MLHM / BPQDs multi-stage structure sodium ion battery negative electrode and preparation method thereof

Abstract

The invention discloses a sodium ion battery negative electrode with an MLHM / BPQDs multilevel structure and a preparation method of the sodium ion battery negative electrode. The MLHM / BPQDs are formed by self-assembly of MXene (MLHM) with a multilayer honeycomb-like structure and black phosphorus quantum dots (BPQDs). According to the invention, a bionic strategy based on double templates and self-assembly is adopted, an MLHM / BPQDs multi-stage structure comprising nanoscale BPQDs, submicron-order cell units and micron-order interlayer channels is prepared, and the MLHM / BPQDs multi-stage structure is a complete negative electrode formed by orderly arranging the cell units, mutually linking multiple layers of honeycombs and tightly anchoring the BPQDs. Compared with a stacked SM / BPQDs electrode, the MLHM / BPQDs electrode prepared by the invention has the advantages that the accumulation of MXene and BPQDs is effectively inhibited, the volume expansion of the BPQDs is better relieved, and the transmission of Na < + > and electrolyte is more quickly realized. By combining the adsorption of an interface Ti-O-P bond on Na < + > and the enhancement of interface charge transfer, the dynamic performance and the specific capacity of the MLHM / BPQDs are synergistically improved. Compared with a coating method, the preparation method of the MLHM / BPQDs disclosed by the invention has the advantages that an adhesive, a conductive agent and a current collector do not need to be additionally added, the process is simple, and the content of active substances is high.

Description

Technical Field

[0001] The present invention relates to the technical field of anodes for sodium-ion batteries, and particularly to an MLHM / BPQDs hierarchical structure anode for sodium-ion batteries and a preparation method thereof. Background Art

[0002] In recent years, phosphorus materials have been regarded as potential anode materials for sodium-ion batteries (SIBs) due to their abundant reserves and high theoretical specific capacity (2596 mAh g -1 ). Among the three phosphorus allotropes (red phosphorus, white phosphorus, and black phosphorus), black phosphorus (BP) with a layered orthorhombic structure is the most stable, and its relatively large interlayer spacing is beneficial to the insertion and extraction of sodium ions. Especially, black phosphorus quantum dots (BPQDs) with nanoscale dimensions in three dimensions have a significant "dimension effect", which can expose abundant active sites, effectively shorten the diffusion path of sodium ions, and is conducive to fully exerting its high theoretical specific capacity and excellent reaction kinetics. However, due to the huge surface energy of BPQDs, they are prone to re-aggregate during electrode preparation and cycling, making the structural advantages brought by their size effect disappear; moreover, BPQDs also have problems of low conductivity and large volume expansion, so it is difficult to use BPQDs alone as the anode material for sodium-ion batteries.

[0003] Loading BPQDs on the surface of two-dimensional MXene materials can effectively improve their conductivity by virtue of the three-dimensional conductive network formed by MXene materials, and to a certain extent relieve the stress problem caused by the volume expansion of BPQDs by means of the flexibility of MXene materials. However, this MXene–BPQDs nanocomposite still has a large surface energy and will still re-stack during use, forming a relatively dense structure, resulting in the re-lengthening of the migration path of sodium ions and the slowdown of the diffusion of organic electrolytes, thereby deteriorating the electrode kinetic performance of the material. Therefore, it is urgent to develop an MXene–BPQDs electrode and a preparation method thereof that can effectively inhibit the stacking of MXene–BPQDs nanocomposites, so as to improve their kinetic performance, better solve their volume expansion problem, improve the cycle stability, and make them more suitable for commercial applications. Summary of the Invention

[0004] To solve the above problems, the present invention provides an MLHM / BPQDs hierarchical structure anode for sodium-ion batteries and a preparation method thereof. The present invention designs a bionic strategy, constructs MXene nanosheets into multilayer honeycomb structure MXene (MLHM) by a double-template method, and uniformly self-assembles BPQDs onto the inner wall of the honeycomb to obtain an MLHM / BPQDs hierarchical structure, so that Na +It can reversibly deintercalate and intercalate in the MLHM / BPQDs multi - level structure like a bee, thus improving its kinetic performance. The object of the present invention is achieved by the following technical solutions:

[0005] A negative electrode of a sodium - ion battery with an MLHM / BPQDs multi - level structure and a preparation method thereof, and the preparation steps are as follows:

[0006] (1) After dispersing black phosphorus in NMP and ultrasonically stripping and pulverizing it, the supernatant is taken after centrifugation, the supernatant is taken again after centrifugation, and finally the precipitate is collected after centrifugation to obtain BPQDs, and it is dispersed into an NMP solution to obtain a BPQDs dispersion.

[0007] In this step, the dispersion of black phosphorus in NMP is to disperse 100 mg of black phosphorus (BP, 99.9% element) in 250 mL of N - methylpyrrolidone (NMP); the ultrasonic treatment is carried out under ice - bath conditions with 300 W and under the protection of an Ar atmosphere for 12 - 24 h; taking the supernatant after centrifugation is to take 2 / 3 of the supernatant after centrifugation at 2500 rpm for 10 min; taking the supernatant again after centrifugation is to take 2 / 3 of the supernatant after centrifugation at 6000 rpm for 10 min; obtaining BPQDs after the last centrifugation is to collect the precipitate after centrifugation at 10000 rpm for 15 min to obtain BPQDs; the BPQDs dispersion refers to a dispersion of BPQDs with a concentration of 10 - 20 mg / ml in an NMP solution. -1 in the NMP solution.

[0008] (2) Slowly add 2 - 4 g of MAX powder to 50 - 80 ml of a LiF + HCl etching solution or an HF etching solution for etching, wash and collect the precipitate, then ultrasonically strip or intercalate and strip, take 2 / 3 of the supernatant after centrifugation and remove air to obtain a MXene dispersion.

[0009] In this step, the LiF + HCl etching solution is prepared by adding 4 g of LiF to 80 ml of HCl (9 mol / L) under magnetic stirring -1)Etchant obtained in solution; the HF etchant is an HF solution with a mass fraction of 49%; the etching is carried out under stirring at 35 °C for 24 to 36 h or in a hydrothermal reaction kettle at 45 °C for 48 to 72 h; after washing, the precipitate is collected by washing with deionized water and centrifuging at 3500 rpm. After repeating multiple times until the pH of the supernatant is approximately 5 to 6, the supernatant is poured out to obtain the precipitate; the ultrasonic exfoliation is carried out by ultrasonic exfoliation at 240 W in deionized water for 1 h; the intercalation exfoliation is carried out by stirring in a 5% tetramethylammonium hydroxide solution at 20 °C for 12 h for intercalation exfoliation; the centrifugation is carried out at 3500 rpm for 1 h; the air removal is carried out by introducing Ar into the MXene dispersion until no bubbles emerge; the MXene dispersion refers to a dispersion of MXene with a concentration of 10 mg ml -1 in deionized water.

[0010] (3) Mix the MXene dispersion obtained in step (2) and the submicron-sized PMMA microsphere dispersion in a certain mass ratio, ultrasonically treat and then vacuum filter to obtain the MXene–PMMA film. After freeze-drying this film to remove the water template, and then calcining to remove the PMMA template, a multilayer honeycomb structure MXene (MLHM) film is obtained.

[0011] In this step, the submicron size means that the diameter of the PMMA microspheres is 0.300 to 0.400 microns; the PMMA microsphere dispersion refers to a dispersion of PMMA microspheres with a concentration of 5 to 10 mg ml -1 in deionized water; the mixing in a certain mass ratio means that the mass ratio of MXene to PMMA microspheres is 1:3 to 1:5; the ultrasonic treatment means ultrasonic treatment at 240 W for 1 h; the freeze-drying means freezing with liquid nitrogen and vacuum drying to remove the water template; the calcining means calcining at 500 °C in an Ar atmosphere for 1 h.

[0012] (4) Immerse the MLHM film obtained in step (3) into the BPQDs dispersion obtained in step (1), and carry out self-assembly by ultrasonic treatment for 1 to 6 hours. After cleaning and vacuum drying, an MLHM / BPQDs multi-level structure is obtained.

[0013] The ultrasonic treatment for 1 to 6 h means ultrasonic treatment at 120 W for 1 to 6 h under the protection of an Ar gas atmosphere; the self-assembly means self-assembly carried out under the action of van der Waals forces; the vacuum drying means vacuum drying at 60 °C for 12 h.

[0014] In the MLHM / BPQDs multi-level structure prepared by the present invention, it includes micron-level honeycomb layers and interlayer channels, as well as nano-level BPQDs loaded on the inner walls of the honeycombs, and the honeycomb layers are composed of orderly arranged open submicron-level hollow spheres. This unique hierarchical structure has the following advantages:

[0015] 1. In the honeycomb layer of the MLHM / BPQDs multi - level structure, the PMMA template induces the close arrangement of MXene into an ordered and open honeycomb - like structure, and then anchors BPQDs on the inner wall of the honeycomb during the subsequent self - assembly process, which effectively inhibits the stacking of MXene and the aggregation of BPQDs, thus shortening the migration path of Na + .

[0016] 2. The multi - layer honeycomb structure MLHM in the MLHM / BPQDs multi - level structure is formed by the induction of a water template to form interconnected multi - layer honeycomb structures. Compared with the conventional single - layer honeycomb structure, it can load more active substances. Therefore, the MLHM / BPQDs multi - level structure can be directly assembled into a battery as a self - supporting negative electrode without the need to add additional binder, conductive agent, and current collector, thereby improving the energy density of the battery. Moreover, the thickness of the self - supporting negative electrode can be conveniently adjusted according to the working conditions by changing the addition amount of MXene.

[0017] 3. The MLHM / BPQDs multi - level structure is a complete negative electrode composed of closely and orderly arranged honeycomb cells, interconnected multi - layer honeycombs, and tightly anchored BPQDs. Compared with the MXene electrode formed by the loose accumulation of MXene microspheres (where the transport of Na + , electrons, and electrolyte is incoherent and the mechanical strength is poor), it has better mechanical strength and a more extensive three - dimensional conductive network. Combining its unique interlayer channels and open space structure, it can better accommodate and withstand the volume expansion of BPQDs and more quickly realize the transport and diffusion of Na + , electrons, and electrolyte. In addition, the loaded BPQDs can provide a higher specific capacity.

[0018] 4. The M–O–P interfacial bonds (M = Ti, Sc, V, Nb, Mo, etc., early transition metals) existing in the MLHM / BPQDs multi - level structure are beneficial to enhancing the adsorption of Na + and interfacial charge transfer, thus improving the kinetic performance of MLHM / BPQDs.

[0019] 5. The preparation method of the negative electrode of the sodium - ion battery with the MLHM / BPQDs multi - level structure provided by the present invention can obtain the MLHM / BPQDs multi - level structure under mild conditions by using a bionic strategy. Through the synergistic effect of the multi - level structure in terms of surface morphology, spatial structure, and surface chemistry, a significant improvement in its kinetic performance and cycling performance can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 SEM cross - sectional view at low magnification of MLHM obtained in Example 1;

[0021] Figure 2 SEM cross-sectional view at high magnification of the MLHM obtained in Example 1;

[0022] Figure 3 SEM cross-sectional view at partial magnification of the MLHM obtained in Example 1;

[0023] Figure 4 SEM cross-sectional view at high magnification of the partial magnification of the MLHM obtained in Example 1;

[0024] Figure 5 TEM image of the MLHM / BPQDs multi-level structure obtained in Example 1;

[0025] Figure 6 HRTEM image of the MLHM / BPQDs multi-level structure obtained in Example 1;

[0026] Figure 7 XRD patterns of the MLHM / BPQDs multi-level structure, MXene, and black phosphorus obtained in Example 1;

[0027] Figure 8 Adsorption-desorption curves of the MLHM obtained in Example 1 and the SM obtained in the comparative example;

[0028] Figure 9 O1s diagram of the XPS spectrum of the MLHM / BPQDs multi-level structure obtained in Example 1;

[0029] Figure 10 Cycling performance diagrams of the MLHM / BPQDs obtained in Example 1, the SM / BPQDs obtained in the comparative example, and bulk black phosphorus at 0.5 Ag -1 ;

[0030] Figure 11 Cycling performance diagrams of the MLHM / BPQDs obtained in Example 1, the SM / BPQDs obtained in the comparative example, and bulk black phosphorus at 2 A g -1 ;

[0031] Figure 12 Electrochemical impedance spectra of the MLHM / BPQDs obtained in Example 1, the SM / BPQDs obtained in the comparative example, and bulk black phosphorus;

[0032] Figure 13 GITT curves and diffusion coefficient curves of the MLHM / BPQDs obtained in Example 1 and the SM / BPQDs obtained in the comparative example;

[0033] Figure 14 SEM cross-sectional views of the MLHM / BPQDs obtained in Example 1 before and after cycling;

[0034] Figure 15Cross-sectional SEM images of SM / BPQDs obtained as a comparative example before and after cycling. Detailed implementation manners

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments and the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the descriptions and the embodiments shown in the accompanying drawings herein can be implemented through various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents the selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0036] Example 1:

[0037] Aiming at the problems that MXene–BPQDs nanocomposites are prone to stacking, resulting in a longer migration path of sodium ions, slower diffusion of the electrolyte, and poorer electrode kinetic performance, this embodiment provides a multi-layer honeycomb structure MLHM / BPQDs multi-stage structure anode for a sodium-ion battery and its preparation method. Through the synergistic effect of the multi-stage structure, a significant improvement in its kinetic performance and cycling performance is achieved.

[0038] (1) Preparation of BPQDs: Add 100 mg of black phosphorus to 250 mL of NMP solution, ultrasonicate for 12 h under 300 W, ice bath conditions, and Ar atmosphere protection, then centrifuge at 2500 rpm for 10 min and take 2 / 3 of the supernatant. Then centrifuge the supernatant at 6000 rpm for 10 min and take 2 / 3 of the supernatant again. Finally, centrifuge the supernatant at 10000 rpm for 15 min to collect the precipitate to obtain BPQDs, and disperse it in NMP solution to obtain a 15 mg ml -1 BPQDs dispersion.

[0039] (2) Preparation of monolayer / few-layer MXene: Add 4 g of LiF to 80 ml of HCl (9 mol L -1 ) solution under magnetic stirring to obtain a LiF+HCl etching solution. Slowly add 4 g of Ti3AlC2 powder to the LiF+HCl etching solution, stir at 35 °C for 30 h, wash the product with deionized water and centrifuge at 3500 rpm. Repeat several times until the pH of the supernatant is approximately 5.5, then pour out the supernatant to obtain a precipitate. Then re-add it to deionized water, ultrasonicate at 240 W for 1 h, centrifuge at 3500 rpm for 1 h and take 2 / 3 of the supernatant, and pass Ar gas into the MXene dispersion until no bubbles emerge to obtain a 10 mg ml-1 MXene dispersion liquid.

[0040] (3) Preparation of MLHM: Disperse PMMA microspheres with a diameter of 0.35 μm in deionized water to obtain a PMMA microsphere dispersion liquid with a concentration of 7.5 mg / ml. -1 Drop 160 mg of the PMMA microsphere dispersion liquid into the MXene dispersion liquid obtained in step (2) of Example 1 under stirring conditions. After stirring for 2 h, ultrasonicate at 240 W for 1 h, and then perform vacuum filtration to obtain an MXene–PMMA membrane. Then, freeze the MXene–PMMA membrane with liquid nitrogen and vacuum dry it for 12 h to remove the water template. Finally, calcine it at 500 °C in an Ar atmosphere for 1 h to remove the PMMA template, and obtain an MLHM membrane with a multi-layer honeycomb structure.

[0041] (4) Preparation of the MLHM / BPQDs multi-level structure: Immerse the MLHM membrane obtained in step (3) of Example 1 into the BPQDs dispersion liquid obtained in step (1) of Example 1, ultrasonicate at 120 W for 1 h under the protection of an Ar gas atmosphere, and finally vacuum dry it at 60 °C for 12 h to obtain an MLHM / BPQDs multi-level structure with a multi-layer honeycomb structure.

[0042] Comparative Example 1: Other steps are exactly the same as those in Example 1, except that no PMMA microspheres and their calcination steps are added in Comparative Example 1. That is, the MXene obtained in step (2) of Example 1 is directly filtered into a membrane to obtain a stacked MXene membrane (SM), and then BPQDs are loaded onto the SM membrane according to step (4) of Example 1 to obtain SM / BPQDs.

[0043] Comparative Example 2: Commercially available bulk black phosphorus, without ultrasonic exfoliation and pulverization, is bulk black phosphorus, named Bulk BP.

[0044] Figure 1 It shows that there are interconnected multi-layer honeycomb structures in the MLHM / BPQDs multi-level structure, which is different from the single-layer honeycomb structure reported in conventional literature. It can be seen from Figure 2 that the channels between the honeycomb layers are micron-sized, which is beneficial to the infiltration of the electrolyte. It can be seen from Figure 3 , Figure 4 and Figure 5 that MXene is closely arranged into an ordered and open honeycomb-like structure, and the size of a single honeycomb chamber is submicron-sized. This submicron-sized open structure is beneficial to the subsequent loading of BPQDs. It can be seen from Figure 6 that BPQDs are loaded on MLHM, which proves the successful preparation of the MLHM / BPQDs multi-level structure. Figure 7 It is proved that no obvious phase change occurs between MXene and black phosphorus during the preparation process of the MLHM / BPQDs multi-level structure.Figure 8 It is proved that MLHM has a larger specific surface area than SM in Comparative Example 1. It can be seen from Figure 9 that there are Ti–O–P interfacial bonds in MLHM / BPQDs, which is beneficial to enhancing the adsorption of Na + and interfacial charge transfer, thereby improving the kinetic performance of MLHM / BPQDs. Figure 10 and Figure 11 It is proved that at the current densities of 0.5 A g -1 and 2 A g -1 , the MLHM / BPQDs in Example 1 have a higher specific capacity than the SM / BPQDs in Comparative Example 1 and the Bulk BP in Comparative Example 2. Figure 12 It is proved that MLHM / BPQDs has a smaller charge transfer resistance than SM / BPQDs and Bulk BP. Figure 13 It is proved that MLHM / BPQDs has a larger ion diffusion coefficient than SM / BPQDs, thus having better kinetic performance. It can be seen from Figure 14 and Figure 15 that after cycling, MLHM / BPQDs (3.1%) has a smaller volume expansion rate than SM / BPQDs (15.7%), and can better relieve the volume expansion of the electrode caused by Na + during the cycling process.

[0045] Example 2: Other steps are the same as those in Example 1, except for step (2): Under magnetic stirring, 2 g of V2CT x powder was slowly added to 50 ml of HF etching solution with a mass fraction of 49%, and the reaction was carried out at 45 °C in a hydrothermal reaction kettle for 72 h. Then the product was washed with deionized water and centrifuged at 3500 rpm. After repeating several times until the pH of the supernatant was approximately 5.5, the supernatant was poured out to obtain a precipitate. Then it was added to a 5% tetramethylammonium hydroxide solution and stirred for 12 h for intercalation exfoliation. Finally, it was centrifuged at 3500 rpm for 1 h, and 2 / 3 of the supernatant was taken, and Ar gas was introduced into the MXene dispersion until no bubbles emerged, to obtain a 10 mg ml -1 MXene dispersion.

[0046] The above are only the embodiments of the present invention, and do not impose any form of limitation on the present invention. The present invention can also have other forms of embodiments according to the above structures and functions, which will not be listed one by one. Therefore, any person skilled in the art, without departing from the scope of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. An anode of a sodium-ion battery with an MLHM / BPQDs multi-level structure, characterized in that The negative electrode of the MLHM / BPQDs multi - level structure sodium - ion battery is self - assembled from multi - layer honeycomb - like MXene (MLHM) and black phosphorus quantum dots (BPQDs). It contains multi - level structures such as nanoscale BPQDs, sub - micron - scale honeycomb cells, and micron - scale interlayer channels, and is a complete negative electrode formed by the ordered arrangement of honeycomb cells, the inter - connection of multi - layer honeycombs, and the tight anchoring of BPQDs.

2. The MLHM / BPQDs multi-level structure according to claim 1, wherein The MLHM refers to multi - layer honeycomb - like MXene prepared by a double - template method based on PMMA and water, and the BPQDs refers to black phosphorus quantum dots prepared by an ultrasonic exfoliation and pulverization method.

3. The multi-layered honeycomb-like structure MLHM according to claim 2, wherein The MLHM is composed of honeycomb layers and interlayer channels.

4. The multi-layered honeycomb-like structure MLHM according to claim 3, characterized in that The honeycomb layer is composed of sub - micron - scale hollow MXene spheres closely arranged into an ordered and open honeycomb - like structure; the interlayer channel is a micron - scale interlayer channel formed by the inter - connection of multi - layer honeycombs.

5. The multi-layered honeycomb-like structure MLHM according to claim 4, wherein The hollow MXene spheres are induced to generate by the PMMA template, and the interlayer channel is induced to generate by the water template.

6. The MLHM / BPQDs multi-level structure according to claim 1, characterized in that In the MLHM / BPQDs multi - level structure, BPQDs are uniformly loaded inside, and there are M–O–P bonds (M = Ti, Sc, V, Nb, Mo and other early transition metals) at its interface.

7. The negative electrode of the MLHM / BPQDs multi-level structure sodium ion battery according to claim 1, characterized in that The negative electrode of the MLHM / BPQDs multi - level structure sodium - ion battery is a self - supporting negative electrode.

8. A method for preparing the negative electrode of the MLHM / BPQDs multi-level structure sodium ion battery according to any one of claims 1-7, characterized in that The specific implementation steps of the method are as follows: (1) After dispersing black phosphorus in NMP and ultrasonically pulverizing it, the supernatant is taken after centrifugation, the supernatant is taken again after centrifugation, and finally the supernatant is taken after centrifugation to obtain BPQDs, and it is dispersed into the NMP solution to obtain a BPQDs dispersion. (2) Slowly add 2 - 4 g of MAX powder into 50 - 80 ml of LiF + HCl etching solution or HF etching solution for etching. After washing, collect the precipitate, and then perform ultrasonic exfoliation or intercalation exfoliation. Take 2 / 3 of the supernatant after centrifugation and remove air to obtain a MXene dispersion. (3) Mix the MXene dispersion obtained in step (2) and the sub - micron - sized PMMA microsphere dispersion in a certain mass ratio, ultrasonically filter it under vacuum to obtain a MXene–PMMA membrane. After freeze - drying this membrane to remove the water template, then calcine it to remove the PMMA template to obtain a multi - layer honeycomb - like MXene (MLHM) membrane. (4) Immerse the MLHM membrane obtained in step (3) into the BPQDs dispersion obtained in step (1), perform self - assembly by ultrasonic treatment for 1 - 6 hours, wash it, and dry it under vacuum to obtain the MLHM / BPQDs multi - level structure.

9. The preparation method of the negative electrode of the MLHM / BPQDs multi-level structure sodium ion battery according to claim 8, wherein In step (4), the sub - micron - sized PMMA microspheres refer to PMMA microspheres with a diameter of 0.300 - 0.400 microns; the freeze - drying refers to freezing with liquid nitrogen and drying under vacuum to remove the water template.

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