A method for constructing a hollow spherical MXene@Bi2S3 / MoS2 negative electrode material based on a lithium ion capacitor

Abstract

The application provides a hollow spherical MXene@Bi2S3 / MoS2 negative electrode material based on a lithium ion capacitor. The composite material is prepared by the following steps: 1. preparing PMMA microspheres as a template; 2. reacting Ti3AlC2 with a mixed solution of lithium fluoride (LiF) and concentrated hydrochloric acid (HCl), and then performing etching for a period of time, separating and purifying the product, and performing freeze-thaw treatment for multiple times to obtain layered MXene; 3. adding the MXene dispersion liquid into the PMMA emulsion to prepare PMMA core-shell microspheres coated with layered MXene through electrostatic adsorption; 4. introducing a bismuth source into the system to perform hydrothermal reaction, and preparing a PMMA@MXene@Bi2O3 precursor; 5. further introducing a molybdenum source and a sulfur source into the system to form a sulfide heterojunction of molybdenum and bismuth on the surface of the PMMA@MXene core-shell microspheres; and 6. removing the PMMA template through heat treatment to finally obtain the hollow spherical MXene@Bi2S3 / MoS2 composite material. The composite material is applied to a lithium ion capacitor negative electrode material, and has high conductivity, excellent rate performance and cycle stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion capacitor technology, specifically to a method for constructing a hollow spherical MXene@Bi2S3 / MoS2 composite material negative electrode. Background Technology

[0002] With the continuous depletion of traditional fossil fuels and the increasingly severe environmental problems, the development of renewable and clean energy has become an important development direction in the energy field, and research on high-performance electrochemical energy storage devices has therefore attracted much attention. In recent years, lithium-ion batteries (LIBs) have been applied in many fields due to their high energy density and supercapacitors (SCs) have high power density. Lithium-ion capacitors (LICs) are a new type of energy storage device that can simultaneously possess the advantages of both, aiming to achieve fast charge and discharge performance while ensuring high energy density. However, since LICs are usually composed of a battery-type negative electrode and a capacitor-type positive electrode, there are significant differences between the two in terms of energy storage mechanism and reaction kinetics. The slow Faraday reaction process of the negative electrode is difficult to match the fast ion adsorption and desorption behavior of the positive electrode, thus restricting the overall performance of the device. Therefore, constructing a negative electrode material that combines high-capacity lithium storage capacity with fast reaction kinetics is of great significance for improving the overall performance of LICs.

[0003] Bi₂S₃, a typical transition metal sulfide, possesses a high theoretical specific capacity and excellent reversible lithium-ion insertion / extraction characteristics, demonstrating promising application potential in the field of lithium-ion storage. However, Bi₂S₃ itself has low conductivity and exhibits significant volume expansion during charge and discharge, which can easily lead to structural damage and capacity decay. To overcome these problems, it is usually necessary to construct composite structures to improve its performance.

[0004] MoS2, as a typical layered sulfide material, has relatively weak van der Waals forces binding the layers together, which is beneficial for Li + Reversible interlayer insertion and extraction also presents promising applications for lithium storage materials. Single Bi₂S₃ materials inherently suffer from deficiencies in conductivity and structural stability. Introducing MoS₂, a material with complementary properties, to construct a heterostructure and achieve a multi-component synergistic composite structure can further enhance the electrochemical performance of the system.

[0005] Transition metal carbides and nitrides (MXenes) are a class of rapidly developing two-dimensional layered materials with high electrical conductivity, good hydrophilicity, and tunable surface chemistry, making them one of the most promising anode materials for lithium-ion capacitors. The introduction of MXenes into the system enables the Bi₂S₃ / MoS₂ heterojunction to achieve uniform distribution, significantly increasing the specific surface area of ​​the material. Simultaneously, MXenes provide abundant electrochemical reaction sites, offering a buffer space for volume changes and effectively mitigating structural stress during charge and discharge processes. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for preparing a multi-level structure for use in lithium-ion capacitor anode materials and its application. This invention overcomes the problems of significant volume expansion of Bi₂S₃ during charging and discharging, which easily leads to structural damage and low conductivity, demonstrating excellent structural stability and electrochemical performance.

[0007] To achieve the above objectives, the present invention provides a method for constructing an MXene@Bi2S3 / MoS2 composite anode structure, comprising: S1. Preparation of PMMA template: A certain amount of initiator and polyvinylpyrrolidone (PVP) were dissolved in methanol solvent, stirred until dissolved, and argon gas was introduced to remove dissolved oxygen in the solution. Then a certain amount of methyl methacrylate (MMA) monomer was added, and the mixture was heated and stirred in a water bath at 55-65℃ for 24 hours. After the reaction was completed, the mixture was centrifuged, washed, and freeze-dried to obtain a white powder. S2. Preparation of layered MXene: A certain amount of titanium aluminum carbide (Ti3AlC2) is added to the solution to obtain layered MXene; the mixed solution is a mixture of lithium fluoride (LiF) and concentrated hydrochloric acid (HCl); S3, Preparation of PMMA@MXene core-shell microspheres: A certain amount of PMMA was dispersed in methanol, and then a certain amount of MXene dispersion was added. The mixture was stirred to fully coat the PMMA surface with MXene nanosheets, and then centrifuged and washed to obtain the composite material PMMA@MXene. S4, Preparation of PMMA@MXene@Bi2O3 precursor: A certain amount of the product prepared in step S3 was dispersed in deionized water, and surfactant, bismuth source and glycine were added. After stirring and dissolving thoroughly, the mixture was transferred to a polytetrafluoroethylene reactor and hydrothermally heated at 140-160℃ for 20-24 hours. After the reaction was completed, the PMMA@MXene@Bi2O3 precursor was obtained by centrifugation and washing. Preparation of S5, PMMA@MXene@Bi2S3 / MoS2 composite material: A certain amount of the product prepared in step S4 was dispersed in deionized water, sulfur source and molybdenum source were added, and after thorough mixing, it was transferred to a polytetrafluoroethylene reactor and hydrothermally heated at 180-200℃ for 20-24 hours. After the reaction was completed, the product was centrifuged and washed to obtain PMMA@MXene@Bi2S3 / MoS2 composite material. S6. Preparation of hollow spherical MXene@Bi2S3 / MoS2 composite material: The product prepared in step S5 was heat-treated under N2 protection at a calcination temperature of 450℃ for 1 hour to obtain the final hollow spherical MXene@Bi2S3 / MoS2 composite material.

[0008] In step S1, the initiator is azobisisobutyronitrile (AIBN).

[0009] In step S2, the molar ratio of LiF to HCl in the mixed solution is 1:4, the etching reaction temperature is 25-35 ℃, and the reaction time is 12-24 hours.

[0010] In step S3, the mass ratio of layered MXene to PMMA template is 1:4 to 1:5, and the purpose of multiple centrifugation and washing is to remove uncoated MXene.

[0011] In step S4, the mass ratio of PMMA@MXene core-shell microspheres to bismuth source is 1:1 to 1:2. The bismuth source is one of bismuth nitrate (Bi(NO3)3·5H2O), bismuth chloride (BiCl3), or bismuth sulfate (Bi2(SO4)3). The surfactant used is hexadecyltrimethylammonium bromide (CTAB).

[0012] In step S5, the sulfur source is one of thiourea (CH4N2S), thioacetamide (TAA), sodium sulfide (Na2S), or L-cysteine, and the molybdenum source is anhydrous sodium molybdate (Na2MoO4) or ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O4). 24 It is one of ·4H2O and ammonium molybdate ((NH4)2MoO4).

[0013] In step S5, the mass ratio of the added molybdenum source to the bismuth source is 1:2 to 1:4.

[0014] The present invention also provides the application of the composite material, characterized in that the material is used as a negative electrode material for lithium-ion capacitors.

[0015] The positive and progressive effects of this invention are as follows: 1. Construction of a highly continuous electron transport network: The successful introduction of an MXene conductive framework between active materials significantly reduces the charge transfer energy barrier at the electrode-electrolyte interface, enhancing interfacial reaction kinetics. 2. Provision of abundant active sites: After sulfidation, the Bi2S3 / MoS2 heterostructure nanosheets uniformly cover the surface of the hollow microsphere structure, providing more active sites for the system, forming a built-in electric field, and accelerating interfacial charge migration. 3. Electrochemical reversibility and structural stability: The organic combination of ternary materials ensures the uniform dispersion of metal sulfides, effectively mitigating the volume expansion effect during cycling and accelerating the Li... + Embedding and de-embedding. Attached Figure Description

[0016] Figure 1 This is a scanning electron microscope image of the hollow spherical MXene@Bi2S3 / MoS2 composite material prepared in Example 3.

[0017] Figure 2 Impedance comparison diagram of the negative electrode material of the half cell in Example 4 before and after the introduction of MXene.

[0018] Figure 3 This is a comparison chart of the rate performance of half-cells under different molybdenum and bismuth source mass ratios in Example 4.

[0019] Figure 4 This is a comparison chart of the half-cell cycle performance under different molybdenum and bismuth source mass ratios in Example 4.

[0020] Figure 5 The graph shows the cycle life test results of the lithium-ion capacitor using MXene@Bi2S3 / MoS2 composite material as the negative electrode in Example 5.

[0021] Figure 6 This is a comparison graph of the lithium-ion capacitor using MXene@Bi2S3 / MoS2 composite material as the negative electrode in Example 5 and the Ragone curve reported in existing literature. Detailed Implementation

[0022] This invention provides a hollow spherical MXene@Bi2S3 / MoS2 composite material, its preparation method, and its application as a negative electrode material for lithium-ion capacitors. To make the objectives, technical methods, and implementation effects of this invention clearer, the invention will be further described in detail with reference to the accompanying drawings and embodiments.

[0023] Example 1 - Layered Ti3T2C x Preparation of MXene

[0024] The Ti3T2C used in this invention xMXene was prepared by a combination of fluorine salt etching and freeze-thaw processing. The specific steps are as follows: S1. Preparation of precursor: Lithium fluoride (LiF) is dissolved in 9 M hydrochloric acid (HCl) solution and stirred vigorously for 5-10 minutes to form a homogeneous solution. Then, titanium aluminum carbide (Ti3AlC2) powder is slowly added, wherein the molar ratio of lithium fluoride to titanium aluminum carbide is 1:7-1:8. The reaction is carried out under 25-35 °C for 24-48 hours with continuous stirring to achieve the etching process. S2. Separation and purification of the product: After the reaction is completed, the product is separated by centrifugation and washed multiple times until the pH of the supernatant is close to 6. The precipitate from the last centrifugation is retained and an appropriate amount of deionized water is added to prepare a suspension. S3, Freeze-thaw treatment: The suspension prepared in step S2 was subjected to four freeze-thaw cycles and centrifuged under a nitrogen atmosphere to obtain Ti3C2T. x MXene dispersion, dispersion concentration of 2-5 mg / mL -1 .

[0025] Example 2 - Preparation of PMMA@MXene core-shell microspheres Preparation of S1, PMMA template: 160 mg of azobisisobutyronitrile (AIBN) and 9.5 g of polyvinylpyrrolidone (PVP) were dissolved separately in 200 mL of methanol solution and stirred at room temperature until completely dissolved. Then, high-purity nitrogen gas (≥99.999%) was introduced to thoroughly remove dissolved oxygen from the solution. Next, 15.8 g of methyl methacrylate (MMA) monomer was added to the system. The mixed solution was transferred to 55-65 °C and reacted in a water bath with stirring for 24 hours. After stirring, the product was washed repeatedly by high-speed centrifugation using water as a solvent, followed by freeze-drying to obtain a white powder product. Preparation of S2 and PMMA@MXene composite materials: Weigh 220 mg of PMMA microspheres prepared in step S1, disperse them in 20 mL of methanol solution, and sonicate for 15-30 minutes to achieve uniform dispersion, obtaining a PMMA emulsion. Then, under continuous magnetic stirring, add 10 mL of Ti3C2T... x MXene dispersion (concentration 5 mg / mL) -1 The MXene nanosheets were slowly added dropwise to the prepared PMMA emulsion. After the addition was complete, the mixture was stirred and reacted at room temperature for 1-2 hours to allow the negatively charged MXene nanosheets to be fully coated on the surface of the PMMA microspheres through electrostatic adsorption. The mixture was then centrifuged, the precipitate was collected, and washed multiple times with deionized water to remove unbound MXene fragments.

[0026] Example 3 - Preparation of hollow spherical MXene@Bi2S3 / MoS2 composite material Preparation of S1 PMMA@MXene@Bi2O3 precursor: PMMA@MXene core-shell microspheres were added to deionized water and sonicated for 2-5 minutes to form a homogeneous dispersion. 100 mg of cetyltrimethylammonium bromide (CTAB) was added to the dispersion and stirred for 30 minutes. Subsequently, bismuth source and glycine were added, with a mass ratio of PMMA@MXene core-shell microspheres to bismuth source of 1:1 to 1:2. The components were thoroughly dissolved and uniformly mixed under magnetic stirring. The mixture was then transferred to a polytetrafluoroethylene-lined high-pressure reactor and subjected to a hydrothermal reaction at 140-160 °C for 20-24 hours in a constant-temperature oven. After the reaction, the mixture was repeatedly washed by high-speed centrifugation using deionized water and anhydrous ethanol as solvents, and then dried in a vacuum oven. The PMMA@MXene@Bi2O3 precursor composite material was obtained after drying. Preparation of S2 and PMMA@MXene@Bi2S3 / MoS2 composite materials: The precursor prepared in step S1 was added to 40 mL of deionized water and sonicated for 2-3 minutes to obtain a uniformly dispersed system. Then, an excess of sulfur source and a certain amount of molybdenum source were added, with the mass ratio of bismuth source to molybdenum source in the system being 1:2, 1:3, and 1:4. The components were reacted under magnetic stirring for 0.5-1 hour until homogeneous. The mixture was then transferred to a high-pressure reactor lined with polytetrafluoroethylene and subjected to a hydrothermal reaction at 180-200 °C for 20-24 hours in a constant temperature oven. After the reaction was completed, the mixture was repeatedly washed by high-speed centrifugation using deionized water and anhydrous ethanol as solvents, and then dried in a vacuum oven to obtain the PMMA@MXene@Bi2S3 / MoS2 composite material. Preparation of S3 and MXene@Bi2S3 / MoS2 composite materials: The sample obtained in step S2 was placed in a ceramic boat and heat-treated in a tube furnace under N2 protection. The temperature was raised to 450 °C and held for 1 hour to remove the PMMA template. After the furnace body cooled naturally, the product was removed, and the hollow spherical MXene@Bi2S3 / MoS2 composite material was finally obtained.

[0027] Example 4 - Assembly and testing of MXene@Bi2S3 / MoS2 half-cells, including the following steps:

[0028] The entire assembly process of the half-cell used for testing the electrochemical performance of the materials was completed in an argon-filled glove box (the glove box required that the water and oxygen levels be less than 0.1 ppm). The batteries used were CR2032 type batteries. The assembly followed the sequence of negative electrode shell, spring plate, gasket, lithium metal sheet, separator, electrode, and positive electrode shell, and was then sealed under appropriate pressure using a clasp sealing machine. After placing the lithium sheet and electrode, a few drops of electrolyte were added before proceeding to the next step. The electrolyte used in the battery was a 1 M LiPF6 solution, with the solvent being a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1.

[0029] Impedance testing, rate performance testing at different current densities, and long-cycle performance testing were conducted on the assembled half-cells.

[0030] Figure 2 The impedance of the half-cell before and after MXene composite is shown. The impedance of the composite ternary material is significantly reduced.

[0031] Figure 3 To illustrate the differences in rate performance of bismuth and molybdenum sources with varying current densities at different mass ratios, samples with bismuth and molybdenum source mass ratios of 1:2, 1:3, and 1:4 were designated as 1-2 MXene@Bi₂S₃ / MoS₂, 1-3 MXene@Bi₂S₃ / MoS₂, and 1-4 MXene@Bi₂S₃ / MoS₂, respectively. Compared to samples with mass ratios of 1:2 and 1:4, and the Bi₂S₃ / MoS₂ sample without MXene, the 1-3 MXene@Bi₂S₃ / MoS₂ sample exhibited superior rate performance.

[0032] Figure 4 For all test electrodes, at 0.2 A g -1 Long-cycle charge-discharge tests were conducted at a constant current density. The MXene@Bi2S3 / MoS2 sample (1-3) also exhibited the best cycle stability, with an initial capacity of approximately 794 mAh g⁻¹. -1 During the initial 30 or so charge-discharge cycles, the specific capacity showed a slight and slow decrease, followed by a gradual and slow recovery and stabilization. After 250 cycles, it still maintained approximately 785 mAh g⁻¹. -1 It has a high reversible specific capacity, a capacity retention rate of 98.8%, and a coulombic efficiency that remains stable at around 100%.

[0033] Example 5 - Assembly and testing of a lithium-ion capacitor using MXene@Bi2S3 / MoS2 composite material as the negative electrode, including the following steps:

[0034] Before assembling a full lithium-ion capacitor cell, the negative electrode material needs to undergo pre-lithiation treatment. The battery used for pre-lithiation is the CR2016 type, and its assembly process is basically the same as that of the CR2032 type battery, but without the addition of spring contacts and spacers. The assembled CR2016 type battery is then subjected to a 0.1 A g... -1 The current density was subjected to 10 charge-discharge cycles, followed by discharge to 0.01 V to complete the pre-lithiation process, and then the battery was moved to a glove box for disassembly to remove the negative electrode. The lithium-ion capacitor full cell is a CR2032 type battery, and the assembly sequence is as follows: negative electrode shell, spring, gasket, negative electrode, separator, electrode, positive electrode shell. After filling with sufficient electrolyte, it is sealed with a clasp sealing machine under appropriate pressure. The lithium-ion capacitor was assembled using MXene@Bi2S3 / MoS2 composite material as the negative electrode, commercial activated carbon (AC) as the positive electrode, and 1M LiPF6 organic solution as the electrolyte.

[0035] Figure 5 For the cycle life testing of the full cell, the assembled full cell was subjected to a 1 A g test. -1 Long-term cycling performance tests at current density were conducted. The results show that after 5500 charge-discharge cycles, it still maintains approximately 73.8% capacity retention, and the coulombic efficiency remains close to 100% throughout the entire cycle, reflecting its good reversibility and cycle stability.

[0036] Figure 6 This graph compares the results of our study with the Ragone curves reported in existing literature. The results show that the device achieves a good balance between energy density and power density, at 218.5 W kg. -1 Under these conditions, it can output 173.7 Wh kg. -1 The energy density, while when the power density is increased to 10.5 kW kg -1 At that time, it can still maintain 78.5 Wh kg -1 The high energy level indicates that the hollow spherical MXene@Bi2S3 / MoS2 composite material has broad application potential in LICs anodes.

[0037] The above description is only a preferred embodiment of the present invention. It should be noted that researchers in this field can make several improvements and modifications without departing from the technology of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for constructing a hollow spherical MXene@Bi2S3 / MoS2 multilevel structure as a negative electrode material based on lithium-ion capacitors, characterized in that, Includes the following steps: S1. A hard template is selected as a sacrificial template, and MXene is adsorbed by electrostatic adsorption to construct a core-shell structure. S2. Introduce a bismuth source onto the core-shell surface generated in step S1 and deposit an oxide layer as a precursor. S3. The product prepared in step S2 is further introduced into a sulfur source and a molybdenum source, and a material with a multi-level structure is prepared by sulfidation and annealing, namely hollow spherical MXene@Bi2S3 / MoS2 as the negative electrode material of lithium-ion capacitor.

2. The rigid template used in step S1 of claim 1 is polymethyl methacrylate (PMMA).

3. The bismuth source in step S2 of claim 1 is one of bismuth nitrate (Bi(NO3)3·5H2O), bismuth chloride (BiCl3), or bismuth sulfate (Bi2(SO4)3).

4. According to claim 1, the vulcanization in step S3 is completed by a hydrothermal reaction at a temperature of 180-200°C for 20-24 hours.

5. According to claim 1, the sulfur source in step S3 is one of thiourea (CH4N2S), thioacetamide (TAA), sodium sulfide (Na2S), and L-cysteine.

6. According to claim 1, in step S3, the molybdenum source is anhydrous sodium molybdate (Na2MoO4) or ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O4). 24 It is one of ·4H2O and ammonium molybdate ((NH4)2MoO4).

7. According to claim 1, the annealing temperature in step S3 is 450°C, and the holding time is 1 hour.

8. A lithium-ion capacitor, characterized in that, Including the MXene@Bi2S3 / MoS2 composite anode obtained by the preparation method according to any one of claims 1-7.

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