 0.5mA, and 1.0mA respectively until the battery short-circuited. The battery was then disassembled, and the porous Ti3C2T was removed. x The thin film was placed in anhydrous ethanol and allowed to stand for 1 hour to remove lithium dendrites, and then ultrasonically dispersed in deionized water. Figure 8 The obtained P-Ti3C2Tx Field emission transmission electron microscopy (FESTEM) images of the nanosheets show that, under different charge-discharge currents, P-Ti3C2T... x The presence of micropores, mesopores, and macropores on the nanosheets indicates that the current density has little effect on the size and distribution of the formed pores.

[0038] To investigate the effects of porous nanosheet structures on the ion diffusion rate and specific capacitance of fibrous electrodes, a three-electrode testing system was used in 1M H₂SO₄ electrolyte to study the effects of porous nanosheet structures on the ion diffusion rate and specific capacitance of Ti₃C₂T₅ electrodes. x and P-Ti3C2T x Electrochemical tests were performed using fiber electrodes. First, the P-Ti3C2T prepared in Example 1 was mixed using a mixer. x P-Ti3C2T nanosheets were prepared by wet spinning in an acetic acid coagulation bath using a plastic syringe after mixing in deionized water for 10 min. x Fiber electrode. Using P-Ti3C2T x A three-electrode testing system was constructed using a fiber electrode as the working electrode, an Ag / AgCl (saturated KCl) electrode as the reference electrode, and a platinum sheet electrode as the counter electrode. This system was used for P-Ti3C2T. x Electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) tests were performed on the fiber electrode. Similarly, Ti3C2T was prepared according to the above method. x Fiber electrodes, in a three-electrode system, for Ti3C2T x The fiber electrode was subjected to EIS and GCD tests.

[0039] Depend on Figure 9 The EIS curves show that, compared to Ti3C2T x Compared to fiber electrodes, P-Ti3C2T exhibits superior performance in the low-frequency region. x The Nyquist curve of the fiber electrode has a steeper slope, indicating that P-Ti3C2T x Ions inside the nanosheets can diffuse to the electrode surface more quickly, generating surface pseudocapacitance. In the high-frequency region, P-Ti3C2T... x The fiber electrode exhibits lower charge transfer resistance, which is more conducive to electrolyte ion diffusion, indicating that porous structures can effectively improve the electrochemical performance of materials. Figure 10 It can be seen that Ti3C2T x and P-Ti3C2T x The GCD curves of both fiber electrodes maintain a symmetrical isosceles triangle shape, indicating that both types of fiber electrodes possess good capacitance characteristics at a current density of 1 A cm⁻¹. -3 At that time, the specific capacitance was 252 F g. -1 and 366F g -1 This indicates that an appropriate amount of pore structure is beneficial to improving the specific capacitance of the fiber electrode.

Claims

1. A method for preparing porous MXene using the principle of lithium dendrite growth, characterized in that, Includes the following steps: Step 1: The MXene nanosheet dispersion obtained by exfoliation is concentrated into a slurry by high-speed centrifugation. The slurry is then coated onto a polypropylene film using a coating tool, or the MXene nanosheet dispersion obtained by exfoliation is directly filtered onto a filter membrane and then vacuum dried to form an MXene film on the polypropylene film or filter membrane. The MXene film is then removed and placed between a polypropylene film and a cleanroom paper to form a polypropylene / MXene / cleanroom paper membrane. Step 2: Using lithium sheets as positive and negative electrodes, lithium hexafluorophosphate as electrolyte, and the polypropylene / MXene / dust-free paper membrane from Step 1 as separator, with the polypropylene film placed on the positive electrode side and the dust-free paper placed on the negative electrode side, assemble a lithium metal symmetric battery; then place the battery in the Blue Electric System for cyclic charging and discharging. Step 3: After the battery is short-circuited, disassemble the battery, take out the porous MXene film and place it in anhydrous ethanol for 30-60 minutes. Then, disperse it ultrasonically in deionized water to finally obtain porous MXene nanosheets.

2. The method for preparing porous MXene using the lithium dendrite growth principle according to claim 1, characterized in that: In step 1, the concentration of MXene nanosheets in the MXene nanosheet dispersion obtained by exfoliation is 10-20 mg / mL.

3. The method for preparing porous MXene using the lithium dendrite growth principle according to claim 1, characterized in that: In step 1, the high-speed centrifugation rate is 13000-15000 rpm, and the centrifugation time is 20-30 min.

4. The method for preparing porous MXene using the lithium dendrite growth principle according to claim 1, characterized in that: In step 1, the vacuum drying temperature is 25°C and the time is 6–12 hours.

5. The method for preparing porous MXene using the lithium dendrite growth principle according to claim 1, characterized in that: In step 2, the cyclic charging and discharging current is 0.2 to 2 mA.

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