A nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nanosphere composite electrode material, its preparation method and application

Abstract

The application discloses a nitrogen and phosphorus double-doped MXene loaded NiCo2S4 nanohollow sphere composite electrode material and a preparation method and application thereof, and belongs to the technical field of supercapacitors.The composite electrode material provided by the application comprises nitrogen and phosphorus double-doped MXene sheets and NiCo2S4 nanohollow spheres in situ loaded on the surface of the nitrogen and phosphorus double-doped MXene sheets, and the NiCo2S4 nanohollow spheres and the nitrogen and phosphorus double-doped MXene sheets form a sheet-ball composite structure.The sheet-ball composite structure effectively improves the specific capacitance and rate performance of the composite electrode, and the specific capacitance of the composite electrode reaches 1 A·g ‑1 The specific capacitance reaches 3300 F·g ‑1 The specific capacitance reaches 5 A·g ‑1 The retention rate reaches more than 85% after 10000 cycles.

Description

Technical Field

[0001] This invention relates to the field of supercapacitor technology, and in particular to a nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material, its preparation method, and its application. Background Technology

[0002] The improvement of energy density of supercapacitors is highly dependent on the specific capacitance and structural stability of electrode materials. NiCo2S4, as a pseudocapacitive material with both high theoretical capacity and rich redox activity, can provide a large electrochemical active interface through its nano-hollow structure. However, the intrinsic electronic conductivity of this material is poor, and the hollow spherical shell is prone to volume expansion and contraction during repeated charge and discharge processes. This leads to the gradual collapse of the shell structure and the continuous deterioration of the electrical contact between the active material and the conductive substrate, ultimately resulting in rapid capacity decay and insufficient cycle stability.

[0003] To alleviate these problems, researchers have attempted to composite NiCo2S4 with conductive carbon materials or two-dimensional layered materials to construct a conductive network and buffer volumetric strain. However, in non-in-situ composite methods, the two-phase interface lacks sufficient chemical bonding, and the active material easily detaches from the conductive substrate during long-term cycling, leading to a significant decrease in specific capacitance. Furthermore, the inherent low conductivity of the active material means that the overall specific capacitance output of the composite electrode is still far below its theoretical value. Existing composite strategies mostly employ multi-step template methods or high-temperature heat treatment, which involve complex process conditions and make it difficult to achieve uniform and stable loading while maintaining the integrity of the hollow structure. The contradiction between insufficient interfacial bonding strength and deterioration in cycling stability has not been effectively resolved, restricting the full utilization of the specific capacitance of the composite material and its capacitance retention capability under rapid charge and discharge. Summary of the Invention

[0004] In view of this, the present invention provides a nitrogen-phosphorus dual-doped MXene-loaded NiCo2S4 nano-hollow sphere composite electrode material, its preparation method and application, which improves the specific capacitance, rate performance and cycle stability of the composite electrode by in-situ loading NiCo2S4 nano-hollow spheres onto the surface of nitrogen-phosphorus dual-doped MXene sheets to form a sheet-sphere composite structure.

[0005] In a first aspect, the present invention provides a nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material, which includes a nitrogen-phosphorus dual-doped MXene sheet and NiCo2S4 nano-hollow spheres in situ supported on the surface of the nitrogen-phosphorus dual-doped MXene sheet, wherein the NiCo2S4 nano-hollow spheres and the nitrogen-phosphorus dual-doped MXene sheet constitute a sheet-sphere composite structure.

[0006] Secondly, this invention provides a method for preparing the above-mentioned nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material, comprising the following steps: MXene was mixed with diammonium hydrogen phosphate in water and then subjected to a hydrothermal reaction to obtain nitrogen-phosphorus dual-doped MXene. The nitrogen-phosphorus dual-doped MXene was mixed and reacted with cobalt salt and 2-methylimidazole in an organic solvent to obtain ZIF-67@NP-MXene composite material; The ZIF-67@NP-MXene composite material was mixed with nickel salt in an alcohol solvent and reacted to obtain the NiCo-LDH@NP-MXene composite material; The NiCo-LDH@NP-MXene composite material is obtained by mixing it with thioacetamide in an alcohol solvent and then carrying out a solvothermal vulcanization reaction.

[0007] Preferably, the mass ratio of MXene to diammonium hydrogen phosphate is 1:(40~100); the hydrothermal reaction temperature is 120~180℃ and the time is 6~24 h.

[0008] Preferably, the cobalt salt is at least one of cobalt nitrate, cobalt chloride, or cobalt sulfate, and the molar ratio of 2-methylimidazole to the cobalt salt is (4~12):1.

[0009] Preferably, the nickel salt is at least one of nickel nitrate, nickel chloride, or nickel sulfate, and the mass ratio of the ZIF-67@NP-MXene composite material to the nickel salt is 1:(0.8~1.2).

[0010] Preferably, the temperature of the mixing reaction in the alcohol solvent is 40~80℃ and the time is 1~6 h.

[0011] Preferably, the temperature of the solvothermal sulfidation reaction is 100~140℃ and the time is 4~12 h.

[0012] Preferably, the mass ratio of the NiCo-LDH@NP-MXene composite material to thioacetamide is 1:(0.8~1.2).

[0013] Preferably, the organic solvent is methanol or ethanol, and the alcohol solvent is ethanol.

[0014] Thirdly, the present invention provides the application of the above-mentioned nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material or the nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material prepared by the above preparation method in supercapacitors, wherein the composite electrode material is used as the positive electrode material of supercapacitors.

[0015] Compared with the prior art, the present invention has achieved the following beneficial effects: The nitrogen-phosphorus dual-doped MXene-loaded NiCo2S4 hollow nanosphere composite electrode material provided by this invention involves in-situ loading NiCo2S4 hollow nanospheres onto the surface of nitrogen-phosphorus dual-doped MXene sheets to form a sheet-sphere composite structure. The nitrogen-phosphorus dual-doped MXene, as a conductive framework, can provide a continuous electron transport path for the composite electrode, effectively compensating for the poor intrinsic electronic conductivity of NiCo2S4. At the same time, the MXene sheets anchor and buffer the NiCo2S4 hollow nanospheres, which can suppress the volume expansion and structural collapse of the hollow spheres during repeated charge and discharge processes, thus helping to maintain the structural integrity and cycle stability of the electrode material. In-situ loading fosters a strong chemical bond between the two phases, preventing the active material from detaching from the conductive substrate during long-term cycling, a problem common in non-in-situ composites, and ensuring the continuous effectiveness of interfacial charge transfer. The sheet-sphere composite structure also fully exposes the MXene sheet surface to the electrolyte, providing an open transport channel for ion diffusion. Simultaneously, the hollow structure of the NiCo2S4 nanospheres provides ample redox-active interfaces. These two factors synergistically maximize the utilization of pseudocapacitive reactive sites, effectively improving the specific capacitance and rate performance of the composite electrode (1 A·g). -1 The specific capacitance reaches 3300 F·g -1 Above, 5 A·g -1 After 10,000 cycles, the retention rate reaches over 85%. Attached Figure Description

[0016] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0017] Figure 1 These are scanning electron microscope (SEM) images and elemental distribution maps of NP-MXene and MXene in Embodiment 1 of the present invention; wherein, (a) is a scanning electron microscope image of MXene, (b) is a scanning electron microscope image of NP-MXene, and (c) is an elemental distribution map of NP-MXene.

[0018] Figure 2 These are the X-ray diffraction patterns of NP-MXene and MXene in Embodiment 1 of the present invention.

[0019] Figure 3These are scanning electron microscope (SEM) images, transmission electron microscope (TEM) images, and elemental distribution maps of H-NCS and H-NCS@NP-MXene in Comparative Example 1 and Example 1 of this invention; wherein, (a) is a scanning electron microscope (SEM) image of H-NCS, (b) is a scanning electron microscope (SEM) image of H-NCS@NP-MXene, (c) is a TEM image of H-NCS, (d) is a TEM image of H-NCS@NP-MXene, (e) is a high-resolution TEM image of H-NCS@NP-MXene, (f) is a selected area electron diffraction (SED) pattern of H-NCS@NP-MXene, and (g) is an elemental distribution map of H-NCS@NP-MXene.

[0020] Figure 4 These are the X-ray diffraction patterns of H-NCS in Comparative Example 1, H-NCS@MXene in Comparative Example 2, and H-NCS@NP-MXene in Example 1 of this invention.

[0021] Figure 5 These are comparison graphs of the electrochemical performance of NP-MXene and MXene in Example 1 and Comparative Example 3 of the present invention; wherein, (a) is a comparison graph of cyclic voltammetry curves at a scan rate of 20 mV / s, and (b) is a comparison graph of galvanostatic charge-discharge curves at a current density of 1 A / g.

[0022] Figure 6 These are comparison graphs of the electrochemical performance of H-NCS, H-NCS@MXene, H-NCS@NP-MXene, MXene, and NP-MXene in Examples 1, Comparative Examples 1 and 2 of the present invention; wherein, (a) is a comparison graph of cyclic voltammetry curves at a scan rate of 20 mV / s, and (b) is a comparison graph of galvanostatic charge-discharge curves at a current density of 1 A / g.

[0023] Figure 7 The graphs show the rate performance of H-NCS@NP-MXene under different current densities in Embodiment 1 of the present invention. (a) is a comparison graph of constant current charge and discharge curves under different current densities; (b) is a comparison graph of specific capacity under different current densities.

[0024] Figure 8 These are the electrochemical impedance spectra and cycle stability diagrams of H-NCS, H-NCS@MXene, and H-NCS@NP-MXene in Example 1 and Comparative Examples 1-2 of the present invention; wherein, (a) is the Nyquist impedance spectrum, and (b) is the capacity retention diagram after 10,000 charge-discharge cycles at a current density of 5 A / g.

[0025] Figure 9These are performance graphs of the H-NCS@NP-MXene / / AC asymmetric hybrid supercapacitor in Example 1 of this invention; wherein, (a) is a comparison graph of cyclic voltammetry curves of activated carbon and H-NCS@NP-MXene at a scan rate of 20 mV / s, (b) is a graph of cyclic voltammetry curves at different scan rates, (c) is a graph of constant current charge-discharge curves at different current densities, (d) is a Nyquist impedance spectrum, (e) is a graph of capacity retention after 10,000 charge-discharge cycles at a current density of 5 A / g, and (f) is a Ragone graph (energy density-power density relationship graph). Detailed Implementation

[0026] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0027] This invention provides a nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material, which includes nitrogen-phosphorus dual-doped MXene sheets and NiCo2S4 nano-hollow spheres in situ supported on the surface of the nitrogen-phosphorus dual-doped MXene sheets. The NiCo2S4 nano-hollow spheres and the nitrogen-phosphorus dual-doped MXene sheets constitute a sheet-sphere composite structure.

[0028] In a preferred embodiment of the present invention, the interlayer spacing of the nitrogen-phosphorus dual-doped MXene is 1.2~1.8 nm, the average particle size of the NiCo2S4 hollow nanospheres is 200~800 nm, and the NiCo2S4 hollow nanospheres are uniformly distributed on the surface and edges of the nitrogen-phosphorus dual-doped MXene sheets.

[0029] NiCo2S4, a pseudocapacitive material possessing both high theoretical capacity and abundant redox activity, offers a large electrochemically active interface due to its hollow nanostructure. However, this material exhibits poor intrinsic electronic conductivity, and the hollow shell is prone to volume expansion and contraction during repeated charge-discharge cycles, leading to gradual shell collapse and continuous deterioration of the electrical contact between the active material and the conductive substrate. Ultimately, this results in rapid capacity decay and insufficient cycle stability. This invention addresses this issue by in-situ loading NiCo2S4 hollow nanospheres onto the surface of nitrogen-phosphorus dual-doped MXene sheets. The nitrogen-phosphorus dual-doped MXene serves as a conductive framework, providing a continuous electron transport path for the composite electrode and compensating for the poor conductivity of NiCo2S4. Simultaneously, the MXene sheets anchor and buffer the NiCo2S4 hollow nanospheres, suppressing volume expansion and structural collapse during charge-discharge processes, thus maintaining the structural integrity and cycle stability of the electrode material. In-situ loading fosters a strong chemical bond between the two phases, preventing the active material from detaching from the conductive substrate during long cycles, a problem common in non-in-situ composites, and ensuring the continued effectiveness of interfacial charge transfer. The sheet-sphere composite structure fully exposes the MXene sheet surface to the electrolyte, providing an open transport channel for ion diffusion. Simultaneously, the hollow structure of the NiCo2S4 nanospheres provides ample redox-active interfaces. These two elements work synergistically to maximize the utilization of pseudocapacitive reactive sites, thereby effectively improving the specific capacitance, rate performance, and cycle stability of the composite electrode.

[0030] This invention also provides a method for preparing the above-mentioned nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material, comprising the following steps: MXene was mixed with diammonium hydrogen phosphate in water and then subjected to a hydrothermal reaction to obtain nitrogen-phosphorus dual-doped MXene. The nitrogen-phosphorus dual-doped MXene was mixed and reacted with cobalt salt and 2-methylimidazole in an organic solvent to obtain ZIF-67@NP-MXene composite material; The ZIF-67@NP-MXene composite material was mixed with nickel salt in an alcohol solvent and reacted to obtain the NiCo-LDH@NP-MXene composite material; The NiCo-LDH@NP-MXene composite material was mixed with thioacetamide in an alcohol solvent and then subjected to a solvothermal sulfidation reaction to obtain the nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material.

[0031] The method described in this invention uses MXene as a raw material. First, nitrogen and phosphorus co-doping is achieved through a diammonium hydrogen phosphate-assisted hydrothermal reaction, resulting in nitrogen-phosphorus co-doped MXene with increased interlayer spacing, more active sites, and improved conductivity. Subsequently, the abundant functional groups and defect sites on the surface of the nitrogen-phosphorus co-doped MXene serve as nucleation anchors, allowing ZIF-67 to grow in situ on the MXene sheet surface. Cobalt ions are uniformly anchored on the sheet surface through coordination with the functional groups on the MXene surface, inducing heterogeneous nucleation and growth of ZIF-67. This method avoids the complex operation of pre-synthesizing templates required in traditional dual-template methods. After nickel ion exchange, ZIF-67 is converted into NiCo-layered bimetallic hydroxide. Because Ni... 2+ With Co 2+ During ion exchange, the difference in diffusion rates between the interior and exterior results in a Kirkendall effect-like effect, giving the product a hollow structure. Finally, a solvothermal sulfidation reaction is performed using a sulfur source provided by thioacetamide. This sulfidation process converts NiCo-LDH to NiCo2S4 while preserving the hollow spherical morphology of the precursor, ultimately yielding a composite structure of NiCo2S4 hollow nanospheres in situ supported on the surface of nitrogen-phosphorus dual-doped MXene sheets. This method requires no high-temperature calcination throughout, with a maximum reaction temperature not exceeding 180℃. The process conditions are mild, the operation steps are simple, and it is conducive to large-scale preparation.

[0032] In a preferred embodiment, the mass ratio of MXene to diammonium hydrogen phosphate is 1:(40~100). Diammonium hydrogen phosphate participates in the hydrothermal reaction as both a nitrogen and phosphorus source. Under high temperature and pressure conditions, the nitrogen- and phosphorus-containing active groups generated by the decomposition of diammonium hydrogen phosphate can undergo substitution reactions with the end groups on the MXene surface or insert into the interlayer of MXene. Typical but non-limiting mass ratios of MXene to diammonium hydrogen phosphate are 1:40, 1:45, 1:50, 1:55, 1:60, 1:65, 1:70, 1:75, 1:80, 1:85, 1:90, 1:95, or 1:100, preferably 1:(60~80). If the amount of diammonium hydrogen phosphate is too low, the doping will be insufficient and the interlayer spacing will be limited; if the amount of diammonium hydrogen phosphate is too high, excessive impurities may be introduced or the MXene structure may be damaged.

[0033] In a preferred embodiment, the hydrothermal reaction temperature is 120~180℃, for example, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃, 155℃, 160℃, 165℃, 170℃, 175℃, or 180℃, preferably 130~150℃. The hydrothermal reaction time is 6~24 h, for example, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h, or 24 h, preferably 10~14 h. Under the above hydrothermal conditions, diammonium hydrogen phosphate can fully decompose and react with MXene, achieving effective doping of nitrogen and phosphorus elements. After the hydrothermal reaction is completed, the product is repeatedly washed with deionized water and centrifuged until the pH of the supernatant drops to 6~7 to remove residual diammonium hydrogen phosphate and reaction byproducts, obtaining a nitrogen and phosphorus co-doped MXene suspension. This washing process helps to avoid the adverse effects of residual impurities on subsequent composite reactions and electrochemical performance.

[0034] In a preferred embodiment, the cobalt salt is at least one of cobalt nitrate, cobalt chloride, or cobalt sulfate, preferably cobalt nitrate. The molar ratio of 2-methylimidazole to the cobalt salt is (4~12):1. 2-methylimidazole acts as an organic ligand, and the cobalt salt provides the metal nodes; the two coordinate self-assemble in the solvent to form ZIF-67 crystals. The molar ratio of 2-methylimidazole to the cobalt salt can be, for example, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, or 12:1, preferably (6~10):1. If the proportion of 2-methylimidazole is too low, the cobalt ions are not sufficiently coordinated, resulting in low crystallinity of ZIF-67; if the proportion of 2-methylimidazole is too high, there will be too many free ligands in the reaction system, which may inhibit the preferential nucleation of ZIF-67 on the MXene surface. Nitrogen-phosphorus dual-doped MXene was mixed with cobalt salt and 2-methylimidazole in an organic solvent and reacted with stirring at room temperature for a typical time of 4–12 h, preferably 6–10 h. After the reaction, the mixture was centrifuged, washed, and dried to obtain the ZIF-67@NP-MXene composite material. The preferred reaction temperature in this step is 20–40 °C, as this temperature range is beneficial for the controlled nucleation and growth of ZIF-67 crystals on the surface of the MXene sheets.

[0035] In a preferred embodiment, the nickel salt is at least one of nickel nitrate, nickel chloride, or nickel sulfate, preferably nickel nitrate. The mass ratio of the ZIF-67@NP-MXene composite to the nickel salt is 1:(0.8~1.2). This mass ratio is typically, but not limited to, for example, 1:0.8, 1:0.85, 1:0.9, 1:0.95, 1:1.0, 1:1.05, 1:1.1, 1:1.15, or 1:1.2, preferably 1:(0.9~1.1). The amount of nickel salt added determines the Co content in ZIF-67. 2+ Be Ni 2+ The degree of substitution affects the composition and electrochemical performance of the final product. If the amount of nickel salt is too low, the nickel content in NiCo2S4 will be insufficient, resulting in a reduction of active sites; if the amount of nickel salt is too high, it may destroy the original ZIF-67 framework structure, leading to morphological collapse.

[0036] In a preferred embodiment, the reaction temperature of the ZIF-67@NP-MXene composite material with the nickel salt in an alcohol solvent is 40~80℃, for example, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃ or 80℃, preferably 50~70℃. The reaction time is 1~6 h, for example, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h or 6 h, preferably 2~4 h. Under these reaction conditions, nickel ions gradually replace cobalt ions in ZIF-67 through an ion exchange process, while Ni... 2+ and Co 2+ Due to the different diffusion rates within the ZIF-67 framework, cavities are formed inside the particles, transforming them into NiCo-layered bimetallic hydroxides with a hollow structure. After the reaction, NiCo-LDH@NP-MXene powder is obtained through centrifugation, washing, and vacuum drying.

[0037] In a preferred embodiment, the mass ratio of NiCo-LDH@NP-MXene composite material to thioacetamide is 1:(0.8~1.2), for example, it can be 1:0.8, 1:0.85, 1:0.9, 1:0.95, 1:1.0, 1:1.05, 1:1.1, 1:1.15 or 1:1.2, preferably 1:(0.9~1.1). Thioacetamide, as a sulfur source, decomposes under hydrothermal conditions to release sulfur (S). 2- Ni in NiCo-LDH 2+ and Co 2+The reaction produces NiCo2S4. If too little thioacetamide is used, the sulfidation will be incomplete, and unconverted LDH phase will remain in the product; if too much thioacetamide is used, excessive sulfur impurities may be introduced.

[0038] In a preferred embodiment, the solvothermal sulfidation reaction temperature is 100~140℃, for example, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃ or 140℃, preferably 110~130℃. The solvothermal sulfidation reaction time is 4~12 h, for example, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h or 12 h, preferably 5~8 h. Under these sulfidation reaction conditions, NiCo-LDH is converted to NiCo2S4, while maintaining the hollow spherical morphology of the precursor, avoiding the collapse of the hollow sphere structure due to excessively high reaction temperature or excessively long reaction time. After the reaction, the material is centrifuged, washed, and vacuum dried (typically at 50~70℃, preferably 60℃) to obtain a nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material.

[0039] In a preferred embodiment, the organic solvent is methanol or ethanol, and the alcohol solvent is ethanol. Methanol or ethanol can effectively dissolve cobalt salts, 2-methylimidazole, and nickel salts, while also facilitating the uniform nucleation and growth of ZIF-67 on the MXene surface. Ethanol, as a medium for nickel salt exchange and sulfidation reactions, has advantages such as moderate boiling point, low toxicity, and high volatility, facilitating subsequent washing and drying.

[0040] In a preferred embodiment, the above method further includes: after the solvothermal sulfidation reaction, cooling the reaction product to room temperature, repeatedly washing it with deionized water and anhydrous ethanol, separating it by centrifugation, and then vacuum drying it at 50~70°C to obtain the final product.

[0041] This invention also provides the application of the aforementioned nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nanosphere composite electrode material in supercapacitors, whereby the composite electrode material is used as the positive electrode material. Because this composite electrode material possesses high specific capacitance, excellent rate performance, and good cycle stability, its application as a positive electrode material in supercapacitors can significantly improve the energy density and power density of the device.

[0042] This invention also provides a supercapacitor comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises the aforementioned nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material. Preferably, the negative electrode is an activated carbon electrode, and the electrolyte is a KOH aqueous solution, such as a 3 M KOH aqueous solution. This supercapacitor, by employing the aforementioned composite electrode material as the positive electrode, achieves high energy density and power density, and exhibits excellent cycle stability.

[0043] It should be noted that the specific process parameters, operating conditions, and raw material types described in the above specific embodiments are merely illustrative examples. Those skilled in the art can make reasonable selections and adjustments within the scope of the technical solution of this invention according to actual needs, and all of these fall within the protection scope of this invention.

[0044] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0045] Example 1 This embodiment provides a nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material (H-NCS@NP-MXene) and its preparation method, including the following steps: (1) Preparation of NP-MXene: 150 mg of MXene powder was dispersed in 40 mL of deionized water, and 10 g of diammonium hydrogen phosphate (the mass ratio of MXene to diammonium hydrogen phosphate was 1:66.7) was added. The mixture was stirred at room temperature for 2 h to obtain a uniform dispersion. The dispersion was transferred to a high-pressure reactor and hydrothermally reacted at 140 °C for 12 h. After the reaction, the mixture was repeatedly washed with deionized water and centrifuged until the pH of the supernatant dropped to 6, thus obtaining a nitrogen-phosphorus co-doped MXene suspension, denoted as NP-MXene.

[0046] (2) Preparation of ZIF-67@NP-MXene: 2 mmol cobalt nitrate and 16 mmol 2-methylimidazole were dissolved in 30 mL methanol. After stirring for 10 min, 5 mL of the NP-MXene dispersion prepared in step (1) was added. After thorough stirring, the mixture was reacted at room temperature (about 25 °C) to allow ZIF-67 to grow in situ on the surface of the NP-MXene sheets. After the reaction was completed, the mixture was centrifuged and dried at 60 °C to obtain ZIF-67@NP-MXene powder.

[0047] (3) Preparation of NiCo-LDH@NP-MXene: 0.2 g of nickel nitrate was dissolved in 40 mL of ethanol, and 0.2 g of ZIF-67@NP-MXene powder obtained in step (2) was added. The mixture was stirred in a water bath at 60 °C for 2 h to convert ZIF-67 into NiCo-layered bimetallic hydroxide. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 10 min, the precipitate was collected and dried under vacuum to obtain NiCo-LDH@NP-MXene powder.

[0048] (4) Preparation of H-NCS@NP-MXene: 0.1 g of thioacetamide was dissolved in 30 mL of ethanol, and 0.1 g of NiCo-LDH@NP-MXene powder prepared in step (3) was added. After stirring for 10 min, the mixture was transferred to a high-pressure reactor and subjected to solvothermal sulfidation at 120 °C for 6 h. After the reaction was completed, the mixture was cooled to room temperature, centrifuged at 8000 rpm for 10 min, the precipitate was collected and dried under vacuum at 60 °C to obtain the nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material, denoted as H-NCS@NP-MXene.

[0049] like Figure 1 As shown in (a), MXene exhibits a unique few-layer lamellar stacked structure with relatively regular interlayer arrangement; while after nitrogen and phosphorus doping modification ( Figure 1 In (b) of the sample, the NP-MXene sample still retains this few-layered lamellar structure, and the layered characteristics have not changed significantly. This phenomenon indicates that the doping of N and P elements has not affected the original two-dimensional layered structure of MXene and has maintained the structural integrity of the material. Figure 1 The elemental distribution diagram in (c) shows that N and P elements are uniformly distributed on the sheets and edge regions of NP-MXene. This phenomenon further confirms that N and P heteroatoms have been successfully doped into MXene material, and the doping process has good uniformity, laying a structural foundation for the subsequent improvement of material performance. Using the hydrothermal method in step (1) to achieve nitrogen and phosphorus doping of MXene can well maintain the structural integrity of MXene.

[0050] X-ray diffraction was used to test the phase composition of both MXene and NP-MXene samples. Figure 2As shown, the diffraction peaks at 2θ = 8.5°, 19.2°, 34.0°, 41.8°, and 60.2° correspond to the (002), (004), (101), (105), and (110) crystal planes of MXene, respectively. The (002) diffraction peaks of the MXene and NP-MXene samples are located at 8.5° and 5.8°, respectively, corresponding to interlayer spacings of 1.05 nm and 1.51 nm. The (002) diffraction peak of NP-MXene shows significant broadening, which is due to the formation of more vacancies and defects within the material after heteroatom doping into MXene.

[0051] Comparative Example 1 This comparative example provides the preparation of pure NiCo2S4 hollow nanospheres (H-NCS).

[0052] In step (2) of Example 1, no NP-MXene dispersion was added; instead, 2 mmol of cobalt nitrate and 16 mmol of 2-methylimidazole were dissolved in 30 mL of methanol, stirred for 10 min, and reacted at room temperature. The remaining steps were exactly the same as steps (3) and (4) of Example 1, and NiCo2S4 hollow nanospheres, denoted as H-NCS, were finally obtained.

[0053] Comparative Example 2 This comparative example provides the preparation of pristine MXene-supported NiCo2S4 hollow nanospheres (H-NCS@MXene).

[0054] The NP-MXene dispersion in step (1) of Example 1 was replaced with the original MXene dispersion of the same concentration, i.e., no nitrogen-phosphorus dual doping treatment was performed. The remaining steps were exactly the same as in Example 1, and the original MXene-supported NiCo2S4 nano-hollow sphere composite electrode material was finally obtained, denoted as H-NCS@MXene.

[0055] Comparative Example 3 In this comparative example, diammonium hydrogen phosphate ((NH4)2HPO4) in step (1) of Example 1 was replaced with the same mass of ammonium dihydrogen phosphate (NH4H2PO4). In step (1), nitrogen-phosphorus dual-doped MXene was prepared, denoted as NP-MXene (ammonium dihydrogen phosphate).

[0056] Test case 1. Morphological characteristics The microstructure of the H-NCS in Comparative Example 1 and the H-NCS@NP-MXene composite sample in Example 1 was systematically characterized using scanning electron microscopy. Figure 3In (a), it can be clearly observed that the prepared H-NCS exhibits a regular nanosphere structure with good size uniformity and an average diameter of approximately 500 nm. When H-NCS is combined with NP-MXene, the... Figure 3 As can be seen in (b), H-NCS nanospheres are uniformly attached to the surface and edges of NP-MXene sheets, forming a stable composite structure; at the same time, slight adhesion can be observed between some nanospheres, and even agglomeration can be observed to form larger spherical aggregates.

[0057] Depend on Figure 3 As shown in the transmission electron microscopy (TEM) images (c) and (d), the H-NCS nanospheres have a distinct hollow structure. After being combined with MXene, the H-NCS nanospheres adhere to the surface of the layered NP-MXene, forming an effective composite structure. Figure 3 The high-resolution transmission electron microscopy image in (e) shows three distinct lattice fringe spacings of 0.33 nm, 0.23 nm and 0.28 nm, corresponding to the (400), (220) and (311) crystal planes of NiCo2S4. Figure 3 (f) is the selected area electron diffraction pattern of the H-NCS@NP-MXene composite material. This pattern exhibits clear diffraction rings corresponding to the (111), (220), and (311) crystal planes, indicating that the sample possesses polycrystalline properties. The elemental distribution of the H-NCS@NP-MXene composite material is as follows: Figure 3 As shown in (g), Co, Ni, S, Ti, C, N, and P elements are uniformly distributed throughout the composite material, with Ni, Co, and S elements mainly concentrated in the nano-hollow sphere region. In summary, the hydrothermal method can successfully achieve the composite of H-NCS nano-hollow spheres and layered NP-MXene.

[0058] 2. X-ray diffraction (XRD) characterization X-ray diffraction was used to analyze the phase information of H-NCS in Comparative Example 1, H-NCS@MXene in Comparative Example 2, and H-NCS@NP-MXene in Example 1. Figure 4 As shown, the characteristic diffraction peaks of H-NCS@NP-MXene correspond to the NiCo2S4 phase (NiCo2S4-PDF #20-0782), where the diffraction peaks with 2θ values ​​of 26.83°, 31.59°, and 55.33° correspond to the (220), (311), and (440) crystal planes of the NiCo2S4 phase, respectively. Meanwhile, the diffraction peak at 5.8° corresponds to the (002) crystal plane of NP-MXene, further demonstrating the successful composite of H-NCS and NP-MXene.

[0059] 3. Electrochemical testing The electrochemical performance of the electrode materials prepared in Example 1 and Comparative Examples 1-3 was tested using a three-electrode system. The electrolyte was a 3 M KOH aqueous solution. The working electrode was prepared as follows: active material, conductive carbon black, and polyvinylidene fluoride were mixed at a mass ratio of 8:1:1, and an appropriate amount of N-methylpyrrolidone was added and ground into a uniform slurry. This slurry was then coated onto the surface of a nickel foam current collector (coating area 1 cm × 1 cm) and vacuum dried at 60 °C for 12 h. The active material loading was approximately 1.0–1.2 mg / cm³. 2 The Hg / HgO electrode was used as the reference electrode, and the platinum sheet electrode was used as the counter electrode.

[0060] (1) Effect of dopant type on the electrochemical performance of NP-MXene To investigate the effects of different nitrogen and phosphorus source dopants on the doping effect and electrochemical performance of MXene, CV and GCD curves were tested on the NP-MXene samples prepared in Example 1 (with diammonium hydrogen phosphate as dopant) and Comparative Example 3 (with diammonium dihydrogen phosphate as dopant).

[0061] Figure 5 Figure (a) shows the CV curves of MXene and two NP-MXene samples at a scan rate of 20 mV / s. It can be seen that the integral area of ​​the CV curve of NP-MXene obtained with diammonium hydrogen phosphate as a dopant is significantly larger than that of the sample with diammonium hydrogen phosphate as a dopant and the MXene sample, indicating that diammonium hydrogen phosphate doping is more beneficial to improving the capacitance performance of MXene. This is attributed to the more suitable nitrogen-phosphorus ratio in diammonium hydrogen phosphate (the N / P molar ratio in (NH4)2HPO4 is 2:1, while the N / P molar ratio in NH4H2PO4 is 1:1), resulting in a more optimized ratio of nitrogen- and phosphorus-containing active groups released during hydrothermal decomposition, which can more effectively regulate the electronic structure of MXene and introduce more active sites.

[0062] Figure 5 Figure (b) shows the GCD curves of MXene and two NP-MXene samples at a current density of 1 A / g. Calculations show that the specific capacitance of MXene is 184.4 F / g, the specific capacitance of NP-MXene doped with diammonium hydrogen phosphate is 793.5 F / g, and the specific capacitance of NP-MXene doped with ammonium dihydrogen phosphate is 445.3 F / g. This demonstrates that the doping effect of diammonium hydrogen phosphate is significantly better than that of ammonium dihydrogen phosphate.

[0063] (2) Electrochemical performance of composite electrode materials To evaluate the electrochemical performance of the H-NCS@NP-MXene composite electrode material prepared in Example 1, CV, GCD, EIS and cycle stability tests were performed on H-NCS of Comparative Example 1, H-NCS@MXene of Comparative Example 2 and NP-MXene and MXene of Example 1 under the same conditions.

[0064] Figure 6 (a) shows the CV curves of the five electrode materials at a scan rate of 20 mV / s. The CV curves of all five samples show obvious redox peaks, corresponding to Ni in NiCo2S4. 2+ / Ni 3+ and Co 2+ / Co 3+ The redox reactions were observed. Among them, H-NCS@NP-MXene had the largest integral area of ​​the CV curve, followed by H-NCS@MXene, and then H-NCS, indicating that H-NCS@NP-MXene has the highest specific capacitance. This is attributed to the stable sheet-sphere composite structure formed between the NP-MXene sheets and H-NCS hollow nanospheres in H-NCS@NP-MXene. The high conductivity of NP-MXene provides an efficient electron transport channel for the composite material, and the defects and active sites introduced by N and P doping further enhance the electrochemical activity. At the same time, the hollow structure of H-NCS hollow nanospheres provides abundant redox active interfaces, and the synergy of the two maximizes the utilization of pseudocapacitive reactive active sites.

[0065] Figure 6 Figure (b) shows the GCD curves of the five electrode materials at a current density of 1 A / g. All GCD curves exhibit nonlinear characteristics and are accompanied by obvious charge-discharge plateaus, indicating that the electrodes exhibit battery-like behavior, which is consistent with the presence of redox peaks in the CV curves. Calculations show that at a current density of 1 A / g, the specific capacitance of H-NCS is 1415.9 F / g, that of H-NCS@MXene is 2365.5 F / g, and that of H-NCS@NP-MXene is 3340.88 F / g. The specific capacitance of H-NCS@NP-MXene is 2.36 times that of H-NCS and 1.41 times that of H-NCS@MXene, respectively. This result indicates that NP-MXene can more effectively improve the capacitance performance of the composite material compared to virgin MXene. This is attributed to the fact that N and P dual doping imparts a larger interlayer spacing, more active sites, and higher conductivity to MXene, thus more fully leveraging the synergistic effect of the conductive framework on H-NCS.

[0066] The rate performance of the H-NCS@NP-MXene composite electrode material prepared in Example 1 was further tested, such as... Figure 7As shown, the specific capacitances of H-NCS@NP-MXene at current densities of 1, 2, 3, 5, and 10 A / g are 3340.88, 2881.96, 2494.98, 1932.11, and 1315.33 F / g, respectively. Even at the high current density of 10 A / g, the specific capacitance of H-NCS@NP-MXene remains at 1315.33 F / g, demonstrating excellent rate performance.

[0067] Figure 8 (a) shows the Nyquist impedance spectra of three electrode materials: H-NCS@NP-MXene from Example 1, H-NCS from Comparative Example 1, and H-NCS@MXene from Comparative Example 2. The impedance spectra of all three samples consist of a semicircular arc in the high-frequency region and a sloping line in the low-frequency region. H-NCS@NP-MXene has the smallest semicircular arc diameter in the high-frequency region and the largest sloping line slope in the low-frequency region. After fitting, the charge transfer resistance (R0) of H-NCS is... ct The R of H-NCS@MXene is 4.012 Ω. ct The Ω is 3.742, while the R of H-NCS@NP-MXene is... ct The resistance is only 1.032 Ω. The introduction of NP-MXene significantly reduces the charge transfer resistance of the composite electrode. This is due to the fact that N and P dual doping effectively improves the conductivity of MXene, providing a more efficient path for electrons to be transported from the active material to the current collector.

[0068] Figure 8 (b) shows the capacity retention of H-NCS@NP-MXene from Example 1 and H-NCS from Comparative Example 1 after 10,000 charge-discharge cycles at a current density of 5 A / g. H-NCS retained 57.57% of its capacity after 10,000 cycles, while H-NCS@NP-MXene retained 85.81%. The cycling stability of H-NCS@NP-MXene is significantly better than that of H-NCS and H-NCS@NP-MXene. The NP-MXene sheets anchor and buffer the H-NCS hollow nanospheres, effectively suppressing volume expansion and structural collapse during long-term charge-discharge processes, thus maintaining the structural integrity of the electrode materials.

[0069] 4. Performance Testing of Asymmetric Hybrid Supercapacitors To evaluate the potential application value of composite materials as energy storage devices, an H-NCS@NP-MXene / / AC asymmetric hybrid supercapacitor was assembled in a 3M KOH electrolyte with H-NCS@NP-MXene as the positive electrode and commercial activated carbon as the negative electrode. Figure 9(a) shows the cyclic voltammetry curves of activated carbon and H-NCS@NP-MXene at a scan rate of 20 mV / s, indicating that there is good voltage window matching between the positive and negative electrodes. Figure 9 (b) shows the CV curves of the H-NCS@NP-MXene / / AC hybrid supercapacitor at different scan rates. Even at higher scan rates, the shape of these curves remains consistent, indicating that the device has excellent rate performance and good electrochemical reversibility. Figure 9 (c) shows the constant current charge-discharge curves of the device at different current densities. At current densities of 1, 2, 3, 5, and 10 A / g, the specific capacitance of the device is 180.73, 151.43, 139.29, 121.43, and 102.86 F / g, respectively. Figure 9 As shown in (d) of the diagram, the asymmetric hybrid supercapacitor has low internal resistance and a relatively fast electron transfer rate. Figure 9 As shown in (e), the H-NCS@NP-MXene / / AC hybrid supercapacitor exhibits excellent cycling stability, retaining 89.42% of its initial capacity after 10,000 cycles at a current density of 5 A / g. Further investigation into the electrochemical performance of the H-NCS@NP-MXene / / AC hybrid supercapacitor device was conducted, and its energy density and power density were analyzed. Figure 9 (f)). The results show that the device achieves an energy density of 50.36 Wh / kg at a power density of 700 W / kg, highlighting its excellent rate performance.

[0070] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material, characterized in that, It includes a nitrogen-phosphorus dual-doped MXene sheet and NiCo2S4 hollow nanospheres loaded in situ on the surface of the nitrogen-phosphorus dual-doped MXene sheet. The NiCo2S4 hollow nanospheres and the nitrogen-phosphorus dual-doped MXene sheet form a sheet-sphere composite structure.

2. The preparation method of the nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material as described in claim 1, characterized in that, Includes the following steps: MXene was mixed with diammonium hydrogen phosphate in water and then subjected to a hydrothermal reaction to obtain nitrogen-phosphorus dual-doped MXene. The nitrogen-phosphorus dual-doped MXene was mixed and reacted with cobalt salt and 2-methylimidazole in an organic solvent to obtain ZIF-67@NP-MXene composite material; The ZIF-67@NP-MXene composite material was mixed with nickel salt in an alcohol solvent and reacted to obtain the NiCo-LDH@NP-MXene composite material; The NiCo-LDH@NP-MXene composite material is obtained by mixing it with thioacetamide in an alcohol solvent and then carrying out a solvothermal vulcanization reaction.

3. The preparation method according to claim 2, characterized in that, The mass ratio of MXene to diammonium hydrogen phosphate is 1:(40~100); the hydrothermal reaction temperature is 120~180℃ and the time is 6~24 h.

4. The preparation method according to claim 2, characterized in that, The cobalt salt is at least one of cobalt nitrate, cobalt chloride, or cobalt sulfate, and the molar ratio of 2-methylimidazole to the cobalt salt is (4~12):

1.

5. The preparation method according to claim 2, characterized in that, The nickel salt is at least one of nickel nitrate, nickel chloride, or nickel sulfate, and the mass ratio of the ZIF-67@NP-MXene composite material to the nickel salt is 1:(0.8~1.2).

6. The preparation method according to claim 2, characterized in that, The temperature for the mixing reaction in the alcohol solvent is 40~80℃, and the time is 1~6 h.

7. The preparation method according to claim 2, characterized in that, The solvothermal vulcanization reaction is carried out at a temperature of 100-140°C for 4-12 hours.

8. The preparation method according to claim 2, characterized in that, The mass ratio of the NiCo-LDH@NP-MXene composite material to thioacetamide is 1:(0.8~1.2).

9. The preparation method according to claim 2, characterized in that, The organic solvent is methanol or ethanol, and the alcohol solvent is ethanol.

10. The application of the nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material as described in claim 1, or the nitrogen-phosphorus dual-doped MXene-supported NiCo2S4 nano-hollow sphere composite electrode material prepared by the preparation method according to any one of claims 2 to 9, in supercapacitors, characterized in that, The composite electrode material is used as the positive electrode material of the supercapacitor.

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