Sulfur-doped three-dimensional wrinkled MXene material and application thereof

Abstract

The invention relates to a sulfur-doped three-dimensional pleated MXene material and application thereof, and the sulfur-doped three-dimensional pleated MXene material is characterized in that elemental sulfur and / or a sulfur-containing compound are utilized to generate alkyl ammonia polysulfide in a compound with an alkyl chain and an amino group, and the alkyl ammonia polysulfide and hydroxyl on the surface of MXene are subjected to electrostatic self-assembly to form a pleated layer; an acid solution is added to provide hydrogen ions, so that sulfur ions are recrystallized and annealed to form a sulfur atom-doped three-dimensional wrinkled MXene sheet layer to modify the self-stacking phenomenon of MXene. The modified MXene shows a three-dimensional fold morphology, the self-stacking condition of the MXene is effectively inhibited, and the electrochemical performance of the MXene is improved by doping sulfur atoms. According to the structure, the specific surface area of MXene is effectively increased, and active sites are increased. The sulfur-doped three-dimensional fold MXene obtained by the method has better sodium storage performance, and still has the specific discharge capacity not lower than 180mAh g <-1 > after 1200 cycles under the condition of 1.0 A g <-1 >.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and in particular to a sulfur-doped three-dimensional wrinkled MXene material and applications thereof. Background Art

[0002] With the rapid popularization of electric vehicles in society, lithium resources have been developed to a great extent, and there is an urgent need to find alternative resources to alleviate the over-exploitation of lithium resources. Sodium-ion batteries are considered to be the most powerful alternative to lithium-ion batteries in the field of electrochemical energy storage. Compared with lithium batteries, sodium-ion batteries have low raw material costs, excellent safety, low electronegativity and large ion size. These characteristics make sodium-ion batteries relatively stable during the charging and discharging process, less prone to structural phase changes, and longer cycle life. However, due to its low energy density and poor rapid charging and discharging capabilities, it is a major obstacle to large-scale application.

[0003] Among the many electrode materials for sodium-ion batteries, sheet electrode materials have high surface activity, which can improve the battery's charge and discharge efficiency and the permeability of the electrolyte. Compared with sheet graphene materials, where charge storage only occurs on the surface of electrolyte ions, the surface of graphene-like structures MXene has many functional groups and terminal metals, which can increase the redox sites on its surface, improve the conductivity of the material, and enable sodium ions to be quickly embedded and detached on the electrode surface. However, in the process of preparing MXene, there is a van der Waals force that causes the sheets to stack, greatly limiting the sodium storage sites between the sheets.

[0004] Tao et al. used the template method to use any one of the positively charged polymers, silica or calcium carbonate as a template to interact with the negatively charged MXene through electrostatic interaction and spatial barrier to inhibit the stacking of two-dimensional sheets; Zhang et al. prepared irregular wrinkles and curls by self-assembly of single-layer MXene and three-dimensional nitrogen material (protonated), avoiding the problem of restacking and collapse of sheets during the preparation process and increasing the specific surface area; Xu et al. first added acid to a mixed solution of sulfur-containing compounds, dispersants and water to obtain a nanosulfur particle dispersion, washed it to neutrality, added a two-dimensional lamella MXene aqueous dispersion, filtered to obtain a two-dimensional lamella MXene / sulfur composite membrane, and removed the sulfur to obtain a three-dimensional porous MXene foam material.

[0005] As mentioned above, the existing public technologies use template method, electrostatic self-assembly and other methods to inhibit the stacking of sheets, but the preparation process still needs to find a suitable template to provide a nitrogen source or a sulfur source and add a dispersant or protonate the template for pretreatment. Therefore, it is still necessary to explore a simple method for preparing three-dimensional MXene materials to inhibit their sheet stacking. Summary of the invention

[0006] The object of the present invention is to overcome the disadvantages and deficiencies existing in the prior art, and to provide a sulfur-doped three-dimensional wrinkled MXene material and its application

[0007] The technical solution adopted by the present invention is as follows: In a first aspect of the present invention, a sulfur-doped three-dimensional wrinkled MXene material is provided, and its preparation method includes the following steps:

[0008] (1) Dissolve the sulfur source in compound A and then drop it into the Mxene suspension and stir evenly to obtain a mixed solution. The sulfur source includes elemental sulfur and / or sulfur-containing compounds, and compound A has an alkyl chain and an amino group;

[0009] (2) Drop an acid solution into the mixed solution in step (1) to obtain an MXene complex coated with sulfur nanoparticles;

[0010] (3) Anneal the MXene complex coated with sulfur nanoparticles at high temperature in a protective atmosphere to obtain a sulfur atom-doped three-dimensional wrinkled MXene material.

[0011] Both the compound containing an alkyl chain and an amino group and the elemental sulfur / sulfur-containing compound can form an open-chain alkylamine polysulfide structure. Among them, the sulfur-containing compound can be selected from hydrogen sulfide, sodium sulfide, etc.

[0012] Preferably, in step (1), the sulfur source includes sublimed sulfur, and compound A includes ethylenediamine.

[0013] Preferably, in step (1), the ratio of the sulfur source to compound A is 0.5-5 mol / L.

[0014] Preferably, in step (1), the mass ratio of the sulfur source to the Mxene suspension is 4-10:1.

[0015] Preferably, in step (2), the concentration of the acid solution is 5-15%.

[0016] Preferably, in step (2), the acid solution includes at least one of hydrochloric acid and sulfuric acid, and the dropping amount is based on the solution becoming clear.

[0017] Preferably, in step (3), the conditions for high-temperature annealing are: 300-500 °C, 1-3 h.

[0018] Preferably, the preparation method of the Mxene suspension includes the following steps:

[0019] Add MAX and LiF to the hydrochloric acid solution, stir for 12-36 h, centrifuge and wash until neutral, and then ultrasonically obtain the supernatant to obtain a two-dimensional sheet-like structure Mxene suspension. MAX, that is, M (n+1) AX n, including TI3SiC2, Ti2AIC, etc.

[0020] Preferably, the mass ratio of the MAX to LiF is 1:0.5 - 2.

[0021] Preferably, the concentration of the hydrochloric acid solution is 20 - 40%.

[0022] The second aspect of the present invention provides the application of the sulfur-doped three-dimensional wrinkled MXene material as described above in sodium-ion batteries.

[0023] Preferably, the sulfur-doped three-dimensional wrinkled MXene material as described above is applied to the preparation of the anode material of a sodium-ion battery.

[0024] The beneficial effects of the present invention are as follows:

[0025] 1. The present invention adopts a simple and rapid template-free method to prepare a sulfur-doped three-dimensional wrinkled MXene structure. Sulfur and / or sulfur-containing compounds are dissolved in a compound with an alkyl chain and an amino group to form an open-chain alkylamine polysulfide. The above suspension is dropped into the MXene suspension, and the open-chain alkylamine polysulfide interpenetrates into the MXene lamellae. Since the alkylammonium ions in the sulfur-containing suspension can have electrostatic interaction with the negatively charged ions on the surface of MXene, it promotes the cross-linking of MXene to form a three-dimensional wrinkled porous shape (3DMXene). Subsequently, an acid is added to provide hydrogen ions, promoting the recrystallization of the polysulfide into sulfur nanoparticles in the voids of 3DMXene, forming a composite of MXene nanosheets coated with sulfur particles. Finally, sulfur atoms are introduced into the MXene framework through a high-temperature annealing process, and the excess sulfur particles are removed at the same time, thereby forming sulfur-doped three-dimensional porous MXene.

[0026] 2. The modified MXene exhibits a three-dimensional wrinkled morphology, effectively suppressing the self-stacking of MXene. The doping of sulfur atoms has an enhancing effect on its electrochemical performance. This structure effectively increases the specific surface area of MXene and increases the active sites. The sulfur-doped three-dimensional wrinkled Mxene obtained in this way has better sodium storage performance and still has a discharge specific capacity of not less than 180 mAh g after 1200 cycles under the condition of 1.0 A g -1 . -1 This sulfur-doped three-dimensional wrinkled MXene structure can better expand the application prospects of lamellar materials in the field of sodium storage.

[0027] 3. In one embodiment of the present invention, preferably, sublimed sulfur is used as the sulfur source. The sublimed sulfur exists in the form of sulfur atoms on the surface or in the lattice of MXene, and further increases the interlayer spacing of MXene during subsequent recrystallization, thereby forming more wrinkles; the sulfur atom doping forms chemical bonds with surrounding atoms, which will improve the electronic conductivity of MXene. By changing the electron cloud distribution of the material through the lone pair electrons of sulfur atoms, the electron transport channels are increased; the presence of sulfur atoms can form weak interactions with sodium ions, enabling sodium ions to find suitable positions more quickly, thereby reducing the activation energy of the reaction and allowing the electrode reaction to proceed at a lower overpotential; it can form chemical bonds with the transition metals of MXene itself to enhance the structural stability of the material and reduce the dissolution tendency of transition metal atoms in the electrolyte. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, obtaining other drawings based on these drawings still belongs to the scope of the present invention.

[0029] Figure 1 It is the preparation process of the sulfur-doped three-dimensional wrinkled Mxene material of the present invention;

[0030] Figure 2 They are (a-c) scanning electron microscope images of MXene; (d-f) scanning electron microscope images of S-3DMXene; (g) high-resolution transmission electron microscope image of S@3DMXene; (h) high-resolution transmission electron microscope image of S-3DMXene;

[0031] Figure 3 They are (a-b) EDX element maps of S@3DMXene; (c-d) EDX element maps of S-3DMXene; (e) TGA and DTG curves of MXene and S-3DMXene; (f-g) adsorption-desorption curves and pore size distribution curves of MXene and S-3DMXene;

[0032] Figure 4 They are (a) galvanostatic charge / discharge curve of S-3DMXene; (b) galvanostatic charge / discharge curve of MXene; (c) cycling performance of S-3DMXene and Mxene at 1.0 A g -1 ; (d) current density obtained by S-3DMXene and Mxene in the voltage window of 0.01 - 3V ranges from 0.1 A g -1 to 10 A g -1Rate performance; (e) The current density obtained by S-3DMXene in a voltage window of 0.01 - 3 V ranges from 0.1 A g -1 to 10 A g -1 galvanostatic charge-discharge curves;

[0033] Figure 5 For 3DMXene, (a) CV curves of the cathode at different scan rates; (b) Pseudocapacitance contribution rate at 1.0 mV s -1 ; (c) Capacitance contribution rate at 0.2 - 1.0 mV s -1 ; (d) Linear relationship between peak current and scan rate; (e) Impedance spectrogram. Detailed implementation mode

[0034] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0035] Example 1

[0036] This embodiment is a sulfur-doped three-dimensional wrinkled MXene material, and its preparation method is as follows:

[0037] S1. Mix 17.5 ml of concentrated hydrochloric acid (37%) with 2.5 ml of water to obtain a hydrochloric acid solution. Add 1 g of MAX and 1 g of LiF to the hydrochloric acid solution, stir at room temperature for 24 h, then centrifuge and wash until neutral, and ultrasonicate for 1 h to obtain the supernatant, thus obtaining a two-dimensional MXene sheet suspension;

[0038] S2. Dissolve 700 mg of sublimed sulfur in 10 mL of ethylenediamine, and add it dropwise to 100 mg of the Mxene sheet suspension and stir evenly to obtain a mixed solution;

[0039] S3. Dropwise add 10% dilute hydrochloric acid to the mixed solution until the solution becomes clear. After the recrystallization of sulfur particles, obtain an MXene complex containing coated sulfur nanoparticles, centrifuge, wash, and dry, denoted as S@3DMXene;

[0040] S4. Anneal S@3DMXene in a tubular furnace at 400 °C for 2 h under an Ar atmosphere to remove excess sulfur particles, and obtain a sulfur atom-doped three-dimensional wrinkled MXene material, denoted as S-3DMXene.

[0041] Comparative Example 1

[0042] This comparative example is a two-dimensional MXene material, and its preparation method is as follows:

[0043] S1. Mix 17.5 ml of concentrated hydrochloric acid (37%) with 2.5 ml of water to prepare a hydrochloric acid solution. Add 1 g of MAX and 1 g of LiF to the hydrochloric acid solution, stir at room temperature for 24 h, then centrifuge and wash until neutral, and ultrasonicate for 1 h to obtain the supernatant, thus getting a two-dimensional MXene sheet suspension.

[0044] Example 1 adopts a simple and rapid template-free method to prepare a sulfur-doped three-dimensional wrinkled MXene structure. Dissolve sublimed sulfur in ethylenediamine to form an open-chain alkylamine polysulfide as shown in the following formula (1). Drop the above suspension into the MXene suspension, and the open-chain alkylamine polysulfide intercalates into the MXene sheets. Since the alkylamine ions in the sulfur-containing suspension can have electrostatic interactions with the negatively charged ions on the MXene surface, it promotes the cross-linking of MXene to form a three-dimensional wrinkled porous shape (3DMXene). Subsequently, add acid to provide hydrogen ions, which promotes the recrystallization of the polysulfide into sulfur nanoparticles in the pores of 3DMXene, as shown in the following formula (2), to form a composite of MXene nanosheets coated with sulfur particles. Finally, introduce sulfur atoms into the MXene framework through a high-temperature annealing process while removing the excess sulfur particles, thereby forming sulfur-doped three-dimensional porous MXene.

[0045] 2H2NCH2CH2NH2 + S x →(H2NCH2CH2NH3 + )(H2NCH2CH2NH - S x - )(1)

[0046]

[0047] From Figure 2 the scanning electron microscope images from a - b, it is observed that the MXene prepared in Comparative Example 1 shows a relatively flat two-dimensional sheet stacking morphology. Figure 2 c shows that the calculated interlayer spacing of the nanosheets is about 1.35 nm, which is the theoretical thickness of the MXene sheets.

[0048] Figure 2 d - f show the three-dimensional wrinkled structure of MXene after removing sulfur particles prepared in Example 1. It can be clearly seen that the stacking of the MXene two-dimensional sheets has been well improved, the stacked sheets are opened, and the previous interlayer attachment sites are exposed. Figure 2 g shows the morphology of S@3DMXene with MXene wrapping sulfur particles of nearly 600 nm, proving that the open-chain alkylamine polysulfide has intercalated into the MXene sheets and can achieve in-situ recrystallization. Figure 2 h is the wrinkled sheet of 3DMXene after desulfurization.

[0049] The distribution of sulfur element in S@3DMXene and S-3DMXene materials was obtained by energy-dispersive X-ray spectroscopy of transmission electron microscopy. Figure 3 b shows the coating state of 3DMXene on sulfur particles. Figure 3 d shows that after desulfurization, sulfur elements are evenly distributed on the three-dimensional wrinkled lamellae. The distribution of S basically coincides with the morphology of the wrinkled layer, proving the successful doping of sulfur elements. The uniform doping of S on the material surface can effectively reduce the energy barrier for sodium deintercalation and intercalation, thereby improving the electrochemistry reaction kinetics and alleviating polarization.

[0050] Figure 3 The thermogravimetric curve in e reveals the mass change of 3DMXene at different temperatures in an oxygen atmosphere. Specifically, before 400 °C, the MXene curve has a depression while the S-3DMXene does not, and at the same time, the oxidation temperature of S-3DMXene also increases. The reason for these phenomena may be that there is partial crystal water in the self-stacked lamellae of MXene, while the crystal water in MXene is indirectly removed during the sulfidation process of S-3DMXene. The increase in the mass of MXene at 452 °C may be due to the oxidation of Ti in MXene to form TiO2. After that, as the temperature continues to rise, the mass decreases because part of the carbon in MXene is oxidized into carbon dioxide and escapes. For S-3DMXene, on the one hand, the mass loss is caused by the oxidation of sulfur to form sulfur oxides, and on the other hand, the formation of titanium dioxide causes the mass to increase. Therefore, the thermogravimetric curve of 3DMXene shows a slow upward process at 452-538 °C. It should be noted that the thermogravimetric curve of 3DMXene shows a cliff-like drop at 541 °C due to the formation of CO2 / CO gas from the carbon in MXene and oxygen. The mass of S-3DMXene is lower than that of MXene at 700 °C due to the loss of S atoms. These results prove the successful doping of sulfur atoms into MXene.

[0051] Figure 3 The adsorption and desorption curves measured in f are completely consistent with the characteristics of layered structure materials, showing no obvious hysteresis loop and a higher adsorption capacity in the region of relatively high relative pressure. It can be obtained through testing that the specific surface area of 3DMXene increases from 7 m 2 g -1 to 20 m 2 g -1Since MXene has the typical characteristics of two-dimensional materials, there is a strong van der Waals force between adjacent nanosheets, which causes MXene nanosheets to tend to self-stack, thus exhibiting a small specific surface area. On the contrary, 3D MXene forms a wrinkled shape, which inhibits the self-stacking phenomenon and results in a large specific surface area. The large specific surface area provides more active sites, which is beneficial to improving the electrochemical performance. Figure 3 g is the pore size distribution diagram of the two materials. Compared with the pore size distribution of MXene, the contribution rate of mesopores in the 3D MXene material is higher, thus providing a fast channel for ion transport and helping to improve the electrochemical performance of the battery.

[0052] The electrochemical performance of 3D MXene was further studied. As can be seen from Figure 4 (a, b), S-3D MXene has a higher initial charge / discharge capacity than MXene. Among them, the initial charge / discharge capacities of S-3D MXene and MXene are 423 / 270 mAh g -1 and 350 / 167 mAh g -1 respectively. The initial Coulombic efficiency (ICE) of S-3D MXene is 64%, which is relatively higher than the 45% initial Coulombic efficiency of MXene. These results can be attributed to the more active sites provided by the wrinkled structure of S-3D MXene, which increases the contact area between the electrode material and the electrolyte, thus improving the reaction efficiency. The wrinkled lamellae can well maintain the ion transport channels during the electrochemical reaction process, reduce the loss of ions during the transport process, and obtain a higher Coulombic efficiency. In addition, 3D MXene also has excellent long-cycle performance, as shown in Figure 4 c. After cycling 1200 times at 1.0 A g -1 , a reversible capacity of 180 mAh g -1 and a stable Coulombic efficiency are obtained. The capacity of MXene is significantly lower than that of S-3D MXene, and it is 73 mAh g -1 at 1200 cycles. Figure 4 d and e are the rate performance and galvanostatic charge-discharge curves obtained at a current density ranging from 0.1 A g -1 to 10 A g -1 in a voltage window of 0.01 - 3 V. The capacities of S-3D MXene at 0.1, 0.2, 0.5, 1, 2, 5, 10, 20 A g -1 are 230, 206, 190, 178, 166, 150, 136, 120 mAh g -1 respectively. As can be seen from Figure 4 d, the capacity of S-3D MXene is higher than that of MXene at different current densities. When the rate is switched back to 0.2 A g -1 , there is still 190 mAh g-1 The capacity shows an upward trend. It can be concluded that S-3DMXene has better rate performance compared to MXene.

[0053] Figure 5 a shows the cyclic voltammetry (CV) curves of 3DMXene at different scan rates. As the scan rate increases, the curves still maintain their original shapes, and the positions of the peaks only shift slightly, indicating that the material has good reversibility and kinetics during cycling. Figure 4 The cathodic (peaks 1 and 2) and anodic peaks (peaks 3 and 4) shown in a enable the main storage mechanism of S-3DMXene to be derived from Equation (3):

[0054] logi = log a + b log v (3)

[0055] where i is the current density, a and b are two variable parameters, and v is the scan rate. The slope of the linear relationship in the log i vs. log v curve ( Figure 5 b) represents the b value, which determines the relative contributions of the above two charge storage mechanisms. When the b value is 0.5, the diffusion-controlled process dominates, while when the b value is close to 1, the capacitive process dominates. The b values of the cathodic and anodic peaks of S-3DMXene are 0.85, 0.75, 0.89, and 0.99, with an average value of 0.87. The b value being closer to 1 indicates that the S-3DMXene electrode has a greater tendency towards pseudocapacitive behavior. The contribution rates of the capacitive and diffusion-controlled processes in the overall electrochemical behavior of the S-3DMXene electrode are further calculated using Equation (4):

[0056] I = k1v + k2 v 1 / 2 (4)

[0057] where k1v and k2 v 1 / 2 represent the capacitive control component and the diffusion control component, respectively. Figure 5 c shows that at a scan rate of 1.0 mV s -1 the capacitive contribution rate of S-3DMXene is approximately 83.7%. Figure 5 It can be seen from c that the capacitive contribution rate increases with the increase of the scan rate, indicating that the reaction mechanism of S-3DMXene is controlled by pseudocapacitance. As is well known, pseudocapacitive behavior usually has fast reaction kinetics, so a larger pseudocapacitive contribution helps the material maintain a high capacity at high rates.

[0058] Figure 5Figure e shows the electrochemical impedance curves of MXene and S-3DMXene, where the slanted line in the low-frequency region corresponds to the diffusion process, the semicircle in the high-frequency region corresponds to the charge transfer resistance, and the intercept represents the ohmic impedance within the battery. It can be seen from the figure that the 3DMXene material has a smaller radius and a larger slope, indicating that the material has a lower charge transfer resistance and ion diffusion resistance, and this pattern still holds even after charge-discharge cycling. These results indicate that a larger specific surface area helps with charge transfer between the electrode and the electrolyte, reduces the charge-discharge resistance of the battery, and improves the rate performance.

[0059] The above-disclosed are only the preferred embodiments of the present invention. Of course, the scope of the rights of the present invention cannot be limited thereby. Therefore, equivalent changes made in accordance with the claims of the present invention still fall within the scope covered by the present invention.

Claims

1. A sulfur-doped three-dimensional wrinkled MXene material, characterized in that, The preparation method comprises the following steps: (1) Dissolve the sulfur source in Compound A and then drop it into the Mxene suspension and stir evenly to obtain a mixed solution. The sulfur source includes elemental sulfur and / or sulfur-containing compounds, and Compound A has an alkyl chain and an amino group; (2) Drop an acid solution into the mixed solution in step (1) to obtain an MXene complex coated with sulfur nanoparticles; (3) Anneal the MXene complex coated with sulfur nanoparticles at a high temperature in a protective atmosphere to obtain a sulfur atom-doped three-dimensional wrinkled MXene material.

2. The sulfur-doped three-dimensional wrinkled MXene material according to claim 1, characterized in that: In step (1), the sulfur source includes sublimed sulfur, and Compound A includes ethylenediamine.

3. A sulfur-doped three-dimensional wrinkled MXene material according to claim 1, wherein: In step (1), the ratio of the sulfur source to Compound A is 0.5 - 5 mol / L.

4. A sulfur-doped three-dimensional wrinkled MXene material according to claim 1, characterized in that: In step (1), the mass ratio of the sulfur source to the Mxene suspension is 4 - 10:

1.

5. A sulfur-doped three-dimensional wrinkled MXene material according to claim 1, characterized in that: In step (2), the concentration of the acid solution is 5 - 15%.

6. The sulfur-doped three-dimensional wrinkled MXene material according to claim 1, wherein: In step (3), the conditions for high-temperature annealing are: 300 - 500 °C, 1 - 3 h.

7. A sulfur-doped three-dimensional wrinkled MXene material according to claim 1, characterized in that: The preparation method of the Mxene suspension comprises the following steps: Add MAX and LiF to the hydrochloric acid solution, stir for 12 - 36 h, centrifuge and wash until neutral, and then ultrasonically obtain the supernatant to obtain a two-dimensional lamellar structure Mxene suspension.

8. A sulfur-doped three-dimensional wrinkled MXene material according to claim 7, characterized in that: The mass ratio of MAX to LiF is 1:0.5 - 2.

9. A sulfur-doped three-dimensional wrinkled MXene material according to claim 7, characterized in that: The concentration of the hydrochloric acid solution is 20 - 40%.

10. The sulfur-doped three-dimensional wrinkled MXene material according to any one of claims 1 - 9 is applied to a sodium-ion battery.

Images